demon 850 idle

[bromurph66]

hi i recently got a 850 speed demon carb from a friend and put it on my 351w engine. but i can barely get it to idle. and if i do get it to idle the acceleration is horrible and usually dies. i have the butterflies set to spec. (looks like a square). idle mixture screws backed out 1 1/2 turns. fuel pressure set at 6... 85/90 jets.6.5 power valve.timing set at 20 total 36. just to get it to idle i have to open up the butterflies and back the idle mixture screws 4 1/2 turns out. it idles at 1000 rpms with 10" vacuum and 7ish in gear. im running my demon 650 vac sec.at the moment and runs perfect. the 850 is probably way to big for my setup. just wanted to see i could feel a difference in performance



my setup
351w bored .030
victor jr intake
procomp 190- 2.02/1.6
comp cam 284/296 .541/.544
10:1 compression
msd ignition
2600 stall converter
3.73 rear gears

[kryptik]

Way too much for that engine...the most I'd go is 750cfm. In fact you are hurting the performance of your engine throughout most of it's RPM range with that oversized carb. And if you are on the street....get rid of that Victor Jr for a Performer RPM/RPM Air-Gap.

[jprewitt]

I second that. Way too big.

[critter]

Third. I had an 850 on my Pontiac 462ci and it was happy there. Idled like a champ. Too much for your smaller cubes and that vacuum level. Wish I'd never sold that Demon...

[bromurph66]

thanks guys. ill probably move to a 750 later on. and it does see more strip than street thats why i got the victor jr. thanks again

[Shaker455]

hi i recently got a 850 speed demon carb from a friend and put it on my 351w engine. but i can barely get it to idle. and if i do get it to idle the acceleration is horrible and usually dies. i have the butterflies set to spec. (looks like a square). idle mixture screws backed out 1 1/2 turns. fuel pressure set at 6... 85/90 jets.6.5 power valve.timing set at 20 total 36. just to get it to idle i have to open up the butterflies and back the idle mixture screws 4 1/2 turns out. it idles at 1000 rpms with 10" vacuum and 7ish in gear. im running my demon 650 vac sec.at the moment and runs perfect. the 850 is probably way to big for my setup. just wanted to see i could feel a difference in performance

my setup
351w bored .030
victor jr intake
procomp 190- 2.02/1.6
comp cam 284/296 .541/.544
10:1 compression
msd ignition
2600 stall converter
3.73 rear gears

Simply put the carb is not properly set up and tuned to your combo...not that it's too big.

That's just and easy reply you can expect to see on the web.

Got to get into tuning and the 850 does require the most attention.

[opnwide]

I'm having problem getting a Demon blow thru 750 to idle on my 462 pontiac. I've got the idle eze cranked all the way down and sealed with teflon tape. (I've poured gas into the area with the carb off the car and it does not leak.) With the transfer slot on prim set to square, and rear throttle closed all the way, my transam idles at 1000rpm warm, sometimes a little higher. It has a .600 solid roller cam, but its only 224 in/234ex at .050 and pulls 11-13 in vac at that 1000rpm. I've checked everything, and it behave like a vac leak, but there is none. I've read somewhere that Demon sets the throttle blades purposefully to not seal and that you shouldn't loosen them up and try to reset them. I'm about to do that, though. My engine really should idle at 800rpm.

I've been able to get a better idle from a 850 holley DP on a 355 chevy, so I don't think its the carb size. I think its the way the carb is made.

If your problem involves idle quality and not idle speed, then I'd take that demon apart and clean it out again. The four corner idle circuit is touchy.

[Shaker455]

Greg,
First test the shafts for vacuum leaks as you may find your problem there.

Carbs with problems I recieve including a recent new Demon 750 get all torn down and cloesly inspected.

In the case of the Demon I worked it had one bent throttleblade exposing too much and resulting in an erratic idle.

[csx4766]

I had a Demon 850 on a motor and it ws a complete mess. Nothing inside was tight. One of the jets was sitting in the bowl. Dumped it for a Holley HP. With that said the carb size and jet size is completely wrong for that motor.

[Scarelane]

Must agree. Seems too big. But i have many friends who have had issues with Demon carbs. Once they get the kinks worked out, they kick ass. They seem to have bad quality control?? I went simple and used a Holley 650DP for my 347 Stroker. Not fancy, but it works well.

Good luck man!!