40TLRD low RPM when warmed up

[CharlieB]

Hi, I have a 2005 40TLRD 2 stroke. It gets tuned in the off season with Yamaha parts & NGK Plugs. Last two years it ran great. The beginning of this year it ran great. I use ring free plus in my fuel with Marine Sea-Bil. Anyway the last month or so (July 2023) after the engine is warmed up, the RPM is limited to 2900 RPM exactly. So for the first 10-15 minutes, I can reach 5000RPM WOT or easily get to 3500- 4000RPM which is the nice cruising range. After that first 10-15 minutes the engine sputters a little then drops to 2900 PRM. It idols fine warm or cold.

The boat is a 13' Boston Whaler.

The things I will try:
Swap fuel tank with a little portable one with new gas - To eliminate the Yamaha water sep filter and the long hose run from the gas tank that sits under bow seat..
Swap the element filter on the engine - this has not been done in 2.5 years of ownership.

Any other thoughts on cause. I suppose the Carbs could be at fault but want to exhaust simpler items.

Thanks
Charlie B

[CharlieB]

One other important thing I forgot to mention regarding this engine. The idle throttle brings the engine up to full operating RPM at any time engine hot or cold.
THanks
Charlie B

[99yam40]

pull plugs to inspect them for anything different or strange.
run a compression test while the plugs are out

I would check to see if the timing is doing anything different when the problem shows up.
Timing light will show that along with if the plugs are all firing steady if you move the pick up to each plug wire.


fuel pump if leaking into #3 crankcase will make #3 overly rich.
make sure the prime start switch is in the normal/auto position.

I would expect the RPMs to be limited to around 2000 if overheat or low oil alarm were to come in, so I do not think that would be the problem. But make sure the alarm system works.

[99yam40]

One other important thing I forgot to mention regarding this engine. The idle throttle brings the engine up to full operating RPM at any time engine hot or cold.
THanks
Charlie B

if you are saying the fast idle lever will bring the motor RPMs up without any load on the motor,
that has no bearing on anything.

[panasonic]

Is your alarm working? Do you know how to test it?

[CharlieB]

Thanks for the reply. Oil Alarm works because it went off last year with low oil. How would I test to be sure it works?

I have noticed oil in the engine bottom cowl which drips into tho boat. I thought it mostly was due to the oil reservoir cap leaking while the engine was trimmed all the way up when not in use. But I did see oil in the bottom carb when I pulled off the intake.

[CharlieB]

OK Ive narrowed it down to Cylinder 3. By process of unplugging each plug individually, I found cylinder 1&2 to drop power level down to a max of about 1700 RPM. While unplugging 3 made no change to the current engine performance (Max PRM 2600). I then swapped out plug 3 for a new plug and same thing. All Disto wires were giving me shock so there is charge going to each plug. All plugs looked exactly the same which is balckend with a little oil. See photo

I suppose its either to much oil or to little gas. Next test will be carb 3 is getting gas.

[99yam40]

reread post #3
about the fuel pump
too much fuel or not enough will keep a cylinder from burning properly.

[ausnoelm]

Getting a shock doesn’t mean there is adequate spark, you need to actually test it, but……first check the fuel pump as mentioned, don’t stab all over the place, be methodical and be 100% certain of your tests, looks/sounds OK isn’t testing, it’s guessing.