Tuning Holley 80551 (4160)

[GA_Boater]

My final settings are: #71 main jets (going to #73 felt less crispy), #35 power valve (did not feel much difference to be honest between stock #25 and #35 but settled on #35 to be consistent with VP), BLACK secondaries spring. These settings give me nice crispy response all the way from idle to about 3000rpm. After 3000rpm she still responds to throttle but more sluggish. I still get a slight surge over the last 1000rpm to WOT as I hear secondaries making the swooshing sound. Looks like this is the best I can do with this carb - still not perfect. I managed to get my hands on another Holley. This one is 750cfm (model #9015-1) . I'll start another thread as I attempt getting better results tuning a bigger carb...

And you would get replies on both threads. Threads merged.

 

[Scott Danforth]

So, I've been thinking about my own question - why my new 600cfm Holley is so much off tune in stock configuration. Holley's answer to this: the carb is sized incorrectly. My choice of 600cfm was determined by the size of the carb I had prior - Edelbrock 1409 (600cfm). (Edelbrock was installed by a mechanic who worked on my engine a few years back). I went to calculate the proper carb size for my engine using online formulas, now I am even more confused. Max RPM for my engine is listed as 5000rpm (in reality I am only getting 4600 @ WOT). Plugging it into the "standard" formula I am getting 430cfm "street" and 550cfm "racing" suggested carb sizes. Either way 600cfm appears to be too big! This would explain the bad tune (for my engine) in stock configuration. An oversized carb would cause weaker vacuum requiring bigger jets and stiffer secondaries spring to restrict air flow - this is consistent with what I am seeing in tuning process. Ant yet, VP puts 715cfm carbs on this engine! Although I see some 5.7 models that had 450cfm factory carb. I am confused... Any thoughts?

Your carb is too small for a 5.7 in a boat

 

[saf]

Update:

So, I spent another day on the water attempting to tune (brand new) 750cfm Holley. The conclusion I am coming to is that 750cfm is WAY too big for my engine (350ci). No matter what I do I cannot get rid of the severe bog between ~1000 and ~2000rpm. Going bigger or smaller main jets only makes the issue worse, different power valves and secondaries springs make no difference as one would expect. The engine would not even run smooth once I attempt to increase rpms beyond 1000. RPMs start fluctuating even to the point of stalling when smaller main jets are used. Looking down the barrels (with flame arrestor removed) I see fuel dripping, which would explain why the engine runs so bad in this regime. Everything goes back to normal once RPMs go above 2000 - good throttle response, good power, etc (good idle too). I am thinking the air flow is just not high enough at low RPMs to atomize the fuel properly. Another sign telling me that carb is too big is the idle tune. I had to go almost an entire turn in on the mixture screws from the initial setting of 1.5 turns out to get the idle right.

I am returning the carb. The 600cfm carb, while not perfect, runs much better.