Demon Carb Question

blykins · #1 (**permalink**) 08-01-2005, 06:16 PM

Guys...I'm running a Mighty Demon 750 on my engine....

I've been noticing that when advertised on TV, they always add that the Mighty Demon should be used on engines with cams over 240 deg duration...

I'm 230.

So what's different on the Mighty Demon (besides the obvious....no choke, adjustable air bleeds, etc.) that makes it ideal for engines with larger duration cams?

Power valve? Squirter sizes?

My engine always idles really rich....I've bored out the idle air bleeds to try to remedy it, but it seems like I just can't get away from it.

Don't know if there's something internally wrong with the carb (bought it brand spankin new) or if it's just too wild for my cam....

Anyway, I need to start looking for a solution....

Dennis Kelley · #2 (**permalink**) 08-01-2005, 06:38 PM

I never could get my speed demon right. It performed good on the dyno but had lousy street maners. I bought a Holley and wouldn't go back for any carb.

later dennis

Tony Radford · #3 (**permalink**) 08-01-2005, 06:46 PM

I'm running a 750 Mighty Demon on my Windsor stroker and have been for a couple of years. I would highly suggest having a dyno tune performed. Mine ran rich enough to eat the piston rings after the intitial build. Though the carb setup remained the same, they had to crank in some additional advance to get the A:F ratio right.

TonyMadrid · #4 (**permalink**) 08-02-2005, 06:13 AM

Check out Don Gould at 4secondsflat , he's the guru of Demon Carbs.

He is always very helpful and helped me solve problems I was having with my carburetion. You can reach him at (503) 627-0728, or e-mail him at bigdaddy@4secondsflat.com

rob frink · #5 (**permalink**) 08-02-2005, 07:13 AM

Tony hit the nail on the head!!! Don Gould knows demons!

I love mine!! There is a very important key to get them to perform. You must use a lot of initial iginition timing if you have a hot cam. Due to cam overlap, the engine doesn't want to idle...so traditionally, we open the butterflies to let in more air and increase the idle speed.....this is not the case with Demons...which rely on vacumm and air velocity to set up the a/f circuits. By increasing the intial timing, you allow more time for the fuel to burn in the cylinders....which increases the engine speed. ...Then slow the engine down by closing the butterflies with the idle speed adjuster screw......now you have vacumm and air velocity through the carb circuits. The low speed torque increases and the throttle response becomes amazingly snappy. With this done...the high speed circuits work properly. Demons are flow adjusted at the factory...and don't really require rejetting. If you are rejetting them...it is likely that something else is out of place. I've been through this with mine...I think there are many dyno-tuners that don't understand these carbs.....they are not the traditional "old-shool" jobs and require a differnt thinking process when tuning them. 4secondsflat had a great tech article online about this......But it looks like it's been taken down due to bandwith limitations.

tried to edit spelling errors

mj_duell · #6 (**permalink**) 08-02-2005, 07:27 AM

Rob nailed it! Don is the guy. After talking with him all of my Mighty Demon issues disappered. I will never go back to Holley again. The engine runs with a nice curve and the idle is perfect. There is a lot more to tuning the carb than the corner screws. First I made sure the distributor was right. I sent it to Don to have it recurved. Secondly, made sure my spark was all it could be by adding an MSD high vibration coil along with my 6AL. Then I triple checked the timing with Don on the phone.

--Mike

blykins · #7 (**permalink**) 08-02-2005, 08:34 AM

Alright...I shot Don an email....hopefully he can give me a hand.

My initial timing is at 12....maybe that's one of the problems.

Goose · #8 (**permalink**) 08-02-2005, 09:16 AM

Brent,
I just thought I'd chime in to let you know that, as others have previously stated, Don Gould is the right guy for the task.

I had a 750 Speed Demon on my 351W that was causing all sorts of problems. With Don's guidance, I now have a 650 Speed Demon and a recurved distributor (initial advance 20deg, 34deg total) that produces a very snappy throttle response, idles great within a few seconds and, with some idle mixture adjustments, is producing clean spark plugs (vs. the previous, black, sooty ones).

Given that your initial advance is only at 12deg, I would suspect that that is part of your problem, but every motor is different. Don had me advance my distributor 2deg at a time, with a warmed up engine, turn off the engine, let it sit a few minutes and try to refire without touching anything. I continued doing this until the engine did not want to fire easily and began to idle rough. At that point, I backed off 2deg to determine my initial advance but could not run it there until the distributor was recurved for the appropriate total advance. Confirm the process with Don; he's a great guy with a lot of advice to give.

Gus

blykins · #9 (**permalink**) 08-02-2005, 09:29 AM

Yeah, I'm looking forward to his reply....

That's what gets me though....the car behaves so beautifully except for a rich idle.

If the engine is warm, you can just reach in and hit the key...it will fire on the 1st or 2nd revolution of the crank....you don't have to give it any gas at all....it seems like it's well tuned except for the sooty plugs.

rob frink · #10 (**permalink**) 08-02-2005, 10:01 AM

Yep!! Ditto on Goose's timing!

I run 20 initial...34 total. No Vac advance. idle speed about 900-1000 rpm. My MSD billet distributor uses different size bushings to limit the amount of mech advance. I had to machine my own bushing to limit to 14. It turned out to be .400" dia.

TonyMadrid · #11 (**permalink**) 08-03-2005, 01:30 PM

Yes, Don had an excellent article on his website regarding correctly tuning carbs. I believe it was called four corner carb tuning. It used be there for free, but Don removed it as a freebie (except for his customers) and now I believe he offers it as a tuning guide for a price.

Nonetheless, I would call him and speak with him. He's a very friendly and helpful guy.

blykins · #12 (**permalink**) 08-03-2005, 01:43 PM

I gave him a call and got a copy of his handbook....

Looks like I need to look at initial timing first....Don recommended 18 degrees for my application....I'm at 12.

Roger Vincent · #13 (**permalink**) 08-03-2005, 07:47 PM

Brent, keep us posted with your results and what you did to get them.

Roger, here in Bowling Green

blykins · #14 (**permalink**) 08-04-2005, 06:42 AM

Will do. I hope to get to it this weekend....

trularin · #15 (**permalink**) 08-04-2005, 07:15 AM

I'll second on Rob's numbers, I run 36 total with NO vac and inital at 18. No problems. Running Mallory.

blykins · #16 (**permalink**) 08-04-2005, 09:11 AM

Don told me to try 18 and 34....so I'll try that first and see how she purrs....

blykins · #17 (**permalink**) 08-07-2005, 05:25 AM

OK....Did a little tuning yesterday...I had a lot on my plate yesterday...I had some new PSE wheels mounted and balanced....I worked on my new sidepipe flanges (no more sloppy slip fits) and I tinkered with the carb.

I set the initial time at 18....then went around the carb again. I stuck a couple new plugs in the engine to see what it was doing....it does seem a little crisper on the throttle response. It still starts on the first whirl when it's warmed up. The plugs seemed like they were burning cleaner, but I didn't get to run it as long as I wanted to with all the other projects going on.

I'll keep you posted.