Yamaha 15hp 2 stroke issue

HI All,

My friends Yamaha 15hp 2 stroke is doing funny things. It runs fine but keeps dropping revs for no reason. It will run for 10 minutes at full revs and then drop to about half power for another 10 minutes then run fine for 5 minutes then drop for 5 etc. It never stops even though it drops revs.

Checked all the throttle cables etc and they all are fine. If it was a 4 stroke I would say it was dropping into safe mode but its a 2 stroke. Fuel has been checked for dirt etc and all is good there. Anyone got any ideas?


Many thanks


Caz

Sounds like crud in the carbs, a blocked jet or suchlike.
It may need a stripping and cleaning....

could even be water in the fuel mix happened to me twice caused by leaving fuel in metal tanks overwinter when condensation get mixed in with the fuel ?
mickser

Thanks for the replies lads. Will let you know if we solve it.

Caz

Is there an inline fuel filter between the fuel tank and carburettor? If there is, it may need cleaning or replacing.

If that's a twin cylinder my money is on one of the plugs not firing.
If he hasn't changed them this year then do it. Our cars are so good on plugs we forget that this engine technology is older and not so forgiving.
Is the oil mix right. Aren't these at 100:1 and how is it being measured.
Is he draining the carb(s) after every use. letting fuel dry out in there just gums things up.
A dose of Redex every now and then would help.
As said filter and every season completely empty tanks and jerricans and dry them out.
Water is everywhere and you have to stay on top of it to stop it accumulating in the system.

Get this sorted yet? I had a similar problem with a big Tohatsu 2-stroke and it turned out to be something really simple - the air bleed on the fuel tankk was stuck shut even when it was screwed wide open. Just slackening the filler cap a tiny bit got the revs back to normal and a new cap/air bleed cured it.

If you are still stuck on this then give John Flood a call in Omagh, they are Yamaha experts:
https://www.floodmarine.co.uk

Couldn't recommend John enough. The last time I had a problem with an outboard I gave him a call, he couldn't do enough to help me - he talked thought the problem exactly and came up with a solution, then sent out a small part to fix it. great job

If that's a twin cylinder my money is on one of the plugs not firing.

Had a similar issue with an older 30Hp Yamaha. Wasn't the plugs, It was the coil pack that supplies the spark. It was starting to act up on one of the plug, so the engine sounded ok but exactly like this, was loosing revs, and felt like it was struggling for power.