I had strange problems with the ignition control on my Microsquirted truck using the GM HEI 7-pin module.
During cranking the engine would buck and pop and fight the starter until it would run properly. At first, it all seemed like a fuel and air based problem.
In an effort to solve this problem i googled the proper method of setting base timing on a 90s GM truck and hit a wasp nest full of problems doing that.
Now the engine would start immediately but after a few seconds of running it would just run badly, pop, rumble and die after a few seconds. On further investigation i found that after the GM bypass (i’ll explain later) is activated by the Microsquirt ECU, the ignition timing is badly retarded and the engine was “running” at about 30-40° ATDC!
Picture courtesy most likely GM. I found it on the Interwebs.
The GM bypass is a little electronic switch built into the HEI module that switches the ignition signal from an internal source – the magnetic pickups signal coming into the module- to the external source – the ECU.
Applying anything from 5v to 12v on pin B on the 7-pin module enables the bypass and the usage of the external timing signal on pin E (or EST).
Back to the initial problem. Engine runs badly on activated bypass. I have gone through every component of the system, checking if the timing mark on the balancer still lines up with the cam (it does), checked whether the reluctor phasing is correct, checked for proper firing order, etc. etc.
The problem usually lies in the last thing you check.
I knew that switching the magnetic pickup leads polarity on the HEI module can cause a lot of trouble with timing but it did not come into my mind until i realized that the problem comes from retarded timing. From there on the solution was a simple rearranging of the plug and voila, it runs nicely.