The standard Gemini in-tank electric pump runs at around 8 volts via a relay and resistor tucked under the dash.  The relay is only triggered when the alternator has an output voltage above a certain level, indicating that the engine is idling.  This is a safety feature that simply disables the fuel pump unless the engine is running.

Modifying Standard In-tank Electric Fuel Pump

When more fuel delivery is required, the standard fuel pump can be made to output more flow and at a higher pressure with some very simply modifications.  One obvious mod is to wire the pump from a 12 volt power source, increasing the pump speed and thus the amount of fuel flowed.  The pressure output is limited by an inline relief valve containing a spring and small rubber valve.  The force of this spring dictates the maximum pressure output of the pump, so if this spring is removed, stretched, and then put back into the housing, a higher pressure will be available.

The return line from the carburetor to the fuel tank should also be restricted to allow more fuel to be available at the carburetor when it needs it.  This restriction can be performed easily with a needle valve placed in-line with the fuel line, for precise adjustment.

EFI Fuel System

This is the fuel system we run in the boot of our gem.  Above the LHS wheel arch is a great location for a custom surge tank, its neat and out of the way, leaving the boot clear for other things.  Although purists might say it promotes a high centre of gravity of for the car, It only holds about 1 litre.  One modified standard pump could be used to supply a surge tank, or an aftermarket pump can be installed if preferred.  Keep in mind that once modified, the standard pump output is quite good, and it only has to supply through a short hose to the surge tank, not all the way under the car to the engine bay and the original carburetor, so restriction is minimal, especially if you install larger hoses.

If you are really keen, you can run two modified stock fuel pumps in parallel for extra flow.  I simply modified the bracket that mounts the stock pump inside the tank so both pumps feed a tee piece then pump fuel up to the outlet.  The 5/16 outlet has also been replaced with a 3/8 copper line.  These pumps feed the surge tank through the Ryco remote filter unit.

The two low pressure pumps in this configuration supply enough fuel for a calculated 450hp at the pressures measured in this set-up.  The VL turbo pump is also rated at around 350hp.  There are plenty of larger fuel pumps available that are similar physical size to the VL turbo EFI pump, but unless your making that sort of power, you won’t need a larger pump, and you’ll actually waste power as they draw more current and heat your fuel up.

As can be seen, the surge tank gravity feeds to the EFI pump which uses standard fuel lines running under the car.  All rubber sections of fuel line should be replaced with EFI hose and decent hose clamps when running EFI due to the higher fuel pressures.

A more simple alternative is to plumb the output of the standard fuel pump directly to the inlet of an EFI pump.  This works well, the only drawback being that if you have a very low fuel level in the main tank, you can get fuel slosh/flat spots/lean outs as the fuel pick up in the tank cannot suck fuel for a short period.  This is typically only a problem whilst circuit racing or cornering hard on the street, with a near empty tank.

The other thing to keep in mind is the importance of running a filter between the low pressure pump and EFI pump, regardless of whether you’re using a surge tank.  You don’t want dirt or other debris getting into the EFI pump, junk in the pump can wear them out pretty quickly, and reduce performance as well.

Blow Through Fuel System

The fuel system for a blow through turbo set-up is simple, but due to the requirements, some people find it confusing.  Its virtually identical to the standard carburetor supply system, except for the one golden rule.  The fuel pressure must remain higher than the boost pressure at all times.  

The reason this is important is simple.  In a blow through carby situation, the air in the fuel bowel of the carby is connected to the air passing through the carby, so if the air pressure is above atmospheric (ie. boost), then the boost pressure in the fuel bowel will push the fuel back through the fuel line and back towards the fuel tank.  As soon as the fuel bowel is empty, the engine will cut-out.  If the boost pressure is higher than fuel pressure, fuel will continue to be pumped into the carby just as normal.

This can be achieved via a number of ways.  The easiest and superior way is to use a fuel pressure regulator that is boost referenced, so that when boost is sensed above the diaphragm, the fuel pressure is increased.  There are many regulators off the shelf that will work with carby fuel pressures, Malpassi is a typical example.

An EFI set-up is usually fine to be boosted because stock EFI fuel pressure regulators are almost always connected to the inlet manifold with a vacuum line.  This vacuum line will also provide a boost signal to the top of the regulator diaphragm and increase fuel pressure when required.

