Question about new motor

[Blue Man]

After my yamaha 90 2 stroke seized, I repowered my 18' Nautica with a yamaha 115 2 stroke. The 90 used to get up to about 36 mph on my gps with a couple guys so I was excited to see what the 115 would do. I took it out and was surprised I could only get it up to 32 mph on my gps (w/ two people). I'm maxing out at about 5200 RPM.

I suspect the problem is the new prop. I didn't measure the prop on the 90 and I don't know the pitch but it definitely had a smaller diameter than the new prop. The new prop has a 13" diameter and there is no indication of the pitch. I'm not an expert and would love to hear your suggestions.

Funny thing is the guy who sold me the motor referred to my rib as a "Raft" and said they have a lot more drag than most boats so need to adjust prop accordingly...

Thanks,

John

[beamishken]

I'd be doing some basic checks as the rpm is in the right ballpark 5200 won't be on the rev limiter I'd expect it to max out at 5500rpm? so the revs are not dragged down by being over propped
I'd check rpm accuracy with a shop tach
Then start with compressions etc
Is the engine the same height on the transom?

[jeffstevens763@g]

i had a 14 x 19 ali prop on my 6 m rib 90 hp Suzuki 4 stroke very best speed perfect conditions 45 mph on the speedo
got much the same results with a 13.5 x 21 stainless steel prop.
i would have a look on the yam site to see the prop range for the 115 not an expert but i would say your down on diameter at 13 inch, 1 inch of diameter is approximately the same effect as 2 inch larger pitch.look on propeller-shop.eu choosing the right propeller
hope that helps

[beamishken]

Before he starts switching props he needs to get to the bottom of why the rev counter says it's spot on for correct set up but not going fast enough
If he's under propped he should be bouncing off the rev limiter at around 5700
Engine is either under power or rev counter is telling porkies

[Pikey Dave]

How does the holeshot compare with the 2 engines, the 90hp could have been overpropped. Are the 2 Yammies the same engine/block? With different badges? Or is the 115 a physically different engine?

[jeffstevens763@g]

1993 Yamaha 115hp 2 Stroke Prop and MAX RPM Page: 1 - iboats Boating Forums | 9918239

might be of some help 13 x 19 s/s prop

[beamishken]

PD the 115 is v4 detuned 140 & 90 is 3 cylinder in line 115 should have massive grunt over 90 as much larger ccs rated prop range on 115 is 4500 to 5500 so rpm is saying prop is spot on so somethings not right

[Pikey Dave]

Originally Posted by beamishken

PD the 115 is v4 detuned 140 & 90 is 3 cylinder in line 115 should have massive grunt over 90 as much larger ccs rated prop range on 115 is 4500 to 5500 so rpm is saying prop is spot on so somethings not right

Ah! I suspected that might be the case, but you never know with the older engines. Summat's not right then, as you say.

[Pikey Dave]

OP, are you using the same instruments & controls for the new engine that were on the old engine, just thinking that the tachometer might need adjusting from a 3 pot to a 4 pot engine.

[Blue Man]

Thanks for all the replies. Both the old motor and new motor are 20" shaft and look the same mounted on the transom (except for the larger girth of the new motor. The blocks are quite different as someone mentioned above. The older was 3 cylinder straight up and down and the new engine is a 4 cylinder v with 2 carbs instead of three.

I'm using the same instruments and harness for both motors. The compressions are all at about 110. The current prop is aluminum; I'll see if I can measure the pitch somehow.

[Last Tango]

I'm guessing the tacho pulse is generated from one of the the AC charge phases rather than ignition, otherwise going from 3 to 4 cylinders would cause it to read high rather than low.

[Blue Man]

I don't know for sure Last Tango but that would make sense.

[beamishken]

Which might explain things if the engine is severely over propped and can't rev out a high idle test would confirm either way
Rev limiter should stop it at 5700 rpm or thereabouts

[Pikey Dave]

Originally Posted by Last Tango

I'm guessing the tacho pulse is generated from one of the the AC charge phases rather than ignition, otherwise going from 3 to 4 cylinders would cause it to read high rather than low.

Which could well be the case, if the tachometer is reading high, I.e 5500 rpm & the engine is only doing 4200, therein lies the problem

[beamishken]

Originally Posted by Pikey Dave

Which could well be the case, if the tachometer is reading high, I.e 5500 rpm & the engine is only doing 4200, therein lies the problem

That's what I was trying to say maybe not very clearly[emoji106]

[jeffstevens763@g]

Yamaha.net

Might be of use to you when you need to chose new prop

[Blue Man]

I used the method here: https://sites.google.com/site/thepro...itch-measuring and it looks like the pitch on the current prop is about 16.3.

[Blue Man]

OK, I think I found the problem, but I'm not sure how to fix it. When I have the controls at full throttle, the carbs only open 2/3 the way. When I put it in reverse and pull the throttle all the way back, the carbs open all the way. Thoughts?

[beamishken]

Possibly just need to adjust the control cable just be cautious of where you adjust as you don't want to upset the relationship between the throttle butterfly's opening and the ignition timing advance linkage
If you can do it at the cable then that should be fine just be careful of adjusting the link rods on the carbs

[Pikey Dave]

Originally Posted by Blue Man

OK, I think I found the problem, but I'm not sure how to fix it. When I have the controls at full throttle, the carbs only open 2/3 the way. When I put it in reverse and pull the throttle all the way back, the carbs open all the way. Thoughts?

Is this going to be the issue though? If you're hitting 5500rpm, you're only 200 short of hitting the limiter, another 200rpm isn't going to get you the extra speed you're looking for.