1989 Yamaha 150 hp max rpm is only 4400

[erock7678]

I was just wondering what the max rpm for my 150 hp motor should be at full throttle it will ony reach 4400rpms. It seems to me it should max out at 5500-6000 range if so what could cause the low top end. The motor doesn't miss and runs strong.

[99yam40]

over propped, rigging issue, or a motor problem not letting it produce the HP needed to turn the prop

[erock7678]

How do I know what size prop to have on it? My boat is a 21 ft. Proline w.a.

[ausnoelm]

We need to know if it ever reached max RPM, or you will be chasing your tail.

[99yam40]

How do I know what size prop to have on it? My boat is a 21 ft. Proline w.a.

See if you can find a rig like yours here, to see what prop they used and how it ran
Outboard Performance, Performance Bulletins | Yamaha Outboards

[erock7678]

We need to know if it ever reached max RPM, or you will be chasing your tail.

Since I've owned it it has never been above 4400 but not sure if the previous owners had it above that. I mean the boat still cruises at 32mph, not sure if it is just because it is an old and heavy boat?

[TownsendsFJR1300]

He's was trying to find out if the RPM's suddenly dropped, a running issue....

Probably not a running issue, but a wrong prop on it....

[ausnoelm]

Yep, that's what I was getting at, no use playing with props if it used to run full RPM, similarly, no use fooling with mechanicals if it's a prop issue.

[Jason2tpa]

How do I know what size prop to have on it? My boat is a 21 ft. Proline w.a.

Look for numbers on prop hub, such as 13 x 21. The first number is diameter, second is pitch. If your props pitch is on high end (21-25) then decrease pitch. Rule of thumb...every inch decreased in pitch increases rpm by 200. And vice versa.

This is all assuming no engine issues.

[Jason2tpa]

Also, and I'm repeating what the pros on here told me, if your engine is not reaching spec rpm range at WOT you've got a problem. Once you check your props pitch, if it's on the low end (13-17) then you can rule out the prop. Bc a lower pitch prop produces a higher rpm. Again, assuming all other factors are normal. Good luck and let us know.

[rodbolt17]

actually??
find a dealership that has test wheel #YB-1626.
install the test wheel and back it down the ramp and see if it will turn it to 4600-4800.
if it will fix your prop/rigging issue.
if it wont fix the motor and retest.

[Jason2tpa]

actually??
find a dealership that has test wheel #YB-1626.
install the test wheel and back it down the ramp and see if it will turn it to 4600-4800.
if it will fix your prop/rigging issue.
if it wont fix the motor and retest.

Testing as you suggested, wouldn't the lack of load produce an inaccurate max rpm at max throttle?

[ausnoelm]

See where he said "test wheel" , guess what a TEST does?

[Jason2tpa]

See where he said "test wheel" , guess what a TEST does?

I've never used one, though the concept makes sense. However, I'm not familiar with how the test prop accurately determines rpm when no load is applied. If you are please explain.

[TownsendsFJR1300]

I've never used one, though the concept makes sense. However, I'm not familiar with how the test prop accurately determines rpm when no load is applied. If you are please explain.

I've never seen or used one.

I gather it puts load on the prop shaft (while still on the trailer, backed up to water) and allows the engine to be in gear at full throttle simulating WOT for testing purposes..(engine is going up in RPM's as if in open water)