The Stovebolt Site Forums The Shop Area Engine Shop Rochester B (261 carb)

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

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.

"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

JW,

I figured that must be the case.

Chris

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

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.

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

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?

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.

If the throttle positioner screw, a.k.a. curb idle screw, is turned in too far; it effectively removes the idle circuit from the carburetor, and the engine idles on the main circuit. If this condition is true, then one can close the idle mixture control screw OR remove it and place it in one's pocket with very little effect.

Your description "We set the idle at rough 500 RPMs".

Does this mean you used a tachometer and set the idle at 500 RPM and the idle was rough, OR you set the idle by ear at roughly 500 RPM?

Jon

I have a tach inside my truck and the tach showed roughly 500 RPMs but was running slightly rough.

A couple thoughts/ questions:

1) To HRL’s comment about the power valve. Doesn’t a bottomed-out idle screw equate to a blocked idle circuit? Thus Pre-68 Dave’s suggestion of wrong gasket is worth a look? (Not being a smart-arse, just trying to learn something).

2) I also fully understand the it about throttle opening nullifying the idle circuit. But I have my doubts about 500 rpm being a number at which that is the case. Unless some other engine condition is suppressing idle speed (timing, some sort of miss, etc)…I’m not convinced 500 rpm would expose the transfer circuit.

Something that would be of NDKid’s benefit, would be to pull the carb in its current “500 rpm” state, without making any other changes, and just see if the transfer slot is exposed.

I once had a BC carb, where the throttle shaft bores were fairly worn. This allowed air to leak around the throttle shaft. The idle adjustment screw made little difference when turned. Something to think about, good luck with it.