The Stovebolt Site Forums The Shop Area Engine Shop Rochester Power Valve.

My 250 has an issue with stumbling when taking off or when the engine is still at a slower rpm right after changing gears.

Is this the power valve hole on the base of the carb?
http://rides.webshots.com/photo/2029599170100720268TyiElo

And how does it get drawn by the vacuum of the engine with no hole in the intake manifold, it just has the little gap at the gasket that holds it up. http://rides.webshots.com/photo/2133706940100720268rthHVr

I did trim the gasket back so that the hold I'm pointing to has a clear shot to the main passage in the manifold. Am I missing something?

Thanks!

Well you got it right, the slot in the gasket is how the manifold vacuum is ported to the hole in the base of the carb. If it doesn’t line up then I would say you’ve got the wrong gasket. Just file or cut the slot over to line up with the hole in the base or better yet, get the correct gasket.
It sounds to me more like you’ve got a bad accelerator pump. Look down the throat of the carb and make sure that your squirting fuel into the venturi when you operate the carb.
Sticking power valve would usually show up with a loss of power when the engine is under load. When the butterfly is open under acceleration or load and the vacuum has dropped to nil the spring under the power valve pushes it up and moves the ball check off the jet in the main well and adds more fuel all during this period of operation. The power valve has very close tolerances and any dirt/sludge/gum/scratches or burrs can cause it to stick, it’s not unusual to find the stem bent also after they have been worked on. The correct spring pressure is critical to its proper operation. If the carb has been worked on, the gasket under the main well is often the wrong one or installed incorrectly causing the main well to sit on the air horn cocked, then the stem on the power valve doesn’t line up properly.
And finally, as the carb king would tell ya, the problem might just be that, it’s a Rochester!!!
Denny Graham
Sandwich, IL

Thanks Denny. I was thinking that was the power valve, but I wasn't sure if that was causing the problem. When I took the carb off, I drained all of the gas out of it by opening the throttle and closing it. It has plenty of power when I pulling a load, it just stumbles occassionally when I take off or try to rev it up.

I've read carbking's opinion of the Rochester, thats why when this summer gets here and I have a job, I'm upgrading to a Clifford head and a Holley 390 4 barrel.

Also check the float level. You might try raising it 1/32 above specs. You may want to go to 1 or 2 thousands larger main jet.

The power valves in Rochesters tend to hang open rather than shut, but they also tend to get a bit stiff and not slide real well in the piston bore. If that happens, lap the piston with some fine rubbing compound in the bore (make sure you rinse it out well) and it should slide and operate a lot better.

I didn't get a chance to tinker with it anymore this weekend. I had to put it back on as it was and move the truck back behind the shop (mom likes parking her car in the garage). I'll keep this stuff in mind when I'm home again in a month after exams.

This Rochester giving me issues constantly is the biggest reason I'm upgrading to something much nicer. I may have to keep it around as a paper weight, or make a desk lamp out of it. A Rochester has to be good for something.