Too Big Carb
 

I currently have a 725 demon carb with vac secondaries on my 306. I have a Crane cam with .50 lift and a edelbrock rpm dual plane intake, mild head work on stock heads, borg warner super T-10 4-speed, tri-y headers with factory shelby racing glasspacks. It is running pretty strong right now. I just ran it at Fun Ford in Houston several weeks ago and did a 14.30 at 96mph. no posi , open 8 inch rear 3:50 gears with stock bfgoodrich comp ta's. I removed my 600 edelbrock and put the Demon carb on and it seems to run a bit stronger, But am I overcarbing it??
I know my next items are a 9 inch with 373 gears and a aluminum driveshaft.
Any reasonable place to get an aluminum driveshaft or 9 inch rear. or maybe a posi 8inch pumkin???
This is all in a 66 mustang coupe with a lot of fiberglass parts,,,hood , trunk, bumper, front valance.
davmust@hotmail.com

Depends on what is all done to the engine and what rpm you plan on shifting at.
But for how fast you are going now...I would say stay with a carb in the 600-650 range and just tune it. Dont just throw another carb on it.
FYI...Demon carbs are set different then Holleys and Edelbrock. Their Carb ratings are considered conservitive in conparison. It's not that they flow more air...its that BG has tailored a different fuel curve and flow design. Kinda of hard to explain but for a Demon Series carb, I would not go any larger then a 650 on a 306 shifting under 7,000 rpms.
My .02's

I'm shifting around 5,800 -6,000. Some friend of mine said if i get some aftermarket heads like Trick Flow or ARE the demon might perform better. I've only had the carb for 2 months and i'm tempted to sell it so i can get a aluminum driveshaft. I paid 325 for it. anyone out there give me 295????
I tend to agree with you now about the 600-650 range on a small block.
Summit wouldn't take the carb back once i put gas in it.
Or does anyone have anything decent to trade?

625
 

I use the Road Demon 625 on my 306 and it seems about right with my pocket ported TFTW heads and other mods. The 750 probably does OK due to the vacuum secondaries, but the 625 is more the right size for your application, And I agree tht you should shift 5500-6000 with the way you have it set up.

Rev

I agree i'll have to get a smaller carb 650 probably. I would return this demon to summit but once there is gas in it they won't take them back. so i guess i'll try and sell it.
I have a high speed vibration that goes away when i put in the clutch, starts around 55mph then gets worse the faster i go, low speed isn'y bad at all, i have all new mounts. new harmonic balancer and supposedly a balanced driveshaft, new u-joints. I'm thinking maybe an aluminum driveshaft would help. any ideas.

u-joint reposition
 

If the u-joints are new and the drive shaft is balanced, you could reposition the rear u-joint 180 dgrees and see if it helps. No new parts needed and only 15 minutes of your time. It's only a suggestion though. Sounds like u-joints to me.

Rev

Thanks Rev
Yeah I forgot to say I already tried it and it didn't seem to change the vibration. it gets worse the faster i go.
I also had the tires balanced several weeks ago and they said my left front wheel is bent and that i would get a vibration from that, but I don't see how it would give me a driveline vibration at high speed. Thanks for the tip anyway!
Were you able to make it to Fun Ford in Baytown?