Problem with Mighty Demon 750

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Whatever your vacuum is in gear at idle I'd get a primary power valve rated as close to 2" lower in vacuum as I could. On the secondaries the same or weaker, just not one that will suck shut at WOT.
 
Not surprised..the Mighty Demon i had was a total piece of ****,all i did was adjust readjust readjust again..got rid of it bought a Quickfuel best damn carb. i've ever owned no issues in over 6 years of racing..
 
Demon is a good carb.
Power valve blocked in the rear is good for street/strip. If you were road race you could install PV in the rear and square jet the carb.
1st thing I would do is get more intial timing 24-26* 34 total mechanical.
Next is check what you have. Previous owner may have changed original settings. Demon jets run bigger than holley. Use one or the other when changing jets. Ex 79 Demon is = to an 80 holley.
72/78 jets are more in line with a demon 650
71/79 is stock jet for 650

My guess is you have a 650. A 750 with 72/78 jets would be lean.
http://documents.holley.com/lit705rev.pdf

Check the airbleed sizes IAB .070 MAB .039
I run 75 PMJ 84 RMJ
Set float to lower line on site glass.
My regulator set to 6.5 psi
 
I took it to work with me today. The primary squirters were plugged, there were a few other ports on the metering blocks that were nasty too. I cleaned it all up. Vac at idle is 13 lbs. So I need to remove the 4.5 and put in a 6.5. It's still rich, but it's able to run without choking out
 
Well now I've got a fuel pressure issue... I've got a Holley 125, brand new. It was pushing 6 lbs yesterday.. 2-3 today. I don't know what the hell is going on. I checked the pump filter screen. It's clean. I've got an Earl's inline filter after the pump as well. Not sure that thing could get plugged without the pump screen not having anything in it. And it's only got about 15 minutes of run time.
 
Check the pressure regulator my pressure was jumping around,thought it was the pump took a shot and changed the regulator problem solved..
 
I agree it CAN BE a good mod when done in steps. But for the OP IMO, right now he needs to get the carb back to stock so he can have a base line and then go from there. Not knowing "where you are" in a tune is a bad place to be.

mderoy340 said:
Demon is a good carb.
Power valve blocked in the rear is good for street/strip. If you were road race you could install PV in the rear and square jet the carb.
1st thing I would do is get more intial timing 24-26* 34 total mechanical.
Next is check what you have. Previous owner may have changed original settings. Demon jets run bigger than holley. Use one or the other when changing jets. Ex 79 Demon is = to an 80 holley.
72/78 jets are more in line with a demon 650
71/79 is stock jet for 650

My guess is you have a 650. A 750 with 72/78 jets would be lean.
http://documents.holley.com/lit705rev.pdf

Check the airbleed sizes IAB .070 MAB .039
I run 75 PMJ 84 RMJ
Set float to lower line on site glass.
My regulator set to 6.5 psi
Click to expand...
 
And I did notice too. The original
20160604_113422.jpg
owner drilled holes in the butterflys.
 
OK... As the car sat and idled. It got up to 185*. I looked at my gauge.... Dropped to 5 psi... My fuel lines are away from heat. Nothing running by the headers at all. Not sure what's up. The pump isn't hot either.
 
Do not put a power valve in the rear.
Set fuel pressure at operating temp.
Close down the butterflies as much as you can with those holes drilled.
 
OK 5 psi for now. 6-7.5 is demon spec.
Take a pic of the rear vent tube area and post it. Is there a #28 cast in the main body?
6.5 power valve is stock.
Did you measure the air bleeds to see what you have?
 
that's a 750
Since holley took the demon specs off their site this is a stock setup:
IAB .070
MAB .039
.031 squirters pink cams
65 PV front, rear plugged
IFR channel .031
PVRC .059
.031 emulsion bleeds 3 each side
PJ 76
SJ 83
 
OK. Good. Currently it has 72 primaries and 78 secondaries. Nozzles are 31's. I need to get the 6.5 PV in.
 
Looking at your pix of the air bleeds, they mains and idles look to be about the same size. That is not normal. The idle air bleeds look way big. Is that the secondary side? Look at the numbers in post #42 for IAB and MAB? They are quite different and the inner bleeds (the main air bleeds) should be much smaller than the outers (the idles air bleeds).

And in post #1, is that your actual metering block? I can't see too well in the pix, but there look to be NO main emulsion tube restrictors in place. Look at the 3 threaded holes in a vertical row on each side; there should be brass 'jets' (emulsion bleeds) in each of them per the info on post #42. What the heck was this set up for previously?
 
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Yes, that's my actual block. I. Not sure of the setup, as I bought it used. If I had the money to go brand new, I would have. I figured I'd have to do some replacing and tuning before it was perfect.....
 
I'm amazed the IAB and MAB aren't numbered. I have nothing that small to measure the inside. Any tips on that?
 
Best to get a set of small twist drills or wires and use calipers to check whatever fits snugly in the bleeds. Any markings would be on the bleeds once you screw them out. I wonder if these have been drilled out.

It looks like someone pulled the restrictors out of the metering block before you got it or drilled the snot out of the holes. See if those vertical sets of 3 holes in the metering block are threaded or of there are tiny restrictors in those holes (that don't show in the pix); if the restrictors are not there.....no wonder it is pouring fuel everywhere.

I'd be measuring every restriction and jet in that thing. I got a carb that had been jetted up a lot...... that's ALL gotta be fixed up to match the numbers in post #42 before putting this back on the car.
 
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I've got it to where it's not puking fuel now. I cleaned the **** out of it at work yesterday. I've adjusted all four corner screws. It's idling pretty good now. My vac IN GEAR bounces between 12 and 8 lbs. I've still got the 4.5 PV in. Not sure if I should throw the 6.5 in or not with the vac reading like that.
 
The 4.5 won't cause excess fuel but might result in a bog when at moderately heavy throttle. IMHO, focus on getting the bleeds and such all checked before worrying over that.
 
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