Suzuki 115hp late 80s

[Crumpet46]

Hiya, I have a 115hp 2 stroke on a ski boat, which from what Ive seen should be revving to roughly 5500rpm WOT but on a flat calm stretch of water on a Savage electra which should be a quick boat with little drag it is struggling to hit 4800rpm. Am I missing something or is this seriously over proped? The next question would be how can I work out how much to take off the diameter/pitch to get it singing? The boat is used for cruising round and soon to be used for wakeboarding (not skiing).

Thanks in advance for any help.

ps, new to the forum...be gentle

 

[jestor68]

Re: Suzuki 115hp late 80s

The general rule of thumb regarding rpm change is 150-200 rpm for each one inch of pitch you change.

In your example, you would need to go up 700 rpm to reach 5500. That means 3.5 inches pitch change. Since most props are available in whole inch sizes, you would do down 3 inches of pitch to gain about 600 rpm. Raising the motor a hole will also gain rpm and speed until you reach the point where the prop ventilates.

The info I have on a Suzuki DT-115 is the WOT rpm range is 5250-5750 rpm using a 2.08:1 gear ratio.

 

[Crumpet46]

Re: Suzuki 115hp late 80s

The prop is only a 17 pitch prop as it is. So taking it down to 14 seems a huge change. Is there a chance the engine is sick which could stop it revving out? It runs smoothly other than being a pig to start first time in the day. After that it seem perfect.

Martyn

 

[steelespike]

Re: Suzuki 115hp late 80s

I agree I think a 17 is probably on the low side and it should rev easier.
We need your wot rpm and gps speed to evaluate your setup.
Some things to check Be sure the throttle is opening all the way.Does it have "whale tail","Doel fin" on the motor.
Are you adjusting the trim up for best performance?
Is it running on all cylinders? You wouldn't be the first to not realize you were missing a cylinder, except for the performance, even those with only 2 cylinder motors.
Is the motor in excellent tune?
Have you done a compression check?

 

[Crumpet46]

Re: Suzuki 115hp late 80s

I agree I think a 17 is probably on the low side and it should rev easier.
We need your wot rpm and gps speed to evaluate your setup.
Some things to check Be sure the throttle is opening all the way.Does it have "whale tail","Doel fin" on the motor.
Are you adjusting the trim up for best performance?
Is it running on all cylinders? You wouldn't be the first to not realize you were missing a cylinder, except for the performance, even those with only 2 cylinder motors.
Is the motor in excellent tune?
Have you done a compression check?

Yes it has a whale tail, and yes the trim is lifted up quite a bit to get it sitting nice and light in the water. I will take the boat on the water to give you a GPS speed on Saturday, and the WOT rpm is 4800rpm.

The engine seems to run fine, on all cylinders other than if its left at 6kts for a while where it will cough, but other than that it seems fine. Is there a 100% definite way of telling if its on all 4 cylinders? Ive raced 4cylinder bikes my whole life so should be fairly in tune but Im just using any suggestions at the moment.

Compression is 120 120 110 115.

I'll check the throttle is opening all the way. Would be a daft reason but would be a cheap fix!

Thanks for all the suggestions!

Martyn

 

[steelespike]

Re: Suzuki 115hp late 80s

If the whale tail is in the water at speed it could cause extra drag and may cause the boat to lay down flat handling oddly and
dragging too much boat through the water.At wot about 1/3 to 1/4 of the boat is in the water.
You need to be careful checking for a dead cylinder. Pull one plug wire at a time ground it and test run If no change then you have found the dead cylinder.Some ignition systems will self destruct if you don't ground the loose plug wire. You might be able to check the plugs for indications its not firing. You could get a spark checker at the auto store. Should jump 3/8" with a good blue snap.
There is a setting called link n sync It involves being sure the stator picks up the carb linkage at the right point as it advances.
Shift into forward and with the motor off advance the throttle and observe the mechanism.
You may have to rotate the prop to get it to slip into forward.
Check the bottom for a hook. Use a straight edge parallel to the keel. Your looking for a depression creating a space between the straight edge and the bottom. Usually just ahead of the transom.