Yamaha misfire when it warms up?

[Blahst]

My 220 Yamaha misses above (roughly) 3000 rpm and surges up and down to 4000rpm or so, but will rev either flat out or just idle.
What makes it a bit tricky is it runs fine when its cold but get progressively worse as the engine warms up.
Could it be that the coils are breaking down as they get hot, or could it be a stator?
Fuel supply etc. and reeds are all fine.

[Ray Neudecker]

Check the plug caps. They should ohm about 5K if they are the gold ones.

[Blahst]

Thanks Ray

[Blahst]

Seems like you shunted me into the right direction to look for the misfire.
I pulled off the spark plug caps and measured the resistance and got the following:
Top Left: 6.81k Top Right: 6.19k
Middle Left: 5.15k Middle Right: 7.15k
Bottom Left; 6.39k Bottom Right: 8.25k

From what you said and what I've found in previous posts the maximum resistance of the plug cap should be 5-5.25k - obviously - they ain't looking good.

I also measured the resistance through the spark leads back through the coil to earth (I didn't cut back any of the lead wire - didn't have time)
Top Left: 2.5k top right: 2.37k
Middle Left: 2.5k Middle Right: 2.34k
Bottom Left: 2.38k Bottom Right: 3.33k

What should the resistance be back up the cable and back through the coil?

After sussing out replacement 5k ohm spark plug caps - NGK seem to make various types but boat shops seem to charge around AUD$20 here - US$15.40.
But if you go to a motorbike shop - they've got nearly the same 5k ohm NGK spark plug cap for AUD$7.50 and it still has the rubber boots to seal the cap. I think I'll go that option and see how it goes. Is there anything I might be missing?

Thanks for your help.

[Blahst]

An interesting tid-bit, the motorbike tuner that I was speaking to said that it was relatively common for the resistor in the spark plug cap to get a bit old and begin to fail in addition to the spark plug lead at the spark plug end to begin to "rot" as he termed it.

He reckoned that if the cable had already been cut back too many times or was "rotton" too far back in the cable, that TIG wire could be fed back into the spark plug lead and it did the same job as a replacement lead.
I guess it'd do the job if you were desperate to get out on the water...