I'm very happy to announce that I found what was causing my Yamaha TLR motor to drop RPM's after a few minutes of running. For those interested on the whole story please look on post named "Yamaha 90 TLR Drops RPM", dated 15 Dec 09.
Based on everybody's recommendation I went ahead and replaced the thermostat, impeller, spark plugs, and lower unit oil.
Guess what, the same problem remain when again I tested it in the water.
I was then convinced that it had to be related with the fuel. Off the water I ran the motor and found that one of the cylinder (bottom one) was not getting warm. Removing the spark plug cable from this plug showed no different motor behavior as it did with the other two, however there was excellent spark on all three.
I removed the plastic cover from the carb's and after revving the motor a couple of times I noticed there was no fuel being sprayed from the jet of the bottom carb.
Bingo! The bowl from that carburator was full of gum and the jet was clogged. Cleaned all carb's but noticed only the bottom carb was actually really bad. Note: I realized also you basically have to remove all three carb's to get just to one of them. There is a metal plate that has to be removed which all the carb's are screw to it.
Finally, tested the boat in the ocean and the baby ran beautiful. I immediately called my wife and informed her we had a good running boat and it would be like that for a while.
Thanks to everybody for the very informative assistance provided.
Last edited by VillaCocos; December 23rd, 2009 at 11:34 AM.
Reason: Word addition.
Thanks for sharing your solution to your problem. I run a 90HP also, so have a keen interest in this. I drain each carb several times per year which is a very quick and easy job on this motor once you do it the first time. Install fuel/water separator, use Stabil Marine and Ringfree all the time, no more fuel/carb problems! Good Luck!
The Yamaha Ringfree is a special fuel additive developed and approved by Yamaha/Chevron to prevent carbon deposits under and around the piston rings. You can buy it on-line, delivered for around $38/quart, but at the "deposit prevention" dosage of 1 ounce per 10 gallons of fuel, it's well worth the price. Good Luck!