TheSamba.com :: View topic - Gas leak from throttle bushing: possible or not?

Mon May 14, 2012 6:08 am

I've studied a lot of schematic and exploded diagrams for my 28 (1963) carburetor.

Then I did not recognize any way for fuel to reach throttle shaft and, via worn bushing, leak out.

I've the classic issue with 28, the start difficulty after 1-2week not running, due to an empty carburetor tank.

I've tried a lot of other carbs (30 pict3, 30 pict2, 31pict4) and NONE of them leaks fuel from throttle bushing, I am sure they have worn bushing, but no fuel from there.

What is different in 28 carbs? Perhaps the "air bypass" idle of newer carbs is a remedy against fuel leaks?

Thank you

Mon May 14, 2012 6:35 am

Its not that they leak gas out, its that they will leak air in which upsets the air/fuel ratio

Mon May 14, 2012 7:31 am

my59 wrote: Its not that they leak gas out, its that they will leak air in which upsets the air/fuel ratio

28PIC often leak fuel out from the throttle shaft.

Air leak is normal on so old carburetors.

I'm trying to understand why in 28pic the leaks are also of gas out (with engine off, of course) and on 30 and further carbs there are no fuel leaks, only air leaks.

Mon May 14, 2012 9:13 pm

herbie1200 wrote:
I'm trying to understand why in 28pic the leaks are also of gas out (with engine off, of course) and on 30 and further carbs there are no fuel leaks, only air leaks.

Shut off valve not functioning properly. Lots of reports that the new valves are junk! Thank goodness have been saving all the used OG ones....

Shut off valve float too heavy or cracked. BTDT with the former.

Crack in the carb body.

New fuel today vaporizes too easily in hot weather. Adding a second gasket between the shut off valve and the carb top can help. This moves the valve closer downward to the float so it pushes harder to close the valve. This worked for us.

All of above have had happen with all the later carbs.

Tue May 15, 2012 9:26 am

I also think a hot-soak after shutoff can heat the gas in the bowl, make it overflow through the vent pipe (especially if float level is a little high and the shut-off is leaky, like most replacement kit units do), and down the carb throat, which can then leak out through the throttle shafts.

Tue May 15, 2012 7:00 pm

KTPhil wrote: I also think a hot-soak after shutoff can heat the gas in the bowl, make it overflow through the vent pipe (especially if float level is a little high and the shut-off is leaky, like most replacement kit units do), and down the carb throat, which can then leak out through the throttle shafts.

If it vaporizes just in the bowl the fuel will not drip out. To do that you need more liquid fuel poured into the float chamber.
Think the fuel in the fuel pump to carb hose gets bubbles of vaporized fuel in it like carbonation bubbles in soda. This pressurizes up the fuel in that line to open fuel shut off valve and pours into the carb float chamber.
As mentioned before adding the second gasket to the shut off valve seems to have solved the problem for us.

Tue May 15, 2012 9:26 pm

Im having this problem right now.
A nice 28pict1 carb in a 63. Carb is in good condition, cleaned with a new gasket kit, and the correct washer under the needle valve. The gas intermittantly pours down the carb causing flooding. It comes out the throttle shaft and drips all over. Put your ear down there and you can hear the gas percolating down the carb.
It must be the float needle valve. Maybe because the new replacement ones are not adequate.
I will try another washer under the needle valve, any suggestion as to what thickness?

Oh and I might add, there is a new fuel pump on there. I hear they can be over 5lbs pressure?

Wed May 16, 2012 6:37 am

Those answers sound strange to me.

You are supposing fuel is dripping off from the ventilation duct of the float chamber (due to excessive fuel pressure or vaporization) and drip into the carb.

But those drops instead of falling into manifold, due to the closed throttle (engine is stopped) exit through the worn bushing on throttle shaft...

Strange, but simple to test: I'm going to remove air filter and (with engine shut off) put a small quantity of fuel directly into carb inlet, and to look if it goes into manifold OR prefer to exit via throttle bushing.

A simple test, which result I will report to you.

Thu May 17, 2012 7:24 pm

Your throttle shaft bushings may be worn, but fuel dumping into the carb will come out of the throttle shaft whether they are worn or not.

I jumped in here because this was happening on my 63 as well at this time.
I tried Eric&Barbs suggestion and added a washer under the inlet needle valve.
It had the correct 1.0 mm thick washer on it already, and I added a .80 mm fiber washer to that. It seems to have stopped the flooding of the bowl with gas and the resultant gas going down the throat of the carb (and out the throttle shafts).

I hope this is a fix.

Wed Aug 13, 2014 9:59 am

I’m rehashing another oldie. I’m pretty sure I’ve got the same leaking issue on my 30Pict-1. It only leaks after the car is shut off. Idles pretty good. I see the recommendation is to add a second gasket to the shut off valve. I know where the shut off valve is, but I’m not real sure where to add this gasket.

Wed Aug 13, 2014 10:30 am

It's #6. You either use two thin or a single thicker washer (depending on your kit) to, in effect, locate the shutoff point lower in the bowl, lowering the maximum fuel level.