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Carb mystery

elyon

elyon

1/2 ton status
Joined
Feb 16, 2012
Posts
214
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Location
Eastern NC
Here's the set up:

383 stroker
Edelbrock 122 e-force roots style supercharger
Quick fuel Q750 boost referenced carb
Edelbrock performer 110 GPH/5 psi mechanical fuel pump
Aero motive fuel pressure regulator set to 5 psi
Poweredex AFR fuel air monitor

The oddity. On a hot day, say upper 80's to mid 90's. High humidity.

After sitting in traffic for a light or something, accelerate, let up in the throttle and AFR will go from around 12:1 to 8:1. It will flood itself out. If I see it happen I can give it a little extra throttle and it will correct itself in a few seconds. It can happen when light throttle cruising and then closing the throttle. I can sometimes smell fuel.

All engine perameters are normal.

This only happens when pretty hot out and high humidity. Any other time, no problem.

Fuel levels are half way up the sight glass on the float bowl as specified. Fuel pressure is below specified 6.5-7.0. Pump only makes 5.

Some kind of super cooling on back side of throttle plates?

I'm at a loss.
 
I would suspect the power valve, might also want to add a fuel pressure gauge you can see while driving, or record it for a drive just to rule it out. The humidity thing makes no sense. And I thought you were going to say it was going lean (possible vapor lock), but going rich suddenly I would suspect the power valve.
 
I would suspect the power valve as well, but I replaced it just to make sure.
 
The mechanical fuel pump you are using is not the best way to go with a supercharger and a Holley double-pumper carb. A high quality electric fuel pump for a carb would be a more reliable fuel supplier.
 
I have been running a mechanical pump for several years with no issues. Just for fun I am upgrading to a higher volume and pressure pump making 130gph and up to 10 psi, regulated back to 6.5.

I did not have this issue with the edelbrock AVS carb which I was running earlier.

This Quick fuel 750 has a boost reference port for the power valve. It receives a vaccuum signal from the back side of the supercharger. Both negative and positive vacuum. I'm wondering if that is coming into play.

Power valve opens at 4.5", cruising down the road, vacuum is typically around 10", when you close the throttle it initially goes to around 19" then settles around 11" with throttle closed.
 
I was wondering about your boost reference system as well. Have you thought about putting in a power valve block only for testing (obviously don't get into it) and see if it does the same thing still? If it does you know where to focus your efforts, if it doesn't, then keep looking. Just either jet up or don't get into the throttle while you run that test.
 
I have considered blocking the reference port off to see if that effects a positive change.
 
So the problem didn't start till you switched carbs?

Does it happen every time the temp is warmer.

If it's an every time problem I might be inclined to say you have a delivery problem maybe fuel boiling in the lines?

If it's not an every time thing it would suggest a carb issue.

In any case I would start like you said with the power valve. Eliminate one possible cause and keep moving
 
I don't think it is a boiling issue. I have a 1 inch spacer between the carb and supercharger. Engine is never over 185 degrees. Really big Griffin aluminum radiator.
 
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