Boiling gas in the float bowls?
 

Now that I finally have my car street legal I have had a chance to go on some longer drives. The other day I came back from a 30 minute spin, and parked for the evening. Later that night I went back out to the garage for something and noticed the smell of raw gas. I looked the car over and there were no leaks, but I noticed I couldn't see any gas in the float bowl sight glasses. After opening to door to air out to garage I turned the key on and the float bowls re-filled.

The next morning the level was still the same (1/2 way) and no smell. So today I go for another drive. When I stop at my destination I pop the hood to verify level, which there is. I come back an hour later and again no gas.

Now I drive it back home and park. Check the levels, yep half way. I feel the bowls and they are nice and cool. This time I leave the hood propped to see what happens. I came back out after an hour or so and again I can smell gas, but not as strong as the other day. This time I look closely and I can see the gas "jiggling" every so often through the sight glasses.

It looks to me like the heat of the engine is boiling the gas out of the bowls once the car us parked. Has anyone else had this problem? Will a simple heat shield from Summit or elsewhere solve the problem? I should mention that the carb is a 650 Speed Demon on a 351W.

You might consider a 1" aluminum or composite type carb spacer.

I always leave the side door to the garage open and the hood up to let it cool down in the garage. Otherwise my house will blow up. I bought the Holley heat shield and spacer. I couldn't go to thick on the spacer. No clearance with the 514.
Scott

I don't think you have a "gas boiling in the carb" problem, just a hunch.

What type of fuel pump are you running (elec/mech, mfr and model)?
Are you running a return fuel system?
Fuel pressure regulator (bypass or single line)?
Fuel pressure gauge?
What fuel pressure are you running while the car's sitting and idling?
Any drivability problems?


Possibilities:

1. trash under the carb's float bowl inlet valve(s).
2. Creeping fuel pressure at idle, overpowering the float bowl inlet valve(s)
3. Leaking carb w/loose screws and/or leaking gaskets.. just to mention a few. Get back to me answering my initial questions and we can pin it down a little better.

Dave

A phenolic spacer is what its called. You can probably get 1/2 inch one.

It is an electric fuel pump (Holley "RED"). There is no regulator or gauge. The holley pump has an adjustable relief valve with a maximum pressure of 7 psi.

There are no drivability problems. Idles fine when warm, and runs strong.

The main reason I suspect the fuel is boiling out is because when I run the fuel pump and fill the bowls when the car is cold the level holds. I would think that if there were a leak the gas would disappear if it were hot or cold. In my case I don't have the problem when cold, or even after short drives. It's only after then engine is fully warmed up and the car has been run down the road for 15 or 20 minutes.

Well you could try setting a small cap with some gas in it on top of the intake and see if it boils off at the same rate as the float bowls. Since your smelling gas I'd guess it's exactly what's happening.

The fill valves are pretty high up in a carb so I can't see it being a drain back issue.

The heat shield suff reminded me of a favorite carb of mine, anyone remember the Thermo-quad? These were really neat carbs that had all metering in the top and a phenolic body that kept the fuel about 15~20deg cooler.

They were standard fare on chrysler big blocks. As far as I know no one offers stuff for them anymore. Carter only offers stuff for the AFB now.

Even if your gas isn't boiling, your garage will still smell like gas when you park inside.
Scott

When you shut your engine off, the heat soaks upward through the intake manifold and heats the carburetor. With the current "winter" gasoline this will cause a considerable amount of the fuel in the carburetor to vaporize and escape through the bowl vents. Since these cars don't have the vapor capture and recovery systems that production cars had after 1968 or so, you will smell it in a closed garage.
Once we transition to lower volativity summer gasoline, it will get better until the weather gets really hot.
A phenolic isolator will probably help, but it will probably not totally eliminate the odor.

Good morning.............I had a variety of irritating "gas" problems........leave it alone.....I finally resolved them by returning to a smaller diameter air cleaner. I think the larger air cleaner trapped more heat which affected the winter gas.
Good luck
Mike

I saw an interesting trick on the "Horsepower Show" on Speed 2 or 3 weeks ago. I have no idea if this will work but it was an interesting idea. They said that some people are painting the under side of their intake manifold with white ceramic engine paint from Dupli-color. The idea is that the white paint will keep the intake cooler. Of course the Air Gap intake will do a better job. It is cheap and easy to do, but will it help?????

Wayne

Same issue for my 418W and Demon carb. I can hear gasoline in the carb boiling shortly after shutting down, and the odor is pretty strong. It is better after we get rid of this crap they call winter gas in Colorado. Now, I leave the cooling fans on (and hood open) for a few minutes after shut down - it helps a lot!

Terry

I had a worse problem, once hot and sat for a while, you'd start the car and gas would dribble out the bowl vents and kill the engine. I have a holley insulating kit installed now, but mostly you need to mix some race gas in with winter gas. A couple of gallons of 100 octane per tank seems to do it for me. I don't have the problem in the summer.

Chuck

Here is a picture, you can see the heat shield under the carburetor.
http://www.chuckbrandt.com/pcv.jpg

I just installed the same kit last weekend, however I installed the phenolic under the heat shield, not on top of it. My theory is that I want the heat shield not to absorb heat from the manifold and radiate it to the float bowls.

If someone has suggested this, then good, but have you checked the checkball in the fuel pump?

If the check ball leaks, the gas will syphon back to the tank.

Hey, it's just an idea.

:D :D

I tried that
 

I tried that but my accellerator pump arms would hit the tin. So I switched it. If it works for you that's probably prefered.

Chuck

Yes, my pump arms hit as well, so I used my dremel to cut small slots in the shield to make room <- actually, scootter told me about this a while ago, so I take no credit here ;)

I'm not sure yet if it works, since I have not had an opportunity to enjoy our recent "spring-like" weather. I plan to driving to breakfast this Saturday, so we shall see (smell) if it works...

I too had the same problem and it was most definetely fuel boiling in the carb after shutting the engine off. I lowered the float levels and installed a half inch plastic spacer purchased from Jeg's( I preferred the black color rather than the typical redish phenolic spacer ).
JEGS CNC Poly-Billet Carb Spacers - JEGS
I have not had an issue since. I was running the cooling fans manually for around 5 minutes as a temporary fix until I installed the spacer and still do so when I park it in the garage.