The Stovebolt Site Forums The Shop Area Engine Shop Air Fuel Mixture Rochester B

Hi Fellow Bolters,
I know the Rochester B is a very common topic here and hope to share some insight and get some too.

I am running a 1956 Chevy 235.
I am also running a new gas tank and and fuel lines with no filter at the moment.
Also running 87 Octane Ethanol free fuel.
Size 58 Main Jet.

I have 4 Rochester B carburetors. I have one that is original as can be with tag for a 216 that came off the original 216 farm truck. I have no doubt it would make a good rebuildable carb. It still has the bronze gold anodizing it good shape

The other Rochester's have the throttle body for 235 and I would suppose
have many mixed parts. The Vacuum powered Power Piston were all different in the 3 other carbs

One had Solid top stem with no tiny hole above the top ridge which would make it a Pre 1953 piston. One has hollow stem with flat opening and a tiny hole above the top ridge. The other has a hollow stem with curved opening and tiny hole above the top ridge.

I rebuilt 2 of the Rochester B and tested them both, trying different air horn/bowl gaskets and the two hollow shaft power Pistons.

Of all 4 Rochester B carbs, none of the Power Pistons slid up and down in the shaft without some sort of binding. This even after rigorous cleaning and emery cloth.

An assortment of throttle body to manifold gaskets making sure vacuum port is open on the bottom. None of the rebuild kits I bought had the right gaskets in them. I had to cut out the vacuum port. This is using the insulator as well.

Float Drop set to 1 3/4" and Float Level 1 9/32".

I found the Power Piston with the Curved hollow top ran the best.
I had this carb. running so well on this 235 I had never heard or seen it run so smooth. The Idle Mixture screw worked very well.

After this run I could not get the motor started again. I figured something happened with the fuel or still dirt in the carb that came loose., I took it apart again and cleaned it and blew with compressed air.

It turns out I had an electrical problem with an inline fuse that would not provide sufficient spark with cranking. I fixed that. She'll start but now the carb hisses or sounds like much air. No vacuum leaks on the outside or intake using carb cleaner spray or unlit natural gas.

She accelerates good with little hesitation.

I tried both carbs and either of them the Idle/Fuel mixture does little to nothing when adjusting it. Seems like they are both running really Lean and not rich.
I have it running RPM wise around 550 and got it to 500 and should be running where the Idle mixture would work. Float levels are right on, I am wondering if the 87 Octane ethanol free fuel requires different measurements in the float.

If the Power Piston is not operating properly, does this bypass the low idle circuit and in turn render the Air/Fuel useless?

There were 2 springs used in the seating of the ball of the power piston stem cap. One is typical straight coil and the other is barrel shaped, tapered on the ends. I found using the barrel shaped ran better, it also has more strength seating the ball.

The Vacuum advance on the distributor is working properly.

I haven't covered everything but I am open to suggestions.

I started to rebuild the correct Carter YF2100s as a replacement but realized it had an accelerator pump housing from a Ford YF carb that is different from the Chevy one. Still searching.

Thank You