Mild Yamaha 90 Outboard Jet

**mitch184** · 02-09-2018

Looking at trying to squeeze a few more HP and RPM out of a early 2000's Yamaha 90 2 stroke. It's going on outboard jet so motor/boat setup is basically already set up. Holeshot is most important but still basically a result of WOT. WOT will be 5500 max with a jet. Usually if you manage to pull more rpm's, you just switch to a bigger impeller. I know they tune props to run at higher rpms, but I've never seen someone do that with a jet for some reason?? I know the larger impeller's are generally more efficient and resist cavitation so maybe that's why.

Motor is stock now with 120psi +/- 2 in all 3. Was looking at doing the Hydrotec Phase kit (Heads, jets, tuner, etc) except use CCM reeds rather than the Boyeson. (My own personal reason for that) Compression with their heads it around 145psi per them. Not sure if decking them more would even gain much on 91 octane. Also looking at cutting/porting the reed cages since it seems fairly straight forward, but wasn't sure how that effects longevity. Also, since the impeller acts as a flywheel, I also wanted to look at lightening the actual flywheel but wanted to make sure it didn't effect anything else I'm not thinking of. I think the motor only puts out 10amp, but I'll need every one I think.

Any guesstimate on what sort of power these parts might put out?

Thanks

**hightide** · 02-13-2018

Where are you located? I'm working in a shop now that can handle all you need done. Your biggest gains without going in to the motor will be getting the compression to 150psi or so and cutting the flywheel

**mitch184** · 02-14-2018

I'm up in Washington state. Thanks for the offer.

I was thinking, since its a jet, that a lightened flywheel would make a larger difference.

**hightide** · 02-18-2018

We have parts mailed in all the time here's a 40 Yamaha flywheel

**johnson60** · 02-22-2018

If your wanting to get hole shot and top speed. you can mod the jet too. with the impeller you can sharpen the blades, polish and lighten. the foot can be modded also. beveling the liner on the bottom for better flow. sharpen the grates. Most people see at least 2 mph gain with those mods.what type of impeller are you running? most people have 3 blade stainless which are awesome for hole shot and top speed but sometimes cant handle a load as good as a 4 blade.

**mitch184** · 02-27-2018

Originally Posted by johnson60

If your wanting to get hole shot and top speed. you can mod the jet too. with the impeller you can sharpen the blades, polish and lighten. the foot can be modded also. beveling the liner on the bottom for better flow. sharpen the grates. Most people see at least 2 mph gain with those mods.what type of impeller are you running? most people have 3 blade stainless which are awesome for hole shot and top speed but sometimes cant handle a load as good as a 4 blade.

I'm pretty familiar with touching up the jet units. Sharpen the and polish the blades, bevel/blend the liner to the housing, teardrop the grates, smooth the casting lines, port the housing, keep the clearance to a minimum, machine the impeller nut.

These all work GREAT and improve the efficiency of the jet tremendously. The only downside is that by making the jet more efficient it loads the motor even more. Standard for the motor is a 3 blade 7 3/8" and outboard jet sizes their impellers to let the engine spin 5000 @ WOT. If you go to a 4 blade and do the above, I would imagine the motor won't even turn 4900 WOT.

One thought I had was stepping down in diameter to the 3 or 4 blade 7 3/16" (~5400 WOT) , then work the housing so I keep the RPM's up a little more in the friendly zone. Jet are definitely a little different as you don't typically want them to spin way up because you'd be more efficient overall stepping up a larger, or steeper pitched, impeller. That being said, to me, if the motor makes max HP at 5500, you'd want to spin up to that at WOT. That doesn't seem to be the line of thinking for the outboard jet manufacturer, but makes more sense to me.

Hightide,
Thanks for the offer. I will keep that in mind when the time comes. Given the jet impeller acts as a 2nd flywheel, the lightened flywheel seems like a no brainer.

**Fastjeff57** · 02-28-2018

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

Jeff

**mitch184** · 02-28-2018

Originally Posted by Fastjeff57

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** · 02-28-2018

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** · 02-28-2018

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.