Carb heat soak: causes, related problems, and cures?

I have also learned the importance of always using the correct took when addressing carburetor issues such as this...

 

I basically had the same problem. Tried all sorts of tricks to keep the fuel cool. Finally I checked the fuel pump pressure-8 PSI. Installed a fuel reg. set at 3 PSI, problem solved.

 

This is good to know, makes me feel I am on the right path.
Did you also have problems with getting the engine to idle once warmed up?

 

What do the plugs look like ?

 

If you park it hot, remove the airfilters, and look in the carb throats what do see? Any gas dripping?

I think there are carb spacers and fuel recirculation in your future. But First I'd double check the float heights, and run them on the low side. With low density gasoline the float will sink a little deeper, resulting in a higher fuel level for a given float setting. High fuel levels aggravate hot soak problems.

Are the carbs old enough to have external bowl vents with throttle activated flapper? I believe some would open at idle to relieve pressure and vent gasoline vapors instead of dumping them down the carb throat.

 

No, idled OK but was a bitch to restart when hot. Could see gas running thru the carb. Assumed it was boiling.

 

BTT!
What ever happened? I've fought the same problem on my 348 for years and wanted to know what you finally figured out that worked. Hopefully worked that is!! LoL!


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Well, I'm not the OP but I can tell my experience.
I added a 1" phenolic spacer from Edelbrock recently. On my Ford 390 I noticed a seat of the pants increase in low end torque and I no longer have to pump the gas pedal to prime the carb at start up. So it's boiling less fuel out of the carb after the engine has been shut off. I need to re-route the fuel line around the engine instead of over it to help keep from cooking the fuel on the way to the carb. Luckily this summer had been fairly mild so I haven't had many extreme heat days to test the spacer in that regard.
I do plan on adding a shroud around the intake and ducting fresh air to it as well to keep the incoming air as cool as possible.

Backstory: the engine has less than 15k miles on a rebuild, a new higher flow pump so that at idle it would move more water since that was when it was heating up in traffic. A new 3 core aluminum radiator was added after I found the old one was nearly plugged and only cooling inlet to outlet 5 degrees. The timing has been adjusted with a vac gauge and it has helped a little as well both in performance and in running temps. The last addition was a radiator cap with built in temp gauge to get temps directly from the FE's expansion tank.


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Just saw this thread, a few observations:

If one is having this issue, the phenolic spacers certainly may help (and may not).

There are a number of comments concerning fuel pressure. The ideal fuel pressure for any carburetor will depend on a number of variables: (1) float buoyancy, (2) float height, (3) orifice of the fuel valve seat, and (4) material used for the fuel valve. In the FWIW category, I run about 7.5~8 psi on the two genuine Carter AFB's on our shop truck.

We have tested quite a few of the A/M inline dial-type $19.95~$39.95 fuel pressure regulators, although none in the last few years. Our findings consistently show these regulators to be fairly accurate AT RATED FLOW (maybe 10 gal/hour) but at idle the output pressure is basically identical to the input pressure.

Last summer we set all kinds of temperature records here in Missouri (of course the rest of the country probably did as well). We had several days when the ambient was above 100 degrees F. The aforementioned shop truck never failed to start when hot (although one must crank the engine WITHOUT one's foot on the footfeed). The engine is a Ford 390 with dual genuine Carter 625's residing on top of an Offenhauser aluminum intake. Carburetor mounting gaskets are 1/16 inch Armstrong N-8090. There are no spacers used.

And some explanation on float/fuel orifice sizes:

Carter used fuel orifices from as small as 0.081 to as large as 0.111 inch on their genuine AFB carbs. They also had at least 5 different floats with difference buoyancy ratings. If someone talks you into a "high flow" fuel valve and a "smaller float to increase bowl capacity" you may expect pressure issues.

Jon.

 

Jon, I bought some rebuild kits for my 348 a few years ago and should probably call you sometime about the carb issues I'm having..... Which are NOT the carbs kits that I got from you, but rather the swapmeet stuff that was in them when I got them. You did save me by telling me to change the air bleeds so it would run, which worked like a champ!
Can you PM me your email or phone number? I seem to have lost it!
Thanks. Jimmy


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