The Stovebolt Site Forums The Shop Area Engine Shop Ran great - then instantly horrible

'52 3100 with 235 and 1 barrel rochester.
Truck wasn't running perfect but OK. The other night I changed the oil and replaced the gasket on the carburetor between the air horn and the fuel bowl (since fuel was seeping out). I reassembled it and could barely get it to run. Took carb off, cleaned it, did minor adjustment to the float, and reinstalled. Idled fine. Took it for a drive around the block and ran good for about a mile, then from a stop I stepped on the throttle a little hard and it back fired, starts coughing, smoking, and have to give it lots of throttle to keep it running. Barely got it home. Got it running the in garage smoking and running terrible, then I took off the air cleaner and it started running more like normal. What happened? Where do I start?

Thanks in advance... please feel free to speak like I know nothing!! That was the first time I've taken a carb off but want to learn.

First, take the carburetor back off. Make sure the holes in the gasket match up exactly with the holes in the base of the carburetor. If not then get your drill and exacto knife out and make the holes match. Put it back on and try again. 🛠

If the gas was leaking out where the top of the carb meets the float bowl body, the problem is not the gasket. I am assuming here that we are talking about a Rochester. You have a case of flooding. The gas level should not get up that high. Also that gasket comes in two different versions and you need to use the correct one.

Yes it is a Rochester. Gas was leaking out at the top of the float bowl body. I adjusted the floats to the proper specs and no leaks now. I took the gasket I installed from another Rochester -- I didn't check every detail. I'm hoping it's just the wrong gasket - I'll check tonight. It just seemed weird that it ran so well for a mile or so then completely horrible.

All I am going to say it is frustrating when one day it is great then it is not.

I had something vaguely similar happen to my Rochester B: a backfire blew the choke disc closed, but it went back in place after a while.
HTH

I had a similar experience last year where mine was cutting out, bucking and backfiring and then running perfectly. It turned out to be a failed condenser. I was able to hook up my timing light, point it at the front window and observe the spark die as the engine was failing.

Check that air cleaner to see if its plugged up. If your carb keeps loading up you are getting trash in the needle & seat from the gas.
George

I believe your Rochester is leaking/seeping because of a warped air horn and this condition can and will cause you to lose vacuum and in turn this loss of vacuum can cause your power piston to fail and make your carburetor run very rich (with no cure). If you want to switch to a Carter, I have one which was rebuilt before I got it and never used. Look in the "parts for sale" section. It is one of two models Carter made specifically for your engine. On the other hand if you want to straighten it, last summer I posted tons of information and images on how to do that. Good luck.

Just an update for anyone curious ...
Took the carb off / apart and cleaned. Reassembled and still not right. I found 2 other Rochester carbs that someone gave me. Took all 3 apart and cherry picked the best parts to make one "good" one. Truck seems to start really hard from cold start, runs great at idle, pretty good running down the road now. Still not 100% right - if I bump the throttle hard I get a back fire. Ordered rebuilt kits for the other 2. Thanks for the help.

This may sound stupid, but have you ensured your heat rise flap is moving? Also, do you use choke to start it when cold? Could your timing be off just enough that when you bump the throttle the advance is not turning it enough? Just thinking outside the box.

Chris

Sounds like the accelerator pump is not working quite right. Rebuild kit should take care of that. It should squirt a pretty good amount of fuel when opening the throttle quickly. It could also be lean and stumbling when opening throttle at speeds. Were all three jetted the same?

I use the choke to start it - It has been starting really hard on cold starts with current jet / setup, great when warm. Not sure on the heat rise flap. Planning on checking that along with timing and vacuum this weekend. The accelerator pump did squirt out some but no idea if enough.

Carb jets -
Original was unmarked.. looks bigger than .5 - similar to .57
.57 was in it for a little bit - started good but didn't run great
.50 is in it now - starts real hard when cold - runs great when warm at low speeds, cruises good, and accelerates Ok

Going to do full rebuild on one of the carbs and then switch. Suggestions on jet size?

Mike a size 50 jet is small for a 235 with a Rochester B and ethanol laced gas. Usually 58 or 57 is what I've seen with a couple of 56 sizes in there when the 235 was a younger engine (maybe 1950 year or so). A 50 jet will make your carburetor run very lean. At this point it is very hard to unscramble the egg, but as I recall the smaller jets were common in the 216 engines...size 51, 52 jets, etc. If I was going to use a Rochester B or BC on a 235, I'd start with a 57 size jet and then increase to 58 if needed.

But honestly before I spent one cent or any more time on a rebuild kit for the Rochester I would consider a Carter. The design is much better and they don't have that wacky power circuit which with unleaded/ethanol gas is just about guaranteed to get stuck in the open position.

If the carburetor you have was really intended for use on a 235, then I'd guess your air horn and possibly float bowl are warped (which is so common with Rochester B and BC it should be automatically expected) and if so, I would bet your power circuit is in the failed mode (meaning your power valve is open all the time). If somebody has put a 50 size jet in there, it could have been because they didn't understand what was happening with the power piston/power valve, didn't know what to do with it and they were trying to get the thing to run leaner. Or it could be that parts have been mixed up a lot in there.

Good luck.

Mike, in case the work I did last summer isn't easy to find, here's a link to where (as I recall) it began: https://tinyurl.com/y4a4bktb

Good luck.

If you're pulling the carb apart again for a rebuild check out this thread:

https://www.stovebolt.com/ubbthread...ster-carb-not-squirting.html#Post1314009

Mine was giving me an absolute fit until i found that the "T" holding down a spring and check ball was installed wrong and blocking a jet.

Mike's Carbs has a great video on YouTube for rebuilding a Rochester. It's like 45 minutes long. He's also where i got all my rebuild parts from.