The Stovebolt Site Forums The Shop Area Engine Shop GMC 270 Updraft Carb

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

New doesn't mean anything these days. Brand new stuff is crap made in China and it's a roll of the dice whether it actually works or not.

Are you getting good spark at the top of the dizzy? At the plugs? What about the condenser?

Check the fuel pump output and pressure.

Thx again all for the direction and guidance. I shut the fuel off, closed the choke, cranked but got no start. I actually had one of the in-line ignition testers that buoymaker recommended. I plugged into several plugs/wires and never got any blinking (indicating an ignition system problem). I'm nearly 100% certain this is distributor related as this is the only part of the whole truck restoration I had trouble with. I know, how in the hell do you have a problem rebuilding a distributor. Let's not talk about it and move along

Now, my question is, instead of dicking more with, should I just replace with electronic ignition? If so, would this kit work? As a reminder, this is a 6V Negative Ground system. Or am I giving up on too easily?

Thx,
Derek

Heck no! You'll be asking for even more problems trying to monkey it up with electronic ignition.

Pull your dizzy if you're sure that is the problem, and send it to one of the rebuilding facilities like Cardone, or Atwater-Kent, or someone else.

Before that, I'd check all dizzy related parts, though. Primary wire connections,points, condenser, cap and rotor, coil, etc.

It should be pretty easy to trace backward to the source of the "no spark".

Do you get a spark from the center wire at the top of the dizzy? If yes, the rotor, cap, wires , or plugs are the problem. If no, then check the primary dizzy wire at the points, the points themselves, the condenser, and the coil.

Is the distributor turning? One broken tooth on the camshaft gear will stop any engine dead in its tracks, and all the electronic gismos in the world won't help.
Jerry

Hahaha, ok guys, thank you! I'll take a closer look and report back. Thx for hanging in there with me

I just clicked on the image in your post. Pertronix is the LAST electronic ignition system you would want to use if you ever go that route. You won't though. Your 6v negative ground system is simple and easy to get functioning properly.

What is the overall condition of this vehicle's electrical system? Was the truck restored at some point? Is it largely unmolested original other than replacing worn parts?

How are the battery ground connections?

When was it last running properly and what symptoms did it exhibit leading to the current state of affairs?

I have seen a '57 GMC 270 that had the distributor walked up "high" enough that it wouldn't turn the oil pump but still engaged the gear on the cam allowing the distributor to turn and spark to happen. Made a mess of the engine.

Just like the Sun test equipment slogan for decades- - - - -"We Test- - - -Not Guess!" Everything is wrong until you prove it's right. Shotgunning parts at a problem is not a solution and usually creates more trouble by exponentially increasing the tendency to induce more errors. Back in the dark ages we called guys like that "wheelbarrow mechanics"- - - - -roll a wheelbarrow full of new parts up to the job and start swapping stuff until something fixed the problem. Of course, they never removed any of those parts- - - - -"It probably need it anyway!" Any new hire that tried that in my shop didn't last until lunch on his first day on the job.
Jerry