Yamaha 90 2 stroke, 88 mod. Lack of RPM in top end.
Just got the engine. It was without prop. Did some calculations and came to an 19" ballistic should fine on this setup. Ibiza 17 sport. But got very surprised when i tested. Only got about 4900rpm at max and 39 knots. Engine othervise runs fine, no missfire or hesitation.
So, borrowed a few other steel props, 17", 18" and 21". Here is when it gets a bit funny.
The 21" balliatic gave 4500 rpm, but 41 knots.
The 18" gave 5100 rpm and 37 knots.
And the 17" gave 5300 rpm and 36 knots.
The previous motor i had was an 55 Hp suzuki 2 stroke, it gave 32 knots!
Sparkplugs are brand new.
Powerhead had a rebuild last year with new pistons, rings and all bearings etc. It has 50hrs since then with no probs.
ignition has been checked, all 100% tip top.
So, i kinda feel it could be a fuel problem. Did today a check on the fuelpump. Took it of the block but did not open it. Tried to pump up the fuelpressure with the hand pump, there is no fuel comming out the vacuum port. So i asume the membrane's in the pump are fine. At least they are not ruptured.
If compression , spark, and timing are all in spec. and your motor set up is correct and the boat is not water logged and too heavy
then you are left with fuel problem is my guess( carbs need a good cleaning)