I've been fighting this for a while but no sure solution yet.
When it's hot out, my idle speed tends to be lower, plus when I'm coming
to a stop the idle will sometimes drop considerably, then come up again
after the stop is completed. In really hot weather, though, the idle
still tends to run 300 + RPM lower than normal, from where it can
ocasionally stall. Other than that, the car accelerates and cruises
beautifully. If I can pick a point where this happens readily, it's if
my water temp is over 200-210. If it's 190 or below (normal highway temp
in hot weather), it doesn't happen.
I have a 289 with a Holley 570 Street Avenger carb. There is already a
metal baffle (stock with this carb) in the front float bowl, just under
the air passage above, that's supposed to keep fuel from sloshing up
there. I've re-checked the float levels in both bowls and the fuel level
is just at the bottom of the sight holes.
I switched out my old aftermarket "cube" fuel pump and installed a
Holley red pump a couple of weeks ago. Fuel pressure is now between 5-6
PSI at hot idle vs the 3-5 when hot with the old pump. This has helped a
little bit with the lower RPM issue when hot, but not completely.
I talked to Holley about this and the tech guy said to check my power
valve size, if the fuel levels were already correct, which they
apparently are. The carb came with an 8.5 valve but I checked and the
idle vacuum is between 15 and 16 so, by Holley's formula, I need to run
the next size smaller, a 7.5, which I've bought and will install this
week. Holley says that the bigger power valve would be open even at idle
and in addition to hurting gas mileage, this could probably cause the
RPM drops at hot idle and coming to stops.
Before I take the carb apart again, does the Holley guy's theory make
sense? Any additional ideas on what could be causing this?
By the way, I installed cruise control last week and it makes an
amazingly good difference for long distance cruising. It was a bear of a
job (for me anyway) but now that it's done it was definitely worth it.
Highly recommended to anyone who's been thinking of it.
Steve Sage
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