I finally have my beast running again, but I'm having some carb/timing issues.

I'm running a Mighty Demon 750 with mech. secondaries. 68 Primary Jets, 78 Secondary, (I think, I'll have to double check that one). I think I'm running a 6.5 lb power valve. I couldn't really read what it was. It's the valve that came with the car. This carb does not have a choke horn, or any provisions to add a choke.

At Idle I'm pulling 14 lbs of Vacuum. My cam is .575 lift and .248 Duration @ 050. This is in a 454 bored .60 over (468).

It is very difficult to start when cold. I usually backfire it through the carb occasionally starting a fire in venturis. After blowing them out, and finally getting it going, it idles nicely. It hesitates on acceleration and misses when it gets to mid-rpm range.

I'm using an MSD distributor with the black advance stop bushing (at Demon's recommendation). I'm using one heavy and one middle spring for the advance springs. I'm set at 18 degrees initial advance and 35 total by 2000 rpm.

I have never had so much trouble getting an engine to run correctly. Maybe it's just the relatively big cam and the oh-so-tinkerable demon carb. It's using the stock pump shot diaphragm, which I think is 30cc. When I give it gas, I can see the spray deflect out of the venturi and mist up above the carb, which seems bad to me. The pump cam is set at the first position, so that there's no delay on the pump shot.


I'd like to start with making it easier to start. I know it's not going to be easy without the choke, but right now I'm afraid to crank it with the hood down for fear of starting a fire. Am I too far advanced? I'm ready to start over.

Thanks

 

a standard 750 mighty uses #75 pri and #83 sec jets while a 750 annular booster mighty demon uses #67 primary and # 75 secondary niether of these match the jet #s you gave

i would look at the engine vacuum if the jets are correct for your model mighty demon, if the needle bounces (or is not steady) on the vacuum gauge at idle or higher RPM's you may have a flat cam or other engine issue
a flat cam or the firing order spark plug wiring being incorrect could cause the engine to fire back thru the carb (or a lean air/fuel mixture under some conditions)

 

Your jetting doesn't sound right, and it doesn't match the BG jetting requirement for any 750 MD carb. Your symptoms are those of a lean condition. Jet the carb to its correct stock specs and then bump it up 2 sizes if you need to in order to cure the backfire. Backfire through the carb (if timing and valve problems are not the case) is a lean condition.

 

I'll bump it up and let you know. The car was gassing out the garage, and turning the pipes orange with the jets it had before, that's why I bumped it down that far.

Thanks

 

FYI a rich mixture (High CO or carbon monoxide) has no smell while a missfire from a rich or lean air/fuel mixture (or fouled plug etc) will create high HC (hydrocarbons) which smell, cause your eyes and lungs to burn.
in my mind the only real way to check the air/fuel mixture is with a exhaust gas analyzer which can check the HC and CO content of the exhaust (plus o2, co2, and noX) or a wide band o2 sensor based air/fuel meter. there are those that say they can read the air/fuel mixture at every operating condition (from idle to 7,000 rpm) by looking at the spark plugs but i think there are only a few people like Jack Roush left in the world with that kind of talent.

 

Steve -
What Henry says is correct - CO is odorless, so you can't smell a rich condition. HC (resulting from misfire) is what smells "rich." A/F-related misfire on a street driven vehicle is most often from a lean condition - not the rich end of the spectrum. Henry has worked with rich-end racecars a lot, so he's seen that situation, whereas I work mostly with mild street driven vehicles, so I see the lean end problems as the common occurrance. Either one can occur, but if you're a couple of jet sizes down form the stock setup, I can assure you that you are not rich - you're getting a lean misfire that makes it appear to be rich (because you're throwing unburned fuel right into the exhaust froim the lean condition, as bizzarre as that may sound). The other thing that a very lean jetted carb will do is cause you to crank the idle speed screw up off the idle circuit. This can cause it to then go rich due to fuel now being sucked out the transition circuit and due to the power valve potentially cracking open. So a lean condition can screw you up all over.

Do you have my "How to Set Up A Demon Carb" article? You need to do a good basic setup on the carb with the carb in its stock configuration as your initial starting point. Only from that point can you then start to make changes based on how the car is responding and what it's "telling" you. If uyou need the article, e-mail to:
V8FastCars@msn.com
Lars