The Stovebolt Site Forums The Shop Area Engine Shop Rochester B Carburetor problems

Ok folks...

Rebuilt my rather leaky Rochester "B" over the last weekend (attached to my 58 235), and had a rather vexing problem after what I thought was a very careful rebuild. Verified my work with the Rochester B manual at the Old Car Manual website.

Idles fine, so no apparent problems in the idle circuit. Accelerator pump is functioning perfectly, verified that visible amounts of gas are being sprayed into the carb throat with and without engine running. Problem is at the transistion from idle, in that it dies under modest acceleration, so perhaps I can expect to see problems in the "Power Piston" system.

However, where else may problems crop up? Checked for vacuum leaks, new insulator properly aligned, no changes in the engine tuning from before the rebuild, no visible external problems with the accelerator pump linkage. Everthing looks bloody glorious, except for the fact that it doesn't run worth a flip.

I have said before that I was at one time a mechanic, but I didn't say I was a good one! :rolleyes:
Any Ideas?

Have you checked the vacumn advance on the dist. to see if it is working?Just a thought.

Well, haven't really touched any other items except for the carb in the rebuild. Did check to make sure that the connection at the base of the carb going to the advance was tight. I will give that a good going over though.

Thanks,

Richard, my son's 50 has a simular problem can't tell you what to fix but will be watching this thread closely.

On my old 64 I ended up polishing the power piston w/000 steel wool-seems clean just wasn't good enough. Did the trick on mine,,Brian

The only circuits that seem to be in this carburetor are the idle circuit and the main, or high speed circuit.

The book indicates any transition from idle to main that reduces manifold vacuum by 5hg will unseat the power valve and give the engine the needed shot of juice to increase RPM.

I think the accelerator pump is for less subtle throttle changes...as in "put yer foot in it".

My though would be something didn't get cleaned properly when the parts were 'tanked'.

As Brain mentioned, the piston which 'is' the power valve must operate freely. It has a spring, but I think it is quite small.

If you carburetor didn't do this before the rebuild, and it does it now, it would be a safe bet something is rotten in Baghdad.

The base gasket must be oriented properly to expose the little vacuum sensing hole.

If this hole were to be blocked, I think the power valve would be in a constant state of OPEN!

Having said all this, keep in mind I was absent from school when they taught 'Rochester's 101'.

Stuart

If the power valve is opperating correctly The float level may too low. Try enriching the idle jet a little. That may help. The good thing about these carbs. is the are so easy to go back into. Try that on aQ-jet.

Before going back into the carb, try readjusting the idle for a leaner mixture; with more throttle opening.

If you have a perfect carb (oops, forgot we are talking about a Rochester B ), and the idle is set too "fat", the vehicle will exhibit exactly the conditions you describe.

Jon.

Allright, my friends,

We shall see tomorrow morning, and I shall report back what I find out.

Jon, I will try your idea first.

THANKS TO ALL!!

Here's the rest of the story...

Jon, I tried your suggestion to no avail, so I knew it was time to get the carb back apart. I removed the air horn part, leaving the rest on the intake.

Took this to the bench and I noticed a slight drag on the power piston, so I took the main well support off and threw everything attached to it back in carburetor cleaner.

Took it all out and washed the parts carefully and it was then I noticed the problem. At the bottom of the main well support, there was a small piece of old gasket material I had not cleaned off. It wasn't much, but it was enough that when I had initially reassembled the carb, the support was tipped back far enough that the top of the power piston rod was scraping slightly against the sides of the support and not cleanly contacting the power valve.

RECLEANED everything for good luck, put it all back together, reassembled the carburetor, and it now works perfectly (OK, Carbking, at least well enough for being a Rochester B). And obsa, you might have your son check the power piston assy in the 50 for some similar problem.

Thanks again everyone. That's why I hang around here. You guys are the best.

Richard - glad you found the problem. With the power valve in the state you found it, the engine would be running rich, especially at idle, with droplets of fuel adhering to the walls of the intake. When the throttle is opened, one then has (contrary to what many believe) an instantaneous RICH condition causing a hesitation. Same symptom as setting the idle mixture screw out too far.

Jon.