Thread: backfire/miss

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Last summer I bought a 350. Edelblrock performer intake, Edelbrock 650 carb. The rest is stock. It ran well for a while, then one day as I was driving up a steep grade, It started popping, everytime I tried to give it more gas. I thought (my little bit of mechanical knowledge) maybe I had a bad ignition wire. I replaced the plug wires with new msd superconductors. I also replaced the cap and rotor. The popping was still there. I thought maybe I had a leak somewhere between the engine and the intake so I checked it with a can of brake kleen and did'nt notice any vacum leaks. Just for fun I adjusted the valves. I thought it was an ignition problem but the distributor and everything else is new. It is not a vacum leak either. I hope its not a flat spot on the cam or a stretched timing chain, since these things cost more money than I have right now. Does anyone know what else this problem could be? Could it be fuel related? I replaced the mechanical fuel pump with an electric unit. I have rejetted the carb one step richer (forgot the actual numbers) also.
Help before I spend more unecessary money

Originally posted by olsenam
Help before I spend more unecessary money

Too late for that! Not trying to sound harsh here, but in guessing you spend money that could have gone toward fixing the real problem.

It's unlikely to be a flat cam if it's only a problem when you push it. Have you stuck a timing light on it to check advance? Could be something as simple as a cracked vac line if you've got a vacuum advance. Distributor may be tired, and yes, maybe the timing chain. Check the timing advance and check back with results.

is it poping through carb or exhaust if carb could be burnt intake valve if exhaust ignition problem or broke exhaust spring. there are other possibilities but not enough info.

I have timed it with a timing light at 10degrees+. Vacum lines all seem to be in excellent shape no cracks or any thing suspect. It does not pop if I am in park revving the engine, it does not pop if I am going down hill and gas it. It only pops when under a load, the steeper the hill the sooner the pop (usually between 2750r's and 3500r's) If I dont let it warm up as long as I usually do (10 minutes or so,) the problem seems worse. It pops through the exhaust. I am running a fairly open exhaust, one shorty glasspack on each pipe, it is a 75 Jeep CJ 5 and I have also run the exhaust in frame, without a crossover. When I adjusted the valves I did not notice any cracked or broken springs. Just today I noticed a little bit (very little) of fuel seepage from the rear of the carb. I already bought a new gasket and will install it tonight after work. I dont think this is the problem, because I never noticed it seeping before and this problem has been an ongoing thing .

Just wondering, did this start after you rejeted the carb?

Just a guess:
You may be running too rich. You say you staged it one step richer already, maybe that was the wrong way to go.
If you say all Vac lines are good and the timing is right with new plug cables, cap and rotor, then I guess it wouldn't be crossfiring either, and therefore probably not a timing problem.
If it runs well otherwise your valves will probably be set OK.
That leaves fuel-related problems as the quickest to find: If it pops through the carb it's often the ignition set too far advanced and the spark ignites the fuel before the intake-valve closes. But you say it's popping through the exhaust. If I were you I would try the following: see whether you can find out if it always pops through the same side or if it changes sides. If it only pops on one side it might just be a wrongly set ex-valve.
But if your running too rich (and the mixture gets even richer under high load, because of the little accelerator pump in the Edelbrock carb) you might be getting too much unburnt fuel out into your hot exhaust pipes. These get even hotter under load so it might just be enough to ignite some vapors here and there. That'll give you the popping (backfiring). A giveaway for too rich mixture is poor mileage (but not only that). Try to find out whether your exhaust vapors "smell" of fuel (be careful doing this and don't take too deep a breath, but if you just stand behind your car and smell, you ought to be able to detect a smell like you get at gas-stations). That would also mean your running too rich. If all this doesn't appear to be the case, your problem might be elsewhere, as I say, I can only give a rough guess with your info.
If you do think this might be the problem get rid of it the following way: get some carb-jets and rods (very easy to swap in an Ede and hardly cost anything) and go 2 steps leaner than you are now (so one step leaner altogether) in the power mode. Check if it makes a difference. While your about it, check your float height (easy to follow instructions on how to do so are on edelbrock's site under technical support).
I think that will be the cheapest (and possibly right) way to start tackling your problem. If all this doesn't change a thing your out of 20 bucks max and an hours work. Cheaper and faster than checking your timing chain...
What happens if you go up a slope and accelerate slowly?
What happens if your stopped, in gear (level ground) and kick it?
What do you have to do exactly to make it pop?
Just my few cents worth...
Let me hear how you're getting along,
Max

The reason I rejetted richer, was because of the lean indications of the spark plugs second to last time I changed them. The pop was present before I rejetted. If it is excess fuel in the exhaust It still does not explain why it seems to cut out and loose power when it pops. (or does it?)
If it acelerates slowly it wont pop as often
If im stopped at ground level I can kick it and it screams with out a pop.
When I am heading up the hill, I drive on my way to work, If I punch it, it will pop/sputter. If It kicks down a gear while passing it will pop/sputter. It definately feels some power loss while it is popping, not just noise.
Maybe this is a dumb question but, I installed a fairly cheap electric fuel pump, and mounted it next to/under the driver seat, I did this so I could hide a switch for anti-theft (dont tell anyone, after all it is a soft top Jeep) I have heard that electric fuel pumps should be mounted as close to the tank as possible. Could this probelm be from to much/too little fuel pressure?. It does'nt seem likely to me but, Im sorta dumb sometimes.

okay heres the skinny....

1) Is the popping consistant? if so it's either a worn cam lobe or a worn off lifter.....If it was a burnt valve it would pop all the time..

2) if the popping is not consistant.... check the plugs....all of them...what color are they.....should all be a lightish tan...

if they are black you are too rich....could cause popping....
if they are white you are too lean.....could cause popping... and burn stuff....hope that helps..


Too check for a worn lobe or lifter..... pull the valve covers.... rotate the engine over and watch the rocker arms...

On a worn lobe....the lifter will usually go farther down in the bore...but not always... check the travel... watch the rockers go up and down.... you'll have almost a half inch of travel.... the one that doesn't move as much is your culprit...

It has a brand new E.G.R. It came with the valve cover kit I bought when I pulled the old ones to adjust the valves.
The popping is not consistent. Ill check all of the plugs for cracks.
Last time I checked, about a month ago, the plugs were not showing any signs of rich conditions. They were a light shade of tan without any blisters or buildup of any sort. I am definately suspecting the fuel pump as the culprit. I'll check it for pressure, as soon as I get a guage. Do I check it at the pump outlet? or do I check it at the end of the fuel line that attaches to the carb? The fuel pressure should be four to six psi right? If not what is the proper fuel pressure. Is there a pump brand that you would reccomend? I really dont like the one I have now, because it is super noisy, and I only paid twelve bucks for it and at that price Im afraid that I got what I paid for. I think the line size I used was 5/16 i.d. that would be big enough right? Oh, maybe it is useless info at this point but, It mostly pops from the passnger side pipe, and If I really get on it, it pops from both pipes. If it is the fuel pump why would it favor one side of the engine? It is significantly worse now in the winter, than it ever was in the summer.
Thank you guys so much for your help.
I really like this page
I really hope that someday I'll know enough to help someone the way you guys do.

check the fuel pressure at the carb. 4- 6 lbs. get a bucket and run some fuel into the bucket, should have a study stream of fuel. you can have pressure without volume. if youve got pressure and volume then the fuel pump is ok. then you need to take another look at the valve springs and lifters.