NEED HELP! Backfiring!

-
2

2doorswinger

Well-Known Member
Joined
May 19, 2014
Messages
512
Reaction score
19
Location
canada
Hey guys, just finishing up the restoration on one of our 1969 swingers and we are having engine issues. We had the original engine rebuilt by a reputable engine shop locally. Engine was broken in and ran fantastic on the dyno. Now that we have it home and in the car, its running very strange. It is backfiring through the carb, and making small back fires (popping sound) out of the exhaust as well. the 600 edelbrock that was on it was used, so i figured maybe the accelerator pump was gone and maybe it was backfiring due to being too lean and needing more carb. As a temperary solution, i put my edelbrock 850 thunder avs on it (i knew it was too much carb, but tried to see if it was a lean situation). It was running really rich, but STILL backfiring. So, i checked timing. Everything seemed spot on and we made sure it was dialed in (pertronix flamethrower distributor), still backfiring. I put brand new matching pertronix plug wires (replaced the new taylor made wires) and still backfiring. Put a new matching pertronix coil on it, still back firing. We ended up buying a brand new edelbrock 750 carb to get the fuel a little closer, and guess what, still back firing. Ive thought about a wiped cam lobe, but it was broken in properly and ran great on the dyno. Could it be a wiring issue in the car itself, an engine issue, anything? I am at a loss and have no other ideas. Has anyone had an issue like this? Thank you for your help in advanced!
 
Yes, starts and idles fine. Not fluctuations in idle when left to idle. Only under hard acceleration. We haven't removed the intake at all, engine builder was the last one to touch it.
 
One thing to try comes to mind, lock out the secondaries so it's only running on the primaries. See if that helps.
 
Robbie2734 said:
One thing to try comes to mind, lock out the secondaries so it's only running on the primaries. See if that helps.
Click to expand...

I hate to "be that guy" but sounds like wiped cam. How many pulls on the dyno? If less than about 12-15 then that is not enough time for the problem to rear its ugly head. Pull the covers and inspect the valves/rockers. A compression test might reveal a cylinder with abnormally high readings compared to the others as well. Also there should be some kind of valvetrain noise present but it could be very slight. Good luck. J.Rob
 
if all else fails I would check the pickup coil in the distributor if it is a different one than used on the dyno .
 
timing and accelerator pump
 
YY1 said:
timing and accelerator pump
Click to expand...

Timing isn't going to backfire through the carb unless it is MEGA advanced which OP says it is not. This is only happening under hard acceleration because cylinder pressure is highest. Then again maybe if we knew intial, total and vacuum advance is hooked up, maybe we could help more. J.Rob
 
What plugs are You running, have You pulled them & inspected them?
 
Killer6 said:
What plugs are You running, have You pulled them & inspected them?
Click to expand...
2doorswinger said:
Hey guys, just finishing up the restoration on one of our 1969 swingers and we are having engine issues. We had the original engine rebuilt by a reputable engine shop locally. Engine was broken in and ran fantastic on the dyno. Now that we have it home and in the car, its running very strange. It is backfiring through the carb, and making small back fires (popping sound) out of the exhaust as well. the 600 edelbrock that was on it was used, so i figured maybe the accelerator pump was gone and maybe it was backfiring due to being too lean and needing more carb. As a temperary solution, i put my edelbrock 850 thunder avs on it (i knew it was too much carb, but tried to see if it was a lean situation). It was running really rich, but STILL backfiring. So, i checked timing. Everything seemed spot on and we made sure it was dialed in (pertronix flamethrower distributor), still backfiring. I put brand new matching pertronix plug wires (replaced the new taylor made wires) and still backfiring. Put a new matching pertronix coil on it, still back firing. We ended up buying a brand new edelbrock 750 carb to get the fuel a little closer, and guess what, still back firing. Ive thought about a wiped cam lobe, but it was broken in properly and ran great on the dyno. Could it be a wiring issue in the car itself, an engine issue, anything? I am at a loss and have no other ideas. Has anyone had an issue like this? Thank you for your help in advanced!
Click to expand...
Could have a couple of plug wires that may have mistakenly got crossed? I seen that do odd things to performance depending on which two are crossed!
 
If you suspect a wiped lobe, you could paint a dot on each of the push rods and crank the engine to make sure they are all spinning properly.
 
If you rule out the electrical find out what's showing up in the oil. Cut the filter open if you need to.
 
Does this happen at all RPM's at WOT or just low or high RPM's or when exactly? Is this with the engine cold or hot or both?

Some thoughts: Backfiring in the carb can easily be ignition crossfiring or misfiring; the mixture tends to get harder to fire the more the mixture is compressed (more throttle make higher cylinder pressures), and that will make it more easily crossfire. How are your wires routed? Do they run closely parallel to each other? They need to be separated as much as you can and cross at sharp angles.

How close is the rotor pointing directly to the proper spark tower when it should be firing, or is it pointing somewhere in-between spark towers? (This would be a rotor phasing problem.) Is this the Mopar distributor?

Intake valves sealing can be another source of backfires. If this is at higher RPM's only, maybe the lifters are pumping up or something like that to hang the intakes open a bit. What is the cam and lifter type and setup?
 
Thanks for all the responses guys! It isn't happening at low or high rpm, only upon quick snap of throttle. If you Rev it up real slow, it will be fine but will get some popping out of the exhaust. The cam was definitely broken in properly (done by a well known shop in Canada called Beatty and Woods). Triple checked to make sure plug wires weren't crossed. I'm thinking now a plug MAY be fouling, but im not sure. If one plug is bad or going bad/weak, could it cause this issue?
 
You must log in or register to reply here.
-

Back
Top