Need Advice - Yamaha 2-Stroke Issue

[crump190]

My 1998 Yamaha SWSII 200hp runs fine, starts fine but seems a little rough at idle, and will stall from time to time. Idle will sometimes drop to around 500. New plugs this year and new fuel/water separator last week. Have not changed fuel filter (hard to get off, but looks clean when I shine a light behind it.) I do not run any additives in the gas. Any advice, would a decarb help?

Thanks

 

[Tuna Man]

Sounds like it may be a fuel problem to me. Perhaps the jets in the carbs are partially clogged and not alowing enough fuel to each cylinder? Have you cleaned the carbs recently? I doubt there is enough carbon in the cylinders to cause these symptoms but I guess you could check that by pulling the spark plugs and carefully looking at the top of the pistons (maybe a photo with a digital camera on super-macro would be better).

Hope this helps.

 

[seasick]

My 1998 Yamaha SWSII 200hp runs fine, starts fine but seems a little rough at idle, and will stall from time to time. Idle will sometimes drop to around 500. New plugs this year and new fuel/water separator last week. Have not changed fuel filter (hard to get off, but looks clean when I shine a light behind it.) I do not run any additives in the gas. Any advice, would a decarb help?

Thanks

Does it idle rough both when cold and when warmed up? If only when cold, choke is acting up probably.

The condition of the motor mounted fuel filter is tough to judge from the outside since the fuel enters through it's center. So if if clogs, it does from the middle out.

Low speed jets could be clogged or low pressure pumps could be bad. Decarb is worth a shot.

 

[no problem]

Had the same symptoms on a friend of mines Yamaha. Do an engine de-carb and use the shock treatment dose of ring free. Most auto parts stores have a carbon remover type product in a can that can be sprayed into the carbs when running. Then I spray it into the spark plug holes and let it soak. We got lots of black nasty stuff out the prop-hub exhaust. With ring free in the gas it runs better after every trip. Ring free seems pricey at first, until you see the results. good luck with it. :mrgreen:

 

[ocnslr]

Check the O2 sensor. You haven't mentioned it, so it is probably dirty and needs a cleaning. Or even a replacement.

No carbs on an OX66.

 

[uncljohn]

Check the O2 sensor. You haven't mentioned it, so it is probably dirty and needs a cleaning. Or even a replacement.

No carbs on an OX66.

What he said (both parts).....

 

[seasick]

Now that I read this again, I see that I have the same issue on a 2000 OX66. When cold, it starts right up, runs at 800 rpm, then over the next 2 minutes, the revs drop. When the revs lower to 500 or 600 the engine sputters, pops and stalls. I have removed, cleaned and tested the O2 sensor and it is fine. I have replaced all filters, cleaned the VST and checked the throttle adjustments. I tested both head mounted thermo switches and checked the continuity of the temperature switch. Both t-stats were changed and checked. Low pressure fuel pumps tested OK also.
So, I am flustered but what I do now is to start the motor. After 30 seconds, I hold the binnacle in neutral but raise the revs to about 900. After 3 minutes, everything is fine. Once warm, the motor runs like a champ and starts/idles normally.
My problem is not fuel I am certain. The only thing I can think of is that the ECU thinks the motor is warm before it really is and that would be the temp sensor that I tested. I didn't test its performance curve with hot water and a thermometer though. I will wait until fall layup and do so.
If you find out what your issue is, let us know.

 

[Walkers Edge]

The OP was referring to the carb'd version of the 1998 SWSII, I have the same engine. OX66 came later.
Coincidentally I had/have the same issue. I used "dunks" seafoam decarb method with good results, the idle improved and the engine runs smoother. With that being said the engine will still exhibit the same symptoms after prolonged idling sessions but returns to normal once I get the rpm's up such as when running fromm spot to spot. I plan on performing the decarb one more time and changing the plugs before I get into the carbs.

 

[seasick]

Re: Fixed - Yamaha 2-Stroke Issue

After trying everything I could think of that might be the cause for my rough idle and stalling when first started, I tried something simple that I didn't expect to work.
I started the motor cold and as usual it idled at about 900 and after a minute or so started to slow down. When it got to about 600 rpm, I adjusted the idle speed screw on the throttle body linkage to raise the rpms to 700. As it continued to warm, I continued to raise the idle to 700.
I expected that as it warmed up, the idle would now now be out of spec ( too high) since I had previously adjusted the warm idle to spec ( approx 730 rpm).To my amazement, the motor now starts cold and doesn't stall and it doesn't idle high when warm.
I don't understand why and still suspect that something is out of whack but heck it's better than it has been for almost a year.
During the winter layup, I will test out the throttle position sensor to see if it is adjusted properly.