Ignition Timing Curve and Fuel Delivery

[CdnCutlass]

Trying to work the bugs out of my car and get it somewhat tuned. The HEI was dropping spark, and after a failed attempt with a Mallory ignition box I installed a Pertronix Flamethrower upgrade kit.

I think I finally got the timing right, where the car seems to run good. I set base timing around 8 degrees, and have 34 degrees all in at 3000rpm. The vacuum advance is set at about 10 degrees. The car will now rev past 5000rpm where before it would drop spark.

However, now it has developed a new issue where it backfires/pops at high RPM under heavy throttle. Would this indicate a lean condition? I don't know if its related to the timing adjustment or not. I had too much timing in it before.

 

[online170]

Pops heard from the exhaust?

Would be an exhaust leak I think right at your "collectors".

 

[pontiacgp]

it wouldn't be a leak in the exhaust causing it under load, it would backfire when you let the gas off, that would suck in air where the exhaust is leaking and cause the backfire

 

[CdnCutlass]

It is backfiring at the engine, heard it come from the carb.

 

[online170]

PontiacGP youre right.

In that case it could be a bunch of things. Valve float, valve springs, ignition issues... Your timing seems just about perfect with the 34* all in so I wouldnt mess with that.


If the backfire happens AS YOU ARE MASHING the throttle, it could be a lean condition (pump discharge nozzle, cam or pump itself bad maybe).

If you mash the throttle, keep your foot in the same place, watch the revs climb, and THEN it starts to misfire (all the while your throttle position hasnt changed), I would guess its iginition related.

Need more clues.

 

[pontiacgp]

if your car developed a new issue where it backfires/pops at high RPM under heavy throttle after you changed the distributor then I'd be looking at the distributor and the installation of the ignition wires

 

[454muscle]

I had similar problems, and I'm still trying to fine tune it. My crate motor had the wrong initial ignition program so the timing was waaay off base. I have a air/fuel ratio gauge as well as a mini display control unit that shows more detail and ability to change air/fuel etc.. -- even still, it's a tough tuning process. When you change the timing, so does your air/fuel ratio -- this is in real time.

Advancing the timing gives me more gas:air ratio and can lead to a rich condition.
Retarding the timing gives me more air:gas ratio and can lead to a lean condition.

So what I suggest as an option, just get an air/fuel ratio gauge and each time you change the timing, adjust your air/fuel accordingly above, it's hit/miss and trial/error but you'll eventually hit the sweet spot. I'm still looking for mine.

 

[pontiacgp]

Advancing the timing gives me more gas:air ratio and can lead to a rich condition.
Retarding the timing gives me more air:gas ratio and can lead to a lean condition.

You have to have the correct timing and advance curve before you take a air/fuel ratio reading. Too much or too little ignition timing can give you a false lean or rich air/fuel ratio reading.

 

[454muscle]

Advancing the timing gives me more gas:air ratio and can lead to a rich condition.
Retarding the timing gives me more air:gas ratio and can lead to a lean condition.

You have to have the correct timing and advance curve before you take a air/fuel ratio reading. Too much or too little ignition timing can give you a false lean or rich air/fuel ratio reading.

Hmmmm. Thanks for the enlightenment Pontiacgp. Not sure how to check / change that. I'll have to contact Edelbrock tech support I guess (it's their Proflow EFI system).

 

[pontiacgp]

I thought those systems took all of the guesswork out of it...