backfire thru carb at 3000 RPM

tiltbilt

Active Member
Hi,
New to the forum.
Hope to receive some help on some items.
I am restoring a 1951 Buick Special Model 48
After assembling the straight 8 and running it, I had a backfire thru the carb at 1800 RPM.
So I rechecked my timing chain marks and found I was one tooth off. Corrected that, reassembled, and now no backfire at 1800, but now backfire starts in at 3000 RPM.
I have checked initial timing, mechanical advance, vacuum advance. All are within specs. Tried 2 different carburetors because the backfire seems to go away when you close the choke plate about 3/4 of the way.
Both carbs no difference.
I have checked valve lash several times.
Tried another distributor with no difference.
Do you think I have a wrong or defective new re-ground cam?
Any suggestions would be much appreciated.
Pulling my hair out, and at 63, there is not much left.
Thank You
Tiltbilt
 

firstofeight

Active Member
Backfiring

Sounds like fuel starvation. Maybe plugged filter. Try using a boat gas can , hooked directly to carb inlet, setting higher than the carb. Or with a low pressure elect pump between the tank and carb. If problem goes away, start chasing the problem back from pump to tank.

Ben
 

tiltbilt

Active Member
backfire

Sounds like fuel starvation. Maybe plugged filter. Try using a boat gas can , hooked directly to carb inlet, setting higher than the carb. Or with a low pressure elect pump between the tank and carb. If problem goes away, start chasing the problem back from pump to tank.

Ben

Thanks for the idea, I will try that tomorrow. But if my memory serves me, I think I checked the carb and it had a full float bowl at the time. (carb has a removable plug to check fuel level.
Has anyone tried running their straight eight at 3000 RPM while sitting to see if this a normal situation?
 

TexasJohn55

Active Member
Is it doing it no load out of gear or under load while driving?

If choking seems to help, that does sound like a lean condition. Are your manifolds sealed up good?

Have you considered points bounce or too wide? Did you have all your valve springs tested?

I can't seem to get my head around the RPM range changing so drasticly. Valve timing I suppose.

Try to pin it down to one cylinder by pulling plug wires. See if plugs show any color differences.
 

tiltbilt

Active Member
backfire

Is it doing it no load out of gear or under load while driving?

If choking seems to help, that does sound like a lean condition. Are your manifolds sealed up good?

Have you considered points bounce or too wide? Did you have all your valve springs tested?

I can't seem to get my head around the RPM range changing so drasticly. Valve timing I suppose.

Try to pin it down to one cylinder by pulling plug wires. See if plugs show any color differences.

Thanks for the reply,
I am in the process of assembling the car after a frame off, nut and bolt restoration, so I have not driven it yet. It backfires thru carb at 3000 RPM in neutral with no load. I have pulled plug wires one at a time while it is doing it, and cannot isolate it to any one cylinder. Engine has new valves and springs. Have tried adjusting the points tighter and another distributor. No vacuum leaks, had the intake and exhaust manifolds machined together as a unit. I am beginning to wonder if this a normal condition, and maybe wait for a test drive, but would like to know before I install the grill and front bumper in case I have to replace a defective reground cam.
 

Jenz38

Active Member
The Problem has changed after your new cam setting ? You done the Job like in the manual described ,you know about the 10 teeth differend ? (Sorry for that ,i hope,stupid questions,but only for my understanding now )
I would try another Ign.Coil and condenser if you hadnt done it with the distributor changing.
You have checkt the firing order? If only 2 cyl are interchanged,you can have that Problem.
You checked the dist cap for Cracks ?
 

tiltbilt

Active Member
backfire

The Problem has changed after your new cam setting ? You done the Job like in the manual described ,you know about the 10 teeth differend ? (Sorry for that ,i hope,stupid questions,but only for my understanding now )
I would try another Ign.Coil and condenser if you hadnt done it with the distributor changing.
You have checkt the firing order? If only 2 cyl are interchanged,you can have that Problem.
You checked the dist cap for Cracks ?

All great ideas. Problem did change after new cam setting. Originally backfired at 1800 rpm, now backfires at 3000 rpm.
Set the timing chain as the manual states, also as the picture in the manual shows. (second time). Originally I was one tooth off.

New ign. coil, also tried another condenser.

Checked firing order about 6 times.
New dist cap and rotor.

Has anyone tried to run their straight 8 at 3000 rpm in neutral to see if it backfires(pops)?
 

Jenz38

Active Member
Another Reason :
sticky lifter ,filled with old burned theavy oil ,witch cant move at higher revs and stop to make it hydraulic.
You can try to give a little more valve lash then zero,like .001. If it starts to get better, you are on the right way.
I think a only regrounded Cam cant be the reason,only then when its wrong inatalled.
 
Last edited:

tiltbilt

Active Member
backfire

OK, Can the shop manual picture be wrong? It shows that there are 10 teeth from the cam gear mark and the crank gear mark. Maybe should show 12 teeth? That would be 10 teeth between the gear marks.
The first time I set it up, I did like the picture shows (10 teeth between gear marks) and it backfired at 1800 rpm. Second time I set it up, I added 1 tooth (11 teeth between gear marks) and it backfired at 3000 rpm.
So now I think I should set it up with 12 teeth between gear marks (that would put 10 teeth BETWEEN the gear marks. Maybe that what the manual means? (to have 10 teeth BETWEEN the marks)?
 

tiltbilt

Active Member
backfire

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/3qj6d-1948-buick-camshaft-gear-crankshaft-gear-timing-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
 
Last edited:

tiltbilt

Active Member
backfire

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

Active Member
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

Active Member
backfire

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

Active Member
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

Active Member
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.
 
Last edited:

39CENT

Active Member
backfire

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

tiltbilt

Active Member
backfire

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
 
Top