E10 ethanol fuel crappy hot idle

[Bailey28]

I have a NOS quadrajet truck carb, 17081290, that I am using on a stock 350 Chevy in my El Camino. 

I am having difficulty obtaining a smooth idle when the weather and the engine is very hot.  When cold, the car starts fine and runs great.  Idle is very smooth.  20" of rock solid steady vacuum. 

I can drive it for 5 minutes or an hour but after it gets it's first "hot soak" the idle turns to crap.  It will idle rougher, like it is misfiring, the vacuum gauge (mounted in the car) fluctuates between 18-20" in neutral. 

[Bailey28]

Carburetor is set up as follows:

75 mains, 51M rods,
APT 1.5 turns up from bottom
DR secondary rods
.420" float level
5 psi fuel pressure with regulator and return to tank
Idle mixture screws are 4 out on each side for highest vacuum

I checked for vacuum leaks and found none. 

Can anyone get a steady HOT idle with E10?

Car drives fine when it's hot, it's just the idle that goes to hell. 

I have never used Ethanol free but I may try a full tank to see if it helps. 

[Bailey28]

Sorry guys, some other things that I thought of that could contribute to or help solve the issue:

Exhaust crossover port under carb is wide open both sides, no restrictor gaskets or plates. 

There is no "EFE" valve after the exhaust manifold, only a wide open spacer where it was.

I removed the hood mat and the cowl seal to let hot air out. 

I am using the stock Chevrolet air cleaner with snorkel. 

There is no smog, A.I.R. or converter on the car, only stock iron exhaust manifolds to dual exhaust

With the hood open in park it smooths out just a bit but with the hood back down shut the car idles rougher soon after. 

I am using a AC Delco PV774C pcv valve. 

Engine temp is 185-190F. 

[Kenth]

Starting a cold engine it is running with enriched mixture due to the choke, with choke off at hot engine it is running 10% lean due to the 10% ethanol fuel. Underhood/engine temperatures has not much, if anything, to do with this. Fan and speed wind cools underhood/engine plenty enough, and hot air escapes under the car driving.

With E10 fuel i would open the idle tube restrictions in your 17081290 from .036" to .040" and the idle needle holes to .090" for better control of the idle/low speed mixture.

[von]

Does the air cleaner snorkel have a valve in it that's supposed to close during engine warm up and open when it's up to temp? If so, is it operating properly? If not (or I'd do it anyway), disconnect or block the vacuum line going to it so it's open all the time.

[Bailey28]

The air cleaner valve is disconnected and it is open all of the time. 

Kenth, when engine is warming up and comes off of the choke it is still very smooth at idle.

 This issue starts after the car as been driven then parked and restarted after a hot soak.   It will also happen after high speed driving for a while on open roads.   Coming to a red light it will act up like it wants to stall. 

I'm starting to think I don't have any type of mechanical problem and that the issue may be fuel quality related.  The engine runs perfectly the way it is set up until it gets very hot.   Perhaps the E10 does not want the exhaust crossover under the intake. 

I live in Florida where it is baking hot right now.  This isn't an issue I have during the winter months. 

[Bailey28]

Today I raised the fuel pressure to 6.5 psi which seemed to help when idling.


Who knows, maybe with lower fuel pressure and the lower .420 float setting the small amount of fuel in the bowl is boiling quickly causing the unstable idle.   Perhaps with slightly more fuel in the bowl it won't percolate as quickly. 

I would imagine if the fuel in the bowl is percolating then the idle tubes would pick up vapor as well as liquid from the well.  I think this percolated fuel passing by the air bleeds then adds more air to the already percolated hot fuel which is what causes the inconsistent idle. 

Next step might be to raise the float level in the bowl. 

[qjetsrule]

Have you considered a thicker base gasket or stainless heat shield?

https://quadrajetparts.com/quadrajet-stainless-steel-heat-shield-thick-p-97.html

[Bailey28]

Great idea on the heat shield..

I am using the 1/4" thick Felpro 60482 gasket as well.

I raised the float level to 9/32" which helped stabilize the idle some. 

Today I filled the gas tank full of E0 ethanol free which helped even more.