Yamer hammer 15 4stroke over revving?

[jeffro897]

Hey guys,

I picked up a slightly used 2003 yamaha 4 stroke 15hp to put on my 15' weldbuilt and it runs good but when I get up on plane and turn it to WOT then the motor starts stuttering at WOT like its hitting a rev limiter? First question is do these motors a have a Rev Limiter cause thats sorta sounds like what its doing? If I back off just a hair it stops making that noise and runs perfect as usual.

Also if this is the rev limiter kicking in what shoud I do to make it where I can run the motor at full WOT and get all my power, cause now I cant? (cant turn the tiller much past the 3/4 mark or it will start doing it.) Dont know anything about prop diameter and pitch but from what I can tell from reading I should be able to get a higher pitch prop and reduce my RPMS by around 300 rpms so that at full WOT my motor will be revving less...correct? Will going up a pitch size fix this?

THANKS FOR THE INPUT,
Jeff

[Redleg Camo]

yes, go up in pitch to LOWER rpm's.

[jeffro897]

Alright Dee, Im gonna check what pitch the stock prop is now and try to get my hands on one a pitch degree higher. So is that basically what its doing though is hitting a rev limiter? Ive had two boat motors before this one, a yamaha 25 2stroke and a 20 2stroke and neither of them had this.(but then again they were 2 strokes and this is my first 4 so...)

Thanks for the advice,
Jeff

[Redleg Camo]

without being there and seeing it myself, that sounds like what it is doing. it may be a carb issue. i would replace the plugs.

but, a 15hp 4 stroke hitting the limiter on a 15ft boat, that kinda sounds crazy.

is it new?

[jeffro897]

Put two brand new NJK plugs in per the manual. And nah its used. Got it from a guy in florida who had a hourmeter installed with it when he bought it. Its got 10.2 hours on it so practically brand new. I had tom hughes clean the carb for me last week and check it over. So im gonna call him back tomorrow and see what he says I guess. Dee why you say its hard to beleive thats it rev limiting on a 15'? Just curious man and thanks for the info.

Thanks,
Jeff

[mud ugh]

You may want to get a tachometer before you start blindly buying props. It can save you from buying the wrong one the first time and having to sell it at a loss or have a prop that you probably won't ever use. I would look again for a fuel/spark issue first because as Tees says, it's mighty strange for a 15-hp to be turning up that high on that big of a boat unless it's way underpropped or your seller lost a hell of alot off the blades grinding through sand in Florida, which is not out of the question. You should have seen my aluminum prop after a week of fishing down there the first time.

[Redleg Camo]

Just a guess, but the previous owner may have used it for a kicker on a big boat and may have put a low pitch prop on it. Regardless, find out what pitch you are running now and let me know.

The yamaha site show a 9 1/4"x10 prop on their test with a 15ft aluminum.

Another thing i am curious about is turning that thing into a 20hp. the 15 and 20 are the same motor. it's probably a restrictor plate in the intake. i bet you would love to get a few more out of it.

[jgdavis123]

i bet you would love to get a few more out of it.

For real. I have a 40 on my 1542 war eagle. What kind of speeds do you get with that 15?

[Redleg Camo]

I think he should run about 20-23 set up properly.

JG, what motor do you have?

[jgdavis123]

Mercury 40hp 2 stroke

[Redleg Camo]

Mercury 40hp 2 stroke

3 cylinder? What year?

[jgdavis123]

Yeah, 3 cylinder. 1998.

[Redleg Camo]

That motor will fly. You need to get up with 1995US1 on the refuge. He knows a few tricks that can be done to that motor.

[jgdavis123]

It moves pretty good. 37-41 with me, the dog, and decoys. The best I have gotten with it was 43 just me and pretty much nothing in the boat. I appreciate the info. I will look him up.

[Redleg Camo]

yea, i would imagine you could run 48-50 empty, 43-45 loaded with the kit from danny. It bumps it up to 60hp.

[jeffro897]

Man I have looked into making mine a 20 but cant figure out exactly what to do. Some say exhaust, some say carb, but cant get definitive answer on what to change out to make me gain some HP!?! I think if the motor was acually not hitting the limiter, and over revving I could acually hit those low 20MPH speeds Dee's. I havent GPS'd it yet but I guess im running about 18 MPH tops with me and my lab this weekend and my fishing gear. The white aluminum prop only has about a 1/4inch silver ring of paint wear so dont think its too wore down. I wil get the pitch size tonight and post it up on here and maybe you guys can help me out some more... Im hoping its just "underpitched" and if so im gonna get me a 9 1/4"x10 prop like the one you said yamaha was running on the test 15' boat...

Thanks,
Jeff

[Redleg Camo]

get the model number while you are at it.