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

Great description. I have a Roch B on my 216 that runs nearly flawlessly (knock on wood) so they do have that capability.

Without an air cleaner on it does make a pretty loud sucking sound down the throat. Cars of that era had special air cleaners that looked like oil baths but the large chambers were for muffling the sound of the carb drawing air. Just thought I'd add that to the convo.

re. fuel/air adjustment inop.
Sounds like the power piston(s) may be stuck in either the open or closed position. With little engine vacuum it's supposed to open and dump more fuel into the venturi (like when accelerating and/or the throttle is opened past idle/low speed to compensate for more engine load).
And at high vacuum (idle/low speed) it's supposed to close and not allow the additional fuel, allowing the idle/low speed circuit to fuel the engine).
I'm unsure what circuit the power pistons fuel travels as I've got Roch B's with differing internals but it could be rendering the idle circuit inop.

Thank You for the info and reply Volfandt!

I am indeed running with out the Oil bath on and I did throw it on there for a brief period and I could not hear that air Sucking noise at all! That makes me feel a bit better. I thought I had the the Power piston moving pretty good up and down in the bore but it did barely have a binding feel at a certain point in the bore. I suppose this is a bigger problem than imagined. I was surprised to see that all 4 Rochester's did not have a smooth pull in the piston bore.

I'll try lubing the stem with gasoline next. I made sure the stem was also straight and not bent.

Look forward to hearing any more comments. Thank You much

OK, not my favorite carb, but maybe this will help.

The vacuum piston is designed to be operated by negative pressure (vacuum). Virtually all pistons will have some binding if pressed on rather than being pulled. Try applying vacuum at the vacuum source.

The float will not need a different adjustment with E-10, but the fuel jets (both idle and main metering) may. We suggest opening the idle jet 0.002 inch, and the main metering jet 0.001 inch.

Once the idle jet is enlarged, the idle mixture control screw may function as designed.

Contrary to popular opinion, the idle mixture control screw does NOT control the idle mixture! Rather, it controls the volume of the preset idle mixture which is determined by the values of the idle jet, the idle air bleed, and the idle bypass. For an analogy, think of a three faucet shower. The hot water would represent fuel; the cold water would represent air; and the big knob in the center that controls the amount of water represents the idle mixture control screw. In the case of the carburetor, BOTH THE FUEL AND AIR INPUTS ARE FIXED INSIDE THE CARBURETOR UNLESS MODIFIED. Opening the idle jet as suggested above does make the mixture richer.

Jon.

Thanks for the analogy Jon.

When you are talking about the Idle Jet, Where exactly are you talking about if not the main metering jet?

Thank You again for sharing your great info!

Update: One for the books

When testing with the the Straight Coil Spring and Barrel Taper Spring that fits on the check ball of the Power Piston stem.

Somehow the check ball made it inside the middle of the barrel spring. The open ends of the barrel taper spring are much smaller than the check ball, I am not sure how it got up there.

Regardless, I fixed this and it still made no difference in the Rochester.

I am running Ethanol free 87 Octane sold by a new Hy Vee Gas Station in my area, which I am ecstatic about. It seems to have a Bluish Purple dye to it that looks much different from the E-10 fuel.

When you refer to the idle jet, are you talking about the passage after the main metering jet?

Appears you are about an hour away from Jon and the Carb shop in Eldon,MO (unless your in the other KC). If I were any where near that close I would go visit him in person. I've visited with him over the phone, and wish I could spend a week following him around. Give him a call.

I keep forgetting that the Rochester B has the idle tube permanently pressed-in, and as such, not readily accessible. Since you are able to purchase non-ethanol fuel, probably better to leave this enhancement to when you upgrade to a different brand of carburetor. It can be modified, but why bother, when better carbs are readily available.

Sorry for the confusion.

Jon.

Thank You Jon for the clarification.

Allen, Eldon is 2 1/2 hours from KC. Jon do you accept walk in customers, say if I needed a kit or a certain part?

Yes, but call first, as we are only open some days; and all kits are made to order. We offer over 10,000 different kits. Try to visualize the amount of shelf space just one of each would require if we had one of each on the shelf.

We also encourage mini-tours IF WE SCHEDULE IN ADVANCE. So bring a couple of your friends and we can talk carburetors. As Monday and Tuesday are communication days, best to schedule a different day. Often, I have a low battery light on my left ear at the end of Mondays and Tuesdays. Unable to really concentrate on in-store customers when I am trying to answer the phone.

I will guarantee to show you carburetors NONE of you have ever seen before.

Jon.

Jon, Awesome! I will call sometime, and hope to get some Carb guys together.