Carb Guru's...Help!

dolfan87 · Dec 13, 2006

Ok, this is my issue. I have a very well built 393W, solid roller lifter cam in the 290's for duration, and .585 lift. AFR 185 heads that have been port matched to my Edelbrock Performer RPM. All valves are set at .020 with the engine at full operating temp.

I have a Mighty Demon 750 carb, and the thing is pig rich, eye watering, black smoke coming out the exhaust. I have checked the vacuum at idle and she sits at 11-12 pounds. The power valve is new, and a 6.5. I have downed the jets from the stock 75's to 70's and it made no change. I have set the throttle plates down as low as I can, and adjusted the idle mixture as lean as I can.

It's still rich, and all the plugs are BLACK.

When I pulled the Demon apart, it had aluminum swag in almost ever orfice left from the CNC machine. I completely cleaned all that out, but it really made no change in the performance of the carb.

Could the carb just be crap? It's my first Demon, and all the Holleys I have ever run worked great.

I have also checked for vacuum leaks, and found none on the top side of the motor. The heads were not milled, but there is a remote chance that it's bleeding from the underside of the intake. Would that cause my rich condition, or just lean out the mixture?

I am about to order a $400.00 Holley 750, and just get it over with, but I am really bummed that my $500.00 Demon is a paper weight!

Any advice, or help would be greatly appreciated.

87

Crs · Dec 13, 2006

Check your vacuum in gear to see what you get. On my 393, i only had 7-8 inches in gear and ended up using a 3.5 power valve.

dolfan87 · Dec 14, 2006

I have a 5 speed...but the power valve is a thought. Good call, I will try a lower one. I am also going to bump my timing up to 18 degrees intial, and see how that does.

Crs · Dec 14, 2006

Play around with the timing around 12-14 initial and 36-38 total. Keep us posted!

bnickel · Dec 14, 2006

there are a couple things i would do.

1st, it seems that demon carbs like the float leverl set a liitle lower than what you'd think it would be if you are used to a holley. mine is set right at the lowest line on the sight glass, that cleaned up the idle mixture a ton!!!!

second, i'd change out the idel air bleed screws to a larger one. i also had to this but i have the road demon so instead of just being able to swap them out, i had to pull mine and drill them then re-peen them into the carb body. the speed demons have replaceable air bleeds.

dolfan87 · Dec 14, 2006

Bryan...I already drilled out the idle air bleeds, and the float level is set right where yours is. Timing right now is 16 BTDC initial. I was told that 18-20 is more on par for my cam.

You get the CD yet?

thehueypilot · Dec 14, 2006

Open the rear throttle plates slightly to allow more air in at idle. Holley has an adjustable throttle stop on the sec. side that you would adjust to tilt the plate open slightly. The next option is to drill .094 diameter hole in each primary throttle plate to allow more air in, but that should be your last resort. If you adjust the air/fuel mixture screws all the way in does it have an effect at all on idle quality?

BlwnGazkit · Dec 14, 2006

Your power valve maybe incorrect for the vaccum you actually have. The Mighty Demon carbs have an adjustable air bleed screw under the airfilter stud. It does the same thing as drilling holes in the throttle plates but lets you keep it adjustable.

If nothing on here seems to help, call Demon Carbs! I called their tech support once and it was very helpful. Somewhat confusing, but the guy was willing to clarify what he meant for me to do. The guy in their tech actually uses these things so he knows how to tune them.

10secgoal · Dec 14, 2006

Make sure the gasket for the PV is correct. Mine smoked and I couldn't figure it out for days. It was leaking by my brand new PV gasket. Stick with holley paper replacements.( If that's what it is)

mstngjoe · Dec 14, 2006

If those files I sent you don't help, look up user name Tech @ BG on www.hotrodders.com

As his name says, he's a BG Tech. He's good about answering pm/email.

