Well I have had a Speed Demon on my 67 for a little over a year now and I have to say it out performs my old Holley hands down. There is something that bugs me about it though, It has never had a smooth idle. The vacume moves up and down from 15 inches to 16 inches and the engine idles up and down about 50-100 rpm. Its a fine point but I wonder if I could get it to idle a little better. The Holley idled very smooth, but didn't run as good as the Demon. Any suggestions? Shes a 396 with a Crane .515 / .515 cam. I like the curb appeal of a slightly rough idle, just wish she would stay steady.

Oldskool67

OK, well since no one had a reply I figure no one had an answer to the problem. If anyone is interested in knowing how to fix the problem, I did figure it out. Just ask and I'll share it with you.

I own a 850 speed Demon and kind of have the same problem. Mine is a little different though. When I set the idle and mixture screws at temp. It would be steady but after driving it a while it will climb on rpm by about 200 rpm and stay there unless I tap it gently on the gas pedal.
It will go down back to the 800 rpm. I tried the ezy idle screw but that does not do much. I'm running a 496 .
I bought a new vaccum gauge so I will set the idle mixture screws again. Anyway let me know what you came up with.
I'm running about a 246 duration.

Share The Info....

67SS138: Sounds like either the throttle plates are binding and it doesn't return to idle all the way or the advance weights in the distributor aren't returning all the way.

I have a holley but my cars idles at 950-1000 and vac is 10 lbs. I tried hooking up the vac advance to the full vac tube at the base of the carb and it would not come down to a normal idle till I put it back to ported vac. my vac advance can comes in at 13 lbs. and when I backed off the gas the high vac kept vac advance on till I churped the throttle then it would come to 1000 rpm idle. cam is 247/251 690 lift. I need a vac can that comes in at about 9lbs.

I have a holley but my cars idles at 950-1000 and vac is 10 lbs. I tried hooking up the vac advance to the full vac tube at the base of the carb and it would not come down to a normal idle till I put it back to ported vac. my vac advance can comes in at 13 lbs. and when I backed off the gas the high vac kept vac advance on till I churped the throttle then it would come to 1000 rpm idle. cam is 247/251 690 lift. I need a vac can that comes in at about 9lbs.
You will need a Crane Adjustable Vacuum Can to get that initial where you want at 9lbs.

I need a vac can that comes in at about 9lbs.

I think you mean 9 inches. The B28 can (NAPA VC1810) is full in at 8" and should solve your problem. This is the can used on factory Corvette 327's with the 30/30 cam. Another possible cause could be too light advance springs. Typically, the lightest springs allow for centrifugal advance to start below about 800 rpm. The increased advance raises rpm which in turn increases advance even further (and the rpm goes even higher).

Well what I wound up doing was moving the vacuum line to the manifold, adjusting my advance with the vacuum hooked up to the distibutor until I achevied the highest reading I could get. I pulled the Vacuum back off and check the timing with the timing light and it was 4 degree's more advance than I had it to start with. The car idles better, is quicker, and starts every bit as good as it did. I have a buddy that worked for Roush and he was the one that suggested adjusting the timing using the vacuum gage. He said most performance motors will benifit by using this method, but you have to make sure you don't go too far and get into detonation, thus confirm with the timing light. He also said I should break torque the car and listen before driving it to make sure it was not too far advanced. Boy you learn something everyday. I now have steady idle, steady vacuum, and better performance!

Thanks for the info DEEBOO and Jerry70. I'll pick a vac can up and give it a try.