Rejetting a carb

I'm looking at going from a single carb to a triple carb set up but I want to keep the same CFM rating. I know the total CFMs I'm supposed to have and how much I should have after I rejet but I can't seem to find anywhere where I can get new or different jets.

Any help?

 

What kind of carbs are they?

 

You can't change cfm by rejetting a carb.

And to make things even more complicated, I believe that the cfm ratings of carbs are set at different vacuum ratings for 4 bbls and 2 bbls. So a 500 cfm 4 bbl doesn't flow the same amount of air as a 500 cfm 2 bbl.
Larry T

Don't know if this'll help or not.
http://www.candsspecialties.com/ratings.html

 

they're single barrel rochesters

 

Why would you want triple carbs to have the same CFM as a single?

Multi-carbs are all about additional air flow.

Simple answer is, get two more carbs like you're running, block off the end carbs.

Be sure you put a couple of blocked off fuel lines on the outer carbs otherwise your fake setup will be easily spotted.

 

I want the engine to run more even. The problem with the single carb being in the middle and the intake not being directly on top of the engine like on a v8 is the middle two cylinders run rich and the outer four run lean. With a V8 the intake sits on the engine and is heater properly so the fuel vapourises more redily. With an inline six the intake is heated less as it's beside the block

just wanted to see if there was a way

 

Like they said, a re-jet doesn't change the CFM rating, only the air/ fuel ratio. Also as said a 2bbl is rated at 1.5" of vaccumn and a 4bbl at 3" of vac - however a CFM rating is a CFM rating. The issue is if you want to compare a 2bbl CFM rating to a 4bbl CFM rating on an apples-to-apples basis. Divide your 2bbl CFM by 1.41 to get a "4 bbl equivelent". That's why at first glance running six Holley 94's looks like overkill, but when you do the math and get it into "4bbl speak" it's not overkill at all.

 

Actually I just posted that backwards: 2bbl rated at 3" and 4bbl at 1.5".

 

How about adding manifold heat? the original intake and exhaust manifolds on a straight 6 have a heat exchanger between them...there's a heat riser valve that forces hot exhaust gas around the bottom of the intake manifold when the engine is cold, then closes as the engine heats up. Many hot rod manifolds don't have this feature, resulting in lousy fuel vaporisation when the weather is cold. What manifolds are you running?

 

3 posts and you still haven't given much info about what engine you're working on. What is it?
Larry T

 

it's a chevy 235 with an old offenhauser three carb

 

If it were me, I'd probably make sure the carbs were all set up the same and put them on with stock jetting (if the throttle bore isn't much bigger than 1 1/2"). The only place I see that you might run into problems would be too much accelerator pump shot. You might have to find a way to lean them out a little or your gas milage might suffer some.
If you're trying to equalize the mixture to all cylinders, you're not going to be able to run progressive linkage. I haven't tried it on 3 1 bbls, but I've run straight linkage and progressive linkage at different times and couldn't tell a lot of difference in performance and milage.
.00002,
Larry T

 

I'd approach this about so, to keep things simple and not have to solve too many problems at once:
Get engine running as well as you can on the stock carb\
Remove it, put on an identical carb, tune that in.
Put both on the end positions of your triple, block off center position with a plate, use straight linkage...most of your immediate tuning will simply be getting idle speed down and even...get idle set with the linkage loose, tighten it on when both carbs have been turned down evenly and are idling happily
fabricate a hookup to warm manifold...insulate or restrict this rig a bit, since aluminum manifold needs less heat than iron

 

Thanks very much everyone. This is exactly wht I needed to get me in the right direction