what's up with the demon?

[beerjet]

bought a demon upon much recomondation , hooked it up and it wont idle
might demon 850 mech. sec. just hope it wasnt a bad choice.

[Moneypitt]

What is it on top of?? 850 is alot of flow.........MP

[beerjet]

a 454 which is also for the most part a big mystery

[scott williams]

I belive the idle is a "four corner" idle setup, very tricky.
both the primary and secondary throttle plates have and idle adjustment, and four idle screws, if you are using even a moderatly radical cam profile, it may be difficult to get a solid idle below 900-1100 rpm, NOT impossible, just tedeous and time consuming. I helped a guy with a 454 tune a demon, tricky! tiny adjustments, never got it to idle below about 975 rpm, cam was hydraulic, some custom grind in the 300-310 "advertised range.
good luck, the carbs seem to be well made, and certainly no more difficult than an equivilent holly with 4 corner idle.

[cruser]

According to the literature that came with my 850 Mighty Demons, you set the throttle plates to properly expose the transfer slots and adjust the idle with the screw in the hole where the air cleaner screw goes. Haven't tried it yet, still putting the finishing touches on the motor.

[scott williams]

Yeah, isn't the center screw an "additional air bypass" the demon version of drilling holes in throttle plates. this is used to increase air flow at idle so you can keep the transition circuit "out of play" at idle speeds?

[beerjet]

I set it up just as the booklet suggested , transfer slots , floats , 1-1/2 turns out on the idle screws and I have about 7 psi of fuel goin to it and it still did no idle . I called there bg tech line and the fellow says that the idle feed restrictors might have been drilled out and if thats not it then its something in the main body and it'll have to be sent back to be gone through again. Now without letting too much on I had already completely taken it apart and the idle feed restrictors were drilled and I had been in every hole and crevis like a one night stand and ironically I did find metal shavings in some of the main body passageways. Was that my problem ? I dont know but it wont help .The hole in the restrictor should be the same as a #62 drill bit so tomarrow I will see what the fuzz. Everyone always has great advice and being new to this as well as the boards is like being a kid in a candy store.
-homer-

[Pee Dub]

Guess I got lucky with the 2 Demons I purchased. Both ran perfect out of the box and were way more responsive than any Holley I have ever owned. I followed the BG recommendations as to matching the series of carb to the @.050 duration of the cams.

[cstraub]

beerjet,
RumRunner on OSO and Speedwake is Doug S, he is head of technical support for BG and a sharp guy. You may want to do a search on either board and PM or e-mail him. Super guy to work with.

[Kurtis500]

My 850 demon would never work properly, even with all the work and assistance I could muster up. I bought a new holley and all the problems went away with the new carb.

[xs ultra]

Bought a Demon 750 speed demon mechanical secoudary for my 454ci
Ran great out of the box. Idle great and top end is strong!

[Sleek-Jet]

Yeah, isn't the center screw an "additional air bypass" the demon version of drilling holes in throttle plates. this is used to increase air flow at idle so you can keep the transition circuit "out of play" at idle speeds?
Yep. And all new Demon mechanical secondary carbs come with them I believe??
My brother and I put a Mighty Demon on top of his built small block, and it took a little tuning to get it set up right. Once it was set though, it has been a fantastic carburator.
But back to the original question. You say it won't idle. Does that mean it stalls and dies at idle speed or does it race and not come back down to an idle???

[beerjet]

I can get it to fire up and then it'll come down and die.

[Moneypitt]

Now we're narrowing it down. Can you keep it rev'd up, and keep it running? How far open are the butterflys being held open by the idle "speed" adjustment? If the butterflys are open you are not using the idle circuit. Is the primary bowl full? Has the ignition timing been set, and where? Was the cam degreed in? And finally, are the valves too tight? Tight valves will cause very low vacuum, late ignition timing will cause low vacuum. All of this may be in left field if this motor ran decent with another carb, but it sounds like you have made several changes in the set up all at once. I will again mention the valve adjustment and how critical it is when you're dealing with solid rollers and performance grinds. If this history of the engine is way off, please give us some background as to when the problem started, was it running decent until the carb change? Any HUGE vacuum leaks? Does it start and die, start and die, or can you keep it running, rev'd up?...........MP

[beerjet]

I can keep it running at 3000 or so but below that if I get on the gas it will just quit. As far as the butterflies they are set according to the instructions and diagram in the booklet. Bowls are at the center or just below the center casting mark by the glass.I dont have the boat at my place cuz I live in an apt. so it stays at my folks place and this is where tuning and wrenchin becomes a battle for time between here and there. Worst case sinario I get a divorce and move back home right.

[Moneypitt]

OK, so it will stay running @3K or more. If you let it down does it die, or only if you try to rev from below 3K? Again, did it run right before the carb change? Or is this the initial fire up after several changes? Do the acc pumps work? I'm sorry to keep asking questions in answer to questions, but there is something missing here........MP
PS: Do you have the old carb? if so, put it on and see if the problem goes away........Also, was the cam degreed? by whom, same for the valves, who set them, and how were they set?.......

[beerjet]

The old carb (750 vac. sec.) ran fine with a dual plane but it really pushed hard around 3000 so I picked up a single tr a while back and thats why I bought the demon . the 750 did the same thing on the tr.

[SmokinLowriderSS]

Ditch the single carb tunnel ram. From what I have seen on here, nobody has had much good luck with them, nobody reccomends one. They are aparently prone to mixture distribution problems at the 4 corners, puddling under the carb, slow airflow velocity at any low rpm's (causing idle and fuel puddling troubles), all sorts of nightmares. IMO, unless you are running a seriously hot motor, stick to a good dual or single plane single carb manifold. I haven't got anything bad to say about my Performer RPM Air Gap. A friend of mine also had a lot of trouble from a high-rpm design Offenhauser single plane till he pulled it off & went Performer RPM, mostly idle quality troubles & having to pedal gently away from idle to avoid stalling. I don't think there's a thing wrong with your Demon, just your intake setup is disfunctional.

[Moneypitt]

Oh, now we're dealing with a tunnel ram? Do you have a vacuum gauge?.......Has this combination, cam, valve setting, ign timing ever worked right?......MP

[RumRunner]

The Tunnel ram, and a large carburetor may be contributing to your problem. There may not be enough air velocity going through the engine at low speed to break the fuel up properly. We may be able to get it better with some tuning, but without knowing more about the engine its going to be a stab in the dark.
Carburetors are extremely simple when you boil them down. If it is the right size, and type for the application (which may be in question here) and it is not running properly it either has too much fuel or not enough. The first thing we need to do is see what your engine wants. If you can keep her running at 3000 RPM do that. Now start to lower the RPM. As the engine wants to die press the accelerator pump arms without moving the throttle linkage. This will add fuel without changing the amount of air the engine is seeing. The engine will either die (too rich) or try to contiue to run (too lean). Then you can try to adjust the idle mixture screws accordingly to see about getting the engine to run properly.
If you're still having problems after that, contact our technical department directly at tech-sales@barrygrant.com

[PC Rat]

The single carb tunnel ram may not be the best performer, but it is doubtful that this is the sole reason that your engine will not run under 3000 rpm.
You did say that you changed to this manifold and tried both a 750 carb and the 850 demon, and this 750 carb worked on the first manifold.
So, since you changed intakes; I would go back and double check that you got your ignition timing back where it was before, and check the intake manifold for vacuum leaks.
Brian

[PC Rat]

I had a single carb tunnel ram on an old boat and seem to remember that it had a couple of large plugs in the plenum. Just want to make sure that you don't have some huge vacuum leak somewhere.
Brian