what's up with the demon?

[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