Demon Carb Flooding After Shutdown - Third Generation F-Body Message Boards

CamarosRUS · 10-08-2009, 10:06 PM

When I turn the car off after it has reached temp, vapors will emerge from the carb. Fuel is seeping into the hot intake causing a fuel 'fog' and also puddling on the throttle blades.

First thinking this fog and my no hot start was some vapor lock issue I now know it is flooding.

What should I be looking at for the culprit? Power valve leaking?? Something to do with the idle mixture circuits??

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five7kid · 10-09-2009, 01:16 PM

First verify the float levels are set properly.

If they are, then what is probably happening is the fuel is boiling in the bowls. When this happens, it forces fuel through the passages that normally need vacuum to pull the fuel into the air path.

CamarosRUS · 10-09-2009, 01:45 PM

Floats are correct. No evidence of fuel boiling in the bowls. I will look much closer, however.

Carb bowls are cool to the touch, stainless fuel lines, while approx 4" from the header at one point, are also fairly cool to the touch. The fuel is leeching from somewhere.

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I have narrowed a few things down, looking for other possibilities than the common fuel boiling.

1. Power Valve (thought it was extremely rare to blow these days) - Could this cause my symptoms anyway?
2. Pressure in fuel system - Car no longer has factory charcoal cannister...will try to vent the gas cap to check for improvment.
3. Dirty Needle and Seat - Car idles and runs fairly rich anyway. But the float levels are correct and constant.
4. Something going on with the idle circuits perhaps?

I've heard Demons are terribly rich many times out of the box, this one is no exception. BUT, there has to be something else going on.

Timing is 20* initial and 34* total.

Keep it coming, I can almsot see light at the end of the tunnel...

EvilCartman · 10-09-2009, 07:54 PM

Mine gets the same vapor after shutting the engine off but not that thick looking. If you can, try a half inch phenolic spacer. Really helped out with the Holley I had and the hot start problem.

CamarosRUS · 10-09-2009, 08:34 PM

I might give that a shot but my carb is not getting that hot and I cannot see any signs of fuel boiling. It would be nice to not throw a bunch of parts at a problem like I have done with this piece of work for 3 1/2 years

Thanks for the advise, it certainly is seeping fuel somehow.

CamarosRUS · 10-13-2009, 10:12 AM

TTT

Excess fuel system pressure is not an issue.

Carb baseplate and 1/2" spacer and not near hot enough to boil fuel.

The fuel itself is cold.

Any other ideas what is wrong inside this carb?

EvilCartman · 10-14-2009, 04:17 AM

Fuel on the throttle blades rules out the PV, that would dump below the blades. Can you actually see the fuel dripping out of the boosters? What have you done to actually tune the carb so it's not so rich? Even mine will idle and not burn your eyes.

five7kid · 10-14-2009, 12:29 PM

The power valve is in the main circuit, which feeds through the booster venturi. The power valve would be open when the engine is shut off. However, there would not be any air flow to draw fuel from the fuel bowls into the venturi.

It still sounds like floats too high, or something causing the bowl level to rise when you shut off the engine.

CamarosRUS · 10-14-2009, 04:52 PM

Floats are just licking the sight window. No evidence of anything rising, you can see through the clear sight plugs. when I get it rebuilt I will pay close attention to the floats when I shut it down. This seems like the logical problem but I have no clue what would cause fuel to continue to enter the bowls.

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For what it is worth...the entire carb is junk until I fix it! I decided to rip everything apart after close inpection found one idle air bleed without a hole!!! All the others were a different size

Shavings all through the carb as well...looks like this remanufactured deal off eBay was just a scam. I bought the remanufactured carb (and even asked specifically about the metal shavings) for a reason; because it had already been rebuilt and cleaned properly!!! I was LIED to.

The metering block to main body gasket was soaked but the baseplate gasket was dry as a bone. I read if the metering block gasket is wet we have a problem. Can anyone elaborate on this? This may be my flooding problem...
Can't tell if the PV is ripped but a new one is in order anyway.

I don't know if this rebuild and resizing of the air bleeds will help the flooding issue but it better make this thing run, and especially idle, MUCH better

EvilCartman · 10-15-2009, 12:48 AM

Wish you had gone through the thing a long while back. I started to go through every carb I got, new or rebuilt to check everything out. If it wasn't right, I'd correct it. The carb isn't junk, just clean it out and make sure all the holes are the size they should be. Get a set of drill bits from a hobby store for the tiny ones. Take the needle and seats out and clean them, even if they look clean already. Check every passage for flashing, burrs and chips left behind. Once you're ready to put it back on the car, set the carb up to stock settings. Once you have the primary throttle blades set for that transition slot opening, don't touch it again. Use the secondary throttle blades for messing with your idle speed. Get a vacuum gauge and set the idle mix screws for highest vacuum. I've found the mix screws on the Demon carbs are a lot more touchy than Holley's so even a small turn will make a difference.

EvilCartman · 10-15-2009, 03:47 AM

Quote:

Originally Posted by five7kid

The power valve is in the main circuit, which feeds through the booster venturi. The power valve would be open when the engine is shut off. However, there would not be any air flow to draw fuel from the fuel bowls into the venturi.

It still sounds like floats too high, or something causing the bowl level to rise when you shut off the engine.

My thought was if the PV was bad, wouldn't it be leaking through to where the passages are for the vacuum and then coming out the baseplate?

CamarosRUS · 10-15-2009, 07:15 AM

Quote:

Originally Posted by EvilCartman

Wish you had gone through the thing a long while back. I started to go through every carb I got, new or rebuilt to check everything out. If it wasn't right, I'd correct it. The carb isn't junk, just clean it out and make sure all the holes are the size they should be. Get a set of drill bits from a hobby store for the tiny ones. Take the needle and seats out and clean them, even if they look clean already. Check every passage for flashing, burrs and chips left behind. Once you're ready to put it back on the car, set the carb up to stock settings. Once you have the primary throttle blades set for that transition slot opening, don't touch it again. Use the secondary throttle blades for messing with your idle speed. Get a vacuum gauge and set the idle mix screws for highest vacuum. I've found the mix screws on the Demon carbs are a lot more touchy than Holley's so even a small turn will make a difference.

You're exactly right. I can not take ANYTHING for granted with this hobby.

I plan on new needle and seats since they should come with the rebuild and I might just go ahead and tap the air bleeds for changeable screw in bleeds.
The Speed Demon is supposedly only "good" to 240 duration so I need all the tuning aids I can get.

EvilCartman · 10-15-2009, 09:47 AM

Kinda sucks that it has to be that way :/ Every carb I've gotten has had some sort of problem, Holleys and Demons.

CamarosRUS · 11-16-2009, 11:10 AM

Update:

Finally got around to rebuilding and installing the carb. Flooding and mystery fog are gone, car will restart hot now!!

The metering block gaskets were soaked so I can only assume I had an internal fuel leak somewhere; the PV did not seem to be blown.

FWIW while I was at it I restricted the IFR's and enlarged the IAB's a bit to help combat the extremely rich idle. Havn't got to tune it yet but it seems to have helped at least a little.