Backfire through carburetor

[Casey Shoemaker]

I have a 350 4 bolt that sat up for a few years and I have been trying to get it back on the road past few months. It has been giving me so much trouble backfiring through the carb. I can't pin down the cause. Once it warms up it seems to be better but will still backfire through carb if you floor it. The accelerator pump seems to be black and smokey Im not sure if thats relevant. I have checked firing order and sparkplugs are tan in color so Im not thinking its a lean condition and I installed new gaskets between carb and intake when I added the spacer and checked around the intake for vacuum leaks and there were none. I have adjusted and played with the air and fuel mixture screws and idle screw. I have adjusted the timing back and forth between 12 and 8 degrees at idle and it seems to like 12 degrees best. It cranks very easily and idles very smooth and runs and really smooth under normal running conditions but when you go WOT thats when it stalls and spits through the carb. It seems like maybe the elec choke is malfunctioning. I drove it around town today and it was completely warmed up when I put it in park it my rpms were reading around 1000 instead of 650. I hit the throttle a few times and it went back down to 650. Is that normal? Would a out-of-tune/ malfunctioning choke cause backfire through the carb? I'm at my witts end any suggestions or advice would be greatly appreciated.


edelbrock 1406 elec choke
weind alum intake single plane
1'' open bore spacer (to eliminate heat soak)
headers
stock 350 heads (462624)
stock camshaft
New plugs, wires, distributor cap and rotary installed
timing set at 12 degrees at idle
sparkplug boots installed thinking maybe the heat from headers were causing a missfire.

 

[82sbshortbed]

Welcome from Texas. Someone on here will help you I'm sure of it. Sorry i couldn't be more help.

 

[Shorty81]

Take the single plane intake off and put a Weind stealth dual plane intake on.

 

[Shorty81]

Single plane intake work better on race motors that live above 6000 rpms. Street engines are more suitable with dual plane intake in the 0-5500 rpm range.

 

[Charlie]

 

[1987 GMC Jimmy]

The choke issue would cause rich running, sooty plugs, etc. No, I think your issue is a lean condition. Since you have a stock cam, you should be running a stable vacuum of no less than 17” up to 21” of Hg on the gauge. The accelerator pump diaphragm or linkage slack at the piston might be at fault. I would watch the fuel discharge inside the carb while moving the throttle to see if there are any discrepancies in it. I can’t imagine the timing at that setting being the cause of it, but if you want to try advancing it a little more you can. As long as it doesn’t ping, it’ll be fine.

 

[Casey Shoemaker]

The choke issue would cause rich running, sooty plugs, etc. No, I think your issue is a lean condition. Since you have a stock cam, you should be running a stable vacuum of no less than 17” up to 21” of Hg on the gauge. The accelerator pump diaphragm or linkage slack at the piston might be at fault. I would watch the fuel discharge inside the carb while moving the throttle to see if there are any discrepancies in it. I can’t imagine the timing at that setting being the cause of it, but if you want to try advancing it a little more you can. As long as it doesn’t ping, it’ll be fine.

Thanks Jesse, Don't have a vacuum gauge on hand at the moment but I ordered one. And I have tried advancing the timing and it doesn't like any advance past 12 degrees at idle. I will check the fuel discharge and inspect the accelerator pump diagram this weekend hopefully and get back with some results. Thank you for response.

 

[Casey Shoemaker]

Take the single plane intake off and put a Weind stealth dual plane intake on.

Thanks for the reply, I have always ran single planes on all my sbc's never had any issues before it got parked a few years ago. Don't really have a dual plane laying around to throw on and try without spending a couple hundred bucks right now.

 

[Casey Shoemaker]

Welcome from Texas. Someone on here will help you I'm sure of it. Sorry i couldn't be more help.

Ha Thanks! I appreciate the hospitality! And hopefully so I just want to get my baby back on the road!

 

[chengny]

I have a 350 4 bolt that sat up for a few years and I have been trying to get it back on the road past few months. It has been giving me so much trouble backfiring through the carb. I can't pin down the cause. Once it warms up it seems to be better but will still backfire through carb if you floor it. I'm at my witts end any suggestions or advice would be greatly appreciated.

