Problem with Mighty Demon 750

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71valiant

71valiant

Make fast, break, repeat.
Joined
Jul 21, 2004
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Location
Columbus, Indiana
I've got a used Demon 750 mech sec. This thing has ruined two sets of plugs. 360 .30 over,MP528 cam, EQ heads with 2.02/1.60 valves, Performer Rpm. Insanely rich. Black smoking. Pissing me off. Primary jets are 72, sec are 78. 45 power valve in front.... Now the rear block has a plug in the metering block instead of a power valve.....Not sure if this may be my problem or what. My fuel bowl levels are good. I can't keep this thing running because of all the fuel. This is the first Demon I've ever owned. Not really digging it so far. Hoping the previous owner just has it out of whack.
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****.... I wanna say 16? I went from a Edelbrock 625 to this. I was wondering if I needed to bump my timing up.
 
What's the rest of the combo? Compression and all that hooha?
 
10.5 comp. Ross pistons, Holley 125 elec fuel pump @ 6 psi. NGK 7373 plugs. Msd ecurve dist, headers. Am I missing anything.
 
71valiant said:
10.5 comp. Ross pistons, Holley 125 elec fuel pump @ 6 psi. NGK 7373 plugs. Msd ecurve dist, headers. Am I missing anything.
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Ok, is the compression ratio an actually MEASURED 10.5?
 
If you are SURE that it's actually 10.5 and not just 10.5 pistons, then the timing might be close. You can try bumping it up around 20 initial but keep the total around 34 all in by 2500 and see how it does. Remember, just because you bought "10.5" pistons means nothing. It all depends on many other factors. Chamber volume, gasket thickness, deck height........If you have not verified the compression by measuring, you could be miles off.
 
The THIN Mopar head gaskets. Can't remember the compressed number. 1.6 roller ratio too. Not sure that matters. Lol
 
They are like .021". That compression is probably close then. I would try a bit more timing. See how it affects things.
 
Gotcha. I'll be back at it after work tomorrow. So the plug in the back end of the carb is normal I take it?
 
I HATE blocking power valves. That certainly could be a contributing factor. Normally, you jump up about ten jet sizes to compensate. But see, that also takes away the fuel curve so to speak. Rather than richening up at a certain amount of vacuum, it is rich all the time. Basically made to run WOT. It's a race mod pretty much. Were it mine, I would install the right power valve and jet accordingly. BUT, remember this too.......the timing needs to be right FIRST or you'll be forever chasing your tail.
 
OK. I'll set the timing then adjust the carb. I'd rather the carb be driver friendly. It's more street than strip. I'd like to maintain as much carb drivability as possible.
 
71valiant said:
OK. I'll set the timing then adjust the carb. I'd rather the carb be driver friendly. It's more street than strip. I'd like to maintain as much carb drivability as possible.
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I've never been a fan of blocking the power valve even on a race only car. I've always thought it was totally unnecessary. Stupid, really.......no offense.
 
I think though, in your case, I would make an exception. What "I" would do would be to get a power valve back there. Get "whatever size" jets back there it came with, THEN get the timing right and then go back to the carb. That way, the carb will not be MILES out of adjustment. At least it will be in the ballpark. I would do the front the same way. Find out which power valves and jets it came with and act like the carb is new and you are starting all over. That way, you will have a true base line.
 
71valiant said:
I planned on that. I'm having a hard time finding info on the stock setting on it.
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Get your model number and punch it on in on demon site and it will tell you everything...Demon site will tell you how to id your carb as well
 
I'd put a proper power valve in there as well. Get your timing set. Rebuild the carb to stock using a rebuild kit. Generally these kits return the carb to exactly how it left the factory. And usually that's pretty close to what you need anyways. Make sure it's all clean and sparkly. Then tune for best vacuum. Now half that number. That's your new power valve number. Get those power valves. Then all you gotta play with is jetting/pump etc. Fine tuning from there basically.
 
Hey man,sorry I guess Holley took them over Demon website had everything you need,,,Holley's is confusing at least as far as Demon is concerned...
My model number is on the left side of carb next to the secondary left barrel.
Look and compare your carb here:
Mighty Demon - Holley Performance Products | Contact Us: 1-866-464-6553
click on your carb and then click on instructions.
 
I've got a 750 mighty demon.. As far as it being a blow through or not..... Idk, where the part number is on the breather gasket base, its been wore off. So I don't see a number. Really hoping it's not a blow through.
 
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