yamaha 2 stroke 25hp questions (safe mode?)

[eight_weight]

Hello all

I just acquired a 2004 Yamaha 2 stroke 25 hp, model 25MSHC.

Motor is pushing a 1973 13' 4" Boston Whaler, weighs about 360# bare hull.

Had it out for the first time last week, motor runs strong for the most part.

motor is on a jack plate and while testing various heights and trim combo's, reached the point where prop was ventilating, motor reved up... cut back on throttle as soon as this would start, no more than a few seconds, and it was never up to WOT when this would happen. after a few episodes of this the motor went into what i assumed was "safe mode". RPM's limited... no problem limped back to the dock, already had what i thought was the best height/trim combo figured out. let it sit long enough to eat lunch and cool off.

Went back out, ran around for another hour and a half did not encounter this assumed "safe mode" issue again. GPS'd at 24-25 mph with 550 lbs of people and gear, 28 mph after i dropped off my father in law at the dock. Dont have RPM's yet, tiny tach is on its way...

did however experience a condition where sporadically while throttling up from idle, motor would bog at 1/2 throttle then "snap" up after a few seconds and take off. figure this is related to clogged carb jets at this point. have not broken them down yet to confirm. gas tank is clean and vent was open, fuel lines and connectors are new as is bulb, which stayed firm when this was happening (checked and it was still firm during the other issue above).

So, i get to looking into "Safe mode" and the particulars of this motor. apparently, the MSHC does not have a thermoswitch. boats.net parts diagram do not have one one the electrical diagram and neither does the service manual. There is no such device on the motor itself, pink wire and yellow/red wire coming from the DCI is not connected to anything... so there is nothing to tell the dci to go into "safe mode" as far as i can tell.

Is there some other way that a "safe mode" can triggered?
did i really enter a "safe mode"
What are the other possibilities that would explain the RPM limiting?

Can i install a thermoswitch and warning light/buzzer? the wiring is there and it looks like there is a spot above the upper spark plug for one.

the DCI unit is labeled 6L2-23 F8T20373 3Y26

any advice is appreciated, let me know if you need more info

thanks in advance

[99yam40]

I would look through the owners manual to see if that said anything about RPM limiting.

A timing light will show if spark is dropping and what the timing is doing when the RPMs drop. Only have 2 cylinders to monitor.

Check fuel pump to make sure it is not leaking into crank case if spark and timing are good while problem shows up

[eight_weight]

I would look through the owners manual to see if that said anything about RPM limiting.

I did. the manual states that it will decrease RPMs to ~2000 upon overheat. However my model does not have the thermoswitch that triggers this function. did not come from the factory with it, and it does not show up in the parts diagrams. does not have the warning light either.

there is a disclaimer at the front of the manual that says not all models covered will have all features listed in the manual...

A timing light will show if spark is dropping and what the timing is doing when the RPMs drop. Only have 2 cylinders to monitor.

Check fuel pump to make sure it is not leaking into crank case if spark and timing are good while problem shows up

Thanks. I will keep this in mind.

[eight_weight]

OK. some new info

Spent the evening reading over the service manual - LIT-18616-01-66

Got to chapter 3 periodic inspection and adjustment

Control system

Check full advanced ignition timing

Set flywheel at 25 BTDC, turned magneto control lever to full advanced stop, aaand... nope, timing indicator does not line up with mark on fly-wheel

marks line up at 19 BTDC


Full retard timing, marks line up with fly-wheel at 3 ATDC as opposed to 7 degrees

Throttle valves are closing evenly...

did not really attempt to examine pickup timing as full advance and retard are incorrect

also throttle roller is not aligned with the mark on the magneto lever arm, barely creeps up over the corner, well shy of the mark


DID NOT ADJUST anything...

Sooo... where to go from here. could this improper timing adjustment be the cause of the problems i experienced?

motor has been operated for probably 3-4 hours in this condition, any potential damage? other than the symptoms mentioned in first post, the engine sounded fine for whatever that is worth... cranked easy, idled fine maybe a little fast..

Next steps?

can try to get pics up of anthying that will help at all...

thanks in advance

[eight_weight]

another update

So a little back story on the motor.

I got it from my brothers father in law.

two years ago he brought to an authorized yamaha mechanic it in for a complete rebuild. He is a crawfisherman and the motor was not ready in time for the season so he just bought a new one. when he go the rebuilt motor back, he had my brother run it one time on his skiff, then it sat in his garage for 2 years before he sold it to me.

I have read that sometimes after a rebuild, timing will be retarded for a short break in period. any truth to that?

[eight_weight]

just an update in case anybody cares

this morning i reset the full advance, full retard and pickup timing, adjusted the throttle roller and the cables per service manual.

