The Stovebolt Site Forums The Shop Area Engine Shop 216 backfiring through carby

HI,
just got an old 40' Chevy running and was running well the few minutes I ran her as still getting new fuel tank sorted.

Anyway she is an Australian truck with rochester carby. I had fuel pump cleaned and pumping well. New fuel filter and just hooked up to a fuel can at present till tank sorted.
The carby was pulled apart, cleaned and checked by my mech and was all gummy, but no carby kit freely available here so tried her as is.
She ran good for a 7-8 minutes (whilst warming her up to do coolant flush)then just stopped. She started again and ran a while then she had a big flat spot if a reved a little and then some serious backfiring via carby so turned her off. This started as she warmed up. She seems to have a bit of a leak too as petrol under carby sometimes.
Now on a side note I have had manifold off recently and was unable to source a new gasket so put her back with old one. Is it possible once heated there was more of an air gap somewhere? causing backfire?
Or is it time to source a carby kit and clean her up? if so should I switch carbys to carter like some people here seem to recommend? or stick to rochester?
I checked float and all good, but there was some crud in bottom of float so I siphoned it out and refueled. She started again today and ran OK for a short time before I got a small backfire and turned her off. She still has massive flat spot though and is a bit hard to start without a squirt of ethnol.
I'll source a manifold gasket and change that too.
j

It sounds like you need a carb rebuild .Maybe a lean pop,you give it a little gas and flat spot and popping though the carb. does it run ok after the flat spot? Or does it pop all the way up on the reving of the engine?

You might also check and see if your vacuum advance is working or not. You might have a hole in the diaphragm which will give you a vacuum leak as well as timing issues.

Kurt

I suppose it is possible for a carburetor malady to cause a backfire through the carburetor (I don't see how); and I have been working on carburetors since 1959 and have yet to see this.

Think how an engine works.

For a backfire through the carburetor to occur, a cylinder must fire while an intake valve is open, creating a path for the explosion to travel through the intake back to the carburetor.

This condition may be caused by:

(A) an intake valve which is sticking partially open or damaged
(B) an intake valve opening at the wrong time
(C) a cylinder firing at the wrong time

Not saying that the carburetor does not need rebuilding; just that other issues are causing the backfire and should be addressed first.

Jon.

Check your timing and use the shop manual to set everything. I had mine set to the wrong mark and I was having similar issues. There's a triangle, a bb and a "TDC" (or something similar, its been a while since I did mine).

Is it a backfire, or a cough? As stated, a backfire is an electrical issue. Loose wire, bad points, condenser, rotor, cap, coil, timing, etc. A cough up the carb is caused by the exhaust valve not opening, and the exhaust gas after combustion comes up the carb when the intake valve opens.

Thanks
She ran fine for about 10 mins just idling very well as sat for years. The dizzy has good cap and has not been moved and even kinda stuck in place by grim lol but will check timing.
My carby needs a kit regardless so will sort that and see.
When i google search about backfiring almost all results talk about carby fueling issues (this would also explain flat spot) as backfiring very uncommon in modern efi cars etc so might start there and also get some new ignition bits anyway as truck very old.
Replies may take a while as have to mail order parts where i live
J

I have to say the comment that something mechanical needs to be wrong for a carb backfire has be baffled.
It is very common for a backfire through the carb when the accelerator pump is not working causing a lean mixture. I have never thought about what causes this but it is not a mechanical problem, except in the carb.

Yeah about 90% of the causes listed on just about everyhelp site i googled all pointed to carby or ignition as primary things to check?
I found a carby kit in oz that i can buy so will find out in next few weeks once it arrives.

I had a backfire/pop through the carb.
Problem was my intake manifold moved out of position
just enough to cause the popping in the carb.

PO did not use the intake sleeves to properly align the manifold.
I installed sleeves, new gaskets and no more backfire/popping.

sounds like a timing issue to me.

Yeah i read that gaskets could cause it and i think it is a likely issue as the old gasket was crap but had no new one at time (just ordering one today)
It makes sense to as it only did it once warmed up after metal expanded.
Either way truck is very old and carby leaks so planning on doing carby and gasket in about a week and will see.

I checked points and very good cond, and bought new condensor too.
J

A vacuum test may show low manifold vacuum- Sometimes.
If so check for pluggage in the exhaust.Sounds like it was parked long enough for serious internal deterioration of the muffler, and the ever present "varments" love to build nests in the pipe--looks just like a mouse hole.
Loosen the head pipe enough to get a sizable leak and see if the popping goes away.
Pull the valve cover and lube the valve stems and other---.Sticky valves/guides can vary as the engine warms up causing the valves to not seat quickly--maybe even throw a pushrod out.

If an exhaust valve is not opening at all, the engine will regularly pop in the intake/carb and more violently as the engine throttle is opened.
If the exhaust valve is not opening enough, as in a badly worn cam lobe, I've had v8's idle perfectly and run good at part throttle then when throttle is opened would regularly pop,pop,pop,through the carb.
Observe the exhaust valve action if the valve cover is removed.
Also you may isolate the offending cylinder by removing one plug wire at a time.
Have seen an engine pop back from a leaking head gasket that burned away between 2 adjacent cylinders and didn't leak water.It sure didn't idle well though.

Just fishing... I wonder if you have a stuck valve. Have you run it with the valve cover off? Any bent pushrods? It's common in old motors that have been awakened from long naps. Something about varnish in old gas gumming up the valve stems.
Good luck with it.

All sorted it was either carby or manifold gasket as replaced gasket (old one was missing a little bit) and had carby kit done...now runs great.