Another way to keep fuel pressure above boost pressure is to switch voltages on the fuel pump, and tune the system pressure by restricting the fuel return line back to the tank.  I’ve used both, but the pressure regulator works much better.

If using an EFI pump for a blow through carburetor set-up, upgraded high pressure rubber sections of the factory fuel lines may not be necessary depending on the fuel pressure involved.

There are various different configurations one can employ to supply an engine with compressed air, and the matching amount of fuel to that air.  These configurations can be divided into two main groups:

1. Fuel is delivered before the compressor (draw through).
2. Fuel is delivered after the compressor (blow through).

Regardless of how the fuel is delivered, whether it be fuel injection, via a carburetor, or from a LPG mixer, the fuel has to enter the airflow in one of the two ways listed above.

When the fuel is delivered before the compressor, such as a carburetor mounted to the front of the turbo, it is said to be in a draw through configuration, as the air/fuel mixture is being drawn through the turbocharger.

When the fuel delivery system is mounted in between the engine, and the turbocharger, the configuration is said to be a blow through, as the turbo is blowing through the fuel delivery system.  Although blow through usually refers to a carbureted fuel delivery system, technically any fuel delivery system can be in a blow through configuration.

There are pros and cons of each configuration.  Typically, and draw through is a simpler proposition in the fuel delivery side of things, as the original fuel delivery device can be moved from the factory fitted position to in front of the selected turbo compressor inlet.  Plumbing to connect the compressor outlet to the inlet manifold must then be constructed.  A draw through configuration is shown in the diagram below.

The benefits of the draw through configuration are primarily the ease of implementing fuel delivery system and the inherent charge cooling from the vaporization of the fuel.

A blow through configuration on the other hand is more complicated from a fuel delivery viewpoint, as boost pressure present within the system will try and force the fuel back through the metering device(s).  Typically, the pressure of the fuel is raised so that a constant level of fuel pressure is used above the level of boost pressure.  This is done with a boost reference fuel pressure regulator.

The benefits of a blow through configuration are many, including the option of placing an intercooler between the compressor and the fuel delivery device to increase air density and reduce temperatures.  The other main benefit is the ability to retain the factory fuel delivery device (EFI or carby) in its original location, giving factory like throttle response and other characteristics.

Carby Mods

The page details the installation of a Weber DGV downdraught carburetor onto the standard inlet manifold of a Holden Gemini, models TX-TG.  There are many different carbys that have been bolted to gem engines over the years, but this has to be one of the easiest conversions around.

The standard Nikki carburetor doesn’t do a bad job in air flow capabilities to allow good torque and fuel economy on a standard Gemini G161 1574cc engine.  But if you’re chasing maximum power, or particularly when you make any sort of exhaust or other engine modification, the Nikki will restrict the engine’s airflow.

The Nikki’s chokes (or venturis) are slightly smaller in size to the Weber’s, as are the butterflies.  This leads to an air restriction as air flow increases.  The small cross sectional area that the air must pass through increases air velocity to a point where frictional losses through the carburetor become unacceptable.  These friction losses effectively limit the amount of air that can pass through the carburetor at a given pressure drop.

The Nikki has a 30mm primary butterfly, with a 22mm diameter primary choke.  The secondary system consists of a 34mm butterfly, with a 29mm choke.  The Weber chokes are slightly larger overall, being 26mm and 27mm for the primary and secondary circuit respectively.  The Weber’s butterflies, at 32mm and 36mm for the primary and secondary throttles, are also larger than the standard Nikki.  This allows for lower air velocities at the same magnitude of airflow, so frictional losses are not as great.

One of the greatest benefits from the use of the Weber carburetor over the Nikki is the greater throttle response offered by the Weber’s mechanically operated secondary system, whereas the Nikki’s secondary system relies on a certain level of vacuum measured at a point below the carburetor.  This means that the Nikki has to wait for a rise in manifold vacuum before the secondary throttle can open, whereas the Weber’s secondary throttle plate is opened as soon as the driver wants.  Because of these differences, the Weber tends to give better throttle response if tuned correctly, whereas the Nikki should be a bit better on fuel usage when flooring the throttle.

