Well after spending $300 bucks on a dyno tuning my speed demon 650ve and another $100 just to get it to idle correctly, thats after having my mechanic clean all the "debris" out of it and and replacing the needle valve, I got my a$$ handed to me racing a dodge avenger because when I hit the throttle at 30mph it just stumbled and missfired. This thing takes 15 minutes to warm up then stumbles everytime I hit the gas off idle or driving. As long as I drive it like a grandma it works fine. It's for sale if anyone wants it, I'm going to donate the money to Barry Grants Quality Assurance program, evidentally they need it. I think I'll go with a street avenger.

 

Sounds like the usual Demon: metal shavings, insanely rich idle circuit, lean out on the top end, and a pink accelerator pump cam. Yuck! Don't make another mistake though, stay away from the Street Avenger. Get a DP if you have a stick or an auto with a properly matched torque converter. If not just get a regular 750vs or a QJet. My

 

Metal shavings, Check
rich idle, no problems here
lean on top end, couldn't tell ya
pink pump cam, same comes with Holley if I remember right
Street Avenger, JUNK stay away from it, never was able to get mine to run right.

Get a good old double pumper and learn how to tune it yourself. Don't fork out any more money to a mechanic. If things get really out of wack, just put it back to stock specs and start over again.

I've had good luck with my old Speed Demon 750 and my current Mighty Demon 750. Both are older models, current one being a used one off of ebay. The Mighty Demon 850 I bought brand new was filled with metal shavings and I had to clean out the passages because of metal flashing. Too much carb for my engine so it's sitting on a shelf waiting for a BBC.

 

I have a speed demon 850 on my chevelle, and it is the best running carb I have ever used. I did find some metal in the bowls before bolting it on, but that is not unusual for demon, holley or edelbrock. I bought a brand new edelbrock manifold a few years back, that, had I not ran compressed air through all the ports before bolting it on, I would have ran a huge spiral chunk of metal through my 468. It is very typical of any aftermarket products to have to clean them first. For what it's worth, Holley is having a rather large problem right now with the zinc coating they put inside their carbs peeling off and plugging everything up, apparently whoever they outsourced it to got it wrong. So it not just a demon problem.

Also if you were putting a demon 650 on the engine in your sig, it is too big. Demons flow considerably more than holleys do. A holley 950HP actually flows 850. A Demon 850 actually flows 850. A 358 with a tiny 262 cam in it needs no more than a 600 on it. Bigger is not better when it comes to carbs. A smaller carb will have better throttle response than a larger carb.

Take some time to actually learn how to tune a carb. It takes a while, but their are some wonderful tuning aids out there, you just have to find them. If you can't tune a demon on your own, you certainly can't tune a holley on your own either. You need to learn the proper procedures and order in which do it. If you are going to run this engine for most of the year, as climate changes, you are going to have to tune a little bit every now and then, it's worth learning how.

Personally, instead of donating the money to Barry Grants quality assurance program, I would invest the money into learning how to tune a car, as the problems you are having are simple tuning issues.

Have you checked the idle mixture? How about making sure your engine matches the factory installed power valve? Did you check to make sure the accelerator pump is set up correctly?, any adjustments you make requires readjusting the accelerator pump lever. How about the correct squirters, and or pump cam? Are both of those correct? Is your timing dead on as well as the timing curve set up correctly, if not their is no sense in trying to tune a carb with the wrong timing curve.

Jeff