The Stovebolt Site Forums The Shop Area Engine Shop 1954 235 rebuilt with Rochester model B

On this old beast we have a issue. my 1954 235 stovebolt with a rebuilt rochester model B carburetor was rebuilt by a local guy who has been doing engine work for the past 40 years. when we got it back and i fired it up for the first time in 12 years, there is this horrible popping that is almost like backfire out of the carburetor that happens at random times. i dont know what is causing it. ive re adjusted the timing about 5 times and it still pops. does anyone know what causes this and how to fix? i have 2 possibilities, that being either still a timing issue or a valve issue.
ill check the adjustment on them eventually but id rather ask the pros first. it has only ran for a total of 3 minutes so if it is a serious problem there shouldnt be too much damage.

on this note, when it "backfires" it spits out the fuel air mixture. when i rev it it gets worse but when its idling it might happen but again ive only ran it for 3 total minutes so idk what the problem is.

First I'd pull the valve cover and check the push rods.....then go a little deeper.

its certainly worth the shot. they might need extra lubrication too

i should also mention that it doesnt have an exhaust and it can still be a timing issue. ill try both things first before continuing. still will try to keep the engines run time at a minimum in case something is being damaged.

Sounds like a sticky intake valve. If the intake valve sticks in the open as the piston moves up and while the exhaust valve is closed, you will have a backfire through the carburetor.
Twelve years is a long time for an engine to sit. Is the there any twelve year old gas in the tank? Varnish from old gas will gum up valve guides and glue valves to the seats.
How often does it backfire? Does it seem like on cylinder?

First, double check the firing order. 1-5-3-6-2-4, clockwise looking down at the distributor rotor. It sounds like one cylinder is firing with a valve open. Tight intake valve clearance or a sticky valve ae also definite possibilities. Check the firing order and adjust the valves before you go into a Phase 10 panic and start throwing parts at it.
Jerry

its been completely rebuilt and i have it sucking fuel from a brand new gas can. i originally thought sticky valve and since everyone here has suggested it, ill go check the clearances tomorrow.

ill take the valve cover off and run it and see if any of the valves are sticking. im still positive its some kinda timing issue because the first time i set it it was popping like crazy even on idle. but now that ive set it yet again it doesnt pop on idle anymore, just on rev up and holding high rpm. ive checked everything and it idles pretty much flawlessly except it might pop like once or twice at complete random. once i get the exhaust on ill give it the break in run that might help too. my best guess is play with the timing again and obviously check the valve clearances and re oil them again. just thought id play it safe and ask around. also the popping is at complete random so i cant figure out which valve is sticking (it might even be more than one)

if i can figure out how, ill record a video and post it up here, its a weird phenomenon that i tried my best to explain. sorry for the grammar issues its way too late and my brain is really out of it.

it could also be timing because when it "backfires" it shoots what is either unburnt fuel back up or it shoots up exhaust which if i could determine, could tell me if its a timing issue or just a valve problem. im also thinking it could be just timing because when it idles it doesnt do anything but when i go to rev it it starts to pop but it isnt shooting flames through the carburetor so i still have no clue what is causing this.

Cracked distributor cap, incorrect gap on points are a couple more things to look at

It sounds like you tried to do an engine run before everything was dialed in and ready. The longer you mess around chasing gremlins and running the engine at idle, the less likely it's going to be that the camshaft is going to survive the ordeal. Are you running solid or hydraulic valve lifters? It could be that you've got a mechanical lifter adjusted too tight or a hydraulic is pumping up and holding a valve open when you rev it up. A few minutes' running with the valves a little loose is preferable to the problems you're having, at least for the break-in run. I set the valve clearance .002" wider than specification for a cold start, and ignore any lifter rattle until the cam run-in is complete. It's worked that way for me for over 50 years.
Jerry

just found the issue, the exhaust valve on cylinder no.2 was stuck shut and i think the lifter got frozen up and only the bottom half was moving. anyway i flushed it out with wd-40 and tapped the adjustment screw and it snapped back. started it up and it runs like a dream. even holds idle with the choke all the way open and its super quiet now..

Congratulations, Mr. Dane

If the exhaust valve was froze stuck, how did you avoid bending a push rod like a pretzel when the lobe of the cam pushed the lifter up on the push rod?

i gave the rocker arm a very light tap and it broke free after flooding the valve with wd-40. apparently the guy, tommy, who rebuilt it forgot to oil the lifters when he gave it back us and it sat for longer than id like to admit before it finally saw oil to the head. the tap was so light i could probably do it to your pinky finger and it wouldnt even hurt.

thank you sir!

check out my reply to 52Carl. also it has hydraulic lifters, for reference.

the lifter was frozen in the open position and was empty.

If you are setting hydraulic lifters with clearance, you have not properly adjusted them. Hydraulic lifters get 0 lash plus a little more. Are you absolutely sure you have hydraulic lifters?

This thread doesn't make good nonsense. It's impossible to have a valve stuck closed without something being bent, broken, or assembled incorrectly.
Jerry