It should be noted that there are many Weber and some Holley carburetors that share the same stud pattern and that can be installed on the standard inlet manifold with a suitable adaptor plate.  On one of my engines, I am using a Weber 38 DGMS that is very similar to the 32/36 DGV, except that the butterflies are both 38mm diameter, and they open synchronously, i.e. both at the same time.  Other examples include a 32/32mm downdraught as found factory equipped to Alfa Romeo Suds, and the Weber 34 ADM as found factory fitted to 3.3 and 4.1 litre Ford Falcons, from XD to XF model.

The results from this carburetor swap are obviously an increase in airflow capability and more noticeably throttle response over the standard Nikki, so more power, acceleration and efficiency can be had from the same engine.

The installation of a larger carburetor is usually associated with a loss of throttle response due to the decrease in air velocity at lower airflows (lower engine speed), giving a boggy feel when flooring the accelerator.  From experience with this carby on a mildly worked 1600, I can tell you that the Nikki carby had no where near as much throttle response as what the Weber delivers.  This would suggest that the Weber is not too large for the engine.  The Weber also offers much more in the area of tunability than the Nikki, as new parts are still available, although expensive.  Most of the tuning components can be modified very cheaply though.  This means the Weber is a better alternative if further modifications to your engine are desired.

This overall is a fairly simple carburetor swap, with the adaptor plates available off the shelf at stores such as AutoPro, Bursons, Autobarn, etc.  The reason for needing an adaptor plate is obvious after examining the picture of the two base gaskets below, the Weber being on the left and the Nikki gasket to the right.

As can be seen, the mounting holes for Weber are spaced further apart on the longitudinal axis than the Nikki holes, so an adaptor plate is needed to make up the difference.  The adaptor plate bolts to the manifold where the Nikki would normally bolt onto, whilst the Weber bolts onto the adaptor plate.

The only problems encountered with this carby swap is the tendency for the Weber DGV to be jetted slightly too rich for a standard 1600.  This is easily rectified, and a car equipped with an incorrectly jetted Weber will usually still be very drivable, however fuel economy will be worse.

The standard Nikki carburetor is simply too small for maximum engine output and efficiency.  The Nikki can be modified to improved its performance, but compared to the cost of a second hand or even a new Weber, modifying the Nikki through a carby workshop is expensive.  And at the end of the day, the Nikki is harder to tune, and parts aren’t readily available for them.

Putting aside its physical dimensional inadequacies, the Nikki is still a reasonably sophisticated carburetor with several features that even the replacement Weber does not have.  The Weber however, has a better functioning fuel control circuit as discussed on the New Setup page.

The Nikki’s list of features include a fuel shutoff solenoid, commonalty called a dieseling control solenoid that stops engine run on after the ignition is switched off, which can occur if enough fuel is allowed to be drawn into the engine during idle conditions.  This solenoid stops fuel flow to the idle circuit.

The Nikki also features a diaphragm operated vacuum secondary throttle butterfly.  This system will open the secondary throttle butterfly of the Nikki when a certain amount of engine vacuum is created and when a certain amount of throttle is applied through the accelerator pedal.  Therefore, only the amount of air required by the engine will be supplied.  This is a very good system for fuel efficiency, but as a certain level of vacuum is required in the inlet manifold for the secondary butterfly to commence opening, the engine can not produce maximum power as it cannot achieve its maximum amount of volumetric efficiency at that rpm.

The Weber on the other hand has a mechanically operated secondary butterfly which begins to open after about 3/4 of throttle applied.  The mechanically operated secondary system is a very noticeable improvement to throttle response, at the expense of fuel economy, but this is also dependant on driving style.

The Nikki also utilises an electric choke system that operates automatically using the temperature rise of a bimetallic strip to represent the temperature rise of the engine coolant.  The Weber DGV has a cable operated mechanical choke, whereas the DGAV variant has an automatic ‘aqua’ choke, which uses the engine coolant itself to control the choke system,  The DGEV uses an electric  choke that works in the same way as the Nikki carby.

