baxsom

my 68 with a bone stock but freshly rebuilt 289 with an edelbrock performer intake and a holley 600 on it and i think it is too much carb
i think this is because it is bogging in the low rpms (and i have been reading here that 600cfm is too much for that motor anyway)

holley's calculator suggests either a 390cfm or the 570cfm
thats a major difference in size. all the calculators i can find for carb size say in the neighborhood of 500cfm is ideal. i guess i could get the edelbrock 500 but all the hoses, wires, and lines are already set up to fit a holley.

should i go slightly larger than recommended and get the 570 or go under and get the 390

thanks

fast66

To be honest, if you already have the 600, dont go spending cash on another carby. I agree the 600 is somewhat big, but can be made to work quite well with some routine adjustments.
I usually recommend a dyno tune to set up a carb the first time, but there are some carby specialists around who can set them up very well by road testing.
Hopefully 67mustang302 chimes in as he has good knowledge of carby tuning. I prefer to let the experts play with mine.
BTW, I have the 570 street avenger on my 289, and after it was properly tuned, I couldn't be happier with it.

Starfury

You'd be better off with a 500, but since you already have the 600, do what you can to try to tune the bog out before forking out more money.

baxsom

its an old carb to start with and leaks like a sieve from the acellerator pump so either way it needs to come off and either be rebuilt or replaced. id rather go smaller than try to jet and rejet until i get it right
carbs are cheap its the hassle of trial and error i want to avoid

Hawkins 812

Can you fill me in on where you get your carbs then? I had to save up for mine .

fast66

Ok, Sound like yours needs work anyway. You could put a full kit through it, and then have it tuned, or replace it.
If you decide to replace it and want a carb that will run ok straight from the box, Im led to belive the Edelbrock ones are the way to go. If you want a carb that will run better than the edelbrock after some initial tuning, go for the Holley Street Avenger 570, and it will also support further engine mods down the track.

67mustang302

On a mostly stock 289/302 you'd be better off with a smaller carb, but it's not enough to warrant getting another carb just to have something smaller(600cfm was stock on the last of the carb'd 5.0's, hardly a "big" engine in stock form). If you have stock heads and a stock/mild cam then a smaller carb would help the problem, but it's mainly a tuning issue. Most bogging issues with carbs are related to accelerator pump and secondary tuning, when you open the throttle you still have to get the right amount of air/fuel in until the primary system comes online. If your carb is leaking etc, you could go the cheaper route of just rebuilding and then tuning it, or if you have the money you can get a newer carb, Holley 570 Street Avenger, or you could go with a Demon, they have a few Demon carbs that are in the 525-575cfm range, but they tend to flow more in actuality than they're rated(the cfm rating is going to depend heavily on the level of depression in air pressure that the carb is rated it, lower depression during testing = lower cfm ratings, higher depression = higher cfm rating on the exact same carb). If your's is old and crappy, and you have the money, then it wouldn't hurt to upgrade to a more modern carburetor.

Either weay it will need to be tuned though, most carbs will run well, even under full power, out of the box if you pick the right carb, but 9 times out of 10 the throttle response will be off somewhere. What kind of bog are you having at low rpm? When you only open part throttle, when you snap it open to WOT? Does it just fall flat then go, or does it spit and sputter, or just not have any power for a few seconds then start running fine? One thing to consider too is if the pump diaphragm is leaking, then the power valve may be leaking as well which could account for any drivability problems you have. Look on the bright side, replacing a carb is cheaper than replacing fuel injection compenents.

As far as cheap carbs, Summit and Jegs are pretty good

my77stang

+1, hell i have one ready to go for mine

67mustang302

I got mine back from Holley a couple weeks ago, and decided to do some spacer tuning....that carb with a 1" open spacer is making a crapload of power, and still has great drivability, part throttle power, and the throttle response is great(just have 1 VERY slight little hesitation when I crack the throttle open at higher rpm when I'm holding in gear). It's a good carb for the price

baxsom

i have the 1"spacer on it as well. as far as what its doing. if you ease the pedal down no problem but if you snap it to WOT, the rpms just stop for a second then sputter sputter roar

i was told by my mechanic guy who played with the timing trying to get it out that it is definelty needs a smaller carb. right now the timing is at 8 degrees at idle with the vacuum unhooked.