Low RPMS's

[jbt083]

I have a 97 yamaha 90 2 stroke on a nitro 640..... WOT its only hitting 4400 with a 21 prop... already changed plugs ....any suggestions??

[esoxfly]

Was it a sudden issue, or did you just notice the loss of RPM over time?

If all seems and sounds well, I'd check compression first and see what that says.

[jbt083]

well i bought it back in april from a dealership..... it only runs about 40(gps) and it should be running about 50..... it has been like that since i bought it and the dealer says maybe i need to drop down in pitch to a 19 but i think that would reduce my speed

[esoxfly]

Well if it came that way, and you bought it used, I'd still have the compression checked. If it's an old motor with low compression, putting a new prop on isn't going to bring back the power. Make sure the motor is sound and then worry about props and set-up and then worry about speed.

[xpress spanky]

Your way over propped!! I am running a 17" turbo on my 90 and turning 5700 rpms. A 19" powertech that it came with was a DOG. You make the peek power of that motor higher up in rpms than what your turning,so your never putting out the 90 HP that you can have by propping it correctly.If you put the 17"turbo 3 blade on your rig you will be amazed at the performance difference you will see. It will turn slighly less on RPM's on your rig than mine because i am running a 10" jackplate and have the motor raised up pretty good.

[jbt083]

but if i am only running 40mph now and i SHOULD be running around 50 wouldnt dropping down to a lower pitch reduce my speed more?

[iron banks]

Something is not right. I had a 90 yammie on a xpress and a twisted a 17 pitch prop to about 45 mph. I believe you are over propped.

[esoxfly]

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by jbt083 &raquo;</TD></TR><TR><TD CLASS="quote">but if i am only running 40mph now and i SHOULD be running around 50 wouldnt dropping down to a lower pitch reduce my speed more?</TD></TR></TABLE>


Not necessiarily. If it allows you more rpm, your motor can come up on the powerband and push you faster.

It sounds like you're overpropped based on the other replies, which is an easy and immediate fix. But I still maintain, I recommend you check the compression before you start swapping props and experimenting. You can experiment all day long, but if your compression is down or a cylinder or two is off, you're wasting your time. Unless you had it checked when you bought it. That's just my train of thought; make sure the motor is sound before you make changes to it.


Modified by esoxfly at 11:25 PM 9/11/2009

[xpress spanky]

Brian,90 hp WILL push a boat faster than 70 hp!!!! At the rpm's your turning you ain't making the horsepower you have!!My boat runs well over 50 mph with the 17.If i tried a 21 my boat would be a friggen slug,it was bad with the 19 and the 17 IS faster than the 19!!! We can make your boat run a hell of a lot better if you just listen to what everyone on here has said,but remember YOU aint close to your horsepower peak at the rpms your turning.You can make a 200 run doggy if you overprop it.A two stroke needs rpm to make power,and if you lug a motor like your lugging that one it will not last.

[jbt083]

oh I agree!! this is my first boat so I am learning something new each day! My first thought was there is something wrong with the motor but the mechanic where I bought it said I need a lower pitch prop also.. He didnt check the compression but he said he could tell if it was a compression problem.. I thought it was the right prop because the guys on the nitro forum with the same boat except they have the merc 90 say that they run a 23 pitch @ 50 mph.. so thats why i didnt think it was the prop... so now im in search of a new prop!! Thanks guys

[xpress spanky]

Merc has a different gear ratio in the lower unit!!!! Thats why they run a lot higher pitch prop than the yamaha 90.