When installing a Weber downdraught DGEV or DGAV onto the standard Gemini inlet manifold on TC-TG models, the choke housing hits the brake fluid reservoir on top of the maser cylinder, so its usually easier to remove the choke mechanism, or run a DGV mechanical choke carby.  On TX models, the fluid reservoir is remotely mounted, so this may not be an issue.  This is further explained within the New Setup section.

The coasting enricher solenoid is another feature that I am led to believe leans the fuel mixture at part throttle in top gear in a manual gearbox equipped Gemini.  This is an economy feature.

To remove the carburetor, you must first remove the air cleaner, then disconnect the fuel lines and accelerator cable. You can now best reach the nuts that secure the base of the carburetor to the studs within the inlet manifold.  After removing the nuts, the carburetor will slide off the studs.  The insulating spacer plate can now be removed from the manifold.  It should be noted that a gasket resides on both sides of the insulating plate.  The plenum (inlet volume for the individual runners) of the manifold should look like this:

If you are after the best from your new carburetor installation, you should modify the plenum area of the inlet manifold for better air flow capability and better mixture distribution.  For details on how to achieve this, go to the Modified Manifold page.

Compared to the standard Nikki, the Weber has a better functioning fuel control circuit, consisting of removable and therefore tunable main jets, air corrector jets, idle/progression jets, emulsion tubes, auxiliary venturis and accelerator pump discharge nozzles.  The fuel level is controlled by bending the float arm in combination with the size of the fuel inlet needle and seat.

Assuming you have already purchased a DGV, you must now make sure it is in a serviceable condition.  If you do not know if the carburetor has been recently running, or if you think that it is generally in a worn out/poor condition, then it is advisable to purchase a carburetor recondition kit, as that sold by Fuel Miser.  These kits are only around the $35 mark, and include all the gaskets necessary, a new accelerator pump diaphragm, a new needle and seat, and several miscellaneous copper washers and componentry.

If the throttle shaft bushes are worn considerably, then it is advisable to take the carburetor to a repair shop, and have the bushes replaced.  A worn throttle shaft bush will result in air being drawn through the throttle shaft, giving a leaner mixture.  In a turbo application where the carby is used in a blow through configuration, then the air/fuel mix will be blown out of the throttle shaft area, not really causing any damage, more of an inconvenience, making the engine and inlet manifold dirty.

The stripping of this carburetor and replacing the componentry with the new items is simplicity in itself.  The top section of the carburetor is removed by removing the six securing screws, and then lifting the top off.  This exposes the fuel bowl, the main and air corrector jets will be visible, as will the power valve.  See picture below for detail.

1. main jets

2. air corrector jets

3. accelerator pump discharge nozzle

4. power valve

5. auxiliary (booster) venturi

6. main venturi (choke)

7. power valve actuator

8. float

9. location of needle and seat assembly (hidden)

10. fuel inlet

11. accelerator pump

12. idle jet holder

13. idle circuit air feed

14. progression circuit air feed

15. progression jet holder

It should be noted what size jets are in what location as they are removed, as the primary and secondary circuits have different jet sizes.  The air corrector jets can be removed exposing the emulsion tubes.  The emulsion tubes can be removed with a length of wire with a very small hook on the end.  The main jets and power valve can be removed.  The idle/progression jets are secured within brass jet holders and are screwed into the sides of the carburetor.  These can be removed.  The booster venturis can also be removed along with the accelerator pump and discharge nozzle.

A can of carby cleaner should be used to blow through every single air or fuel passage that can be found.  A air compressor should then be used to blow through the passages.  This should remove any blockages or sediment buildups.  The carburetor can now be reassembled paying particular attention to the location of the jets.

The top section removed from the carburetor holds the float and the needle and seat.  It should be noted that the replacement fuel inlet seat is not cross drilled like the original Weber item.  If installing this carburetor on an engine with a power rating of around 95 flywheel horsepower or greater, then it is recommended that the new needle is used in conjunction with the original Weber (cross drilled) seat, as the Weber seat will allow much greater fuel flow into the fuel bowl.  I have experienced occasions where a 9.3:1 compression 1600 with mild cam, port work and extractors has emptied the fuel bowl by 5000rpm in third gear from a standing start with the Fuel Miser fuel inlet seat.  Installation of the original Weber item cured the problem.  This was even after upgrading fuel pump voltage from 8 volts (standard) to full 12 volts.

