1998 yamaha 115 hp false rpm causing rpm reducer to kick in?

[CHEWBAKA1973]

My 1998 115 hp engine is giving me an issue. My last outing i ran 35 miles and had zero issue. That was about 3 weeks ago. Today i put my boat in the water, engine started up no issues. Went on my way at slow rpm until out of the no wake zone area. Powered up the engine and ran fine. Was running it around 3500 rpms no issue but I did notice my tach spiking up a few times. The engine continued to run fine so i figure a bad connection or bad tach. Continued on my way and went a little faster to about 4000rpm. Tach spiked again off and on. Then all of a sudden the engine Boggs down. I ease back on throttle and wait a few mins at idle. Then try again. Ran fine up to 3500 rpms pushed it faster. Same thing.
So my question is, I know there is a rpm reducer but what could cause a false rpm reading and that reducer to kick in?
Or is this a completely different problem?
Also note that after i idle for a few mins I was able to run the engine at 3500 and at 27 mph to my fishing spot and back to the ramp. Only at higher rpms do i have this issue.
Thanks for the help in advance. And please let this not be a costly repair

[99yam40]

the bogging sounds like a fuel issue,
Monitoring fuel pressure/vacuum should help troubleshoot.
2 stroke motors do run a little higher RPM when they lean out.

Might be sucking air into fuel system at a connection also

[CHEWBAKA1973]

Perhaps it is a fuel issue but i don't think so. Can someone explain to me how the rpm reducer would behave? When is it supposed to kick in and what part controls the reducer function ?? When this happens the engine sounds like it running on half cylinders until i let it idle for a couple mins.
If its a fuel issue why are the rpm spiking on the tach but the engine sounds steady ??

[99yam40]

need to do a better job of explaining RPM spiking.

Not sure on that 115 , but some Yamaha's reduce RPM to about 2000 by retarding the timing when low on oil or overheat alarm comes in

[CHEWBAKA1973]

need to do a better job of explaining RPM spiking.

Not sure on that 115 , but some Yamaha's reduce RPM to about 2000 by retarding the timing when low on oil or overheat alarm comes in

Ok sorry. By spiking rpm's I mean that when the engine was running around 4000 RPMs and I was watching the tachometer it would spike up to nearly 5000 or more and that's when I thought the RPM reducer kicked in brought it down to about 2000 RPMs. I premix gas and oil So that shouldn't be a problem. Overheating I don't know? I could see a strong stream of water coming from the engine.

[99yam40]

posting your model # helps others know what motor you have
Is this a C115?
or did you chop up the oil injection?
test the overheat alarm to see if it sounds the buzzer
Do you have gauges that will show if an alarm comes in?
If so watch the gauge also while testing

Pull and test thermostats or replace if you have not lately
When was the last water pump replacement?

Might want to have a shop tach hooked up to make sure your tach is not the problem if motor sound is not changing but RPM is moving around

[CHEWBAKA1973]

posting your model # helps others know what motor you have
Is this a C115?
or did you chop up the oil injection?
test the overheat alarm to see if it sounds the buzzer
Do you have gauges that will show if an alarm comes in?
If so watch the gauge also while testing

Pull and test thermostats or replace if you have not lately
When was the last water pump replacement?

Might want to have a shop tach hooked up to make sure your tach is not the problem if motor sound is not changing but RPM is moving around

I believe its C115TLRW, i say i believe because the decals have faded beyond readable. I recently purchased this motor so i do not have much of history on it.
The previous owner told me that he ran it 50:1 oil mix. Not sure if it ever had oil injection. He changed the water pump last year and always did on a yearly basis.

I do not have gauges other than an aftermarket Telefex Tach.

How do you test the overheat alarm and where would the buzzer be located? I have a 704 control box and the standard Yamaha ignition.

How do you test the thermostats?

If it was only a faulty tach, why would, the what I believe is RPM reducer, kick in? I dismissed the tach jumping because the engine was running fine until it bogged down. Then after idling a few mins it seemed to run fine unless I pushed it close to 4000 rpms.

What can cause a faulty RPM reading? And if it is truly a faulty RPM what tells the engine to kick in the RPM reducer. Maybe I m chasing the wrong thing here by trying to figure out why the RPM reducer is at fault.
Would a “faulty” over RPM cause the reducer to kick in or is it only for overheating and low oil?

