Carb tuning

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jtschug
Posts: 862
Joined: Fri Sep 11, 2015 1:33 pm

Carb tuning

Post by jtschug »

My engine is pretty far from stock and I've got a Holley 4150 750 carburetor on it. Last year was the first full season, and it smelled pretty rich and generally got about 10MPG. Over the winter I decided to put a AFR gauge on he column so I could check, and yes. It is way rich, frequently dipping below 11.

I've also mocked up a vacuum gauge. My plan has always been to go MPFI, but there are higher priority repairs, and so I'm going to have the carb for a while, so I might as well get it running right.
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64ZCODE
Posts: 254
Joined: Wed Mar 20, 2019 9:41 pm

Re: Carb tuning

Post by 64ZCODE »

Hey jtschug. Your post didn't say what your engine displacement is. I'm running a Holley 670 cfm carb on my '64 390 and the engine seems to love it just fine. My driving style opens the secondaries often and I'm averaging 11-12 mpg. Everything is stock except an electronic ignition system, free flowing air housing and filter, and Magnaflow exhaust.

Check your engine oil. If the engine has been running rich for awhile, the oil may be diluted with gas which can damage the bearings. If the oil looks diluted or smells like gas, recommend you change it out.

I don't know if a 750 cfm carb is too big for your engine or not, others may be able to comment on that. But for sure, check the power valve to make sure it is correct for the amount of manifold vacuum you have at idle. If the power valve is too big, it'll dump too much gas too soon into the throat of the carb. A friend of mine had that issue with a 351 Cleveland and the thing would barely run and stunk of gas when it did run. He's running a radical cam and so the manifold vacuum is pretty low at idle, around 8 inches of vacuum. He ended up settling on a 4.5 power valve to get it to idle and run right.

Good luck and please let us know what you end up doing.

Doug
Life is short, my friends, so take my advice
Pull down your pants and slide on the ice

jtschug
Posts: 862
Joined: Fri Sep 11, 2015 1:33 pm

Re: Carb tuning

Post by jtschug »

It is a 449 inch 428 stroker with a medium roller cam.

64ZCODE
Posts: 254
Joined: Wed Mar 20, 2019 9:41 pm

Re: Carb tuning

Post by 64ZCODE »

Then I would think the 750 would be fine, a larger cfm car would probably work also. What is the manifold vacuum at idle?
Life is short, my friends, so take my advice
Pull down your pants and slide on the ice

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