Rochester B

[Walker]

In adjusting things up in the engine bay, I noticed my carb, a Rochester B, would increase the idle when the throttle adjustment is screwed all the way in. I will be cleaning it out but was wondering if there is anything in particular to focus on for this problem. It's on a rebuilt 2.25, '72 Series 3. Thanks.

[jonnyc]

I may not quite understand your question, but isn't it supposed to? Screw goes in, throttle goes back, more fuel flows, idle speed increases. Are you asking about the mixture screw?

[Walker]

Yes, the one near the base of the carb. I screwed it in as far as it would go without forcing it. If memory serves, that should have leaned it to the point of starving it, but the rpm's just kept increasing.

[east high]

You're right. It should have stalled with the screw all the way in. You mave have an air leak.

[SafeAirOne]

You're right. It should have stalled with the screw all the way in. You mave have an air leak.


I'm not familiar with the theory of operation of any of the particular carbs that work on the 2.25, but if it's that's the idle mixture screw that you're turning in to the point of cuttiing off the idle fuel supply and the engine is running higher and higher RPMs while you're adjusting it, then I'd look for fuel leakage on the primary fuel supply circuit that is continuously supplying fuel to the engine when it shouldn't be, for whatever reason.

I'd check to see that the fuel float isn't sticking open, forcing fuel through the system, linkages/cams for correct adjustment, etc.

What do your spark plugs look like? Black/sooty?

Again, this is based on the theory of operation of OTHER carbs plus your description of the issue, so it may or may not be applicable to your situation.

[jonnyc]

Hooray, the Rover is much better, and the wife is much happier!!!
I'm referring back to the original reason for this thread; major vac leak(s), really rough idle, violent backfiring. Rebuilding the carb and sealing every possible vac leak spot seems to have helped loads. I now can adjust the idle and mix screws and actually get the truck to run differently. Vac pressure is now a steady 19-ish. I think I still need some fine tuning, however, so here are a few questions:
1. On a Roch. B, putting the mix screw in makes it leaner, correct?
2. For better, smoother acceleration, do I want it on the leaner or richer side of things?
3. The linkage all seems solid and the springs good, but the idle speed sometimes (too often) hangs too high and doesn't drop down. This is a fairly recent (the last 6 months). Lifting the pedal or pushing up on the throttle arm will drop it down, but that doesn't really help while on the road. Any thoughts or suggestions?
4. Will somebody please start making new Rochesters so we don't have to keep working on 1950s carbs?
Thanks for the past, present, and future help.

[east high]

Glad you got it fixed. You might try a stiffer throttle return spring if your idle's floating; however, my Rochester exhibits a slight floating, too, so maybe it's an old as dirt Rochester thang?

[jonnyc]

Richened it up a bit and it's running OK. Still a little "fluttery" with some hesitation in 2nd gear, but definitely drivable. Just ordered the two return springs and should get them installed over the weekend.
My fingers are crossed.

[east high]

You'll want to run the mixture screw in as far as possible while still maintaining a smooth idle. Run it in until the engine starts to stumble, then back it out a quarter turn or so. Blip the throttle a few times and see where the whole thing sits.

Also make sure it's jetted properly for your altitude and you have the right plug for your compression ratio.

Where'd you get your rebuild kit?

[Walker]

I just picked up a rebuild kit at the local NAPA. Just tell them it's for a '60s era Chevy 6. The jet that came with it was a '53' and mine should be a '51' so checking the jet is a must.