[ausnoelm]

forget the RPM reducer business, get it right out of your head, you did not tell us when the tach "spiked" did the motor sound different and speed up or slow down? or did the tach just read a "spike" and the motor sounded the same?

[CHEWBAKA1973]

forget the RPM reducer business, get it right out of your head, you did not tell us when the tach "spiked" did the motor sound different and speed up or slow down? or did the tach just read a "spike" and the motor sounded the same?

ok its out of my head, i dont know thats why im asking here because my lack of experience with yamaha engines.
i saw the tach spiked while the engine was running just fine below 4000 rpms. I dismissed it because the engine didnt slow or rev up or anything. When I sped up and reached around 4000 rpms is when the engine bogged down and i had to let it idle for a few mins to be able to get it back to running normal around 3500 rpms. while running below 4k around 3500 i could still see the tach spike occasionally but no effect on the engine or performance. Just before reaching my dock, i tried pushing the speed up to above 4k and the same thing happened. engine bogged down, ran rough, and i had to let it idle for a few mins for it to run smooth again.

[CHEWBAKA1973]

anyone have any ideas? or will i have to take my motor to the local mechanic?

[Seacraft12]

If it goes into safe mode, it will not operate beyond 2k rpm. This is do to either low oil or overheat. If it goes past 2k this is not the problem. Check connections to back of tach for tightness then clean the carbs. Might solve the problem, might not, but it's good to do anyway. I did it and I'm not mechanical at all. I have a 2001 115 TXRZ. Sounds like fuel to me.

[CHEWBAKA1973]

If it goes into safe mode, it will not operate beyond 2k rpm. This is do to either low oil or overheat. If it goes past 2k this is not the problem. Check connections to back of tach for tightness then clean the carbs. Might solve the problem, might not, but it's good to do anyway. I did it and I'm not mechanical at all. I have a 2001 115 TXRZ. Sounds like fuel to me.

so the only 2 reasons it would go into safe mode is low oil or over heat. Low oil isnt possible since i pre mix oil/fuel.

over heat is possible i guess but the water pump is pushing good water from the pee hole. So could a faulty thermostat trigger the safe mode ? any way to test the thermostats or should i just go replace them?
And if that is the problem, how do i explain the tach RPM spiking without the motor actually reeving up??

[Seacraft12]

If guess you have two problems. One being a faulty tach and the other being fuel. If your engine operates at anything above 2k while not running right then the limp mode is not an option. If you run at 4k than the engine bogs down and only operates to 2k than I would suspect high temp. But an engine in limp mode will not run past 2k. We're you running at 4k, than the engine bogged down and only operated normally at 2k or below? Or we're you running at 4k than the engine bogged down but operated normally at 2100 or above? My guess is carbs are dirty. Mine was acting similar. Carb cleaning did the trick

[CHEWBAKA1973]

If guess you have two problems. One being a faulty tach and the other being fuel. If your engine operates at anything above 2k while not running right then the limp mode is not an option. If you run at 4k than the engine bogs down and only operates to 2k than I would suspect high temp. But an engine in limp mode will not run past 2k. We're you running at 4k, than the engine bogged down and only operated normally at 2k or below? Or we're you running at 4k than the engine bogged down but operated normally at 2100 or above? My guess is carbs are dirty. Mine was acting similar. Carb cleaning did the trick

Great two problems. lucky me. The engine runs perfect under 4k. i push it over 4k and it seems to run in "limp" mode or i think its that. i let it idle for a few mins and it runs perfect unless i push it past 4k. when i push it past 4k, it doesnt act like a slow down because of fuel, its like an abrupt stop down to 2k. thats why i suspected some kind of mode kicking in but I dont know why it only does it above 4k. i can run the engine at 3500 all day and 27mph with no hesitation. does that answer your question?
when you say high temp, you mean that the thermostat(s) may be at fault or the engine is actually over heating?

[Seacraft12]

Yep, sounds like a temp issue. 4000 rpm sounds like the trigger point where I assume the temp exceeds parameters and it tries to save the engine from damage and drops it to 2000. I would replace thermostats as they are designed to open at a certain temp, maybe 4000 rpm it reaches that temp but fails to open some or at all. Good to do and fairly cheap and easy as well. Try to get a used tach or borrow one to see if it does the same thing as your current tach.