Popping/backfiring through carb. Help?

[dmw319]

So one day my truck runs fine and the next its popping and backfiring through the carb (I can see it when I take the air cleaner off) and its loud!

70 c10... 350 motor, 1405 Edlebrock carb, HEI distributor. Plugs/wires changed about 2 years ago. Running 4.5lbs fuel pressure through a mr. gasket regulator. It appears I may have a slight fuel seepage from the pump/fuel line connector so I'm going to check the pressure on it tomorrow night and make sure its not the culprit (its pretty slight, and been this way for a while). My timing was set to 20 degrees, which I thought was a bit high, so I took it down to 12 but still having the issue. It starts really rough (have to pump the gas a bunch). Once it starts, it idles OK, but driving it seems like its misfiring and not responsive at all when I press the gas and bogs when I'm not giving it gas. So, I have just been letting it sit the last 2 days.

Any ideas what could be causing this?

[homemade87]

Sounds to me like you have a valve spring broken or failed stamped steel rocker . My guess would be a broke intake valve spring . Easy enough to check .

[A1971Blazer]

possible bad cam lobe(s)...although it would likely do it constantly....original engine?
pull valve covers and start it up...you can easily tell if one or more are bad...

[61K10]

Org motor-miles? timing chain?

[dmw319]

Not the original motor. It was an 1980s motor that was rebuilt about 10 years ago.

[kazoocruiser]

If you can, you might want to pull the distributor cap, and put a wrench on the crankshaft bolt and rotate the engine back and forth to see if there is any lag time to the distributor rotor turning. If there is, it might be time for a timing chain.

[Dead Parrot]

Check for leaks/holes on the suction side of your gas lines. Too little gas can cause a lean backfire. Might also look at it running in the dark. Might spot ignition spark leaks. Bad sign if your dizzy cap looks like one of the Borg recharge stations from Star Trek.

Initial timing of 20 seems high but also possible the timing marks don't perfectly match true TDC. Once your problem is fixed, I would put it back if it liked running that way.

[Feel the 'Pane]

I always go easiest fix 1st. Carbon tracking on the cap?

I'd change the cap and rotor & if still backfiring, I'd bust out my vacuum gauge.

[cadillac_al]

I'll 2nd post # 2 & 3

[68gmsee]

I also go for the easy stuff first. Ignition is the primary culprit and then fuel. Do the checks mentioned above for dist., cap and rotor, mist the plug wires with water spray at night to check for arcing and check the ground strap on the distributor coil HEI to make sure it's not rusted or corroded.

A write up from this site below: (http://www.badasscars.com/index.cfm/...d=95/prd95.htm)

Having timing that is too late (retarded) can cause this same thing sometimes if your engine is running too rich. When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust system and causes a loud bang.

Back firing can also be cause by a cracked distributor cap, or one that has carbon tracking inside which causes cross firing between the terminals inside, which in turn, sends spark to a spark plug that isn't ready for it yet.

Backfiring through the carb (spitting or coughing) usually occurs in the morning when a carb's air/fuel mixture is a bit too lean. This usually goes away once the engine warms-up. It is also commonly caused by the accelerator pump inthe carb not squirting enough fuel before the main jets start working. If you spit the instant you blip the throttle it is probably the accelerator pump in the carb not working, plugged up or out of adjustment.

As with backfiring, coughing or spitting can also be caused by a bad ignition system, such as cross firing, which sends a spark to a cylinder that has the intake valve open. When that plug sparks out of turn, it lights the fuel in the cylinder and the pressure has to go somewhere... so if the intake valve is open, it goes right back up through the intake manifold and out the carb with a "spit" and sometimes even a flame.

[paul mora]

If it was sudden and alway firing bad you may have a timing chain problem

[dmw319]

Wouldn't a timing chain issue make it run rough at idle as well? it seems fine at idle and rough under a load when I put it in drive.

I will check as much of the things mentioned above tonight and report back! Although I am assuming my cap/rotor/dizzy/timing chain are fine and there's also no arching issue. I suspect fuel related, but we'll see!

[68gmsee]

Originally Posted by dmw319

Wouldn't a timing chain issue make it run rough at idle as well? it seems fine at idle and rough under a load when I put it in drive...!

I agree here. If the timing chain was the problem it would not go away. I've had that issue before. The last two paragraphs on the write-up above give good info re. bogging down when stepping on the throttle.

[paul mora]

A timing Chain problem is alot like a distributor in one notch off it will start and run but once you start stepping on the pedal you can reall notice it. Keep us posted.

[dmw319]

thanks everyone! the carb is less than a year old, so not sure why the accelerator pump would be jacked up - but I'll take a look! Going to hook a fuel pressure guage up tonight to test that as well.

[dmw319]

I got nowhere last night trying to diagnose it.. fuel pressure was good, plugs were good, accelerator pump on the carb was good, couldn't find any vacuum leaks, timing was still at 12, etc. Took it to a local good ol boys shop for them to help diagnose..

[Myself]

Originally Posted by cadillac_al

I'll 2nd post # 2 & 3

And I'll third this.......your boys will find a problem in the cam or valvetrain.

[cadillac_al]

It's pretty easy to remove a valve cover on a '70 C10. The boys are gonna appreciate that job.

[dmw319]

well the stuck valve spring is an easy fix, so hopefully that's all it is. It was 30 degrees when I was working on my car last night outside so... i'll let them do it

[68gmsee]

Originally Posted by dmw319

well the stuck valve spring is an easy fix, so hopefully that's all it is. It was 30 degrees when I was working on my car last night outside so... i'll let them do it

Long time ago I had a broken valve spring on a Dodge van with a 318 and you could definitely hear it and feel it in the way it ran. Also had a worn cam love on a 350 engine I had on a stand. There was a very noticeable lope at idle and at diminished at higher speeds. This is opposite of what you metnioned above. Hopefully they find it's an easy fix. Keep us posted.

[dmw319]

thanks 68gmsee! ill let yall know

[dmw319]

So I just got the call. They said my distributor cap had a crack in it causing the main issue, but also the plugs were slightly fouled. They replaced the cap and rotor button and plugs and retuned the carb and reset the timing. They said the carb I have is a bit too much for the motor, and they had to lean it pretty far out to get it to run smooth. Another thing they said was my wires were getting too close to the manifolds and causing them to get super hot and recommended to get some wire loom to hold them up out of the way.

Anyone have a recommendation/link to one they would suggest?

Going to pick it back up soon and hopefully it drives a lot smoother.

[Myself]

I have a hunch there are still deeper issues. A bunch of misfiring can definately foul plugs but having to lean a 600cfm way out on a 350 engine just sounds wrong. Either the carb is flooding badly with a stuck float or something else is still wrong.

[dmw319]

seemed to run fine the past few days

[Feel the 'Pane]

Glad to hear it was an easy fix.

My daily driver started ticking yesterday and from the increased volume of it by the time I got home, I envisioned a bent valve or pushrod...

Turned out to be a few loose header bolts. It's always something!

At least it's an easy fix!