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I have a 600 cfm 4 barrel carb on my 289. Obviously the carb isnt tuned properly, even causes the car to die at times. I have no idea how to tune it, and neither does my brother. The screws on the carb are supposed to be out what like 2 and a qaurter? To get the car to start the screws have to be out 6 and a quarter! This cant possibly be normal right? The engine has been bored 30 over. My main question is, i have a 500 cfm 4 barrel carb that will probably make the car run better but how bad would it effect my performance from going from a 600 cfm to a 500 cfm? My brother said since its a 4 barrel it shouldnt really effect anything, but i disagree, the bigger the cfm the more performance correct? Im likey gonna upgrade to a bigger carb down the road, does anyone have suggestions on the size i should get when i do? Any help would be greatly apreciated.
Thanks alot.
Shawn

:By the way, the 600 cfm carb likely needs rebuilt, but not sure what to do performance wise. Thanks again.
 

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Get a Vacumm gauge or borrow one. Hook it to manifold vacumm should be around 20 inches of vacumm.
Then turn those air fuel screws in and out slowly watching the gauge keep adjusting back and forth till you achieve the most vacumm
Now check timing (at the correct idle speed for your trans )and adjust as ness
Next recheck those screws.

If vacumm jumps all over and wont settle you may have a cam problem or a vacumm leak.

Good Luck.
 

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If you have to back the screws out that far then there's either something major wrong with the carb or you have a vacuum leak... and I'm guessing it's a vacuum leak. Make sure you don't have any open orifices on the carb or intake that don't have a hose or plug on them... also could be bad carb or intake gaskets... easiest way to check that is to get a can of carb clean or something similar and give a quick little shot at the base of the carb, each of the intake ports, etc... and listen for the engine RPM to rise briefly... if it does then you've found your leak.
 

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Make sure your ignition system is 100% perfect before you start chasing the carburetor.

A lot of problems that seem like carb are really ignition, and vice versa. Getting the ignition just right cuts out half the possiblity for error.

Also, there are a lot of good books out there on rebuilding Holleys and tuning them. Read up and follow the directions, you'll be fine.

Good luck!
 

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Oh and as far as your original question, I'd have to agree with FE and say if the 500 doesn't have problems and the 600 does, the 500 will feel more powerful overall. Plus if it's a fairly stock 289, there wouldn't be much difference between a 500 and 600 anyways.
 

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CFM DOESN'T = Performance - there's a calculation of required CFM based on engine displacement and RPM you want the power in.

I'd say the 500cfm unit should work great on your 289 if it's mildly built and you run it up to around 4800/5000 rpm - if this is a high winding drag motor regularly going to 6500 yes, the 500 is too small.
 

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The 500 would be great if your 289 is fairly stock. Although more cfm will allow more power, if its steup properly for the right engine. I run a 650 on my 302 and it works great, although it puts down 300 rwhp. In my experiance your problem sounds like a vacuum leak but it could be lots of different things. A vacuum gauge can tell you a lot of things. As for the carb reccomendation we need to hear more about your combo.
 
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