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Adjusting air bleeds

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Hi,

I've got a Pro Systems built Holley HP 750 double pumper carb on a SBC road race motor. Running 76 jets all around. Looking at my O2 traces from the last track day, the jetting is good but it get's a little leaner as the RPM's increase. The carb had size 40 air bleeds on both the primary and secondary. The builder suggested I bump those down to 33 on both the primary and secondary. Taking it around the neighborhood it looks to be a little to rich now as the RPM's come on. But I didn't get a really good test at high RPM's and the motor wasn't all the way hot. I'll have to wait until I get to the track to really see how the change worked.

Question - if 40 air bleed is too much and 33 is too little... any reason I can't swap back to the 40's on the secondary and keep the 33's on the primary. That would be a quick and easy change I can make at the track. I figure that should give me something in-between having it 40 or 33 all around. Running a high rise single plane Holly strip dominator intake.

Thanks!
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Here you are at the consequential problems of carburetors short of Weber DOCE’s and similar types. Where the circuits are divided more finely this is a more discreetly managed problem of isolating high speed mixture control from that of lesser speeds.

With the Holley as with most any domestic or most foreign carbs for that matter the main metering system covers a very long cycle with pretty large differences between highway cruise and peak sustained power. Given the engine will come apart when the mixture is too lean at high speed the typical is to be a bit rich on the top end and live with the lower RPM consequences.

That said the Holley 4 barrel does offer the opportunity to jet (air correction and or main metering) on the secondary side to be more rich while leaving the primary side closer to cruise or lower power AFR. Plus you can get secondary metering blocks that use a power valve.

The advice you received concerning air correction jets is correct but as always you’re going to have to tune into what the engine wants so finding your self in a range of 40 to 33 with something close to perfect as these things get might be inbetween. You can also fiddle the curve by changing the emulsion tube jets, they all don’t have to be the same size. Not that while Holley usually uses 3 emulsion jets the aftermarket more commonly uses 4 to 5 which lets you dink with the fuel curve pretty seriously.


The link shows some of what’s available

Bogie
 

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So why not go to a 35 or 36 all around?? There is no reason you HAVE to use just 33 or 40....they make them in a full range of sizes you know.

33 in front and 40 in rear means the secondary end of the manifold/rear cylinders will tend lean and the front of the manifold/cylinders will tend rich....and a collector mounted O2 A/F meter will only display the average, after the fact.
 

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In theory there should be enough turbulence in the plenum that the mixture will blend out to an average ratio. A big part, if not the biggest, of tuning is finding what solutions you can find that work best with the equipment you have. This brings us back to Smokey’s “cut and try” process. The book learning and theories well only take you so far, the rest is experimentation.

Bogie
 
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