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Demon Carb tuning question (continued from my Paxton Thread)

T

toddwallace

Member
Ok.. Went and bought smaller jets (.067 Primary and .089 Secondary) to try and fix my problem with the car die'ing. I am not sure I put them in correctly as the car seems to be starved for fuel when I excelerate. Car seems to cut out when I accelerate... I pulled the bowls..replaced the gasket..screwed the jets all the way down tight.. I am not sure if I was supposed to screw them down and back it off, but I did not. The car does start and idle better now..

Talking to Keith at Speed Tec, he did not think my problem is a JET problem.. He thought it was a heat problem boiling the gas, which might make sense. The car does get hot, but not overheat.

Observations..
looking at the clear glass Gas filter, the filter does not get full.. it seems to bubble through.. when I turn the car off, I see gass bubbling through.. When it is running, gas seems to slosh through..

One thing that happened once (so I am not sure if it is a regular thing).. After I turned the car off, I heard gas pouring and noticed that gas was pouring out of one of the secondary venturi bores.. Just one of them.. It filled the bore.. The car would not start for a good 15 minutes which is the symtom I am trying to fix. I took the bowl back off and pulled the jet and replaced it and did not see this again. Not sure if this was a one time occurance or what my problem is.. It is hard to tell as the bonnet makes it hard to tune..

Right now, the jets are replaced.. car starts and idle's fine. It does not have power when you punch it.. Not sure if it dies when I drive it far as I don't want to get stranded..

Thoughts??
 
Fuel dumping into the venturis is a float level problem. That float needle is/was stuck or is set too low and not closing completely.

The jets get tightened down completely and snugged good. They should not be backed off.

You acceleration problem sounds like either an accelerator pump not giving enough squirt, wrong sized squirters, or undersized power valve. I'd start with a size or 2 bigger squirters, then maybe a little more pump shot if that doesn't fix it. Changing the jets likely will not help WOT very much.
 
An easy fix for the fuel boiling problem is a phenolic spacer, you can get them in 1/2" if you have aircleaner to hood clearance issues or 1" if you have the space. I put one on mine with a Demon Jr., w/o spacer I could barely touch the carb = was as hot as the engine ~200*+? After installing a 1/2" phenolic spacer I could leave my hand on the carb. I used an IR temp probe to check temps before/after but forgot to write the diff. down, want to say it was at least a 40* temp drop.
OPEN:
http://www.summitracing.com/parts/SUM-G1405-1/
4 hole:
http://www.summitracing.com/parts/SUM-G1402/

I had a glass fuel filter years ago and want to say it did the same thing running, bubble/percolate, spoke to someone recently who put there's between the frame fuel line and the FP inlet (horizontal) and he said it stays full of fuel now. Being at a vertical tilt causes air bleed down or something.
Jon
 
The air in the clear filter is perfectly normal. I wouldn't go re-doing things to try and fix that. I have clear filters on both of my cars. They are never full, and never will be. The filter is bigger than the line. There's just nowhere for the extra air to go when the float needles are closed.

Both of my cars have seen lots of miles and lots of drag strip time with the air in the clear filters. Never had a fuel problem at all.
 
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