Installed tiny tach.

took the boat out this afternoon and gave it a work out for about an hour.

bogging upon throttle up in the mid range is gone. very smooth and responsive now. Engine sounds grrreat

did not get a repeat of the phantom overheat "safe mode"

Some numbers:

WOT - 5900 RPM - 27 MPH - this is with about 100 more lbs in the boat (and probably 100 lbs shy of what my heaviest load will be) and much choppier water than what i had for WOT speed in my first post... wish i would have had the tach for that but oh well... did not run light today but i am betting i would have got close to 30

Currently running a 10 1/4 diamater 12 pitch solas SS prop. It does blow out in hard turns...

The last 400-500 RPM only give me 1 MPH. not sure whats up with that.

[99yam40]

my thoughts are,
that motor Should only run 5500 WOT
5900 is too high and you have the wrong prop.
I could be wrong, you need to see what the WOT RPM is suppose to be in the manual

[eight_weight]

my thoughts are,
that motor Should only run 5500 WOT
5900 is too high and you have the wrong prop.
I could be wrong, you need to see what the WOT RPM is suppose to be in the manual

The owners manual and service manual say WOT range is 5000-6000 RPM

agree on the prop though and i am looking into my options.

Its just what came on the motor so that is where i am starting...now that the engine is set up correctly.

Thank you for your thoughts 99yam40

[Dashunde]

Your figures are showing about 16% prop slip.
Try lowering the motor just a bit more.

No idea how old that stainless of yours is, but I bought a modern aluminum Spitfire 4-blade 23p for my 200hp Merc that outperformed my older 3-blade ss and 5-blade ss in every way, including torque steer and holding in turns at all speeds.

[eight_weight]

It's actually kind of raggedy... Has a few nicks and I can see where it has been hammered out in a few spots. It's getting replaced soon and will serve as a back up, but its the only one I have right now.

I was going to jack it down a notch or two but I left the wrench in the truck... Lot of good it does there right...

It's got a permanent home in the bow locker now.

[Dashunde]

That being the case, on a new/modern prop you might see better hook-up, less slip, lower revs yet still get same or more speed.

The old ss looks like its allowing plenty of slip and might be running up against the rev limiter.

Ideally, you want as little slip as possible and have it get close to max revs only when your alone and empty.

But then again...
I did have a 4th prop for my Merc 200.. a big ear chopper... it was horrible out of the hole, but it hooked up solid on plane and turned in 4-5mph more on 500 rpm less rpm than the others, but the torque steer, prop walk, holding in turns... it was all a mess and not fun to drive.
It was good for only one thing... Cruising at 50mph covering long distances on glassy water with the steering friction knob turned tight.
It would do 65'ish, but it wore me out wrestling the squirreliness.
So getting top speed and top revs isnt everything...

For our larger I/O's I have always sought props that are good out of the hole, hold in turns and have minimal slip at cruise speeds (~75% throttle) resulting in more go on less fuel.

[99yam40]

The owners manual and service manual say WOT range is 5000-6000 RPM

agree on the prop though and i am looking into my options.

Its just what came on the motor so that is where i am starting...now that the engine is set up correctly.

Thank you for your thoughts 99yam40

If the manuals call for 6K on that motor then 5900 is good.
New prop in better shape may help

The old 35 Johnson I had did the same thing when the prop got chewed up, the last bit of throttle did not do much if any at all

On my 16' Xpress SV I am running at Yamaha C40TLRX with a SS 10 1/8" X 13 Ballistic.
I get 30 at 5500 , But will get to 5900 at wot, not sure of speed.
Too much slip I guess and too high of RPMs as it is rated for 5500,
but normally cruse at 4500 and Is OK for what I do.

Thought the prop being old and worn was the problem but the new ballistic 13 is getting the same. Not sure if I want to buy a 15 right now or wait and verify the tach readings some how

[Dashunde]

On my 16' Xpress SV I am running at Yamaha C40TLRX with a SS 10 1/8" X 13 Ballistic.
I get 30 at 5500 , But will get to 5900 at wot, not sure of speed.
Too much slip I guess and too high of RPMs as it is rated for 5500,
but normally cruse at 4500 and Is OK for what I do.

Thought the prop being old and worn was the problem but the new ballistic 13 is getting the same. Not sure if I want to buy a 15 right now or wait and verify the tach readings some how

Assuming your tach is somewhat accurate and your final drive is 1.85, those figures are showing a 18% slip.
Thats a fair amount of fuel getting spit out.
Not sure a 15 would do you any good, might slip even more, and be worse out of the hole?
Maybe go up a little in diameter if you have the clearance?
With a little wider bite it should slip less on the same 13p, and pull your revs down a little.
Might even have a better hole shot and turning hook-up as a bonus.

[99yam40]

Thanks for the info, something to look into and think about

[Dashunde]

Anyone pondering their prop options should check this out.