The Stovebolt Site Forums The Shop Area Engine Shop Rochester BC question

Can someone help unfrustrate me with this carb? Its on a 235 installed in my 53. It was running rough but reliably rough so I installed an overhaul kit but now it starts good and runs relatively smooth, but after warming up and driving about 20-30 minutes it all changes. Won't idle and quits, then won't start without a little shot of either, bucks and jerks at low speed. I've torn it apart a half dozen times checking/rechecking the float level and making sure I installed everything correctly. The float level is at 1 9/32, drop is 1 3/4, choke rod 5/64 and unloader at 15/64 as per the service manual. I thought it was the choke so i advanced/retarded the coil well beyond the recommended 2 notch's with no improvement. Let it set over night and starts fine then same thing after driving a while. This thing is pretty simple, what am I missing ?

Is the fuel pump keeping up?
Check your timing... a bit to far advanced?
What does 'rough' mean?
What kind of carb?

CASO, rough as in i think a 235 should be smoother. mostly at idle although i can feel some roughness, almost like a miss but not a miss at cruise speed. that was the "bucks and jerks" i was talking about. i expected it to run smoother after the kit but that didn't happen.
its a Rochester BC off a 55 Chevy car.
i'll check the FP and recheck the timing soon as i can.

thanks

As a test, next time it starts acting up immediately remove the aircleaner and look to see if the choke is closing up. If it is you should also see black smoke.

You could also be getting vapor lock on the fuel line where it runs from the fuel pump to the carb. Make sure the line isn't touching the block/head/valve cover etc., where heat coould be getting transfered to the line. May even wrap some aluminum foil around the line where it's close to a heat source for testing.
One other cause could be the vent is clogging up in the gas cap. To test for this remove the cap just when it starts acting up to see if it clears. I've had this experience on several old tractors but not in an auto, but it's the same principle.
Be careful w/the ether as an engine can get addicted to it...

Are you sure its the carburetor giving you trouble and not the ignition failing?

An old timer once told me in an effort to keep it simple, an engine only needs atomized fuel/air, spark, and compression to run. He said to start with the easiest to check/fix first. He said that he saved the carb for last because it is the least likely culprit and the most difficult to fix. Is your intake heat riser functioning properly? If it is stuck in the heat position you will have poor performance once the engine is warmed up. I would next look at all of the plugs to see if they all look the same. If one or two look bad then you may have a problem with those particular cylinders (IE rings, valves, valve clearance) and not a fuel delivery problem. Have you checked the condition of the points? Have you checked for vacuum leaks? Bottom line, start with these easy checks then move on to the more difficult checks if necessary.

This problem is something that is degrading from heat probably. Probably not the carb. Many good ideas have been mentioned. Sometimes a coil will do that, wires will do that, valve train will do that, heat riser, vapor lock as mentioned,.. if it's the carb at all, it's a piece of dirt floating in and out of somewhere. Keep thinking and testing ideas. It can even be a weak battery discharging too low, then magically rejuvenate upon "settling", I know sounds crazy. Watch a few UTube videos on BC rebuild to be sure you got all the right balls in the right hole and you didn't leave a stuck ball in there.

Well...that's a lot of good advice. I'm not sure of anything right now so guess i'll start all over with the basics, not carb, and work from there. I did find a clogged accelerator pump orfice. Cleaned it up, took her out tonight, thought i was on to something but 30 minutes later...the same thing. I am big on the KISS principle (keep it simple stupid)so i'll check everything else. Nothing sounds crazy at this point and may be something else like ignition, compression, valves, etc. Oh well....more to come.
Thanks everyone

Oh yeah, if anyone thinks of anything else while i'm checking, let me know....

Just one more thing, What temp thermostat are you running? I used to run a 195 degree thermostat and had bucking, [censored] and moaning after warm up. I then changed to a 180 and every body lived happily ever after. I think I was experiencing fuel (Ethanol) boiling and vapor lock. Keep us up to date.
Jerry

This is great. I needed this to get my focus off the carb, even though that seemed to be when it all happened but i was experiencing symptoms before and why i overhauled it in the first place. Maybe the old carb was masking something else. I've made a checklist of everyones thoughts, wish i had the day off to jump into it. I have checked the choke when hot, its open. The heat riser is free and I put some C5 on it. The vacuum holds at 21. No other apparent leaks. I'll work thru the rest soon as i can. Got to be one of them !!!(i think)
Thanks

I would at least be carrying a spare coil with me if I were you. The alternative is just to put a new one on.

It is good to hear that your heat riser shaft is free. Is the bi-metallic spring moving the shaft as the engine heats up? Have you checked the condition of the spark plugs as soon as the engine starts messing up. This could tell you a lot.

Yes the spring does move the shaft. At lunch I did a compression check. It's 150 on all 6. Well, +/- 2. The plugs are sooty (rich). I leaned the idle mix a little but will set it to the vac gauge later. I'm picking up a coil tonight. Maybe weak spark. Just working down the list, we'll see.

EEEHAAA, OK so I been working down the list of possibilities when I had a dream to go back to the carb. Been there done that at least a dozen times but oh well. Now I'd got my overhaul kit from NAPA, you know, the guys with the good stuff. So with no better idea's I put the old needle and seat back in, took her down the road and she runs like a champ. Apparently it was not shutting off completely and causing my problem. You know what they say when you assume !!!! Life is good again..
Thanks everyone.

Congratulations! You can now consider yourself an expert in early 235 diagnostics. Especially since you were able to put aside everything which we suggested was your problem and went back to the carb to find the cause. Now that is top level diagnostics.

Thanks but i think it was lucky frustration. I did most of the suggestions and wasn't nailing it so took a shot....

you guys had some good stuff to remember though...

The sooty plugs would have led me back to the carb as well. Learning to read the plugs will often lead you down the right path and they are the easiest thing to check.

Your right and that's probably what took me back to the carb. but what a relief it was to see the compression good. I bought the engine thru a friend while i was rebuilding the truck. He said it was strong so i figured that was one less thing to worry with. But not tearing it down, you just don't know for sure. I think that was in the back of my mind the whole time. Looks like it is as it was advertised.