backfire thru carb at 3000 RPM

[Jenz38]

you must hold your finger on the cam mark then start to count and move to the next pin if you on ten
there must be the crank mark.
here it is good explaining with pics i find


http://www.justanswer.com/buick/3qj6...ing-marks.html

[tiltbilt]

you must hold your finger on the cam mark then start to count and move to the next pin if you on ten
there must be the crank mark.
here it is good explaining with pics i find


http://www.justanswer.com/buick/3qj6...ing-marks.html

Thank you for the article and picture. That is exactly how I set it up. But now what? It still backfires.
This morning, I was wondering if maybe the original muffler that I re-used was partially plugged, causing back pressure, so I took it off. Still backfires, so put it back on.
Also warmed up the engine, took it up to 3000 rpm and sprayed aerosol carb cleaner down the carb while it was backfiring, and the backfire went away. So I am thinking lean? But I have tried 3 different carbs and all the same result. No leaks at intake that I can find. No change in backfire when you pull one plug wire at a time.
Any ideas?
Thank You

[Bob]

It's hard to suggest on a fresh rebuild, but what about cam lobe failure, broken valve spring...

[tiltbilt]

It's hard to suggest on a fresh rebuild, but what about cam lobe failure, broken valve spring...

I have cranked the engine over by hand and watched all the rockers. Cannot notice any flat lobes or see any broken valve springs. And if it was a flat lobe, or broken or weak spring, wouldn't you find that cylinder when pulling plug wires while it is backfiring?
Please keep any and all suggestions coming.
Again, I ask, has anyone taken their straight 8 to 3000 rpm in neutral to see if it pops thru carb?
Could this be normal and maybe not noticed while driving?

[Jenz38]

May be your mixture is too lean,means when engine ishot your gas is boiling in rhe carb ? the reason can be a sticky exhaust flap for prewarm the carb in cold condition. The counterweigjt of the flap must look to the block,then its ok.
Another reason: your fuelpump deliver enough gas for idle and midrange but for for WOT too ?

[tiltbilt]

May be your mixture is too lean,means when engine ishot your gas is boiling in rhe carb ? the reason can be a sticky exhaust flap for prewarm the carb in cold condition. The counterweigjt of the flap must look to the block,then its ok.
Another reason: your fuelpump deliver enough gas for idle and midrange but for for WOT too ?

Great idea !
In just went to the shop and tried the exhaust flap in both positions, same problem.
The exh heat valve is brand new and it is sticking badly ! That will be another issue.
Also removed the brass plug in side of carb bowl to watch fuel level when problem occurs - carb bowl is always full, so that should eliminate fuel deliver issues.

[Jenz38]

hmm... it sounds from here always again like timing problems ..
You can ask the camgrinder what he done ? (degrees ?,more lift ?.... )


you have loosen your Valve lash ? only for a trail give it a bigger gap,because if your lifter are sticky and dont close the valves complete.
A gap of .01 i would try ..

it fires only in intake or in exhaust too ?

It run smooth at Idle and whats your revs ?
May be both carbs do you use are by chance have sticky powersystems ?

You try this all with or without Aircleaner ? if yes,maybe your choke flap are suck in under revs and you cant see it ?

Exhaust,you told is free ?

[TexasJohn55]

Crossfire? If pulling one plug wire at a time did not stop backfire, maybe it is more than one, or two cylinders crossfiring from blown head gasket between cylinders. Did you start torque procedure in the center and work your way to each end? Fire from adjacent cylinder can ignite the charge even without spark. Just a thought, can't seem to think of anything else that has not already been covered in discussion. Try pulling off 2 adjacent wires at a time. Don't know if it is possible to remove exhaust manifold if the manifolds are conjoined. You might find it by using an infrared heat gun to see if one or two cylinders drop temperature while backfiring. Shoot the intake runners also, temperatures could give you a clue. If it is a blown head gasket, it may or may not be bad enough to show up as lower compression on two adjoining cylinders. Pressurizing each cylinder with adjacent spark plug out might hiss enough to find it, piston at TDC with valves closed, listen at carb and adjacent cylinder. ?????????????? That's all I got.

[39CENT]

Dont know if this has been asked? are you hooked up to carburetor or manifold vacuum, or none?

[tiltbilt]

hmm... it sounds from here always again like timing problems ..
You can ask the camgrinder what he done ? (degrees ?,more lift ?.... )


you have loosen your Valve lash ? only for a trail give it a bigger gap,because if your lifter are sticky and dont close the valves complete.
A gap of .01 i would try ..

it fires only in intake or in exhaust too ?

It run smooth at Idle and whats your revs ?
May be both carbs do you use are by chance have sticky powersystems ?

You try this all with or without Aircleaner ? if yes,maybe your choke flap are suck in under revs and you cant see it ?

Exhaust,you told is free ?

Thanks for the idea.
valve clearance specs is 017 so I tried 020 Still backfires thru carb not sure about exh
smooth at idle 450 rpm
disassembled both carbs and sprayed carb cleaner thru all passages and cleaned power valves
all things have been tried without air cleaner. choke flap is ok.
exhaust is clear