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Hey all,

I’ve got a GMC 270 with the Carter B&B1 Updraft carb in a 1951 Chevy COE. The carb was completely rebuilt but it’s leaking out the bottom of the bowl every time you open the throttle. I’ve got a video of it which I’ve tried attaching here with no luck. Before I pull the carb, I wanted to see if perhaps there may just be an external screw that just needs to be adjusted or something? Thoughts?

Thx in advance,
Derek

Float level adjusted too high or something blocking the needle and seat keeping too much fuel in the bowl.

Does it leak out after shutoff?

Read here (https://forums.aaca.org/topic/298573-carter-bb-1-fuel-leak/). It may be related.

That's pretty cool. I wonder why the carbs weren't set up like that to begin with.

The marine versions were set up in this manner.

About 65 or so (too lazy to count them) different BB-1 updraft carbs were produced. Depending on the version Derek has, it may be too small for a 270.

Jon

Thx all, really appreciate the quick responses, thoughts and direction. Interestingly in that reference from carbking (edit: didn't realize carbking was one of the people that replied lol) it says that the area is subject to engine vacuum. I took a vacuum line off from the wipers to try and push some gas into as I’ve not successfully gotten it to start yet after complete rebuild. I don’t know if that might be affecting it or not but I’ll put that line back on tomorrow and try again. It ran before rebuild without the “drip line”, but if the vacuum line doesn’t fix it, I’ll fab one of those up. I’ll keep you all posted in the hopes that if I get this all fixed, it will help the next person!

Edit: the number on the carb tag is 745S.

Thx again,
Derek

The 745s was used on a 235.

If fuel is running out, then obviously there is fuel in the carb.

Turn off the fuel supply, then close the choke, and crank for a few seconds. If it doesn't fire, probably not the fault of the carburetor.

Jon

Thx again Jon, will give that a shot...

Yeh, as it had a lot of the stuff from the original 235 on the 270, I thought I'd try to keep it as original as possible (lol)! And since I knew it ran fine with all of that stuff on it, I didn't think there was any risk in keeping it.

Edit: again interestingly, after looking at more pictures of my setup, the port where the drip tube is attached to the carburetor at the top near the idle adjustment screw (in your post), actually runs over to the vacuum on the distributor of my motor. That's the way it was when I bought it, so that's the way I put it back together. That make sense?

Derek

If the distributor has a vacuum advance, then the distributor will require vacuum.

The drip tube should be connected to a full vacuum port with a small orifice; as if it were connected to a say, 1/4 inch hole, it would create a huge vacuum leak.

The purpose of the drip tube was to collect fuel in suspension above the throttle plate (upper portion of carb, intake manifold) which would (gravity) drop when the engine was turned off, and the fuel was no longer held by engine vacuum. The collected fuel in the drip tube would then be pulled into the engine when the engine was cranked for starting (kind of a "starting circuit" in the carburetor (common on some older imports).

The drip tube was never designed to collect fuel leaking when the engine was running.

If the compression is good, then cranking the engine will put fuel in suspension above the throttle plate. Discontinuance of cranking would allow that fuel to drop.

I'm guessing the leak is a result of defective ignition, not a defective carburetor; but tests are in order. Guessing costs money; and often only results in frustration.

Jon

You're saying that even while cranking, sufficient vacuum should exist such that a drip tube should not be necessary. Ok, I think that makes sense as when I crank the engine, it doesn't leak. It only leaks when (1) cranking and pushing on the accelerator...AND...(2) when not cranking, pushing the accelerator alone makes it leak (because there's no obvious vacuum).

So that's a 'no' to the drip tube...but I'm definitely gonna reinstall the vacuum line that I had disconnected that was running to the wipers. Then I'll do some more testing. I know there's good vacuum in that line because as I said earlier, when I dump fuel into that line while cranking, it's on the verge of starting.

When you say defective ignition, whatcha thinking? Rotor, points and condenser are new. The ignition switch, wiring, plugs, wires, cap, and coil are all, also new. Fuel pump is mechanical. Everything else electrically works as it should. The only things different now than they were prior, are the size of the fuel lines