i know it's a tired subject.. carb choices?

[83GT289]

So i rebuilt my holley 600cfm and i'm still not very happy with it. It just doesn't feel very smooth through the rpm range and especially lacking at higher rpm. Idle isn't worth a dam and the exhaust reeks of gas. According to this article http://www.bob2000.com/carb.htm the carb was designed for applications using smog (the idle mixture screws work backwards). He states these carbs do not perform very well on modified engines. Obviously there no smog or anything on my rig.

Using 5sp with 3:73's, 30 over on my 289 block, pretty big cam (not sure of the specs, trying to find out). I have decided when i can afford it i'm going to get a speed demon. I'm trying to decide between the 750 vacuum sec, and the 650 mechanical sec. What does anyone think? Too much carb?

J.

 

[85_SS_302_Coupe]

I'd think that a 650 is all you're ever going to need on a 289, especially through a Demon 650....those carbs are badass.

 

[408stroker5.0]

Don't even consider a 750, it would be way overkill and would cause your engine to run even more poorly. If you have a pretty decent cam, compression, intake, ported heads etc. I would go with the 650 Mechanical or Vacuum Secondary Speed Demon. You would get a bit more power and response out of the mechanical secondary, while the Vacuum Sec. will be a bit more economical. keep in mind Depending on what's actually done to your engine a 289 would be pretty happy with a 600cfm carb as well....bigger isn't always better.

Shon

 

[83GT289]

yes, i know bigger isn't allways better. If the engine can take a 650 with mech secondaries it should be able to handle the 750 with vacuum secondaries. while i know it won't really use the 750 it gives me room for improvement engine wise without upgrading the carb again. If i'm using vacuum secondaries it shouldn't bog down much if any at low rpm's because the secondaries won't necessarly open just because your at WOT right? I suppose if it was too rich at lower rpm's i could just jet down the mains a little. If it's a really big mistake then i'll stick with the 650 mech secondary. I just figured it wouldn't hurt to much and like i said, leave me that extra little buffer. For me it's a pricey component and i'd like to be happy with it for a long time.

J.

 

[408stroker5.0]

NO..... like I said don't even consider a 750 for a 289. A 650 Mecanical or Vacuum Secondary speed demon will keep you and your engine happy through many more engine modifications. Keep in mind also Demon carb's are underrated in term's of flow...Hell a 650 will be plentiful for your 289....Trust Me. Go with a 650 or smaller...period.

Shon

 

[83GT289]

allrighty. I think i'll be looking at the 650 VE. It's vacuum sec with electric choke. They also have a 625 i've been looking at and it's about $40 cheaper. One of the two it is. Thanks! I was also thinking a 600-650 would have been plenty, but i read some article's last night about using bigger carbs with vac secondaries. I'm also trying to consider the wieght of my car and that it has a pretty big cam, enough to give it a bit of a rough idle. Thanks again! I can grab that 625 VE for about 339, excellent deal i think!

J.

 

[Dark Knight]

the vac secondary carb opeing only when the engine needs it isnt really right... it opens when the vacum drops.. with a bigger carb, the vacum drop will happen sooner, and the secondaries will open.. you can tweak the spring to open them at a certain point by putting a stiffer spring in.. but if it's to much, they wont open all the way. A smaller carb will be more efficient..

BTW, there are places like The Carb Shop that will tweak the factory holley to work alot better

http://www.customcarbs.com/

 

[83GT289]

BTW, there are places like The Carb Shop that will tweak the factory holley to work alot better

Yeah i was looking into Bigs. $289 to do my carb??? hell i can get the demon for $339. The carb i have i'm pretty sure is the factory carb of some ancient machine. I'm guessing it's late 70's ? Really not sure. I am pretty sure the guy i got the car from switched carbs on me before he sold it. I know the car used to run a hell of a lot stronger. The rebuild made a good bit of difference. The floats were adjusted so far down on the secondaries you couldn't even adjust them up again because the threads had lost contact. When i popped the bowl of i noted the float was jammed all the way down against the bottom of the bowl and the needle valve stuck shut. Looked like the seconary metering block hadn't seen gas in years. On top of that it was assembled without the secondary metering block plate in place, and the cork gasket had expanded to fill all the passages. very nice. I will look into the carb shop and see what kind of price i'm looking at there. If i could find someone who really knew these carbs and could get this one working very well perhaps i wouldn't drop on a new one. But if i'm talking about $200+ to get it done i'd just as soon buy the demon.

J.