The Stovebolt Site Forums The Shop Area Engine Shop Rochester BC Choke question

Hey Bolters, I had a little time off and spent a little time on the bus. Specifically rebuilding the carb, a Rochester BC. One unusual thing, and something that was commented on, is the extra port on the choke body. It has the normal "hot air tube" port on the one side coming up from the throttle body portion of the carb. But then, directly opposite of that is another port. Looks like 1/8 NPT male. There was nothing connected to it and nothing that looked like it was supposed to in the engine bay.

As you will see from the picture, mine is broken, and looks like it has been for some time. If it is important I could try to drill and tap a new fitting in. Looks like there is enough meat on there to try.

Any ideas as to what that is?

While I am asking questions, I also seem to be missing the internal Vent Tube in the air horn. Does anyone know if they always had them? All the pictures show them but mine just has the angled hole in the horn.

But, not all bad news. I did the rebuild anyways and put it back on. What a difference! No more backfires, the engine ran smoothly and sounded great. Still having some issue with fuel supply but I suspect the pump may be the culprit.

That broken male threaded fitting is in fact the choke stove heat tube port that feeds hot air from your exhaust to the automatic choke mechanism hiding inside that round dial shaped housing with the black plastic adjuster disc.

The heat tube would have had the other end attached to a sheetmetal shield surrounding a horizontal leg of the exhaust manifold forward of the carb location. It fed hot air as the engine warmed up to actuate the automatic choke as your engine warmed up. Perhaps you can take more pictures of the engine manifolds to see what is there. It is possible that the BC carb you have may not be the original carb for the bus. It could be a transplant and your engine may have had a manual choke instead of an auto choke, but we need more pics to try and figure that out.

Note - Without the heat tube your automatic choke won't function normally. It may be that its adjusted to keep the choke off all the time, which woud lead to poor starting and warm up operation, but once it's warmed up would run mostly okay.

Without knowing more details on the specific BC carb you have, I can't help on the internal vent tube. You should look at the carb closely as there are some casting numbers on the main bowl and throttle base that may help ID it, guessing that the original carb ID tag is missing.

Here's a universal fit choke stove that clamps onto the exhaust manifold (or the exhaust pipe just below the manifold) and routes heated air to the choke housing. Simple to install, and the heat tube can be attached to the broken choke housing with J-B Weld instead of a compression nut.

www.ebay.com/itm/316043628428?

Jerry

I thought it might be the hot air but I was looking through the service manual in the tech tips and Figure 7 Choke System has the other tube listed as the Hot Air Tube and shows the connection to the manifold on the same side. Reading further though it tells me that is the choke suction tube which I thought originally. If it is just the hot air connection I think there is enough threads on it that I can make a connection and use that kit Hotrod Lincoln linked. (Ha. Lincoln linked. Mind of a child I have)

There was a casting number and I thought I got a picture when I had it apart but apparently the phone decided that one didn't take. I think it was on the bottom of the bowl. I'll have a look. I'll have to pull the carb anyway once I get some new throttle plate screws. One was missing. 😬

Had a look at lunch and there is a casting number on the bottom of the bowl.

7003536 which is not on any of the lists in the Tips forums but there is THIS post that I'll have to read through tonight.


But I'm also posting a picture of the exh manifold. Looks like there is a spot where a hot air tube was. Anything wrong with making my own from some steel fuel line if I can find the right diameter? I can bring it up to a flare fitting and plumb it into the carb from there?

You found the heat tube mounting end - Yay! You can certainly form your own tube, but it will be easier to use Nicopp tubing (Nickle-Copper) of the right diameter as it its much easier to bend/form and less prone to kinks. I've used the same type tube for brake lines and fuel lines with great results.

On the carb identification, that bowl number is one piece to the puzzle. Look for a stamped number on the throttle body base. IIRC there is more info on the combined numbers in a posted list that could help pinpoint the ID. Let us know what you find.

You can actually just flare a piece of copper tubing and use the manifold appliance you have. As Jerry said you can use JB Weld on the carburetor. If you want to have something removable on the carburetor, JB Weld a piece of copper tubing into the heat port and then use a tube to tube compression fitting. As long as you get hot air into the auto choke you'll be okay.