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Rochester B (261 carb)
#1435713 Sun Jan 02 2022 02:39 PM
Joined: Jan 2013
Posts: 2,087
'Bolter
Gents,

I’ve had my Rochester B rebuilt at a reputable company recently. We set the idle at rough 500 RPMs and started to adjust the fuel air mixture. We can turn the needle all the way to shutting off and it still runs. Shouldn’t it sputter and die? The RPMs should be low enough so fuel doesn’t dribble down the throat. We checked the float level and it is correct.

When the engine idles it is close but not perfectly smooth as I have heard other Stovebolt’s.

Thoughts?

Chris

Re: Rochester B (261 carb)
ndkid275 #1435726 Sun Jan 02 2022 03:50 PM
Joined: Jan 2014
Posts: 1,965
J
'Bolter
You are getting fuel via some other circuit or internal leak. (I suspect you already know that).

Other, smarter, people will have to chime in, but you could:

* Fuel pressure test (doesn’t take a lot to overcome the needle)
* If you have some means of applying pressure via a hand pump, could see at what pressure the needle pops off.

Re: Rochester B (261 carb)
ndkid275 #1435729 Sun Jan 02 2022 04:22 PM
Joined: Feb 2004
Posts: 24,252
H
Kettle Custodian (pot stirrer)
"The ABC's of carburetion are Always Buy Carter". Use the Rochester as a stinky paperweight!

It's possible that your "professional" set the float level too high and gasoline is spilling over into the airstream from the venturi. Look down the carb throat with the engine idling, and see if there are drips of liquid gas coming from the middle off the carburetor. That shouldn't be happening. It's also possible that the cover is warped, and gas is being pulled into the airstream by engine vacuum that's getting into passages where it doesn't belong.
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Re: Rochester B (261 carb)
ndkid275 #1435774 Sun Jan 02 2022 11:06 PM
Joined: Jan 2013
Posts: 2,087
'Bolter
JW,

I figured that must be the case.

Chris

Re: Rochester B (261 carb)
ndkid275 #1435784 Sun Jan 02 2022 11:40 PM
Joined: Feb 2004
Posts: 24,252
H
Kettle Custodian (pot stirrer)
A Carter YF calibrated for a 258 cubic inch AMC/Jeep 6 cylinder should be an excellent match for that engine. There are usually several of them available on Ebay. Use a real Carter, not a new offshore "clone"!
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Re: Rochester B (261 carb)
ndkid275 #1435797 Mon Jan 03 2022 01:17 AM
Joined: Jan 2001
Posts: 5,197
P
'Bolter
Is the base gasket correct to allow vacuum to get to the power valve port?


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Re: Rochester B (261 carb)
Pre '68 Dave #1435799 Mon Jan 03 2022 01:25 AM
Joined: Jan 2014
Posts: 1,965
J
'Bolter
Originally Posted by Pre '68 Dave
Is the base gasket correct to allow vacuum to get to the power valve port?

Ooooh. There’s a good theory and would be a quick win if that’s the case.

Re: Rochester B (261 carb)
ndkid275 #1435801 Mon Jan 03 2022 01:49 AM
Joined: Feb 2004
Posts: 24,252
H
Kettle Custodian (pot stirrer)
Unless the idle passageway is also blocked, a power valve in either the rich or lean position won't affect the idle mixture. If the throttle blades are open far enough to expose the part throttle transfer slot, the idle mixture screw will have little or no effect. Slow the idle speed down as low as it can go and still keep the engine running, then remove the carburetor and look at the relationship of the throttle blade and the idle jet and part throttle ports. It might be necessary to drill a small hole in the throttle plate to allow the plate to close far enough for the idle needle to work properly.
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Re: Rochester B (261 carb)
ndkid275 #1435805 Mon Jan 03 2022 02:08 AM
Joined: Jun 2008
Posts: 1,700
S
'Bolter
I remember someone (Carbking?) said you shouldn’t use a paper gasket if the carb is sitting on top of a phenolic spacer. Chris’ carb sits on a spacer. So wouldn’t using both cause a problem?


Chuck
1950 Chevy 1/2 ton (all original)
1951 Chevy 1/2 ton (future streetrod)
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1938 Chevy coupe streetrod
Re: Rochester B (261 carb)
ndkid275 #1435808 Mon Jan 03 2022 02:17 AM
Joined: Mar 2010
Posts: 10,076
5
'Bolter
I would start out with trying something easy and free.
Return the mixture screw to the correct starting point.
Start the engine. Let it reach operating temperature.
Turn the idle down to 400.
Now see if turning the mixture screw has any affect.
At 500 RPM, the throttle plate may be too far open, exposing the hole in the throat which will add extra fuel, making the mixture screw adjustment non-functional.

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