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My boy has a nice older restoration stock 51GMC 1/2 ton, it always ran great. the other morning he started it to take it to work and the carb is spewing gas out. he pulled the top off the carb but couldn't find anything wrong, its nice and clean inside the needle looks good, floats are good. so what would cause it to all of a sudden start spewing gas out of it? what should we do?
Jay D.

Even a tiny piece of debris under the needle valve will cause flooding. Is there some type of fuel filter between the fuel pump and the carburetor? If not, try installing an "in line" filter. The small clear plastic filters with nothing but a piece of screen mesh inside are virtually worthless- - - -get one with a metal housing a couple of inches in diameter.
Jerry

I didn't here from him tonight but he was going to replace the filter that's on it.
Jay D.

whats this jet setup for ? I don't remember seeing it on chevy carbs
Jay D.

That is the power piston/power circuit. It will often be stuck in the down position (yours looks like it is in the right position, though), When it is in the down position, it pushes down a ball in the bottom of the mast which lets more gas in and makes the carburetor run too rich at all speeds. Make certain it slides freely in the bore. When vacuum drops (in acceleration) a spring allows this piston to drop down and open the power valve. When vacuum returns this piston hopefully goes back up and lets the engine run at the normal fuel/air mix.

Jon would the power valve get stuck open and cause the problem we are having? he says the gas is pouring out the bottom of the carb. i just talked to him and it started but gas is still pouring out.
Jay D.

Hi Jay,
Well, probably not. It would be unlikely in my opinion. The main thing (primary thing I should say) that will cause this is a faulty needle/seat and/or float. When the fuel level rises in the float bowl (exactly like a toilet bowl), the float comes up and pushes the needle against the seat (stopping the flow of fuel from the fuel pump). As Jerry said above, anything caught between the needle and seat will cause this condition. A broken (leaking) float will do the same. Or a float which is rubbing against the float bowl. I've seen some carbs where a clumsy person bent the float and caused it to hang against the side of the bowl. And believe it or don't this can happen sometimes and not at other times, all depending on a couple of factors. At any rate, Jay...your son is getting too much gas in the bowl and it appears nothing is stopping the flow. I think I'd take the carburetor off, drain it and then sit down and watch the float movement carefully to be sure there's room to move without striking the bowl. Next, check the floats to be sure there's no gas leaking in (shake gently and you'll feel/hear it). Then finally take a look at the needle and seat. See if it needs replacing or if anything is caught in there. Good luck!

THANKS, Jon for the reply. he has done all of above but I'm going to have him run through it again. he said the needle looks nice and smooth no abnormal deformity's, the seat is clean out to the opening where the fuel line connects. he did say the floats fit real snug in the bowel and bent them in just a tad. I also had him check the floats to see if any gas was in one or both, there is none I'M GOING TO HAVE HIM RECHECK THIS. it almost has to be in the float circuit, that's the valve for gas to enter the carb. there is really no other way for gas to simply leak out of the float bowel? its just not shutting off.
do you or anyone else know what the float level should be and how you set it, measure it? it always ran perfect, its strange that it would all of a sudden start spewing gas out. I can only think of two things that could have changed 1 the needle and seat are bad or has something in it or a float has a leak and has some gas in it.
Jay D.

The settings should be float level--1 9/32" and float drop 1 3/4", gasket in place...level checked with air horn upside down and drop checked with air horn held right side up, float hanging free.

Note: we think this is for a 235, but we're sort of assuming that. If the floats fit snugly in the bowl, my guess is you found your problem. They should fit right in the center of the bowl...same distance on each side of the float, but yes...the float & needle/seat is what will stop the incoming gas. Good luck.

Has an electric fuel pump been added? If so, maybe a regulator that failed???

And just curious, why does an "older restoration" 1951 GMC have a Rochester B? The first GMC to use a Rochester was in 1973, and then only on the motorhome with a Chevrolet engine.

GMC used Carters, Strombergs, and Zeniths, with an occasional Holley.

Jon.

thanks Jon and Jon apparently 2 jons. I don't think its a period correct resto, thus the different carb and I do know its 12 volt. nice truck it has the standard mechanical pump no vacuum.
though.

