Backfiring

[Clive]

My exhaust is backfiring on slow down from speed in 4th and when I change down into third or second. Has a single barrel Weber carb.
Replaced the manifold and inlet/manifold gaskets a couple of weeks ago and it seems tight (it was leaking previously), and backfiring then. I think the mixture is OK...., and I can't see any cracks in the exhaust manifold - can anyone suggest a likely cause? I would like to avoid pulling that manifold again... many thanks.

[a109]

You may have a small exhaust leak beyond the manifold and a bit of a rich running carb. On the overrun the small leak could allow sufficient oxygen to enter the sysyem to reignite the exhaust gasses giving the backfire.
John

[Clive]

Thanks - I had a leak between the manifold and down pipe! Fixed that, but the truck still backfires on deceleration, although not as badly. I have noticed that there is sometimes a strong smell of gas from the tail pipe (esp when accelerating the slipstream sometimes blows it back, my truck has a soft top that is often off ) and I am wondering if I may have a carb problem. Have set the mixture, it idles fine but makes no difference to the backfiring. Occasionally difficult to start when warm, as if flooded. How can I test the carb? Its a Weber single barrel. Advice appreciated - Thank you in advance.

[a109]

Warm start problems are common with the single barrel Weber.
Start by checking the float level settings as this could give the rich running condition you seem to experience. At the same time be sure the needle valve is free and sealing well.
I found an improvement to the warm starting by adding a small inline fuel filter at the carb that had a tank return line fitting with a new return line. This had the effect of dropping the pressure in the fuel line when the engine was off so there was no fuel leaking past the needle valve and prevented vapour lock in the fuel line.

John

[J!m]

I found an improvement to the warm starting by adding a small inline fuel filter at the carb that had a tank return line fitting with a new return line. This had the effect of dropping the pressure in the fuel line when the engine was off so there was no fuel leaking past the needle valve and prevented vapour lock in the fuel line.

John

Good idea!

Do you have part numbers and/or sources of supply?

[Jeff Aronson]

Any inline filter, even from NAPA, will do fine.

The backfiring is not uncommon with today's gas and old age of the motors. I, too, run the Weber and have some backfiring once is a while. If you have no leak at the manifold, then you may have a leak at the exhaust where it bolts together.

The smell of gas is another clue; the Weber runs lean so it shouldn't happen. Are you running the idle mixture screw too far out? That will cause it to run rich under low load [deceleration] and you'll smell the gas. If you've adjusted the mixture to help it run better, make certain first that the points are clean, gapped correctly, and check the timing. THEN adjust the carb. Sometimes it's easy to fiddle with the carb when the problem is at the points.

Also, check your PCV or gulp valve for any tear in the rubber diaphragm.

Lastly, if your valves are out of adjustment, you can get backfiring on deceleration.

Good luck,

Jeff

[Clive]

It has Petronix electronic ignition and I have checked the timing with a timing light and it appears fine...also the PVC diaphra is new from Rovers North, as are the pipes. Could it be a sign of the Petronix electronically controlled ignition going out? Or a maladjusted or worn carb? mixture seems OK when set by ear.
This morning on acceleration it sort of felt like it was being held back but at idle runs smoothly.

[J!m]

Any inline filter, even from NAPA, will do fine.
Jeff

The point of the return line was, it will prevent fuel from getting past the needle valve... Also will reduce the vapor pressure in the line. As the fuel boils sitting on a hot engine and not flowing, the vapor has somewhere to go, rather than just creating a void with no liquid fuel in it (AKA Vapor Lock).

There is no reason not to apply some new technology to these older engines... A needle valve (or regulator) on the return line would possibly be needed to regulate the fuel pressure the needle valve sees, or starvation under full throttle may occur (not that anyone would notice...). This is a particularly nice mod if you have an electronic fuel pump, as they often supply too much volume and/or pressure for a 40-year-old carburetter...