LA360 Vapor Lock, Heat Soak, OR??

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AlaskaJeff

AlaskaJeff

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The LA360 in my 70 Duster has recently developed a strange condition this summer after running well with no major issues last summer. The Holley 750 dual feed was rebuilt and a new 2 piece (aluminum outer & phenolic inside) spread to square bore spacer/adapter was installed. It replaced an open plenum similar 2 piece spacer/adapter for better low end. The manifold is a spread bore modified LD340 with Chrysler part # on it, same as last year. 727 trans.
After normal starting & warm-up it is idling & running fine initially. The condition begins around after 30 minutes of driving, it doesn't want to idle when I come to a stop and dies. It will start right back up but I have to keep the go pedal pressed above idle to keep it running. It dies as soon as I let off the gas. IF I stay on it, the motor runs OK at speed, just dies as soon as I let off the gas. Nothing else was changed from last year. The engine temp stays constant around 185. Is the new spacer not as good of a heat isolator as the old one?
Any suggestions or ideas?
 
AlaskaJeff said:
But wouldn't it happen right away?
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Probably not, because when an engine is cold, it needs an overly rich mixture. As it warms up, it no longer has the need for the extra fuel. That's the purpose of the choke. Does it have an operational choke?
 
RustyRatRod said:
Probably not, because when an engine is cold, it needs an overly rich mixture. As it warms up, it no longer has the need for the extra fuel. That's the purpose of the choke. Does it have an operational choke?
Click to expand...
Currently no. It's a manual choke that is not hooked up. So it is very cold blooded when it first starts. I've got to keep it rev'd up a bit until the engine temp coms up then it starts to smooth out and idle properly.
 
AlaskaJeff said:
Currently no. It's a manual choke that is not hooked up. So it is very cold blooded when it first starts. I've got to keep it rev'd up a bit until the engine temp coms up then it starts to smooth out and idle properly.
Click to expand...
Have you physically looked at it to make sure the choke plate isn't partially closed?
 
RustyRatRod said:
Have you physically looked at it to make sure the choke plate isn't partially closed?
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I have, but will again. I know on initial first start-up earlier in the summer I did check and moving it closed even when the engine wasn't up to temp would start to load up the engine pretty quickly. So I didn't think it would be something to do with the choke. I guess I'll need to give it another look.
If I let the car set for 10-15 minutes after it happens, even with the engine temp still up, it will fire right up and idle fine again for a time. And then it starts acting up again after 20 or so minutes........IT'S VERY confusing & frustrating! :mad:
 
Swap on another carb for diagnostics.

Process of Elimination

No Choke in Alaska?

Next move on to a good clean fuel source. Ethanol fuels draw moisture, lots of moisture.
 
Nothing changed? But what about the fuel? The fuel is boiling at a lower temperature. If you have a dual feed line install a return regulator and get a line that accepts one. You will see a big difference in performance once you do this. If you keep the fuel flowing it doesn't boil.

Your rich issue is the floats not working properly due to the fuel trying to vaporize. It is worse today with the new fuel mixtures then it was a couple years ago and is getting worse. They want these cars off the road. Thank your Democrats!

Here is what I did to correct your same problem.



Before
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After V

upload_2022-5-20_7-4-22.jpeg


upload_2022-5-20_7-5-21.jpeg


upload_2022-5-20_7-6-28.jpeg


Remember most of our fuel is imported now. You don't think they are giving us their best do you?
 
My thinking is... Undo what you did one part at a time.

Or...

It could be that your new setup just needs a faster idle?
 
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AlaskaJeff said:
I have, but will again. I know on initial first start-up earlier in the summer I did check and moving it closed even when the engine wasn't up to temp would start to load up the engine pretty quickly. So I didn't think it would be something to do with the choke. I guess I'll need to give it another look.
If I let the car set for 10-15 minutes after it happens, even with the engine temp still up, it will fire right up and idle fine again for a time. And then it starts acting up again after 20 or so minutes........IT'S VERY confusing & frustrating! :mad:
Click to expand...
Sounds like carb ice. Whats the air temp when this happens?
My old 318 did it. 55 ish degrees and high humidity. I watched frost form on outside of carb. I repaired the carb heat parts to make it right again.
 
“The Holley 750 dual feed was rebuilt” There is a bunch of ways that it could change how it runs. Idle mixtures not the same setting. Idle speed not the same. Float levels not the same. I don’t think it is boiling the fuel. The four hole will increase the velocity of the air. More velocity, more fuel. The four hole will probably want different idle mixture and idle speed settings. You should use vacuum gauge and reset idle mixture. Well at high alt it will boil easier.
 
You usually cannot dead end the fuel at the carb with ethanol fuel. You need a return. No matter how you accomplish it The fuel must circulate to keep it from vaporizing. . Take your float adjustment sight caps off and see if you have fuel coming out when the car is running while its stalling. That will tell you if you have fuel problem shut of problem at the needle.
 
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