carb sizing help
Race Director
Joined: Oct 2000
Posts: 14,997
Likes: 79
From: central FL, near the beach.
1st, as this is a BUDGET build, i'd give the guy $20 for the "free" carb so he never gets it back.
i'd rebuild it and remove the choke.
then if by some miracle the combo pulls 2 or 3" at ~6200
i'd port the secondary out to flow whatever it took to get the vacuum down to 1-2, drag testing it. If it kept going faster & faster
but still had too much vacuum and stopped responding to the porting, then i'd buy a BASEPLATE from a larger carb, maybe the 700 or 750(depending on how good the crack u r smoking is, for this fantasy miracle). They cost ~$100. This would give a very crisp throttle with the tiny primary venturi, like my spreadbore 650, but flow as much as it needs at WOT. this gives it a sleeper 600 look and saves $$. 
i'd rebuild it and remove the choke.
then if by some miracle the combo pulls 2 or 3" at ~6200
i'd port the secondary out to flow whatever it took to get the vacuum down to 1-2, drag testing it. If it kept going faster & faster
but still had too much vacuum and stopped responding to the porting, then i'd buy a BASEPLATE from a larger carb, maybe the 700 or 750(depending on how good the crack u r smoking is, for this fantasy miracle). They cost ~$100. This would give a very crisp throttle with the tiny primary venturi, like my spreadbore 650, but flow as much as it needs at WOT. this gives it a sleeper 600 look and saves $$.
Last edited by Matt Gruber; Sep 29, 2009 at 10:30 AM.
Pro
Joined: Feb 2002
Posts: 709
Likes: 10
From: Carlsbad CA
Hoo boy.
If you want to use the carb for break-in, the 600 isn't going to make your engine explode
The rule of thumb - it ain't a "formula" as it's not always correct - is CFM = ((CI x RPM)/3456) x VE, with VE at 100% around 1.4 HP/CID. Generally a smaller carb will give better off-idle and low-speed throttle response at the expense of a couple % HP up top for a street engine because of the venturi size impact as noted above. If you're one of the "racing engine on the street" guys it's a different deal completely.
Without more specs it's hard to say, but I think you'd find a combination of a good dual-plane with a 700-750 vacuum-secondary carb to be a better overall combo unless you're going drag racing.
Otherwise, you can't really over-carb with a vacuum-secondary carb, and given the choice of a single-plane manifold off-idle and low-speed driveability probably is already out the window to some extent.
So, if you're buying a carb a 750 CFM vacuum-secondary would be my suggestion. But you can make the 600DP work with what you've got. Invest in a little dyno time and let us know if it's too small
If you want to use the carb for break-in, the 600 isn't going to make your engine explode
The rule of thumb - it ain't a "formula" as it's not always correct - is CFM = ((CI x RPM)/3456) x VE, with VE at 100% around 1.4 HP/CID. Generally a smaller carb will give better off-idle and low-speed throttle response at the expense of a couple % HP up top for a street engine because of the venturi size impact as noted above. If you're one of the "racing engine on the street" guys it's a different deal completely.
Without more specs it's hard to say, but I think you'd find a combination of a good dual-plane with a 700-750 vacuum-secondary carb to be a better overall combo unless you're going drag racing.
Otherwise, you can't really over-carb with a vacuum-secondary carb, and given the choice of a single-plane manifold off-idle and low-speed driveability probably is already out the window to some extent.
So, if you're buying a carb a 750 CFM vacuum-secondary would be my suggestion. But you can make the 600DP work with what you've got. Invest in a little dyno time and let us know if it's too small
This what I said many posts ago. It really isn't that difficult.
