Doug (Login 6T9Stang) Member from IP address 22.214.171.124
I should say right off the bat I don't have much experience tuning carbs. But I like to learn & can be pretty methodical when I have to.
My main question - does anybody have a link or know off the top of their head what parts will exchange between Holley & Demon carbs?
Basically I'm wondering if I'm going to be able to find all the parts I need to tune the Demon (Speed Demon 750 with vac. secondaries) correctly since Barry Grant is now defunct. I'd hate to spend $$$ and then have to turn around & buy a Holley anyway (although I have been eyeballing the Ultra Double Pumpers).
My other concern is whether the 750 Speed Demon is just too big for a 357" motor. It idles rich & occasionally will diesel a little at shut-down when it's hot. I didn't realize until after I bought it that BG rates their carbs differently because of the wet/dry flow differences. The only adjustments I've attempted so far are the 4-corner idle screws because we barely got the car on the road before time to put it up for the winter, plus I knew I'd be swapping the Weiand X-celerator out for an Edelbrock Performer RPM this spring.
In case it helps:
Aussie 2V heads
Comp 282S Magnum cam
I dont know about what will interchange I only run Holleys, I think the 750 Demon carb should be fine, the cam is making the idle rich, you will just have to get used to it, and a high idle will make it run on after you shut off the engine, do you run premium gas? If you dont try it, it will help.
Holley & BG are rated at different specs, a BG 750 flows a lot more than a Holley 750 at the same depression, Mighty Demon 750 is said to flow 952 cfm
you may have a hard time getting it tuned, if at all?
as you're saying idle will be rich and mid-full power will be lean due to poor signal. you'll have to lean the idle circuit at the idle restrictions and richen the jets until it AFR's right and even then you'll still have transition problems
pretty sure a good old 650 double pumper will rock
there'll be phantoms, there'll be fires on the road... and the white man dancing
Dan - I only run 93 octane in this one. Idles nice @ 800 rpm after it's warm, and will even go as low as 600. Just smells of unburned fuel & might make your eyes sting a little if you're in the garage & there's no breeze :o
>There is very little that interchanges. Time to shop for a new carb IMHO, or start buying up old Speed Demons so you have a parts pile to draw from::
I think you'll find a fair bit is interchangable, with atleast aftermarket parts if not Holley. With the 750 Demon i have, the only original parts are body and baseplate, rest is bolted on stuff when some fiddling was done to it, boosters everything changed. This is my 750 Demon:
I've done quite a bit of reading here on the forum over the last few days, searching through a lot of the 'carb recommendation' threads, and my head hurts!
I've seen some people say Clevelands like more carb than your typical small block, and others say most people over-carb. The 'carb chooser' on the Holley site picked me out a 650 VS, yet I've seen it posted by a respected contributor here that a 700 DP was too small for an engine very similar to mine (same displacement, cam, & aussie heads).
I think the bottom line is I'm asking a question that has no clear answer. Since I don't own a speed shop & a dyno, it looks like I'm just going to have to wait until I get the intake & dizzy swapped, and then let the engine tell me what it wants. Which means I'll probably have a couple of hundred invested in an Idle-Eze baseplate, power valves, and jets... and still end up needing a different carb. Or just spend the $$$ on a new carb & wonder if I really needed it :P
The deal on carbs can be simplified somewhat - LOL.
As long as the carb will meter fuel correctly for the engine, it is not "too big".
For a street or street/strip car, err on the small side. A 650 will easy support over 500 HP. People like to put big carbs on the motor, yet the guys that build alcohol carbs for the huge HP engines around here like to use 700DP Holleys as the base carb.
A VS carb can be larger than a recommended DP because the secondaries only open when there is enough air flow. You can put a 750VS on a 260, it just may not open the back barrels until 9000 RPM. I have personally tested a tuned 750VS against a tuned 650DP on a specific engine combo and found not difference in ET or MPH at the track. And I have found a difference of .02 and 2 MPH between a 600VS and a 600DP on another engine combo. It's all dependent on the build, but in general the VS carbs are more forgiving.
DPs come in 50 CFM sizes because a DP is more sensitive to the combination than a VS carb.
"carb response" is affected by many variables. Smaller primary throttle bores and better boosters drive on the street better. An 850 DP on a Torker will be lazy on a street car where a 600DP may well be rather snappy. A larger cam and still trying to run an advance curve can make a carb harder to tune. It's a lot easier to tune a 4 corner carb on a motor with a decent overlap solid lift cam when the timing is locked at 36.
Idle rules are different for a 4 corner idle carb - start at 3/4 turn out instead of the old 1 1/2 turns out for a two corner.
My personal likes:
Street 302~351 - Edelbrock carb 600
Street/Strip 302 - 600DP Holley
Street/Strip 351 - 650DP Holley
Strip 351~393 - 750DP 4 corner idle custom build, 750HP Holley or 950HP Holley
>The only adjustments I've attempted so far are the 4-corner idle screws because we barely got the >car on the road before time to put it up for the winter
It takes a while to get the carb set up on a cleveland. There are a bunch of things that could be giving you the rich idle. Not enough initial advance, power valve opening at idle, leaking accelerator pump discharge nozzles, etc.
I don't think the carb is too big, it just needs tuning. That's the fun part
I was at the speed shop just last week and one of the guys was freshing up a 750 Demon He said there are ports in the base plate that don't match holley gaskets he opened the hole in the base gasket to line up! If these are not lined up the carb won't work right. I can't tell you were to look because this is second hand info but it is some thing you will have to look at!