The choke system on a properly tuned Weber is rarely needed, and the removal of the choke blades will also increase the airflow capability of the carburetor.  At worst, in a cold start situation, the accelerator pedal may need one pump to get enough fuel into the manifold to allow initial combustion.  As the engine is cold, it will idle at a lower engine speed until normal operating temperatures are achieved.  It may be necessary to hold the engine at a fast idle speed (1800 rpm) for 10-20 seconds until the carburetor idle circuit will sustain combustion.  The picture below shows a Weber with the choke blades and shaft removed and the linkage access hole sealed.  This can be done with silicone (as in this case) or another material such as body filler.

After you have removed the standard carby, and hopefully ported the inlet manifold as described on the Modified Manifold page, the adaptor plate can be ported to match the butterfly location of the carby.  The gasket that seals between the carby and adaptor plate should also be trimmed to match the ported adaptor plate.

The plate can now be bolted into place as specified in the instructions that are supplied with the adaptor plate.  Attention should be paid to the gasket between the adaptor plate and manifold so that there are no restrictions to the airflow.  Also note how the inlet manifold has been ported to try and coax the flow to enter the inlet runner for cylinder 4.  The same has been done to the runner for cylinder 1, as these two cylinders tend to run leaner during operation due to their extra length and integrated bend.

Sometimes you may want to put the adaptor plate on the other way around.  This might depend on the brand of adaptor plate you purchase.  Always keep in mind, the idea is to have a smooth transition from the carby butterflies to the inlet manifold.

For ease of installation, the standard accelerator cable can be retained through the use of the standard linkage system.  The disk shaped linkage as seen in the following picture should be removed from the Nikki by undoing the specified nut, and bolted to the Weber carby.

The throttle bracket that originally secured the accelerator cable adjusting section from the Nikki should also be removed.  This bracket can be cut, drilled and bent so that it can screw to the Weber, using the threaded holes where the choke mechanism was located.  The bracket along with the throttle disk linkage is shown in the photos below secured to the weber.

Once the bracket has been made and attached to the Weber along with the disk shaped linkage, the Weber can be bolted to the adaptor plate using the nuts and star washers that came with the adaptor plate.  Its a downhill run from here, slotting the accelerator cable in the slot on the disk, and using the bracket to set the adjustment.  Most Weber DGV’s incorporate a fuel return line, just like the standard Nikki so hooking up the fuel lines is identical to how you removed them from the Nikki.

In the instance of obtaining a Weber with only a fuel inlet, you can either block the return line off completely, use a brass tee-piece from after the fuel filter, one line going to the carby and one to the return line, or another option is buying a fuel filter with twin outlets such as Ryco part no. 9702??.

As always, your fuel mixtures should be analysed by a dyno workshop with a wide band oxygen sensor.  This is relatively cheap to the cost of either wasting fuel and power with a rich mixture, or wasting power and blowing  motors with a lean mixture.  Expect to pay around $50-$150 to get the carby checked, with possibly a change in jet sizes.  The benefit of this is that you can also get a horsepower readout at the same time.

Most likely, the jetting will be slightly rich for a 1600, as the 32/36 DGV is factory fitment on 2 litre engines.  This can either be corrected by smaller main jets, or slightly larger air corrector jets.  Smaller main jets is the better alternative, but they will have to be soldered up and re-drilled to the desired size.

If you want to get the best results from your engine and new carburettor combination, the standard inlet manifold must be modified.  The standard entrance to the plenum volume is designed specifically for the butterflies of the Nikki carburettor, and not for the use of a Weber on an adaptor plate.  The picture below shows the two cut-outs in the manifold to suit the size of the Nikki’s butterflies.  The Weber’s butterflies are larger, and spaced further apart.

If you choose to use the Weber on an original unmodified inlet manifold, then as the air/fuel mix exits the butterflies, it will hit the flat surface around where the holes are.  The following diagram shows the gasket for the Weber base, with the locations of the holes in the Gemini manifold superimposed on top of them.

To overcome this problem, the inlet manifold should be modified to suit the ‘outlet’ of the adaptor plate.  The following diagram shows what metal should be removed in red.

The gasket can be used to scribe the area that should be removed.  The tools you will require are a die-grinder or electric drill with the appropriate metal grinding attachment as shown in the pic below.

