I wrote an article for the Inliner's magazine about 8 years ago about my carburetor experiments on a 261 Chevy engine.This engine had the usual modification;M4F cam, Fenton exhaust,848 head with some milling on the block to give a measured 9-1 static actual compression ,and upgraded ignition.The engine was in a 3000 pound 37 Chevy pu truck ,3 speed manual,3,36 gears.This was a strong running engine according to several Inliners who drive it .Nothing hi tech about about the testing,full throttle acceleration in second gear from 2000 -4500 rpm using a marked distance to measure acceleration on a back road.
I tried a variety of carbs on a Clifford 4 bbl intake,time was spent to jet the carbs for best acceleration and part throttle driveability.Carbs used were:390 Holley 4bbl,500 2 bbl,400 cfm Carter AFB,500 cfm Edelbrock 4 bbl and a Holley 600 4 bbl.I bought the carbs on Ebay used,rebuilt them,then sold them after the tests,Cost me a summer of messing around and about 300 bucks.
The best results? The Carter 400 cfm AFB and the Holley 390 4 bbl.Larger cfm carbs offered no increase in performance in my tests.I also found while jetting these carbs for my modified 261 that the proper full throttle mixture for power when the throttle was opened full at 2500 rpm became over rich at over 4000 rpm. And of course when it was jetted properly redline power,the midrange went a little lean.So compromise jetting was used.It's my opinion that the power valve port channels on a 390 Holley were too small to deliver adequate fuel at wide open throttle in mid speed ranges.I believe an engine requires the richest mixture at maximum torque,which was about 2500-3000 rpm on my engine.The 500 cfm Edelbrock 4 bbl had a slight bog when the throttle was fully opened at 2500 rpm.No amount of jet tuning solved this problem completley.My conclusion was the 261 engine I had only needed a 400 cfm rated 4 bbl for best acceleration.
A few years later I built a GMC 302 about in the same state of tune as the 261 Chevy.For sure the 302 had a lot power,but I got the same results from messing with several four barrel carbs.400 CFM AFB ran as good as a larger carb.On the 302 I drilled out the power valve channels in the metering block from a .038 to .045 and that helped the midrange lean out.I think part of this was the Clifford intake port runners are too large for some engines.
Of course,your engine may be different,your seat of the pants judgements may differ than mine.
