Problems w/ Yamaha OX66 250HP

[TwoWisemen]

I've taken alot of information from this forum as I've spent the last year rebuilding my '76 23 CC. Now I have mechanical issues. I've been back on the water for two weeks now and I've noticed an engine irregularity that I have played with and it is 100% the same everytime.

I have a 1998 Yamaha OX66 250. Cranks & idles perfectly. I bump it into gear and it runs rough, like it's misfiring. I slowly give more throttle and it immediately clears up from 500 rpm to 1400. Then it begins to "misfire" again. As I slowly give more throttle, fuel consumption triples from 1400-1700+/- rpm as it feels rougher and rougher. Then all of a sudden, the motor shoots to 3200 rpm and runs smooth all the way to WOT. As I come back down off of the throttle, the smae thing happens in reverse. Nothing between 3200 and 16-1700 rpm and then it's rough back down to 1400 rpm. Also, the first time I took it out, WOT was 5800 rpm. The second and all times since, with less weight, WOT is 5100 rpm. O2 sensor? The motor has always been run w/ Ring Free or Sea Foam. Any help or ideas are much appreciated.

[BA17]

Sounds like the o2 sensor. Sometimes you can clean them up a bit, but once you have it out you may as well change it.

BA

[Old'sCool]

The big block Yam OX66's run on 6 cyl. at idle and above ~1900 rpm. Everywhere else it drops two and runs on 4 cyl. I would suspect the throttle position sensor from what you have described.

[eggsuckindog]

I kinda agree with the last guy, not familiar with that sensor, but sounds like its dropping and picking up cylinders though.

[TwoWisemen]

reelescape,

Is the throttle position sensor in the binnacle control or on the motor somewhere? I haven't gotten the complete mechanical manual for it yet...

[Old'sCool]

It's on the engine. Try asking this same question on TheHullTruth.com

SIM is the Yam guru and should chime in. After re-reading your question, you may have more than one problem but the TPSensor would be one of my first choices.

[Fishjack]

Having the same engine I have had a similar issue before and here are some of the items I checked/did to correct the problem -

1st - Like BA said check/clean the O2 sensor -

O2 Sensor Info

Might want to check the "spark plug caps" I had 2 of the 6 spark plug caps that were reading open - they are resistor caps and should read 4-6kohms - If you have problems with one or more I would suggest replacing them all with the NGK Spark Plug resistor cover - PN LD05F - 8060

NGK

They work well and are a LOT cheaper than the yamaha OEM caps. This cured a low speed (idle - midrange) miss I had for a while.

You could also have thermostat problems - I had one that was stuck open. If you have not checked or replaced them it is an easy project and they are pretty inexpensive.
Make sure and get the thermostat gaskets if you replace them.

I have never fooled with my TPS but that might be worth looking into as well as the tiny fuel filter in the fuel injection pump.

You can go here -

Yamaha message board

do a search using OX66 etc. - there is a ton of good advise on that site.

Obviously there are lots of things that you can check and could be causing the problem - please keep us posted on what you find.
Good luck.

[Capt_Mac]

If you were at all working on the motor, you may have a loose ground wire. I had a problem similiar to yours and it turned out to be a loose bolt holding several ground wires to the block located above #1 or # 2 piston(can't remember exactly). You may want to also check the compression in all cylinders, that could cause serious lose in rpm's or a lower idle such as 400 rpms.

[TwoWisemen]

A little update:
Compression on all cylinders is between 101 and 105 psi. Even though they are all w/in 10%, is this low?

I took the O2 sensor out, soaked it in seafoam up to the threads, scrubed it with a toothbrush, and blew it out with compressed air.

All new sparkplugs. Still runs rough until 3200 rpm. I think before I spend more replacing parts that may or may not be bad, I'm going to take it to the mechanic.

[Capt_Mac]

The compression is find, it could be your ecu. These motors also shut down 2 cylinders at low speeds so maybe the ecu isn't sending the right signal. I would recommend taking it to a yamaha certified mechanic, they'll most likely be able to solve the problem in minutes.