The entire area could simply be removed with the grinding bit, but this would be a very laborious process.  It is much simpler to place to cuts in the following positions with an angle grinder and 4 inch cutting blade.

The following photo shows how the manifold can look with the adaptor plate bolted on.  Note:  The adaptor plate has also been ported significantly to improve transitional flow between the carburettor and inlet manifold.

Another small modification should be made to the adaptor plate so that it can clear the vacuum lines going from the manifold.  The following photo shows is taken from the cylinder head side and shows where a small notch should be ground away to clear the hose.

An intercooler cools the inlet air after it’s been compressed by a turbo or supercharger.  This has two benefits.

1. The cooler air exiting the intercooler is less likely to induce engine destroying detonation.
2. The cooler air is denser than warmer air, meaning there is more oxygen for a given volume.  If the correct amount of fuel is matched to this oxygen rich air, then more power can be developed.

There are several intercoolers from factory cars that can be made to fit in a ‘front mount’ position on a Gemini, such as Toyota Supra and Mazda RX7 units.  The Mazda RX7 unit is quite good for moderate power applications and is one that I have installed before as shown below.  As it is originally designed as a top mount intercooler, the air outlet must be cut off, and a new outlet welded to the side.

There needs to be some metal cut from the radiator support panel to allow the installation of this intercooler.  The metal removed from the nose cone is more cosmetic than structural, and the cuts are covered up when refitting the front bumper bar and grill.

It is important that the metal you remove does not structurally weaken the radiator support panel, as the panel also serves to help stiffen the front structure of the car and keep the chassis rails in their correct location.  This means the suspension will work optimally as well, and the car wont flex itself to death.

It took a while to make sure it looks neat.  The metal I chose to remove serves two purposes.  The main one was to allow the physical fitment of the unit, whilst keeping the radiator in the factory position.  The second is to increase air flow through the cores of both the intercooler and radiator, which is why I took some off the front face of the nose cone.  I also moved the number plate up so its on the front of the bumper bar, instead of hanging underneath it blocking flow.

The actual metal that will need to be removed with the particular intercooler you want to install will be different to what I have removed, but these are the general cuts that can be made.

The plumbing to and from the intercooler also has to go somewhere.  On an intercooler such as the RX7 unit, where it is modified to have the inlet and outlet on opposite sides, there are small squarish holes in the sheet metal next to the headlights that can be opened up further.  This one is larger than needed for the rubber bend alone as it feeds air to my air filter which sits behind the headlight.

There are various ways that the intercooler can be hooked up to your induction set-up, below are some photos of various combinations I’ve had with the same intercooler in the same location.  The first is for a blow through carburetor set-up.

The photo below shows final plumbing from the intercooler to the throttle body made from 2.5″ exhaust pipe using mandrel bends.  I would have preferred aluminium, but aluminium mandrel bends were pricey and hard to get hold of at the time.

In an attempt to increase the efficiency of my intercooler install, I painted the cooler black and added some air guides.  Black is the best colour for heat transfer, however I’m not sure if the coating of paint acts as insulation and this cancels out any benefits of the colour.  The air guides are to increase the pressure immediately in front of the core, forcing more air through and increasing the cooling effect.  You may notice that the guides are pointing inwards, opposite to a funnel.  This is intentional, as a higher pressure level is achievable this way.

To get the maximum cylinder pressures occurring at the right time, it is critical that the ignition of the air/fuel mixture by the spark plug occurs at the right time.  To get a more detailed view on why this is critical, you can read this ignition timing background information.

I will talk about four different things;

– static timing

– mechanical advance

– vacuum advance

– total timing

The Gemini is usually set with a base timing figure of 6deg BTDC.  This base timing figure is called the static timing.  This figure was chosen by the Isuzu engineers for low emissions, however the engine will perform better if this value is increased (advanced).  An old theory that works pretty well is that when idling and you advance the ignition timing, if the engine speed increases, then the engine likes the extra timing and is more efficient at that setting.

