The Stovebolt Site Forums The Shop Area Engine Shop Rochester Model B Questions

I have a '58 3200 that I am currently restoring. I have just dug into the carburetor. I started with three Rochester Model B cores. I figured between the three I should end up with one solid carburetor. I have thouroughly cleaned all three carburetors and ensured all the passages are clean. I have a fresh rebuild kit. I feel like I am really close, but I have some questions.

Of the three bases with the throttle valve only one was good. The other two were rusted around the opening for the idle circuit which enlarges the opening causing them to be impossible to dial in at idle. Is this a common issue? Can the other two bases be repaired and is it worth it?

Each of the three donors had different main jet sizes. They were 50, 55 and 56. When the throttle valve is closed I am getting fuel that is not atomized (i.e. liquid fuel) coming out of the crossbar and the carburetor make a gurgling sound. I have read that the correct jet size for a 235 in a '58 truck is a 44 or 46. I am at 6000ft elevation and so I usually use a smaller jet. I have ordered a 42, 44 and 46 and I am waiting on them to arrive. When I went from the 56 to the 50 main jet the gurgling and amount of fuel dropped a lot. Do you think that having the correct 44 or so jet will eliminate this problem? I am not sure why the fuel is coming out of the crossbar with the throttle valve closed.

Finally, when I was using one of the carburetors I was having it bog like the accelerator pump wasn't working. I took it apart and started troubleshooting. All of the ports were clear and the pump was pumping fuel, but there was no pump jet in the throat of the carburetor! The casting for the air horn was flat where the port should have been. I checked the other two and found one other was the same. I ended up using the third that has a pump jet and the accelerator pump issues and the bogging went away. I can't figure out why these two carbs don't have pump jets. Are they meant to be secondary carburetors or are they meant to be used on generators or other implements where an accelerator pump would not be needed?

Thanks,
Chad

There were huge numbers of the Model B's made, and about as many different versions of them. Getting the correct number for your truck would be the best option. Switching parts from one to another is a good way to get of them not working.

The metering is set up to go with the air flow of each carb, you can usually jet up or down a couple of jets without issues, jumping 10 or more sizes may not work.

Do you have the carb numbers of the three you have?

Jon.

Jon,

Thanks for the info. On the pump jet, the passage and the jet are simply not there. As in where the passage should be is completely smooth with no way for the fuel to get up to where the pump jet should be. Then, there is no pump jet with no hole or passage. I can't understand how these carbs could be used on an engine where acceleration would be needed.

Thanks,
Chad

Chad - I have no record of a B ever being used without a pump.

Try making a carb stand to hold the base of the carb, bolt the base onto the bowl, and put these two sections on the stand. Now fill the accelerator pump well to just below the slot (assuming it has a slot), put the accelerator pump back into the carburetor and press it down. See where the fuel exits the pump cylinder. There should be an inlet (somewhere in the bowl), and an exit (somewhere in the venturi area).

Jon.

Jon,

I have done what you suggested and the fuel comes out past the check ball and spring that are held in place with the little metal "T". That is how it is described in the manual as well. There is no passage in the air horn casting that corresponds to the outlet. It is the weirdest thing. I guess at this point it is just academic, although I like for information to be available to anyone in the future that may encounter this and would like to know what the deal is with a Rochester with no pump jet.

I started with 3 numbers matching carbs from swap meets and I have found that two of them don't have pump jets and bog down because of it and the idle circuit is rusted out. I am left with one carb with a pump jet that runs great except that it has non-atomized fuel coming out of the main jet at idle even though I can confirm the throttle valve is closed and within spec. Which means the idle circuit should be used and not the main jet.

I really appreciate all of you help and information and I could never finish this truck without all the great people like you at The Stovebolt.

Thanks,
Chad

I forgot to mention that these weird carbs are the variety that have the screen in the bottom of the bowls and use the aluminum check ball for the fuel inlet into the accelerator pump. The one good carb I have has the slot in the bowl and doesn't use the check ball.

Chad

Joe,

Sorry, I didn't catch your post until just now. I don't have the numbers on me but I will get them tomorrow.

Chad

Chad, I have a Roch B with the exact same symptom you describe, i.e. I could find no exit port for the accelerator pump to squirt fuel into the venturi. The carb has the wide bolt base for a 235.

I was trying to get two Roch B's to work on a 235 dual carb setup and finally punted.
The other B's pump port was clearly visable in the venturi and I could static test it and clearly see the fuel squirt, whereas the other didn't squirt when assembled.
The non-squirting Roch B fills the accelerator pump from the top, not through a screen and check ball at the bottom, it doesn't even have a screen and check ball nor a passage to fill from the bottom.

The Roch B that does squirt also fills from the top. But it has another problem, the throttle shaft has a massive vacuum leak and needs bushings.

When I set the 235 up for a single Roch B, carb "A" would idle fine but stumble on acceleration due to no squirt.
Carb B would accelerate fine but I couldn't get a steady idle or low speed due to the throttle shaft vacuum leak.
Danged if I did and danged if I didn't....

Oddly, the Roch B on my 216 performs great....

Volfandt,

Thanks for the info. I am glad I am not crazy with these carbs with no visible pump jet. I have only rebuilt a few carbs in my life and am no expert. I find these little Rochesters really interesting. But, with so few parts it is frustrating and a little baffling when something isn't working the way it should.

The only thing I haven't checked is the power valve because I can't find my vacuum pump. It seems to stick a little, but I don't see how that would cause raw gas out the main jet at idle and besides it sticks closed and not open. I am going to polish it this weekend and try to smooth it out and see if it helps.

