Street demon 625 for 383

[mpro69rr]

69 RR 383 with a nice CAM, Edlebrock RPM heads, .40 over. I have a street demon 750 and its too rich, would a 625 be to small? My 750 is pretty beat up by backfiring through the carb and something may be wrong with it. I think I fixed the problem with backfiring with correcting the timing.



Thanks!
Mpro69rr

 

[moparedtn]

Sounds like the power valve in the 750 is blown out; might want to check that.
The 750 ought to be fine for a warm 383 if jetted properly and that's the thing - that 750 isn't "too rich",
it's probably just in need of a proper power valve and some jetting work.
The 625 would work also - up to a point, anyways.
It would wind up with the same jetting as the 750 to get things optimized - but it's theoretically possible
for that 625 to "run out of breath" at the top end.
You'll know if it does - the car will literally stop accelerating further and stay at the max RPM/CFM the carb can flow.

 

[Rusty knuckles]

I wouldn't run the 625. It's too small. Fix the 750. The popping through the carb is a lean condition and as moparedtn said, change the powervalve.

 

[beanhead]

Agree with above. Too small for your build....will "it work?" Yes but it will be a big chokepoint. Fix what you have to it's potential first, your drive will be much more fun!

 

[Garceau]

If I remember right the demons flow quite a bit more than advertised. With that said for a practical application and the most top end HP the 750 most likely will make more. But that smaller carb may be a little cleaner running carb around on the street.

But the 750 shouldnt be overly rich if set up right..... as other mentioned, take it a part look it over good. Make sure PV isnt blow out and dumping fuel past it, make sure all air bleeds etc arent clogged putting the idle circuit in to rich mode.....

 

[66chargerpat]

Do street demons have powervalves or metering rods.

 

[4speed68rt]

Do street demons have powervalves or metering rods.

It seems like I remember the older Demons that look like a Holley are called Street Demons, but also the newer square looking ones are also called Street Demons. I have one of the older, Holley type ones that is rated at 650 cfm, and it makes the 440 run clean and smooth. More power than I could ever use on the street. I think they flow more than they are rated. Probably up around 700-725 cfm. Just my opinion.

 

[WileERobby]

If you were stock, the 625 would be fine. But with cam & heads, I'd do the 750 properly.

 

[mpro69rr]

Do street demons have powervalves or metering rods.

Good question, I think they have metering rods.

 

[Geoff 2]

Yes, they do not have the dreaded PV. I have installed two of them, a 625 & a 750. 625 was ok, 750 on a 400 Pont engine had a flat spot that was never cured. Owner rang Holley, & when the Holley clerk ran out of 'fixes' on his computer screen, my friend gave up; re-fitted his trusty QJ.
These SD carbs are cheaply made & designed. If you want a good carb, get a TQ.
If the intake is a dual plane, the 750 is ok. If single plane, 625 would be better.

 

[joe smith]

What do you do with the car? Race it or is it your street driver?? I would put on the 625 and see how it ran... Might surprise you.... Some people still think " bigger " is better with a carburetor... Read some articles about.....

 

[mpro69rr]

What do you do with the car? Race it or is it your street driver?? I would put on the 625 and see how it ran... Might surprise you.... Some people still think " bigger " is better with a carburetor... Read some articles about.....

Its just a street driver, I won't be racing it. I ordered it before I posted this thread, I will try it and if I don't like it I'll send it back.

 

[mpro69rr]

I got the street demon 625 cfm on Sat. and installed it on Sunday, and WOW what a difference! There was definitely something wrong with the 750 cfm. This thing hauls *** now! It idles at a nice 850 rpm and two presses of the gas pedal and it starts, doesn't smell rich. My 750 would take forever to start, backfired and wasn't as fast. I know I will lose some power at high RPM but I'm never going to race it. If I got another 750 I'm sure it would have been the same and ran really good like this one, I just had a bad one. I could have tried to fix it but I'm just not good at doing carbs, tried before. I think I will give this one a little more time but most likely will keep it. Anyone want to but my 1 1/2 year old street demon 750 for cheap, it probably can be fixed. Thanks for all the advice guys, I really appreciate it!

 

[Splangj]

My 750 speed demon ran great on my 383 Mildish cam. I did make sure that I had the right power valve for my idle vacuum. When I installed it on my 496, it backfired And generally ran like S**t. I put an 850 mighty demon annular on that engine and never looked back! Demon tells you what camshaft duration their specific models work with, seems accurate to me. With only a power valve to change for part throttle enrichment, you MUST have a properly tuned/calibrated carb for your engine. With a Carter type, you can make them work on most any engine with all the rods, springs and jets.