If there was no such thing as ignition advance, then the timing would be static all the time.  However the Gemini distributor houses a mechanical advance mechanism that uses centrifugal weights and springs to move the ignition points so that it advances the timing as the revs increase.  This is to make the spark happen sooner as the revs increase, because there is less time for the mixture to burn, so it must happen sooner to get peak cylinder pressures at the right time.  By around 3500rpm, the timing has increased from 6deg BTDC to about 30deg BTDC.  This means there is 24deg of mechanical advance being applied.  It looks a bit like the following graph, but I am going on my sketchy memory, so the RPM points may not be accurate.

As load is increased, the ignition timing needs to be retarded and vice versa.  The Gemini uses a vacuum advance diaphragm which references inlet manifold pressure.  Under light loads, a vacuum is present in the manifold which causes the diaphragm to advance the ignition timing by spinning the breaker points base plate.  As the load increases, the vacuum drops and the pressure increases to atmospheric pressure, where the diaphragm stops advancing the timing.

The vacuum advance diaphragm is referenced from a pressure source above the butterflies in the carburetor.  This is called ported manifold vacuum and it means the only time a vacuum signal is present is when the throttle is lightly to moderately applied.  This was done for emission reasons again, and a smoother idle and better throttle response can be obtained by connecting the vacuum advance directly to a manifold vacuum source.  This means that at idle, you will now get more ignition advance, and you will probably notice that the revs increase, which means the engine likes it!  So you will have to reset your idle speed with the carburetor adjustments.

Total timing is the amount of ignition timing advance that the engine needs to make maximum torque.  This is a sum of the static timing and the mechanical advance.  Keep in mind that when aiming for maximum torque, your throttle will be at Wide Open Throttle (WOT) and there will be no vacuum present (hopefully), so the vacuum advance does not play a part.

A stock Gemini G161 will be running around 30deg total advance, consisting of 6deg static and 24deg mechanical advance.  This may be good for a stock Gemini engine, but if you alter the compression ratio, increase the amount of air flowing into and out of the engine (bigger cam, extractors, etc), or turbo the engine for example, the amount of total timing required will change.

Usually, anything that increases cylinder pressures will mean less total timing is required, and vice versa.  So a compression ratio increase would require less timing overall (total timing), but the engine might like more static timing, and if you set static timing higher, you will get more total timing, right?  That is right, unless you modify the mechanical advance unit.

If you get this far, you have disassembled your distributor and you will find the mechanical advance unit.  You will see the unit can advance until the pin hits the end of the slot.  The size or length of the slot determines the amount of mechanical advance you will get.  The size of the weights and springs determine when the advance will occur, ie. at what RPM.  This mechanism spins the base plate that the points attach to, since it is when the points open that actually determines the ignition timing.

By making the slots smaller, you will reduce the amount of mechanical advance the distributor supplies.  For example, if your target is to achieve 28deg of total timing, and you want to set your static timing to 16deg, then you need to make the slot smaller so that 12deg of advance is delivered (16deg static + 12deg mechanical = 28deg total).

If you want the distributor to advance sooner or later than it currently does, then you need to change the springs, which is something I have not played with before, and I suggest you contact an ignition specialist as they will have to equipment to select the springs correctly for your application, and measure how your distributor is behaving.

The ignition curve would now something like the following.

You should get more performance throughout the rev range with a custom ignition curve.  You will get more torque at low rpm, and you will have optimum torque at higher rpm which suits your engine combo more ideally

The torque of an engine is the turning force generated by the burning of an air/fuel mixture in the cylinders.  This burning creates high pressures in the cylinders, the burning mixture expands and fills the cylinder and continues to expand as it forces the piston down in its bore.  The force on the piston acts throughout most of the stroke of the piston as it travels down, subsequently spinning the crankshaft.

Maximum torque/horsepower is generated in the engine by making sure that peak cylinder pressures are generated at the right time.  Too soon, and the cylinder pressures want to force the piston down in its cylinder before it reaches the top of its stroke.  This is obviously bad for torque as the piston tries to make the crank spin in the wrong direction!  This causes a form of engine knock which is destructive and can damage engine internals.  This is similar to detonation which is caused by the uncontrolled burn of the air/fuel mix.

If peak cylinder pressures are reached too late, then the cylinder pressure acts down on the piston as it’s already traveling down.  The pressure quickly drops because the volume of the combustion chamber is rapidly increasing, therefore the cylinder pressure has a reduced affect on torque and therefore much of the pressure is wasted.

