1993 Yamaha 40hp 2 stroke lacking RPM

[Dadnova]

1993 Yamaha 40hp 2 stroke outboard jet drive lacking RPMs. The compression is 120lbs on all 3 cylinders, all carbs have been cleaned and checked, new fuel pump, new Boseen reeds. Mix is 50:1 But I am only getting 4780 rpm out of it, should be 5500 rpm. Timing is dead on. Any idea's as to why it's not revving up like it should??

 

[mphelle8vld]

I used to own both the 40 and 50 hp Yamaha jet drives, neither of which would pull 5500rpm with the medium pump, 6 1/8in impeller. Are you saying that your motor used to spin 5500? Lots of variables on a 27 year old motor.

 

[QBhoy]

It would be an incredible thing for your engine to exactly make its factory rpm rating on every particular boat it was bolted onto perhaps.
is your impeller all good and intake ?

 

[Dadnova]

Need HELP with my puzzle, lack of RPMs

1993 Yamaha 40HP 2 stroke with jet drive, flat bottom river boat, testing done on lake, for the life of me I can only get 4600RPM out of this engine, should be 5500 RPM, New fuel pump, plugs, coils, everything omed out within specs. Carbs clean adjusted, even installed a totally different set of carbs, same thing, the impeller is not touching, shaft bearing checked and good. Timing is dead on perfect. What could it possibly be??

 

[mphelle8vld]

The factory rating of 5500rpm is based on the prop motor, when they bolted on the jet unit it had a fixed load based on the pump and impeller size. Your 40hp is doing alright getting 4780rpm, mine was getting right at 4900. When I bolted on a set of 50hp carbs and a 50hp exhaust tube it raised the rpm to just over 5200.

 

[mphelle8vld]

I'll try asking again...(Are you saying that your motor used to spin 5500?)

 

[Dadnova]

I'll try asking again...(Are you saying that your motor used to spin 5500?)

No, since I bought it used it doesn't rev properly

 

[Sea Rider]

Those jet things have a larger HP CC powerhead although being 40-50 HP cowl stated to compensate performance loss. Is it possible to raise/lower the motor say one notch and check if gained a higher wot rpm ? Those jet things should benefit running less size, lightly loaded boats to reach their max wot rpm range.

Happy Boating​