Car backfiring on Number 4 Cylinder

[Nick]

My car started backfiring progressively getting worse as it warms up suggesting maybe an expansion / breakdown issue somewhere. After some investigation I found if I disconnected the plug lead to cylinder NO4 the backfiring stopped. So far, I have changed the plug, plug lead, did a cold compression test, and swapped the jets between 3 and 4 on the same carb, this did not cure the problem. I did notice today that the exhaust pipe on 3 is a lot cooler than the others and this is fed by the same carb as 4.

The only thing I can think of now is an air leak on the exhaust cylinder head flange, or a carb issue.

Any advice gratefully appreciated

Thanks

Nick

 

[JP Verweij]

Have you checked if the carb is still in sync with the others.
And if the carb is still in sync with both the throttles of its self.

 

[David Garton]

Nick when you say backfires do you mean pops out of the intake or exhaust? If number three tube is not hot it's more than likely not firing. Pull that ignition wire while running and see if it changes. If it is popping out of intake or exhaust it could be a broken valve spring or sticking valve. Please let us know your findings.

 

[Paul T.]

Nick, not knowing what type your ignition system is. Sounds like it could be a cracked distributor cap.

 

[Howard Jones]

Intake manifold leak at the corresponding intake port. Super lean and not going bang in that cylinder.

 

[Randy V]

Need more info on your induction system and carburetor(s) etc..

 

[Adam C.]

A loose exhaust rocker will still pass compression check but cause a nasty intake backfire

 

[Bill Kearley]

Adam, Do you mean tight rocker !!!!
Or as David said, a sticking valve.

 

[Adam C.]

No if the exhaust valve doesn't open, the cylinder will be under pressure the next time the intake valve opens.

 

[Bill Kearley]

That should be recognized quickly with a lot of valve train noise and or a dead miss

 

[Nick]

Thanks for all the advice I have a few things to think about, should also have said the backfire through the exhaust is intermittent.

 

[Rob Klein]

make sure you have the correct plug, and gap. I was given incorrect plugs at a Napa store.

 

[Randy V]

If you are using Webers, you can increase the idle fuel mixture for that cylinder and it will help cover up a lean condition. The idle fuel slot is a constant contributor to the overall fuel/air mixture whether at idle or WOT.