My throttle shaft seems alright. I spray a little ether on it while running and it has no effect on the engine.

I have not cut any corners on the complete frame off restoration on this truck and I don't plan to start now. That being said, I like to do all the work myself. That means I need to learn about a lot of stuff as I go.

It is great having such an amazing community around here and I hope that asking a lot of questions and documenting what happens will contribute to the body of knowledge.

Chad - not all of the various B's have the same casting characteristics.

GUESSING: probably at some point in the life of the two with the non-functional pumps, they have been "rebuilt" by a commercial rebuilder. The commercial rebuilders "group" the parts together from many different carbs (whether it will work or not), and remove the tags so intelligent customers don't know what they are getting (or not getting).

This really underscores why I commented early in this thread that "marrying" parts from multiple carburetors generally results in disasterous results.

Some limit their interchanging only to those castings with EXACTLY the same casting number; but even this is questionable, as the casting number is the number for a "blank" casting. The blanks can then be machined in a number of different ways. The only really safe interchanging of castings is from two carbs with EXACTLY the same identification number AND engineering status code on the tag.

Just in the "for what its worth category", we have records of 137 DIFFERENT Rochester model B carburetors.

Jon.

Happy Days! I have found the issue with the pump jet in the two carbs. There is no passage into the air horn casting like in the other carb. There is a very small jet machined into the top of the well with the little metal "T". It was plugged despite a week of soaking in a can of carb cleaner. After clearing to blockage with a wire and putting a 55 jet in it is running like a top.

I notice that the jet and the corresponding squirt is much smaller on the models without the jet in the air horn casting. It still runs great with no bog on acceleration.

For setting the idle mixture I used a vacuum gauge on the manifold and set it for max vacuum. I notice there is a fairly large range where the vacuum is steady at max (it stays around 15.5 for around 3/4 turn of the mixture screw). Should I shoot for the rich end, the lean end or right in the middle of max vacuum?

Also, I am having an issue where after running the engine for a while and then shutting it off it seems like pressure is building in the fuel line (probably from the heat) and this is causing fuel to push through the float valve and overflow the bowl a little. Not a major issue for drivability, but it leaves a gas smell behind. Does anyone know of a way to prevent this issue? I was thinking about insulating the fuel line or putting a tee fitting in the line with a pressure regulator returning to the input line to the fuel pump to allow the pressurized fuel to vent back to the tank over a certain pressure (like 8 psi).

Thank you so much to everyone for the help.

Set the idle screw towards the lean side. 1 to 1 1/2 out from seated is about right.
As for the heat soak, do you have a thick isolator gasket under the carb? It will help, you can also lower the float ever so much so the fuel has room to expand without running into the motor.

Joe

Joe,

Thanks for the reply. I have the large plastic insulator between the carburetor and the manifold. So, how low can I safely set the floats without having fuel starvation issues? I set the floats based on the sheet included with the rebuild kit. Are we talking an extra 32nd or like an extra 8th or ...? Adjusting the floats sounds much better than one of my rigged solutions.

Thanks again,
Chad

As long as you don't exceed 5,000 RPM at the drag strip, you can get by with adjusting the float level (not float drop) 1/8" more than what the spec sheet calls for. This should give you the extra space for fuel expansion after shutdown. If that doesn't solve your problem, you have other issues in the carburetor, in that the extra fuel may be leaking internally, or taking a wrong route due to blockage.
Carl

I will give it a try this weekend and report back. I suspect I am operating in worst case conditions with it reading 105F on the thermometer where I am working on the truck. So, if I can get it working now, it should work great when it is only 100F around here.

Thanks,
Chad

A permanent fix is to install a restricted "vapor return line" from the entrance to the carburetor back to the fuel tank. As the line is restricted (smaller line), the carburetor gets first choice at fuel, and all excess fuel is shunted back to the fuel tank.

The manufacturers did this in the early 1960's with air conditioned vehicles for the same reason.

Engineers REALLY do things for a reason.

Jon.

Jon,

The feed line is 5/16 from the tank to the carburetor. I have some 1/4 line. Is that small enough? I was thinking of putting a "T" where the line goes into the carburetor and run the steel line back to a "T" right before the input to the fuel pump. That way any excess fuel will be returned to the feed and mixed with cooler fuel from the tank. And, after the truck is shut down any build up will vent back to the tank through the feed line. Do you see any issues with that?

Thanks,
Chad

Chad - running the line to the input to the fuel pump may work, but will require check valves along with the "T". You might also dump some air bubbles with hot fuel into the inlet stream for the pump, and the pump does NOT like air.

The easy way is to get a three way fuel filter with a vapor line and install right at the carb. With 5/16 fuel line, the vapor line will probably be 1/4, and run it all the way to dump into the top of the tank. Lots of folks find it fairly easy to dump into the filler neck on the tank.

Have never had any feedback on just going to the input side of the pump, so cannot give a definative answer.

Jon.

Get a fuel filter for a mid 1970's AMC Pacer. It has two 5/16 lines (one inlet, one outlet) and then the return outlet to the tank is 1/4 inch and has a restriction built into the filter to allow a small amount of fuel to circulate back to the tank.

Put this filter in the line as close to the carb as you can.
It will have a small barbs to use rubber hose and clamps.

Thanks everyone for the help. I am going to try tweaking the floats first. If that doesn't work I will plumb the return line to the filler neck with a filter and restrictor. Thanks again to everyone for all the help.

Chad