Mild Yamaha 90 Outboard Jet

[Fastjeff57]

I suspect the manufacturer deliberately restricted the rpm to improve fuel mileage--these things are gas hogs.

Jeff

[mitch184]

I suspect the manufacturer deliberately restricted the rpm to improve fuel mileage--these things are gas hogs.

Jeff

I'd agree to a point. If the motor is struggling and can only turn 5000, to me it seems like it's going to struggle all through and RPM range and possibly use more fuel and be harder on the motor.

In my head its logical to make WOT right around the rpm it produces max horsepower. Which I believe on a Yamaha 90 2 stroke is 5500 or even higher. If it doesn't reach that RPM, you're not even using the 85 or so actual HP it has.

That's my opinion based on the fact that the Merc 90 2 stroke of the same era makes its peak HP at 5000 and is noticeably stronger with a jet.

[johnson60]

Stepping down in size will help with your RPM gain. I have a 70hp Johnson that I run a 6 7/8 stainless 3 blade. I turn around 5400 WOT. I have a 6 7/8 aluminum that I have ran before and it will turn 6000 but is 3-4 mph slower. material makes a big difference. there is a guy here in southern MO that does impeller and shoe work and consistently sees 2-3 mph gains with RPM gains.

[mitch184]

Stainless is a must no matter what. For performance and durability. 3 vs 4 blade and 7 3/16" vs 7 3/8" is more complicated with the smaller motors.

MPH and holeshot gains are usually had from working the shoe. However, rarely have I ever seen the RPM go up. I'd be curious to see what he's doing to the shoe to see any increase in RPM's.

[johnson60]

I never seen one that he has done in person but I know beveling the bottom is one thing but I have seen him talk about unshrouding the impeller. it where they cut the top of the liner to match the top of the impeller blades. I have not done this but planned on trying it on a old liner just to see what happens. The RPM gains he talks about comes from doing the impeller and shoe at the same time.

[ferdthe4]

I imagine if he is cutting the liner to unshroud the impeller this is how he is getting rpm up. Kind of like how now the new 6 7/8" stainless 3 blade impellers which mercury/quicksilver makes the new ones are cut back on the top to help the 4 chokes spin up better as they couldn't spin the old style ones meant for 2s which made more power.

[mitch184]

The only real way to increase rpm's, just from working the pump, are to either cutback the impeller, induce slip, lighten the impeller or handwork the foot to "scoop" or increase the ram effect of water on plane. The first two ways, ultimately reduce performance.