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8hp misses

M

mpeterson

New member
I just picked up a 2004 8hp Merc. It starts up and idles good. When run the Thorold up it starts missing. The gas is fresh and put in new plugs.
What could be wrong?
Thanks,
Mike
 
Two stroke or four stroke motor?

You are posting up a model year where both were produced. Could make a ton of difference to the diagnosis.
 
First thing I would look for is a cracked spark plug coil - that's what I would "hope for" since it is a potential cause and it's a relatively cheap fix. If you remove the top cowl you can usually hear a snapping sound if you have cracked coil - if it's a little dark out you can see the electricity arcing from the coil to the block (or wherever it's grounding out).

Second possible cause is a toasted stator - that is more costly and far more work to get at and replace.

The stator on this model has two windings (coils), one is the "low speed" winding - it provides ALL the power when the rpms are roughly 3000 rpms or LESS, and they provide "some" of the power above 3000 rpms - but not enough to sustain the ignition system all by themselves.

The second winding is the "high speed winding", below 3000 rpms it produces almost "no power" and does not really contribute to the ignition, but once the rpms increase above 3000 you absolutely need the high speed winding or you are going to have "insufficient power" to sustain the ignition.

And one of the ways this initially manifests is with a "high speed miss".

And while a bad spark plug coil "usually" would cause a miss at all rpms, it's still possible that it is the cause. But if I had to bet, I might tend to put my money on a toasted stator - the spark plug coils usually are fairly robust for a number of years and your model is a little younger than I typically see them cracked - but to be fair, I rarely see a bad stator on these small models.
 
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