Yamaha 250 OX66 1999 - WOT loss of RPM

[funtastic]

I picked up a new used Pursuit 2470 with a Yamaha 250 OX66 99'. The engine SN# is 61AX102114U with 250 hours. Owner used Ring free and Yamalube which I too have used for 1st season. Put 60 hours on engine last year. ISSUE - At times I experienced a loss of RPM when I went WOT quickly....esp, when the engine was not running for a long time previously. Normal RPM is about 5600 RPM. When you Hole-shot or jump on trottle from low RPM..it hits about 5300 ..and then cuts down to 4200 to 4500 and seems to alternate. I can then back off throtle for a few secs and then "slowly" go up to WOT and it will run at 5600 for hours. I have never had any other RPM issues...rough seas etc. My secret was a "slow" transition to WOT which works everytime.
Also, I have noticed that at lower RPM & Idle that the engine seems "smokey" (from what I've read) a little more than what it should? I've adjusted the Oil Linkage rod ....but still need to cutoff/shorten steel rod to fully adjust to recommendations.

 

[cousinabe]

Re: Yamaha 250 OX66 1999 - WOT loss of RPM

as a start; clean O2 sensor and draw tube. clean VST.
your problem sounds like a fuel restriction issue.

There are many posts here on the OX66 motors regarding maint items.

What maint items have been done to the motor so far? what is the actual model number?

here's a good thread on maint items for OX66.
http://forums.iboats.com/showthread.php?t=289856

 

[funtastic]

Re: Yamaha 250 OX66 1999 - WOT loss of RPM

The engine is S250TXRX build 4/98, but a 99. Used it 1 season 60 hours and know the following. Spark plugs, Fuel Filter<external> changes, Water Pump changes 2 years ago, Compression check = no more than 5psi diff between all 6 cyls. Adjusted Oil Linakge, ordered new one as cracked. Spring 2010 - Will repalce all 3 LP pumps, will test Spark Plug wires, check O2 sensor voltage <.3- .6> and remove check value between Engine Fuel filter and VST. Have seen zero crud or water on any of system. Use 100% of time Ring free, Yamalube and StarTron treatment.(TIP - Use the Diesel version as the only difference is Diesel is 2times more concentrated. This was confirmed by employee at Startron). Trying to find how to know if the O2 Draw Tube in this engine had a update to new tube??? I will Clean it if it a new version, to be sure. Many thx to other "Key" suggestions I SHOULD be addressing for a 10 + year old engine.

 

[rodbolt]

Re: Yamaha 250 OX66 1999 - WOT loss of RPM

by looking at the engine you cant tell which block you have,only by removing the O2 sensor holder and looking.
look at your dealer for bulliten number O2002-007.
it explains it.
if your tube was updated there is a bulliten on that as well but I dont see it offhand.