engine or prop causing no mid rpm range

[yootles]

I have a new yahama 150 hpdi 2 stroke engine on a 1988 18.6 boston whaler outrage. I can not get the engine to hold rpm between 2000 and 2900. top end is 5800 rpm and otherwise the engine runs fine. with the 17 pitch prop i kinow i am a little under proped but is it normal that under propping won't allow the engine to hold a certain rpm. The boat jumps out of the hole and settles at 3100 or so . if you pull back on the throdle to say 2800 it holds for a minute AND then slowly drops the rpm to 2000. this makes it impossible to run rough water off a plane. The mechanic says i need a 21 pitch prop , it just doesn't make sence to me. help!!! Is this an engine or computer problem or what

 

[99yam40]

Re: engine or prop causing no mid rpm range

I would monitor fuel pressures to see if any of them drop when problems shows up. hooking up a computer with help a lot.
Going to a larger pitch prop would not help in my opinion also.
May need to find a different shop

 

[multimech]

Re: engine or prop causing no mid rpm range

Going from a 17 pitch to a 21 is extreme, you are talking about 800 rpms off the top of W.O.T. I call BS. At 2800 rpms I would say you are lugging the engine and the boat is not on plane, or is just trying to get on plane or drop off to displacement running, it will drop off naturally. Sure, hook up the computer, but boats are not like cars and they will not run steadily from idle to w.o.t. without encountering problematic areas. I think this is normal and you need to find a better rpm to run low speed.

 

[yootles]

Re: engine or prop causing no mid rpm range

Thanks All for your sugestions. Let me complete the story to see if it opens up any new ideas. First off I have a 20' whaler with a 200 hpdi and have no problem running at any rpm regardless of the attitude of the boat. The 2005 150 hpdi was purchased new but it was old stock and had bad fuel in it which required service on the fuel pumps. It is under warrenty. The condition now is such that in rough water I either have to be planning at a high speed literally flying out of the water at 3000 rpm or run at 2000 rpm at 10 mph or so. There is no way to achieve a bow high fast plow speed. No matter what I do The engine won't hold any rmp between 2000 and 3000. Doesn't make sence the 18.6 whaler is light, capable of planning at a much slower speed and by the sound of the engine it is not overloading but slowly after a minute or so the rpm drifts back down to 2000. In a previous life I had a 20' seacraft with a 235 evenrude turning a 21 pitch prop. It ran fast in calm water but couldn't climb waver in following seas, so I know the feel and sounds of overproping. The dealer has stated it needs a 21 pitch rather than the 17 and that the lack of force on the prop fools the throddle computer and makes it slow down. I dont want to waste money on a new 21 pitch prop and have the net result zilch and possibly put too much load on the engine as well which I know is damaging. any thoughts

 

[99yam40]

Re: engine or prop causing no mid rpm range

Sounds like the motor has issues.
If WOT RPM is 5800 then it is a little under propped as I believe the top should be 5500, but never heard of under propped would cause motor to load up like that. When it starts plowing it does load the motor differently, but the motor should not drop RPMs like that.
Can you bump them back up a little and hold there about 2500 or does it just head to 3K if you bump throttle a small amount?

 

[yootles]

Re: engine or prop causing no mid rpm range

even the slightest bump of the throttle will jump the rpm to 3000, and if I slowly pull back to 2800 within 30 seconds or so the rpm drops off to 2000

 

[99yam40]

Re: engine or prop causing no mid rpm range

give a good explanation to Rodbolt17 on that other web site you posted, he is a Yamaha master tech and he should be able to help.
Sounds like a problem with motor to me