I had an idea, but I don't know if it's a good one. In tuning my 600 cfm 4160 holley in my 71 Ford F-100 (302, AOD, 4.11's, 30" tires), I wanted to really isolate the mains circuit in the primaries, and lean them out until it became apparent that they were too small, then step back up a couple sizes. I'm tuning for part throttle as well as full throttle, this is not a race only application.

So I tied the secondaries shut and plugged the power valve. I put the jets and everything else back to Holley's spec for the carb (66 jets and 25 nozzle). It ran fine, and I'm working my way down the jet sizes, so far I'm on 63 jets. The 25 nozzle was not great and bumped up to 28.

Results have me a little confused.

Engine vacuum is up an average of 5 inches at cruising speeds. In my test drives, I cruise at 30, 45, 55 and 70, then from a stop mash the throttle and take it to about 4,500 rpms. If the jets were too small I think it would show in the last test. What I don't get though: if I'm cruising at 60 in overdrive (about 1,800 RPM), and give enough throttle that vacuum drops to pretty much 0, it really won't accelerate at all until I close the throttle enough to get some vacuum back.

So the question - is this a lean condition? In neutral and hard acceleration it performs well up to around 4500 RPM, but that low RPM hesitation has me confused.

 

It sounds like a lean condition for sure I would bump back up a couple jet sizes and recheck but also pull plug or plugs now and after and read the plug them are the best indicator for a lean or rich condition plus make sure ur fuel pressure is staying stable as well and u also may need to play with the timing that will affect throttle responce when changing jet sizes

 

Thanks for the input. Fuel pressure is regulated at 5 lbs. I've set the initial advance to 10 degrees - I know that the timing curve is an area that I need to check but don't have good tools to do it the way I want. A freind will help with this later, I'm doing the carb best I can for now. I swap plugs with every jet size change. Doesn't look lean yet - its actually starting to look pretty good.

I think I will start over at the 66 jets and see if I can duplicate the result. I don't understand how part throttle at 1800 RPM could cause lean condition but full throttle at 4,500 RPM would not...but don't want to take the chance.

Any other input? Is it possible that the lack of vacuum is just causing poor cylinder filling?

 

The condition you describe when you accerate on the hwy is why carbs have power valves. Holly does have power valves of various vaccumn settings. That is they open at different vaccumn levels. Find one that has a very low number. Another thing you can try is something I had to do years ago.

My friends and I build a semi-race 2.0L OHC Ford Pinto engine. We also installed a 500 cfm Holly 2bbl carb. You would think that carb is way to big for 122 cid. But jetted right if ran strong. One thing we had to do to get it to accelerate was to plug the fuel meetering holes in the carbs power valve with solider. We then drilled much smaller holes in the plugs. This allowed a much smaller amount of fuel to the engine without flooding it with to much fuel that the stock metering holes allowed.