MGSFJET

Replaced the tired Holley 750 DP with a Mighty Demon 750. The Demon came with 75/83 jets...my Holley was setup 72/82 and ran rich. This lead me to change the jets in the Demon to 71/81. Starts right up...idles with a little black smoke...hesitates when the throttle is blipped...lots of black smoke at higher RPMs.

70/80 next?

383
Heads - Wedge/Fully ported, Large Valves
6way comp valve job
11:1
Holley 750 DP Mech.
Weiand Stealth Intake, match port
Running 2:1 Cam2 110
Hooker headers, 4" sidepipes, STS Inserts (caps off)
Crane Roller cam, Scorpion 1.5 roller lifters
0.050 lift, Int lift 0.561, Exh lift 0.561, lobe center 116.0, Overlap 22.0, Int Dur. 250.0, Exh Dur. 258.0
Minimum RPM 3700, Max. RPM 7200, Valve float 7800
500 HP +/-
NOS Cheater 250
TH350 HD w/ shift kit...3.08 rear end

467-Ratman

I don't know how close the Holley and Demon are , but we just changed jets in my Holley 850DP for my BB, I have 76 up front and 77 in back with a 5.5 power valve. No more black smoke, and seems very responsive at take off. I think 80 is too much for the secondary, with your 383 I would guess a 77 or 78 should be plenty.

LAvetteman

:iagree:

82 maybe too big for the secs, but if you had that before, hmmm.

One very important thing on the demons, you want the right amount of transfer slot exposed on both pri and sec, and the pri and sec idle speed screws adjusted the exact same amounts when adjusted. Look for Lars' Demon tuning paper, its a great starting point.

IRON WILL

I have a smaller demon carb on my car and I run 76 jets in the rear. The car does run alittle rich so I wouldn't be surprised if you need to come down more with your jetting some. Granted with a bigger carb you will need bigger jets. If you have black smoke your are deffinately still to rich. Get the float levels set so that the fuel is showing in the lower half of the sight glass. Also make sure you have the correct power valve in. I think with a 5.5 though you should be ok. I thought I have my carb all messed up, but after I talked to Demon they told me that the cam I am running has too much duration. I should have the Speed Demon series. 650 or 750 CFM. I don't remember exactally what the reason was but... my complaint was the car is running very rich at idle. They told me that because of the duration of my cam it was pulling in more fuel at idle than it should. It would run fine above idle though. What they told me makes sense, cause that was exactally my problem. In you case it sounds like jetting. I am not a carb expert but I hope this helps alittle.

gkull

We have somewhat simular motors other than I use a single plane and SR cam. I have 1/8 inch holes drilled in each primary throttle plate to lean out the idle. i have drilled out the squirter for a bigger faster shot. 8.5 power valve to richen faster on vacuum drop. Because I would get an occasional lean pop

I Now have 74 and 80. When I pulled my heads a few months ago it indicated rich. So I tried 72 and 73 and it got the lean pop again. Your dual plane would not have those problems. Jetting doesn't car how many cubic inches you have. the carb only senses vacuum. Your cam is fairly hot which would tend to have less vacuum which requires more jetting.

It is best to only change one thing at a time. If your rich at WOT go down on the secondaries.

dman535

With the LT-1 cam and a 750 Speed Demon I had to go to a 81 primary and a 7.5 power valve to get things running right.

I am not impressed with Demon technical support!