Tohatsu M115A issues

I have a 115 hp Tohatsu outboard for a pontoon that is giving me an audible alarm at or around 2500-2600 rpms. I get no other alarm on my gauges for low oil or temp. I have installed correctly a new impeller kit and get good flow from the telltale. Oil pressure/pump is good also. Checked and cleaned the thermostat (opens fully at 150 degrees). Could this be nothing more than a sensor going bad? I have yet to diagnose sensor problems but will try that next. Hoping someone could give me a little more guidance to what I'm overlooking!!

That RPM is where either cooling flow (not temp) or low oil level in the tank can trigger an alarm. If it's due to water flow, no lamps will light. You can DL your owner's manual (with schematic to check sensors) at http://www.tohatsu.com/tech_info/own_man_pdfs/Toh115A.pdf

Thanks for your time in responding. I did get your link for the manual. I am fortunate to also have a service manual in my arsennal. I took what you stated and spent the better part of this morning back flushing the cooling paths. Re-assembled everything and took it out for test & tune. Unfortunately, that didn't solve it. I did manage to get 100 more rpms out of my work before it alarmed @ 2600. I can rule out your suggestion on the oil sensor. It's at recommended fill. Here's a better description on what I'm experiencing: Out of the dock, no wake speed (700-900 rpms) till motor comes up to temp. Ease into throttle to around 1200 rpms. My test & tune was going into full throttle from previous rpms till vessel planes out. When I reach 2600-2700 rpms now, motor shakes and sounds if it is starving for fuel and will not run efficient in that range. Also thats when I get the steady alarm (with no other lamps/alarms). Ease back into "paw-paw" mode and it's smooth. I failed to mention before that I increased my fuel line from a remote 5/16" (primer bulb) to a 3/8" line with an in-line fuel filter. Should I have stayed with the smaller fuel line? Read this in some of the other threads that you responded to. You seem to be the go-to guy. Thanks for sharing your knowledge with us!!

Did you have the problems before changing the gas line? 3/8 line is fine, but your filter could have too much restriction, or you may have an air (vacuum) leak in the hoses and clamps or at the quick connectors. Either could make you run too lean at higher speeds.

Likewise, if the ESG is faulty, engaging at too low an RPM, that would cause staggered ignition, which could present symptoms such as shaking, etc. The ESG is built-in to the CD, but IIRC, it can be disabled by disconnecting a pair of wires at the CD. That technique is typically only for testing, as you do want the rev limiter to work.

You could also have a bad prop, with a damaged hub, causing odd behavior... but that usually will not limit RPM -- usually the opposite -- RPM rises, but boat speed doesn't.

Did you run the electrical tests (in the Factory service manual) on the oil and water sensors? Can you go to more than the stuttering RPM, or is that the limit?

Good morning. I'm going to put my nose in my service manual and test the electrical this morning, then after that I will determine if I do have a vacuum leak in my fuel lines.

I inherited the boat from my father in law and it had set for about three years. When the last time i ran, it ran fine and strong. The only issue we had was it never got "blown" out from time to time.