Good luck.

bnickel · Dec 14, 2006

dolfan87 said:
Bryan...I already drilled out the idle air bleeds, and the float level is set right where yours is. Timing right now is 16 BTDC initial. I was told that 18-20 is more on par for my cam.

You get the CD yet?

total timing should be around 36 degrees ro so and be all in by at least 3000 rpm, probably more like 24-2600 with your cam, so set the timing accordingly.

how old is your demon? if it's more than a couple years old it won't have the Idle-EZ feature with the adjustment screw under the air cleaner stud.

try removing the idle air bleeds completely and see how that affects it, they may still be to too small.

the CD came yesterday but i haven't had a chance to listen to all of it yet. the first song rocks though, i really like that one. we spent all day Christmas shopping yesterday otherwise i probably would have been able to listen to the whole CD. so far i really like what i hear!!!

10secgoal · Dec 14, 2006

If it smoking at idle, I don't think the EZ adjustment would work. Sounds like a definite internal leak.

dolfan87 · Dec 14, 2006

Ok, I changed all the plugs (waste of money because the new ones are black now) replaced the rotor and cap because they were both messed up bad, advanced the timing to 25 degrees intial (it's a mechanical distributor, no vacuum advance, comes in full at around 38 degrees at about 2500) and it seems a lot better.

I am going to go drop a 4.5 power valve in it here in a few.

Once those plugs are black, they stay black right?

dolfan87 · Dec 14, 2006

Power valve made no difference.

I hate this Demon.

bnickel · Dec 14, 2006

dolfan87 said:
Ok, I changed all the plugs (waste of money because the new ones are black now) replaced the rotor and cap because they were both messed up bad, advanced the timing to 25 degrees intial (it's a mechanical distributor, no vacuum advance, comes in full at around 38 degrees at about 2500) and it seems a lot better.

I am going to go drop a 4.5 power valve in it here in a few.

Once those plugs are black, they stay black right?

yep. if they're still black and you changed the PV it wouldn't help. are you running an MSD unit? what kind of coil? IIRC Demon assumes that most people running one of these carbs will also be running some sort of CD ignition.

also Demons are very picky about the transfer slot being set just right. pull the carb and set the transfer slot according to the demon specs front and rear and adjust the idle mixture screws so that they are 1.5 turns out from seated. if your carb has a choke set it to the leanest setting. also make sure the fast idle adjustment is set pretty close.

the demon is definitely not a ready to go out of the box carb, but once you get it set it right they are awesome.

i'd also recommend gapping the plugs at .030 to start and no bigger than .035 unless you're running a CD ignition

dolfan87 · Dec 14, 2006

I am running the Davis Unified Ignition from Performance Distributors. They recommend .050-.055 gap.

Ok, I'll re-set everything back to baseline on the carb, and if that doesn't get it, I am ordering a Holley.

69Rcode_Mach1 · Dec 14, 2006

Do not ditch the carb. I went through the exact same thing as you did for weeks, and didn't get to drive it cause of it. I am going to try to dig up my stuff for you, and see what we did to get it running. I am really busy right now otherwise I would post here.

10secgoal · Dec 15, 2006

If you have an electric pump, this will tell you. A little harder if you have a mech, but can still work.
Take the carb off the motor, leave fuel line connected. take the nuts you use to bolt down the carb and run them down the carb studs enough to put the carb on top of them and run fuel pump. Look for leaks coming from the carb.

bnickel · Dec 15, 2006

ok, this one is more related to your dizzy than the carb but when you wired the dizzy did you bypass the stock resistance wire or did you you just the stock ignition wiring? if you used the stock wiring you aren't getting enough spark energy to light the mixture. the stock wiring is a resistance type wire and drops teh voltage down to around 6-8 volts that DUI you have should be using a full 12 volts. just a thought, but worth asking.

Steve69 · Dec 15, 2006

If the engine is idling and you don't have a lot of drive time your going to have
black plugs. I would drive it around and put some miles on it. Mine also smoked
and it burned my eyes and after a couple hundred miles it went away.

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