750 Mighty Demon Question
Thread Starter
Drifting
Joined: Nov 2003
Posts: 1,874
Likes: 7
From: l In
750 Mighty Demon Question
This is my setup and I have a carb problem
383 CID
750 Mighty Demon, mechanical secondaries
Weiand air strike dual plane intake
AFR 195 Eliminator heads
Comp Camp XS-276-HR cam
MSD 8360 dist.
9.5/1 comp
timing 36 total
Having a hugh problem sooting the plugs, running very rich, carb has no stumble and runs like a striped azz ape, just doesn't idle worth a dam.
I pulled the carb apart tonight and cannot for the life of me find what size the power valve is, but the primary jets are 67, and 75 in the rear.
this carb probably only has 1000 miles on it and it ran rich with my other setup.
Carb is clean as new inside.
float level is set at the lower line, as Barry Grant suggests for street motor.
Any ides ?
Thanks,
Phil
383 CID
750 Mighty Demon, mechanical secondaries
Weiand air strike dual plane intake
AFR 195 Eliminator heads
Comp Camp XS-276-HR cam
MSD 8360 dist.
9.5/1 comp
timing 36 total
Having a hugh problem sooting the plugs, running very rich, carb has no stumble and runs like a striped azz ape, just doesn't idle worth a dam.
I pulled the carb apart tonight and cannot for the life of me find what size the power valve is, but the primary jets are 67, and 75 in the rear.
this carb probably only has 1000 miles on it and it ran rich with my other setup.
Carb is clean as new inside.
float level is set at the lower line, as Barry Grant suggests for street motor.
Any ides ?
Thanks,
Phil
Melting Slicks
Joined: Oct 2008
Posts: 2,154
Likes: 69
From: Regina Sk Canada
Yep sell it as fast as you can and buy a Holley. I dont mean that in a sarcastic way either. Get rid of it. i had an 850 Mighty demon and it was full of aluminum when i took it apart out of the box. I could never get it to work on the car, the idle ease would not adjust, and the car was lazy. i sold it and put on reworked Holley from Patric at Pro Systems and the car ran so much better.
BG has terrible tech support and was of no help to me. This is just my experience.
BG has terrible tech support and was of no help to me. This is just my experience.
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
I was running a similar setup before FI. I did manage to blow a few power valves and it made it run extremly rich at idle. It would burn your eyes out. I ran a 6.5 PV and had no problem when they were good. I would blow about 1 a year.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Yep sell it as fast as you can and buy a Holley. I dont mean that in a sarcastic way either. Get rid of it. i had an 850 Mighty demon and it was full of aluminum when i took it apart out of the box. I could never get it to work on the car, the idle ease would not adjust, and the car was lazy. i sold it and put on reworked Holley from Patric at Pro Systems and the car ran so much better.
BG has terrible tech support and was of no help to me. This is just my experience.
BG has terrible tech support and was of no help to me. This is just my experience.
This is discouraging to hear, as I've been planning on an 850 Mighty Demon for my solid roller 427 ever since I decided against going with a 4500 based carb. But, I have definitely noticed BG isn't exactly what I'd call responsive in handling inquiries I've made.
Melting Slicks
Joined: Oct 2008
Posts: 2,154
Likes: 69
From: Regina Sk Canada
Get yourself a nice 4 corner idle 850 HP holley you will be very happy with it.
Le Mans Master
Joined: Oct 2006
Posts: 7,123
Likes: 433
From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Everyone has a different experience. I've got an 850 Mighty Demon and it's perfect. Idle-eze works great. After hearing all the stories I pulled the float bowls to look and everything was spotless. No stinky idle, no bog, stumble, or soft spots.
It did take a bit to figure out how to get it all setup (longer than a Holley IMHO) but once I got it figured out it's a snap.
It did take a bit to figure out how to get it all setup (longer than a Holley IMHO) but once I got it figured out it's a snap.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Everyone has a different experience. I've got an 850 Mighty Demon and it's perfect. Idle-eze works great. After hearing all the stories I pulled the float bowls to look and everything was spotless. No stinky idle, no bog, stumble, or soft spots.
It did take a bit to figure out how to get it all setup (longer than a Holley IMHO) but once I got it figured out it's a snap.
It did take a bit to figure out how to get it all setup (longer than a Holley IMHO) but once I got it figured out it's a snap.
Sorry to the OP if this qualifies as having 'jacked your thread. Just got me thinking here...
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Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I like Demons too. I have a 825 Drag Race Demon that works perfect on the street on my 427ci small block. I did take about 2 years to figure out how a carb ( Holley / Demon ) works though, not as easy as one might think 
You really need a wideband O2 sensor to tune one properly to your motor
You really need a wideband O2 sensor to tune one properly to your motor
Last edited by MotorHead; May 22, 2009 at 12:02 AM.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
IMO - Any carb can be made to run.
All the aftermarket Holley types are all modded. I did everything i could think of to my 750 Demon. drilled all four plates................