In light of the fact that the engine wasn't operated for a few years, it might be worth your while to pull the valve covers and check for a sticking intake valve. You seem to have eliminated the other two most common causes for a backfire condition (an extremely lean A/F ratio and highly advanced ignition timing).

Improperly adjusted carburetors/large air leaks can create a lean condition during acceleration. This lean A/F ratio will sometimes burn so slowly that combustion is still taking place during the exhaust stroke. If lean enough, combustion may still be in progress even after the intake valve opens. In that case, with the intake valve open - and combustion still not complete - the flame front can propagate up the runner and enter the intake. Once the flame front enters the manifold, it can ignite the fuel contained in the incoming combustion air...the resulting explosion then travels out of the carburetor and air filter.

If the timing is too early, the spark plug fires - and initiates combustion - before the intake valves closes. This allows the flame front to travel up the runner and enter the intake manifold - with the same results as noted above.

A backfire caused by a sticking intake valve is closely related to overly advanced timing - combustion occurs before the valve has had a chance to close. But in this case, the condition is simply due to sluggish valve operation. That may be because of a weak/broken spring, bent stem or deposits/rust on the stem.

Once it warms up it seems to be better but will still backfire through carb if you floor it.

I think we all know that sluggish valves often get better when the engine warms up and yet somehow seem to get sticky at higher RPM's.

 

[Casey Shoemaker]

In light of the fact that the engine wasn't operated for a few years, it might be worth your while to pull the valve covers and check for a sticking intake valve. You seem to have eliminated the other two most common causes for a backfire condition (an extremely lean A/F ratio and highly advanced ignition timing).

Improperly adjusted carburetors/large air leaks can create a lean condition during acceleration. This lean A/F ratio will sometimes burn so slowly that combustion is still taking place during the exhaust stroke. If lean enough, combustion may still be in progress even after the intake valve opens. In that case, with the intake valve open - and combustion still not complete - the flame front can propagate up the runner and enter the intake. Once the flame front enters the manifold, it can ignite the fuel contained in the incoming combustion air...the resulting explosion then travels out of the carburetor and air filter.

If the timing is too early, the spark plug fires - and initiates combustion - before the intake valves closes. This allows the flame front to travel up the runner and enter the intake manifold - with the same results as noted above.

A backfire caused by a sticking intake valve is closely related to overly advanced timing - combustion occurs before the valve has had a chance to close. But in this case, the condition is simply due to sluggish valve operation. That may be because of a weak/broken spring, bent stem or deposits/rust on the stem.

Once it warms up it seems to be better but will still backfire through carb if you floor it.

I think we all know that sluggish valves often get better when the engine warms up and yet somehow seem to get sticky at higher RPM's.

Thanks for the reply, So you are suggesting to pull the valve covers and check the friction of the rocker arms and see if one is loose and needs readjusting? Will I be able to see if its sticking by cranking it and watching it idle or is by pulling the head the only way to tell if one of the valves is bent/stuck etc?

 

[Colin]

Backfiring thru carb, only under load, would also make me want to check that the ign coil is not getting
weak and breaking down, not producing the higher voltage required for spark.

 

[Matt Pope]

Jeep had the same problem. Pull the valve covers and have someone crank it with fuel disconnected while you watch the exhaust valves. On mine one of the exhaust valves didn't come up all the way so under load it would backfire thru the carb

 

[1987 GMC Jimmy]

I have not personally seen a factory coil breakdown, but I have seen the conductive surface on them get dead spots, which could be a precursor to that? Who knows. Here’s the test for that.

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[WesN]

With valve covers removed and coil unplugged have somebody crank it over while you watch the rockers. I’m sure you will find one or two that don’t move as much as the others. This means you have been bit by the notorious GM small block worn cam lobe problem. If it has been doing it for a while be prepared to have a fight to get the lifters out when go to change the cam because they could be mushroomed slightly. Thinking it is exhaust lobe and opens enough to let exhaust out at idle and slow rpm but the cylinder can’t clear at high rpm so when the intake opens guess where it goes, pop up the intake and out the carb. Again thanks GM