The Stovebolt Site Forums The Shop Area Engine Shop Carburetor backfires through carb

My 54 235 with a stock 1 barrel with auto choke backfires through carb when the idle is low. Tried tuning, compression check, choke setup, rebuilding carb and a new carb. Problem persists. Any suggestions would be appreciated.

Set timing to factory specs.
Check your points, cap, condenser. The condensers available today are all absolute crap from China. They fail in a few hundred miles if they work at all. You should get one of the 200,000 hr condensers made by one of the members of this forum.

https://www.stovebolt.com/ubbthread...55045/the-newest-condenser-update-8-2024

What carburetor do you have? Rochester B? Carter YF?

I had backfires through the carburetor at idle before I rebuilt a proper Carter YF. It was the accelerator pump leaking. Rochester carbs are prone to warping causing the accelerator pump to leak.

That member is Jon G a moderator for the Electrical Bay forum and a big contributor to our Tech Tip section on this Forum.

Contact Jon with a PM to purchase one of his condensers. I highly recommend it.

Hi Dave and welcome to the place! As has been mentioned, backfiring through the carburetor is usually (or I should say often) a sign of improper ignition timing, but it can also be caused by a sticking intake valve. Particularly at low rpm. What happens normally is a spark plug fires while an intake valve is still open...hasn't yet closed. Usually you'll notice the engine isn't running right if a valve is sticking but in any case if a plug fires while any intake valve is still open it allows fire to by pass the valve and ignite the air/fuel mix above that valve (all the way into the carburetor). The wrong mixture can also cause this, Dave but it sounds like you've already taken measures to remedy that. Often it is a very lean mixture that causes backfiring but not always. A vacuum leak can cause an overly lean mixture but with the 235 it won't be the vacuum line going between the carburetor and the distributor because at idle there should not be vacuum present in that line. However you mentioned an auto-choke carburetor. Those will have some vacuum present at idle (to pull back on the auto choke piston as the engine warms up) and a problem there could be working against you. Those are some possible problems.

As has been mentioned a bad condenser can definitely cause backfiring. It will make the spark pattern unpredictable. Please contact me if you need a new one. If you can't use the PM function yet, you can reach me at [email protected]

Do a cylinder leakdown check- - - -it sounds like you might have a sticky intake valve, something a compression test won't be able to find.
Jerry

Thanks for all the replys. I failed to mention i am running a pertrinix ignition and added a PCV valve. Is there any easy way to check for a sticking intake. Could a timing advance help

Any idea how to find a sticky valve?

Do you have a vacuum gauge?

Pertronix- - - - -PCV- - - - - -"The thick plottens!" Now we've got three suspects in the whodunnit! I repeat- - - - -do a cylinder leak test. Air pressure in each cylinder with the crankshaft locked and both valves closed- - - -listen for air leaks at the carburetor, tailpipe, and oil filler cap.
LOL!
Jerry

pertronix...................did you upgrade to an alternator ?

Jon

Hi Dave,
You weren't here when I did it, but around a year ago I showed with incontrovertible proof how the Pertronix device fails. The round part which fits over the distributor cam becomes more and more de-magnetized until it finally loses ability to do anything. It is possible that is part of your problem as it is possible portions of that part have begun to lose magnetism. Actually the Pertronix device is just garbage. Sorry...I don't know any other way to describe a product that causes so much trouble and a product that even people who use it take along a spare or a points distributor with them if they leave home. I've also shown on this site how to convert an HEI distributor made for a 230, 250, 292 into an HEI distributor that works fine on the 216, 235 or 261 engine using common tools and an hour or two of time. That's what I have in mine. All GM parts or copies of them and less than $100 will take care of it. The Pertronix unit might make the spark pattern unpredictable.

You asked if a timing advance would help. Do you mean a replacement timing advance vacuum canister or just advancing the timing? The vacuum canister will not help with a low rpm miss. At low rpm the Stovebolt engine relies on centrifugal advance only. It isn't until after around 800 rpm that the vacuum advance kicks in and starts affecting things.

However I still think (as I originally said) that you have an intake valve sticking. A vacuum gauge will show this, but be sure to attach to manifold vacuum and not the vacuum line going to the vacuum advance canister. If the needle drops to a low position and then returns to normal and then repeats the drop in a moment (probably coinciding with the backfire) it is certain you have a sticking valve...but which one? Jerry has told you the best way to single out the valve with a problem (leak-down test). Hopefully you can do that.

I know about the round part that goes over the points lobes, but that design hasn’t been used in years.
The style I installed for many years was called “lobe sensing” and does not use that piece.
I never had a complaint from any of my customers.

I installed a Pertronix unit into a '60 Buick Invicta, and it didn't have the plastic rotor thing, just seemed to read off the lobes. That was IIRC in 2005.