Boiling Gas on shutdown

[dlphil10]

I have a 69 7029262 on a 68 350 pontiac V8 that I rebuilt and tuned thanks to your book, parts and help. Overall it runs pretty well and will pull hard through 5200rpms.

Lately I've noticed I have to be quick to get a box fan on the engine after a long cruise in the heat to keep the gas in the carb from boiling. I can sometimes see the air bleeds bubbling which concerns me about oil dilution. This is also annoying but also not practical if im not going out and coming back home immediately.

The car typically runs 185F going down the road, but will creep up to 200F idling for extended periods of time. It quickly goes back down after take off. It gets a healthy diet of 93octane vpower from shell.

I have a new fuel pump with a return. I went through 3 due to high pressure and then put a Holley regulator on it get 5psi. Before this I had nozzle drip and I know the throttle angle isn't too high.

Any thoughts or tips to help fix this? I have a standard paper base gasket under the carb now. Maybe the gasket with plastic insulators around the bolts would help?

[Brian B.]

FWIW....I use a 1/4" base gasket with the plastic grommets inder my Q-Jet on my 455 Pontiac.  My cruise temps are 180 to 190 without A/C operating and 220 while using A/C.  I have no fuel boiling issues.  You might try one too.

[quadrajam]

100% what Brian just said. That may be all you need.

[Cliff Ruggles]

Any idea why it's running so hot?

During the rebuild did you measure "squish" distance or quench?

What is the compression ratio and camshaft specs.

Pontiac engines are a bit sensitive to a quench and when you get a lot in them they can have running hot issues like creeping up at stoplights, or running past the thermostat set point on hot days, etc.....

[dlphil10]

Good questions and thanks for the replies!

Unfortunately this was my first build and I was nervous about taking so much off the block. I had 23 thou taken off and quench is not ideal at roughly 0.062". Compression ratio is 9:1 with a pontiac 350 and a crower 60240 cam 210/221 at 50" lift (yes I know its a little big). Still makes 16.5inhg at a 750rpm idle.

Honestly, I've been thinking about a putting on a new fan clutch. It doesnt seem totally shot, but could be a little looser than new. Seems like cooling system is up to par of no issues cruising, just not enough air at idle. I suppose quench could wreck the equation..

I'll add that it typically hits 225F a few minutes after its shut off and the heat has nowhere to go. Ive been told thats not atypical by other pontiac owners.

[2Low8s]

May be unpopular opinion, but I eliminated temp creep at idle in a couple cars/trucks with electric fans. 

[Cliff Ruggles]

.060" quench is far from ideal and Pontiac engines with that much quench it them will transmit a lot of heat to the cooling system.  The temps typically "creep" up at idle as well.

Since I learned about this I started getting the quench on all my engines under .040".  Doing this IMMEDIATELY stopped any overheating, running hot, or creeping up idle.  Matter of fact those engines ran very cool and I even had trouble getting heat in them in cool/cold weather.

There are other things that contribute to the cause, like a fan clutch that's not locking up when it needs to, using a stamped steel impeller in the water pump instead of an OEM type cast iron one, advanced or retarded cam timing, etc......

[dlphil10]

Understood thank you. Its not a problem worth a rebuild at this point. I'll try a ticker base gasket with plastic inserts to see if that reduces carb temps after shutdown. Thats really the only time there's an issue.

The other variables you mentioned are fine. Cam is degreed per the spec, flowkooler pump with properly clearance divider, so on. I also have issues getting the car up to temp when its 70F or below. Typically it runs at 175F and eventually gets up to 180F.

[quadrajam]

The thick gasket is the way to go nowdays with this pee-pee gasoline
that's being refined. Its not like gas in the trans-am and firebird days.
Todays gas is intended to be under pressure and injected into an engine.
It is formulated to boil at much lower temperatures now days. Old school gas boiled at 150*
Now it's lower. Don't blame the ethanol it boils @ 170*.
The refiners have no sympathy for us carburetor guys.
Pluss it has less BTU energy and it's harder for a fuel pump to pull it.

Edelbrock had the same boiling fuel problems with their carbs and came up
with a 3/8 inch base gasket. Cured that problem.

Anyway that's my rant. yall carry on.....

[Brian B.]

Phil.....please let us know how you get on with this issue.