Back fires under load

[Curtis]

My 85 is back firing out the exhaust when I shift the car with the engine under load. It does it worse before the engine is up to temp. Also seems to back fire when down shifting putting the engine under load. The car is running pretty well and pulling hard, no pinging, but something just isn't right. Timing is set at 12 degrees, engine is stock with the exception of BBK underdrive pulleys, BBK catted h pipe and 50 series flow master cat back.

This winter I had my stock 4180 rebuilt buy pony carbs and I replaced my timing chain and water pump. The car is running much better after the rebuild with the exception of this issue. Not sure if this is timing, carb setting, just don't know and I'm far from a tuning expert.

Please delete my double post!

[bob85gt]

as a general rule back firing out the exhaust is rich and back fire out the carb is lean. my guess is to check your plugs and see what color are they? if black (soot) you are rich, common trbl is float level, blown power valve. has this carb beem modified? the stock carb is pretty good. do you still have the dash pot hooked up?

[MY 85 GT]

Could also been weak ignition too...When was the last time you did a tune -up? how the Coil? when was the last time that was replaced....

[Curtis]

as a general rule back firing out the exhaust is rich and back fire out the carb is lean. my guess is to check your plugs and see what color are they? if black (soot) you are rich, common trbl is float level, blown power valve. has this carb beem modified? the stock carb is pretty good. do you still have the dash pot hooked up?

The carb was rebuilt by pony carbs this past winter, I don't beleive they do any major mods. The do drill out the idle mixture plugs. Not sure what the dash pot is, but everything is hooked up as far as I know except the choke. All emmision systems are in place and I replace every vacuum line on the engine. Like I said the car is running much better after the rebuild except for this issue and it is definitely worse when engine is not up to operating temp. I'll check my plugs to see what they look like.

[Curtis]

Could also been weak ignition too...When was the last time you did a tune -up? how the Coil? when was the last time that was replaced....

New plugs and coil winter of 2007, about 1000 miles ago. It has the original ignition control module (duraspark II).

[JaredG]

I had a similar problem with the '85 I used to own. I'm going back about 10+ years here, but it seemed to be a problem with the air injection (thermactor) system. I think it was injecting air into the heads all the time which I don't believe is correct.
You may want to look in that direction. Good luck.
Jared

[Syco Stang]

Double check your ignition timing setting, it is possible that the stock balancer has shifted slightly and you have set your ignition timing too retarded. This causes unburnt fuel to burn in the exhaust, and this is usually accompanied by extremely hot exhaust headers and popping/back firing in the exhaust system.
You will have to manually bring the #1 cylinder up to TDC, using a long screwdriver or other tool inserted in the spark plug hole to see when the cylinder is indeed at TDC. Then you compare this with your crank balancer markings and go from there. Of course, this is all hypothetical because it's damn near impossible to diagnose a car over the net. And, obviously, this is more of a 'last on the list' thing after checking the rest of the ignition system and making sure all the other basic things are in order.
SS

[bob85gt]

sorry about the dash pot. that was a different stick shift car. a dash pot was used to slow the closing of the carb so it would pass emissions during shifting. when the carb closes quickly it causes a rich mixture. what is your air/fuel mixture? my guess is too rich. cars generaly run better on a slightly rich mixture. so when you back off the mixture goes just a little to rich (imo) and backfire through the pipes. i put an o2 sensor on mine to get the carb set.