Grounding circuit, HEI ignition module to a dirt bike.

Thread Starter


Joined Feb 10, 2018
Good evening.
I have been working on adapting a General Motors HEI ignition module to a dirt bike.

Preliminary testing has been successful. Nice hot blue spark, runs well.
Only one more hurdle to overcome.

A two-stroke dirt bike needs to retard the ignition timing in the higher RPM range.

The GM module does have the pin for grounding (Pin E, circled in blue), which will retard the timing 10 degrees. Normally the vehicle's computer (ECM) will do the grounding when needed.

My question is this: is there some recommended circuit I could build that will ground Pin E at a certain pick-up coil speed or voltage? I'm thinking maybe a transistor circuit that sends Pin E to the ground at a certain point, maybe? Up to this point, Pin E is simply open. If I can develop a circuit to ground and open Pin E at certain points, this project will be deemed a success. Any help would be greatly appreciated. Thanks
GH HEI with EST.jpg


Joined Nov 6, 2012
Abruptly Retarding the Timing is probably not what You want,
that could make riding the Bike a real disappointment.

You need to find the exact Curve that the Factory used on that particular Model of Bike,
and post it here.
The Factory knows what they are doing.

Generally, the Timing will start-out around ~10-ish degrees for
nice stable Idle-Manners, and easy starting,
then, starting from around ~1500 RPM,
it will immediately jump up to around ~20-ish degrees by ~2000 RPM,
and then remain there until around 4500 RPM,
at which point it will then smoothly start reducing Advance,
by around ~10 degrees by the time the RPM gets up to ~8 or ~9000 RPM.

With your setup You could handle the Cranking/Idle Retard,
but you wouldn't be able to "smoothly" Retard Timing as the RPM increases.

I have no idea how your Bike might respond to full Advance at high RPM.
It might be a screamer, or it might get into heavy Detonation and destroy your Engine.

One solution to this would be to run a "Zeel-Computer" on your Bike,
but they're over ~$300.oo,
and I don't know if you have the Cash to layout on such a purchase.
They are extremely slick though, You get to make your own Timing-Map,
and it will control Electric-Exhaust-Power-Valves, and Electric-Carb-Enrichment, etc..


Joined Jul 29, 2018
I'm not familiar with the GM module, but it looks like you could add an external (micro) processor to monitor the R-pin reference signal, create the proper curve-derived timing into the E-pin, and enable the E-pin (when all is well with the E-pin signal) by controlling the B-pin. Processor requirements are not particularly demanding, but it best have a really good timer module and fast interrupts.


Joined Oct 6, 2013
I didn't notice the first time I looked at this, but you need the 5 pin module to get the spark retard. The 'E' on the 7 and 8 pin modules is a square wave from the ECU to turn on and off the coil. When the 'B' pin is at 0 Volts the spark timing is derived from the distributor pickup only and the 'E' terminal is ignored. When 'B' is at 5 Volts the pickup is ignored (other than the reference signal to the ECU) and the timing is controlled by the 'E' pin. You will get a signal on 'R' (generated from the pickup) no matter if 'B' is powered or not.

I couldn't find the information, but I do know the 5 pin module has a pin that will retard the timing for easier starting.


Joined Oct 6, 2013
@shortbus thanks. Several years ago I had an old Ford that either had good low end power and no top end, or spark knock at low end and screaming at top end. No matter what I did with the distributor weights and springs I could not get the curve right and so I decided to make my own controller. It worked beautiful on my table, but once installed there was some electrical noise I could not get rid of and it never worked out. Since I just used it to haul wood and pull trailers around the back roads I dialed it in for low end and usually shifted around 2500. Ran like a champ otherwise.

@RPLaJeunesse comment made it click again. I haven't worked much with GM, but I'm guessing they probably only used the 5 pin version for some carbed Vettes since at the time that was really their only high performance car that would need the extra help to get started.