I'm trying to get this ski going again. The carb was rebuilt by previous owner, so I can't be sure the correct pop off spring is in there. I have all 4 of the mikuni's available, and currently have the weakest in there- it pops off at about 28 or so with a 1.5 needle and 65 gram spring. it has 120 pilot and 115 main jets.

It was running okay, not perfect up until about 2 weeks ago, then power went down and wouldn't idle. I had it with the carb and ordered a Mikuni BN40-44-43 which is supposed to be correct for this 650cc craft, but it had a 115 gram spring in it which gave a 54 pop-off. While I was at it, I checked the compression, which is good at 157 or so both cylinders.

Well, I then found out the real problem was a crinkled reed and a broken reed. I replaced the reeds, carefully replaced and sealed the manifold gaskets, replaced the fuel lines, and installed the new carb.

I can get it to idle pretty well at 1/2 to 3/4 turns out on the idle mixture screw, but the darn thing "stutters" above about 1/3 throttle, and just bogs above about 3/4 throttle. I started with 1-1/8 turns out on the high screw, but it doesn't seem to make much difference whether I screw that in or out 1/2 turn from that- the problem won't go away, or even get a little better- prob gets worse much away from 1-1/8. I then changed from 115 g spring to 65 g spring- no help.

I replaced the fuel filter, cleaned out the selector valve, the vent valve on the tank is installed correctly and tests okay. I replaced the coil, plugs and ends for good measure- the spark looks strong.

My next step is to pull the exhaust manifold and carb, seal them up with rubber plates and pressure test the engine to make sure I don't have a vac leak.

This seems to me like fuel starvation, but what do you all think?
And does anyone know what is the correct spring, and or pop off spec for this- how about the needle arm height above or below the carb body?

Thanks for reading and any suggestions you might offer,