Jon G. Thanks for the specs i'm going to have him try some setting on the float and see what happens.
Jay D.

so I've been doing a lot of reading about the Rochester model B and the warping issue. so were is this warping happening between the bottom of the float bowel and the iron throttle base? can float bowel gas leak out that sealing area, like if the gasket were bad or possibly a chunk of it gone? the reason I ask is the boy says when its running it looks like the gas is spewing out from the bottom of the float bowel. could this happen? he hasn't taken the carb off as we were just work with the float system. it really sounds like we should start looking for a different carb.
Jay D.

If the truck ran fine prior to this recent episode, I would not change the carburetor. Just make sure that the float doesn't hit the sides of the bowl and make sure that you have an adequate fuel filter in the fuel line.
The warped Model B disease will not cause gas to spew anywhere. The leak is internal and will cause the engine to run too rich and stumble.

THANKS FOR THE REPLY Carl. yep i'm not sure what he is going to do. i'm 200 mi. away from him and just talked to him, I think hes going to take the carb off tomorrow and with me on the phone were going to look at it. may buy a kit and throw in it. its been years sense I've work on one of those and he never has. but I'm wondering if gas can leak out of the bottom of the float bowel thats were he says the gas is pouring from. and yes this was the sweetest running motor ive seen in a long time. idling with the hood up you can't hardly hear the motor running and it runs so smooth.
Jay D.

Jay about 2.5 years ago (summer of 2018) I did a study on the Rochester B and BC models. Maybe you saw it or some of it if you were searching on this site. This link should take you to part of it:

https://tinyurl.com/vpk6a2x

Toward the end of the study I showed everyone how you could straighten warped castings using a gas bbq grill, a 4" x 6" flat piece of 3/8" thick steel and 4 bolts/nuts/washers. I got most of these back to within .0015"~.002" variance between air horn and float bowl and then I rebuilt & sold all of them here on the forum for just what I had in them...or less in a couple of cases. It isn't the float bowl to iron throttle body that warps. It is the bowl to air horn (the top plate on the carburetor) that warps and when that happens it causes loss of vacuum (which makes the power piston fail) and it will add to the seepage problem at the top...where fuel dribbles out of the joint. What Carl said is correct and I wouldn't make changes if none are needed either. Good luck!

Jay - possibly some additional clarification might be in order.

"Pouring out the bottom of the float bowl"

Does this occur when the engine is running? Does it not occur when running, but when the engine is shut off?

Is it possible that the fuel is NOT coming from the float bowl, but from the cast iron throttle body where the throttle shaft extends from the throttle body?

I am having a difficult time visualizing how fuel can come out the bottom of the fuel bowl. My first thought was the fuel level in the bowl was too high, and fuel was exiting through the pump operating rod slot, but the top of the slot is higher than the air horn / bowl seam; so fuel would be exiting there before going through the slot.

And no offense meant by the next comment, just trying to make certain everyone is on the same page with semantics:

Air horn - top casting made of zinc alloy, a.k.a. pot metal (contains choke)
Bowl - center casting also made of zinc alloy
Throttle body - lower casting (cast iron, and contains throttle shaft)

Jon.

He tells me the leakage is coming from under the float bowel or that area of the carb. I'm wondering if it is running down the pump slot as you said it might. it does it only when running and when its running its running rough, flooding out. i'm still with the idea that the fuel isn't being shut off as I should. and over filling the float bowel and dumping fuel into the engine and out the pump slot where he sees it as coming out the lower part of the carb. its not just seeping out its like a steady stream of gas running out pouring down to the ground. I don't know what else to think. yep we know what the air horn, float bowel and throttle body is, no offence taken. i'm going to see if he can shoot a vidio of it running so I can post it here.
Jay D.

HERE ARE PICTURES[img]https://www.stovebolt.com/ubbthreads/ubbthreads.php/ubb/download/Number/17390/filename/IMG_2610.jpg[/img] It wouldn't let me post the moving picture of the gas coming out, anyone know how easily. Jon I CAN SEND IT TO YOUR EMAIL.
Jay D.

Jay, videos require a YouTube link or something similar. You cannot post videos directly to your thread.