Yamaha 90 2 stroke, 88 mod. Lack of RPM in top end.

[Rune Wilthil]

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.

Any other tip?

[99yam40]

http://www.yamahaoutboards.com/owner...bulletinsCheck to s

See what prop they used on a rig close to yours and what speed and RPM it ran at.

If yours cannot get there then you have a problem with motor or the set up.

Have you checked compression?

[Rune Wilthil]

19" should give 5600-5800 rpm on this setup.

Comp is about 8.5 bar/123 psi on all. Also monted Lift yesterday to elevate the motor in propper hight.

I have T-ed inn an gauge to measure fuelpressure from pump. Have not tested yet. Anyone that have an idea what pressure to expect?

Going to take apart the carbs as well to check the float and needle.

[99yam40]

this should help
http://forums.iboats.com/engine-frequently-asked-questions-faq/how-testing-your-boats-fuel-system-283363.html

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)