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MOTORHEAD 08-06-2021 10:49 AM

weber backfiring
Trying to get webers jetted correctly, got idle/low speed ckt pretty good , but constant backfire thru exhaust when off throttle. (decelerate).
Rich, lean, doesn't seem to matter, Sounds like auto weapon firing.
Anyone familiar with this problem have a cure ?


1795 08-06-2021 10:58 AM


Are both sides backfiring or just one? Check your plugs and plug wires. What do the plugs look like?


jhv48 08-06-2021 11:18 AM

Failing exhaust manifold gaskets will result in decelerating backfires.
Usually one side only though occasionally both get loose if a new install.

MOTORHEAD 08-06-2021 12:31 PM

Both sides the same.
I'll pull some plugs.
Have AFR gauge on drivers side, off idle is running about 12-13.
Slip joints, no gaskets. Have slight leakage on one pipe.
Looking thru the old Weber posts, I've read some statements saying backfire thru exhaust
is too rich mixture, but that is not holding true..

bobcowan 08-06-2021 12:48 PM

How's the balance. If it's not perfect, you'll get all sorts of weird symptoms.

1795 08-06-2021 01:07 PM

Check the plugs. I had a bad idle jet holder that was plugged and soaking the plug and fouling it out. That then would send unburnt fuel into the exhaust. It could be happening on both sides. When the pipes get hot that could lead to backfires. Loose spark plug wires could also lead to intermittent spark that would dump unburnt fuel into the exhaust.

t walgamuth 08-06-2021 01:58 PM

if it is all cylinders acting up I would suspect ignition timing.

MOTORHEAD 08-06-2021 04:13 PM

No exh manifold leaks.
Balance good at idle, will ck at higher rpm.
Plugs all clean,almost too clean, but last run was only 10 mi.
Never had this problem with the single holley, got to be related to the weber set-up.
Let off on the gas pedal and it backfires, then push in the clutch and it instantly stops backfireing

1795 08-06-2021 06:02 PM


I cannot recall from your earlier thread about switching to the Webers, seems to the best of my recollection you had started with the Webers, went to the four barrel and then back to the Webers. Is this correct? Is this the same cam and timing that you had previously with the Webers initially (if my memory is right), or did you change the cam and timing for the four barrel? What is your fuel pressure at. If it is doing this while running lean, that would seem to be odd, unless the cam and the timing are mismatched for the Webers. What is your timing currently?

I am by no means an expert, learning quickly as I go, but my sense is that this is not a jetting issue. Too clean on the plugs could mean that the timing is off and they are getting wet. Even after ten miles they should show some evidence of combustion. Do you have a laser temp gauge? Check the temps on the header tubes while it is running and see if the temps are hot and consistent. If the temps are cool then it might be ignition related. If they are inconsistent, then the cooler ones are not firing all the time, or possibly at all.

Trying to consider all possible scenarios. Good luck.


MOTORHEAD 08-06-2021 06:57 PM

You are correct regarding the cronology ,webers-holley- webers.
cam is for webers, was not changed for 4 bbl.
Timing is18deg initial, + 20 deg advance , all in by 2500.
Fuel press 2.5-3 psi.
I would feel it if I had a miss. Have already done the IR temp ck's.
My brain is fried, time for some TV & sleep


1795 08-07-2021 05:25 AM


I hope that you had a restful night. I hear your frustration. As I thought about it last night, this is what I came up with regarding things to consider based upon these premises: 1. When you had the Webers on previously the backfiring was not a problem, or else you would have mentioned it. 2. The backfiring was not a problem with the Holley.

What have you done to the engine in order to put the Webers back on, you had to remove the Holley and intake manifold and the distributor and re-adjust the fuel pressure. I doubt that the issue is with the intake manifold having a leak, as if it were sucking air it would also be sucking oil from the gallery and you would notice that on the plugs.

Did you thoroughly clean and inspect all of the jets, air correctors and holders? It does not take more than a small speck to clog or interfere with their function. Drain your fuel bowls and see if anything comes out.

Another possible culprit is the distributor. Are you using a bronze gear on the shaft at the cam? Bronze gears wear (that is their primary purpose, to sacrifice themselves instead of the cam gear) and they really do not last that long. Drag racers replace them often as they rebuild their engine often. On my recent build with the road race engine the bronze gear was showing significant wear after only 4 hours of run time. The engine builder stated that it was affecting the timing. It may be possible that it is slightly off on the camshaft. I would pul the valve cover and check to make sure that you are at TDC on #1 and see where the pointer is showing for timing.

Good luck, hopefully someone with more knowledge than I will chime in. Those seem to be the mostly likely culprits, as those were the things that were changed.


MOTORHEAD 08-08-2021 10:36 AM


I went thru some of my old posts from the archives, and I did mention backfire from the
right side. This was before I had found the intake sealing problem on the left side, which I did not mention anything about backfire from the left side.
Yes, I was very careful regarding the jets, as some were "filled & drilled".

Will be checking timing and plugs tomorrow, and calling Inglese Tomorrow.


1795 08-08-2021 10:41 AM


Good luck. Looking forward to hearing what you find. I found some small specks in the fuel bowls when I drained them after they had sat for about a year. Was having some backfiring at Pitt and turned out that a couple of plug wires had loosened. Pressed them on firmly and that went away......


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