[mtcamp]

2004 Yamaha, 8 hp. 2 cycle.I run this motor about 6 to 8 hours at low speed around 1200 rpm. And the longer it runs, the worse it runs. The top cylinder seems to misfire about every 5th or 6th stroke and is most noticeable at idle speeds. Trouble shooting: Tested by the dealer and myself numerous times at about 1500 rpm with engine cold or hot. - When the top cylinder plug wire is removed the motor will run fairly smooth indefinitely on just the lower cylinder. But when the lower cylinder plug wire is removed the top cylinder runs very rough, hits about 5 to 6 times, and dies every time. I can reverse the plug wires on this motor because it fires at the top and bottom of each stroke. This does not change any thing as the top cylinder always runs bad.A local dealer found the same situation with a brand new, out of the box, 2005.Yamaha said that it uses a Siamese” ignition and that’s the way it runs. Sounds like a fish story.

[Grant S]

If it is still under guarantee it is Yamaha's problem. I have an older model of these and they are usually great motors, look at how few postings there are about them.Have you tried swapping the plug as well as the leads?(had to ask).Given that you seem to have eliminated ignition problems on that cylinder , it must be something else, inlet or exhaust tract problems/blockages perhaps. Did the local dealer say what the problem was , on the one out of the box? 2005?-are you a time traveller?A siamese ignition just means that they are paired and perform together, like you stated.