Backfiring 1972 351C FMX Mach 1 H code

[griffbl]

Gents (and Ladies) my high school car from 1978 has some backfiring issues. 351C FMX has a very mild Holley 4BBl with edelbrock intake. Headers. Internally stock, with a very tall rear end ratio of about 2.8 to 1. Not a hot rod. We have been through it. No vacuum leaks we can detect. 12 degrees BTDC timing. VERY cold natured. Feel lean. backfiring through the carb. Just odd. idles fine. I thought about leaving the aluminum 4bbl intake on and trying an adapter with a re-built Holley 2150 IF I can find one...otherwise, I might try a different 4 bbl carb, OR a 500 CFM Holley 2 BBL carb.....we are stumped at this point. Pertronix under the distributor cap. Wires, plugs, voltage regulator all fresh. Any help much appreciated. A great day to all. Thank you in advance for your help and suggestions.

 

[Steve McMahan]

Tell us more under what conditions the backfire occurs. Of particular importance, only while idling in the driveway, only while driving on level surfaces, only while driving under a load, etc. Did it suddenly start to do this? If so, what was the last thing you did to the car prior to this?

 

[griffbl]

Tell us more under what conditions the backfire occurs. Of particular importance, only while idling in the driveway, only while driving on level surfaces, only while driving under a load, etc. Did it suddenly start to do this? If so, what was the last thing you did to the car prior to this?

Steve thanks for the response. I re-purchased the car a couple of years ago, but have driven it very little since then. VERY slow to warm up, and it is blistering hot right now in Dallas. Idles well. Does NOT backfire when idling. When you put a load on it, it will backfire. And by "load" I mean straight and level driving upon normal acceleration....not flooring the accelerator pedal. We are checking the jetting for a lean condition, but, again, this is a very mild, stock engine. No vacuum leak we don't think, as it idles well. Hope that helps.

 

[boilermaster]

griffbl,
how fresh is the fuel in your car ?.
fuel system is a likely culprit here, I would check fuel pressure and volume to the carb first, then float levels on the carb.
You could post the list number of the carb and determine if you are getting a good accelerator pump shot from the carb.
list number of your carb will help determine what pump nozzle was supplied with your carb and if it is adequate for your combination .
Boilermaster

 

[Steve McMahan]

I’m with Boilmaster, sounds like a fuel problem. Do your self a favor, start at the fuel tank, drain it, check the fuel inlet sock for damages, make sure the main fuel line is clear and clean-ish, etc all the way to the carb. I would pull the carb to make sure there isn’t junk in it. That’s what it sounds like to me.

 

[Cleveland Crush]

When you're done with the above mentioned stuff, ensure the choke is operating properly. Check your spark plugs also to get an idea of what's going on.

 

[Kilgon]

When I hear the word backfire, I associate that with an ignition source occurring when it shouldn't. Other word's a spark of some type at the wrong time either due to timing, foul plugs, or burnt valves. Since yours is occurring while driving and you already think you have a lean condition, I would be pulling the plugs and checking their color to see if they verify it. Is the backfire somewhat sporadic or is it constant? If you start the car and drive it right away does it do it then, or only after it warms up? A compression test can help rule out a valve issue. I would rule out timing if it idles fine and runs fairly good.

 

[midlife]

Another possibility is a slipped harmonic balancer. You can double-check it by using a piston stop on cylinder#1 instead of a spark plug to find TDC and compare to the reading on the HB.

 

[Sheriff41]

Advance your initial timing to 16° (with the vacuum tube removed from the vacuum advance unit and plugged) and reset your idle speed by backing off your idle set screw to see if that helps.

 

[boilermaster]

When we look at the actual term (backfire) it means that the ignition is occurring somewhere else, other than in the combustion chamber.
When we get a backfire thru the carb, this means that ignition is occurring while the intake valve is still open or the engines previous cycle on that cylinder did not support combustion and has contaminated next incoming air/fuel charge.
Ignition timing and thus valve timing do come into play here.
Boilermaster

 

[1973grandeklar]

My two cents:
1) Timing chain worn and stretched causes this symptom while you drive but runs fine in idle condition.
2) Harmonic balancer slipped and the timing you indicate is not accurate.
3) Worn distributor shaft/gear. This causes the Pertronix to see erratic lobes on its pickup.

 

[griffbl]

Thanks to all for the replies! We have many things to check again now. Thank you. Again, It idles smoothly. We replaced the timing chain, and the car seems very slow to warm up. No backfiring at idle.

 

[kcmash]

I had a lot of problems with mine backfiring. You said you went Pertronix, so I won't bore you with the condenser details. Mine ended up being a defective coil. I had an original coil, and after reviewing the symptoms I decided to go with a new coil. No backfiring since I did that.

kcmash