To get the maximum cylinder pressures occurring at the right time, it is critical that the ignition of the air/fuel mixture by the spark plug occurs at the right time.  This is determined by the ignition timing.  Ignition timing is referenced from the top of piston # 1’s stroke, which is called Top Dead Center (TDC).  The crankshaft spins 360degrees to perform one revolution, and typical ignition timing will be set to operate in the range of 5 to 35 degrees before TDC is reached.  Before TDC is abbreviated to BTDC.  The terms “advanced” and “retarded” ignition timing refer to making the spark happen sooner or later respectively.  If the ignition is altered from 10deg BTDC to 25deg BTDC, then it is said to be advanced 15deg because spark will occur 15deg sooner.

An engine will produce optimum torque and power for a given engine speed and load at a specific ignition timing setting.  Advancing the ignition timing past this level usually leads to detonation and engine knock, and retarding the timing leads to a reduction in torque.  In general, assuming all other parameters are correct like air/fuel mixture, an engine will make maximum power by advancing the ignition timing to the point just before detonation.

The ignition timing required for maximum cylinder pressures to be delivered depends on several factors.  The main ones we can cater for are engine speed, engine load, air temperature, and water temperature.

Only a computer controlled ignition system can offer some reasonable level of control of the ignition timing with respect to air and water temperature.  In general, an engine will perform optimally at a particular water temperature.  This may be 90deg C for example.  If the water temperature starts to climb for whatever reason (broken water line causing loss of water, etc) then you can alter timing accordingly.  Increased engine temps increase the likelihood of detonation, which can be countered by retarded ignition timing, so a computer can control this variable.  Retarded timing is also useful for increasing the amount of heat that is absorbed by the cooling system, so retarded timing can be used to reduce engine warm up times.

An increase in air temperature will also increase the likelihood of detonation, so you may want to retard timing at high air temps.  Low air temps may mean that you can advance the timing more to make more torque and power, because the detonation threshold has been increased due to the cooler air.

Engine management computers can control ignition timing with respect to engine speed and load, but this can also be done successfully by mechanical means such as with a distributor.  The Gemini obviously uses a distributor to control the ignition timing and to ‘distribute the spark to each spark plug from the ignition coil, hence the name.

As the rate of the burn of the air/fuel mixture is fairly constant, the spark must occur sooner and sooner as the engine’s speed increases (when the revs increase), so that maximum cylinder pressure happens at the right time.  If the spark didn’t happen sooner, peak cylinder pressures would occur as the piston is well on its way down the cylinder, and as described above, this leads to a reduction in engine torque.  This is why we need to advance the ignition timing as engine speed increases.  As an example, the Gemini is usually set with a base timing figure of 6deg BTDC.  This base timing figure is called the static timing.  The Gemini distributor houses a mechanical advance mechanism that uses centrifugal weights to moves the ignition points so that it advances the timing as the revs increase.  By around 3500rpm, the timing has increased from 6deg BTDC to about 30deg BTDC.

Engine load is the last variable we can tackle, computers can deal with this by measuring the engine load by either reading the inlet manifold pressure, or by measuring the amount of air that is flowing into the engine.  Either method can give accurate data on the load of the engine.  Typically, as load is increased, the ignition timing needs to be retarded and vice versa.  This is because at low loads, the mass of air/fuel drawn into the cylinder is less than at high loads.  The lesser amount of air/fuel mix takes longer to burn as the molecules of the mixture are spread further apart in the volume of the combustion chamber, so the burning of the mixture takes longer.  Because it takes longer, the spark must occur sooner so that peak pressures happen at the right time.

The mechanical method to achieve this is with a diaphragm which references inlet manifold pressure.  Under light loads, a vacuum is present in the manifold which causes the diaphragm to advance the ignition timing.  As the load increases, the vacuum drops and the pressure increases to atmospheric pressure, where the diaphragm stops advancing the timing.  This can be taken one step further in turbocharged cars where the boost pressure in the inlet manifold acts on a diaphragm to further retard the ignition timing in an attempt to limit maximum cylinder pressures and to cater for the very densely packed (boosted) air/fuel mixture that will burn relatively fast.