Carb tuning issues with a 720

[Sebastian Bluecrew]

Current build......
1995 XP 720, Bored 1.5mm, Pro-X pistons, Factory Performance Pipe without ECWI, dual 40mm carbs 140/75 jets, 40-ish pop-off, stock airbox. Motor is complete rebuilt all new gaskets and orings. Compression 150psi both cylinders. Rotary valve timed exact.

Skip the grey fuel lines and carb rebuild answers. It has all been done. And we went through the carbs multiple times. They are spotless.

Runs great on trailer, no high speed or WOT after 4000 rpm in the water under a load. Will stand up when you stab it from an idle then falls on its face.

PLug chop indicates it is light brown color, but not wet.

So I tried adjusting the high speed screw from 1/4 open to 2 full turns open. Made no difference at all. Should I go up on the jet to a 142.5 or a 145? Or am I missing something. I know it is lean because if I hit the primer while holding the throttle it will jump for a second then fall on it's face again.

 

[Matt Braley]

Must be a fuel supply problem then. Does the fuel line in between the baffle and the carbs hold psi? The fuel valve can draw air in or leak between channels and leaks in the filter/separator need to be ruled out also. The valves will sometimes only hold 5 psi but that's enough. If it's not that then my guess would be fuel line routing.

 

[JeremyD615]

Didn’t you have an aftermarket head on this motor, or was that another ski? If so, that 150 psi would be low...

I’m with Matt, it sounds like a supply issue. In addition to the separator and selector valve, you might check the fuel pump side of the carb. I’ve had issues recently with diaphragms/gaskets not sealing well, and they showed up as a high rpm lean condition as well.

 

[Sebastian Bluecrew]

The aftermarket head was the Buckshot I am selling, it is not installed currently. Stock head.. 150PSI.. We already replaced all the lines and checked fuel selector.. We decided we both ordered a set of Super BN44 Mikuni carbs with Mikuni carb kits, and Ocean Pro vortex air cleaners, I have my set of 44s going on a WestCoast intake manifold and he has a Rossier manifold... The question is, what will the jetting be for the Factory pipe, I guess the 40mms with 140/75 at 44PSI was not enough fuel. Should the 44s be jetted lower than 140/75 due to the increase in flow?

 

[R-u-n2it]

A little late but..... a 720 is like a short stroke 785, same bore (isn't it?) so it will want to rev, if it falls on it's face no matter if you gradually increase throttle or just nail it I would lower the pop-off pressure, since no float bowls the pop-off has to be low enough so no fuel starving but high enough to prevent loading up/flooding. Since you don't describe anything like loading up lowering pop-off will help, if that's the only problem lower it till no hesitation. It could be any of the things Matt, Miki, etc said but 44 seems high, I run 12-13lbs as an example. Partially block your intake to test, that increases manifold vacuum which drives the fuel pump more to offset pop-off, if better remove restriction and drop pop-off pressure.

 

[POPPS]

I agree with n2it, for a boat that definitely is in the race boat category, 40 pop off seems high.