My final solution was to have a Carb dyno shop do the total air bleed.............and run it on a test mule dyno circle track motor of equivalent HP. The few hundred bucks made my system run.
You start thinking about spending $12,000 on big inch 434 stroker small blocks and just a few hundred investment is worth every penny. I wished I would have done mine years before.
I deal with this local guy in Reno Nevada. All he does is carbs from all over the country. total flow bench and everything
All the aftermarket Holley types are all modded. I did everything i could think of to my 750 Demon. drilled all four plates................
My final solution was to have a Carb dyno shop do the total air bleed.............and run it on a test mule dyno circle track motor of equivalent HP. The few hundred bucks made my system run.
You start thinking about spending $12,000 on big inch 434 stroker small blocks and just a few hundred investment is worth every penny. I wished I would have done mine years before.
I deal with this local guy in Reno Nevada. All he does is carbs from all over the country. total flow bench and everything
Drifting
Joined: Jul 2002
Posts: 1,254
Likes: 1
From: Edinburgh
Bought a 750 Speed Demon a couple of months ago, checked it for cuttings/filings and it was clean as a whistle. Checked the floats & air bleed as per the instructions bolted it on and fired it up. Did 3 or 4 runs to set up the 4 mixture screws, job done! Compared with the Edelbrock that came off it, it was chalk and cheese
Very very happy with my BG carb.
Graeme
Very very happy with my BG carb.
Graeme
Last edited by Graemeinvette; May 22, 2009 at 03:23 AM. Reason: Add sig
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
start with drilling a .110 hole in each primary throttle blade if your carb does not have the adjustable air.
Do all the normal throttle blade .020 on the x-fer slot and fuel level about 1/2 up the sight glass. use the secondary blades to bring the idle up. set the 4 corner idle screws with a vacuum gauge at @ 1100 rpm and then turn down the idle speed when you are done.
Tons of write ups on the web about how to tune holley carbs. The BG is the same.
Do all the normal throttle blade .020 on the x-fer slot and fuel level about 1/2 up the sight glass. use the secondary blades to bring the idle up. set the 4 corner idle screws with a vacuum gauge at @ 1100 rpm and then turn down the idle speed when you are done.
Tons of write ups on the web about how to tune holley carbs. The BG is the same.
Last edited by gkull; May 22, 2009 at 11:24 AM.
Team Owner
Joined: Feb 2003
Posts: 30,183
Likes: 313
From: Lake Norman NC
I had the same problems with my Mighty Demon 750 on a 383...even had it sent to lars for tune....still ran rich at idle...a 5.5PV helped, but i ended up screwing with it so much it ran like crap, so i pulled it off and sold it. Got a Holley and never looked back.
CF Community Team
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From: Cape Cod, Mass.
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Yep sell it as fast as you can and buy a Holley. I dont mean that in a sarcastic way either. Get rid of it. i had an 850 Mighty demon and it was full of aluminum when i took it apart out of the box. I could never get it to work on the car, the idle ease would not adjust, and the car was lazy. i sold it and put on reworked Holley from Patric at Pro Systems and the car ran so much better.
BG has terrible tech support and was of no help to me. This is just my experience.
BG has terrible tech support and was of no help to me. This is just my experience.
The Speed Demon 650 with mechanical secondaries that I put on my 61 with a 383 has been flawless since day one. Once I set it up about 6 years ago I haven't so much as adjusted the idle. Great carb.
Bottom line: You can get this metal shaving problem with Holleys or Demons. It appears to me to be incomplete machining/improper cleaning at the factory on random carbs.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Interestingly I had the same situation with a Holley spreadbore I put on my 68 BB. Great carb once I got all the metal shavings cleaned out.
The Speed Demon 650 with mechanical secondaries that I put on my 61 with a 383 has been flawless since day one. Once I set it up about 6 years ago I haven't so much as adjusted the idle. Great carb.
Bottom line: You can get this metal shaving problem with Holleys or Demons. It appears to me to be incomplete machining/improper cleaning at the factory on random carbs.
The Speed Demon 650 with mechanical secondaries that I put on my 61 with a 383 has been flawless since day one. Once I set it up about 6 years ago I haven't so much as adjusted the idle. Great carb.
Bottom line: You can get this metal shaving problem with Holleys or Demons. It appears to me to be incomplete machining/improper cleaning at the factory on random carbs.
Safety Car
Joined: Oct 2000
Posts: 4,282
Likes: 172
From: Lake Wylie, South Carolina
850MD here. Runs a little rich but Idles around 900 with a big solid flat tappet cam and I have had zero problems with performance. have not had it apart but probably need to tinker with the PV.
Wade
Wade
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
i bought a demon 750 double pump. bolted that sucker on 2 years ago. set my floats,idle, mixture and been flogging the hell out of it ever since. changed jet to a 73 from 75 just to clean it up a bit and it has never looked back.looks tricky as well.
