Heat Soak

[70bigblockdodge]

What kind of ignition do you have? Chrysler electronic ignition pick up coils in distributor seem to have issues with heat, I'm sure the newer Chinese ones only made this worse. Timing will give you fuel mileage are you sure your vacuum advance didn't crap out. I cannot see Dave setting up a carb that would lose you that much mileage.

 

[Gerald Morris]

What kind of ignition do you have? Chrysler electronic ignition pick up coils in distributor seem to have issues with heat, I'm sure the newer Chinese ones only made this worse. ...

Good question and observations! Iron core inductors invariably become more apt to hysteresis when hot, reducing their polarity and effect. Those Hall Effect reluctors doubtless suffer as they heat up, and yes, inferior sino-iron will more than good, hard North American or European iron will. This holds for the ignition coils themselves also. I replaced the older Taylor I was using the previous 3 yrs after the little engine fire, even though I got the engine through Emissions with both toasted coil and carburetor as things turned out, as the motor clearly didn't run so well post-toasty as prior. Askarel insulated coils might also degrade more with heat, as the oil breaks down more. I advise folks to buy epoxy insulated coils when operating in hot environments for this reason, and I have with good result.

 

[fury fan]

Please stop the bs about blocking the crossover - it works! What a bunch of nonsense. Try using a wide band a-f gage instead of old wives tales

I've had 2 LAs that plugged the heat crossover with carbon and both ran crappy from it, so to say it's an old wives tale across the board is also BS.
When you say blocking it works - is this on a race car or street car?
Did you get better MPG after blocking it?

 

[CBODY67]

The local small town Chrysler dealer sold a good many LA273 Belvederes to older ladies in town. After they got a few years old, I'd see them in the service dept with the intakes off. I thought that odd and asked about them. Seems they only drove short trips and never really got hot enough to fully cook things out, so they were in to get the carbon chiseled out of the heat crossovers. Of course, when things got that coked-up with hard, black carbon, the automatic chokes never came off, which is why they came in. I saw a few like that over a few years.

THEN, after I got my '80 Newport 360 2bbl, I realized just how much cooler they ran than B/RB blocks. Had no issues with the heat crossover, but the pcv hose would hang a bit low in the middle, and in the cooler months, it would accumulate gunk in it, so replacing the hose fixed that. Then, I noticed a white gunk on the top of the engine dipstick. I was not driving it short trips and it always got fully up to temp with a 180 degree F thermostat. So, yes, LA motors run cooler than B/RB engines.

Part of the '80s emissions system included a Charge Temperature sensor in one of the intake manifold runners. One evening at my shade tree shop, I was messing around with it and had it running, fully up to operating temperature. I felt the manifold near that sensor and the runner was definitely COOLer. Then I realized that even if the ambient temperature was 85 degrees, the engine fully at 180 degrees F, with an intake temperature of about 90 degrees with the air cleaner off, that as soon as the air went through the venturi, gaining speed as it did, then getting misted with a gasoline spray, that THAT cooled the air/fuel mixture a lot, enough to physically cool the cast iron factory intake manifold. This after it had already passed through the plenum "hot spot".

I have not checked the intake manifolds on other cars I have, but suspect they might be similar. So, at least on Chrysler LA motors, keep everything open and fully up to operating temperature for a good 10 miles each time the engine starts in order to minimize the condensate in the crankcase and other places in the engine.

From my experiences, yours might vary.
CBODY67

 

[patinated_imperial]

I am a little late to the game, but I have a few numbers - for reference concerning the effect of a carb spacer:

On a 440 RB with the heat-cross over still active, I added a 5/16" fibreglass spacer between manifold and AFB carb. It dropped the temperature of the float bowl from 127 F to 107 F
(53 C to 42 C). Measurements taken with hot engine idling. (Yet another "erratic at cruise")
I don't know if that is enough to decrease heat soak as it did not solve my drivability issues.

I later changed to a Holley I was almost gifted.
With the Holley and also a 5/16" fiberglass spacer the car idles at AFR 13-14. When it has been parked with hot engine and allowed to heat soak it will start up with AFR at 15.5 - 16.5 and will drop back to normal after driving 5-10 minutes.

These numbers are with a return style fuel pressure regulator.

When driving for several hours in the summer I can see the AFR climbing due to the fuel in the tank heating up and being less dense. Of course more evident with little fuel in the tank. At one point last year I actually pulled over and adjusted the idle mixture screws because I was worried it would stall in city traffic. Refueling at a gas station with an underground tank will drop the AFR back to normal.

Hope these numbers are some sort of help.