The Stovebolt Site Forums The Shop Area Engine Shop I'm singing the Rochester B-lues!

Took a spin in the '47 yesterday and it was running really nice. Pulled into the driveway and smelled gas. The throttle body was soaked and with a pump of the accelerator, gas squirted out of the ends of the throttle body shaft.

Incorrect float height, bad float, bad needle valve, or a bad Rochester B? Let me know if you have any ideas!

Thanks,

It is completely normal for gas to leak out around the shaft when the throttle if opened with the engine off. Without manifold vacuum the accelerator pump squirts down on the shaft and then the gas easily leaks out the shaft ends.
If there is leaking with the engine running the float valve may not be shutting off completely, or the float is set to high. Most likely float valve though.

You probably have some foreign particle in the needle & seat.

Thanks for your replies and the information. I will be pulling the carburetor off and having a look inside. If a rebuild is in order, can you recommend a good source for a carb kit? BTW, anyone know what the float height and float drop are for the Rochester B?

Thanks again,

I just set the float in mine, in preparation for it's break in run.
I don't have the paperwork in front of me but IIRC float height is 1 9/32", float drop is 1 3/4" Both are measured with the gasket in place.

Likes been said, you either have some crud in the valve keeping it open or the float is not set correctly. Another but less common cause is one or both of the float chambers has a leak and is too heavy to close. There should be NO gas in the float(s).

Good luck
Dave

Before you pull the carb:

http://www.thecarburetorshop.com/Troubleshooting.htm#Fuelleak

Jon.

Per Wrenchbender's comment, I've had the problem of getting a bit of trash stuck in the needle valve causing the carb float bowl to overfill and overflow. Not a good thing - big fire hazard. I ended up using a good quality aftermarket fuel filter in the line right before the carb. Problem solved.

Update:

Well, with the help of all of you, I think the problem is solved..... It appears that my new unvented gas tank came with a gas cap that 'had' a spring loaded relief valve. Once the tank was properly vented, no more leaking.

As always, thanks for the help!

So Andy, I'm trying to figure out why an unvented fuel tank would cause a fuel leak at the carb. Much like filling your lawnmower with one of those unvented plastic tanks, when gas comes out, without air taking its place you have a vacuum, causing a gulping action. In your truck, this vacuum would try to pull gas back through the pump into the tank. Right? Why would that cause a leak?

If its hot out and there is no vent, air pressure builds up in the tank and puts excessive pressure on the needle and seat until gas forces its way through.
I need to get a new cap on my '50 which will actually vent (I think my "vented" cap is defective) because when I take the cap off, I get a major rush of air out of the filler neck. I never have more than a quarter of a tank of gas in it unless I am going on a long trip. I suspect that adds to the significant amount of pressure in the tank (more air, more potential for air expansion).
Carl

Makes sense Carl. My riding mower has a tiny hole on top of the gas cap. Sometimes my dirty hands clog the hole, and my mower dies as the gravity drain feed can't overcome the vacuum. But it can work both ways as you indicate, tank needs to breathe.

Allen,

I appreciate and agree with what your analogy of the unvented plastic gas can. I'll admit I was a bit confused on the issue and was certain that it would have to be a vacuum condition as opposed to a positive pressure condition. Carl nailed it with his explanation.

I found this web site and read through the troubleshooting section. Scroll down about half way down the page to FLOODING(OVERFLOWING).

The carb isn't leaking and all I did was make sure the tank was vented through the cap. All is well that ends well!

Thanks,

"I found this site"? Hey Andy, you should have found that site a
week ago. Jon linked you to that web page which is on his site
very early in this discussion, that was on 05/13/15.

You guys are talking about an un-vented fuel tank on an AD,
right??? Vacuum in That tank should very quickly over come the
ability of the pump to supply fuel and the engine should die.
A stock AD fuel pump is only capable of supplying around 4psi.
As Allan posted and as Jon has warned on his site, a plugged
vent would work both ways, but I don't see how this could
be and ongoing problem for very long, nor do I understand how
anyone could get past the first tank of gas without the engine
stalling them along side the road from fuel starvation.
The only way an un-vented tank could build pressure would be
if the tank were filled and the truck not driven or only
driven a few blocks a then parked. If you drove it long
enough to completely warm the engine that should be plenty
of time to create enough vacuum to stop the flow of fuel.
dg

It could be that the faulty gas cap is vented in only one direction, allowing air into the tank but not out of it.
A hot day with only a 1/4 tank of gas (3/4 of a tank of air) with no way to get out will likely cause some problems (like forcing fuel past the needle/seat).
I do not know how the vents in these gas caps are supposed to function. Do they allow air both in and out.

Carl

Well, lets apply a little common sense to this. We hear the
story repeated often of guys topping off their AD with gas
that was in the ground at 55°, then parking it in the sun
without burning off any of it and the fuel puking all over the
side of he truck and running board. That would indicate that
it is vented in the positive direction. On the other hand we
know that a plugged or un-vented gas cap will, in short order
starve the engine of fuel, that would indicate that it must be
vented in the negative direction also. So it would appear
that the caps would have to be vented in both directions.
Which brings us full circle and we're back to fuel percolation
when the engine is shut off hot.
dg

Okay Denny, I stand corrected. Yes, Jon deserves the credit for my solution with his carburetor troubleshooting information. Thank you Jon!

Any way, it took me a few days to get back out in the garage and get after the problem and post the update. Let me set the stage for you. The gas tank hasn't had but few gallons of gas in it. I just completed the installation of the 16 gallon gas tank kit last winter. On several occasions I noticed the 'swish' sound when I removed the gas cap, which did seem odd to me, assuming that I had a vented gas cap. So when I took the truck for a drive, about a mile, it was probably 75 to 80 degrees outside. I'm sure that there wasn't much more than a gallon of gas in the tank. When I stopped the truck it was running rough and the engine stalled. So I popped the hood and saw that gas had leaked from the throttle shaft ends and the intake and exhaust manifolds were soaked too. That's when I turned to the knowledge base here on the 'bolt'.

The gas cap had a spring type one-way relief valve in it which should have alleviated a positive pressure condition. However, it must not have worked correctly. I too believe that if a vacuum was created, the truck would not run for very long. The over pressure issue does not seem to be a problem now. Perhaps the little amount of run time I've had with the truck and this gas tank configuration speak to the vacuum issue not fully presenting itself?

So, that's my story and I'm sticking to it. Once again, I am amazed and most grateful for the knowledge and expertise demonstrated here on the Stovebolt.

Thanks,

As a side note; however, probably related in some way....the heat riser valve was stuck in the hot position. Apparently, stuck heat riser valves are common on these old Bolts.

It's in the cold position for now. My plan is to rebuild it this fall with a stainless steel shaft and bushings.

UPDATE:

The heat riser is fixed! A new shaft and bushings have been installed. Thanks Achipmunk !!

No more Rochester B-lues! I took some sound advice from Hotrod Lincoln and located a Carter YF. Thanks Hotrod Lincoln !! After a good cleaning and a carb kit, the GMC 228 runs like a clock and idles steady at 450 rpm!!!

As Phil Robertson would say.....I'm happy, happy, happy

"The ABC's of carburetion- - - - -Always Buy Carter!" That was Carter's slogan for a lot of years, and it's just as true now as it was 60 years ago when I first heard it!
Jerry