2005 Mercury 8hp long shaft

Hello all,

I have a 2005 Mercury 8hp 4 stroke long shaft that I have been having a problem with. The motor's upper spark plug fouls after 8-10 hrs of trolling. I have never had a problem with the lower plug but I seem to replace the upper plug quite often. It is fouling with gas not oil, runs up to full throttle no problem but at idle it will start running rough and sooner or later it will start to die all the time. I have had the motor looked at and everthing seems to check out fine and they can't find anything but this can't be a normal problem to have. Has anyone ever had this problem or could shed some light on what to check? The motor always starts the first or second pull everytime and the rough idle problem seems to start when it is warmed up.

Thanks,

Those motors usually just chug beautifully at low RPM for days.
Using a good mix, I am sure....no ethanol.

Since it happens after warm up, makes me think that it could be thermal expansion working its evil ways.
Been checked for running at proper temp ?

With a digital ohm meter ($12 @ Sears) you can check the resistance in your plug wires to verify that they are good.
Check the lower one (known good) for a baseline.
The top should read similar if they are of similar length.
Put the wires into different positions and check.

I have not checked running temps. I did check the resistance on the coil and it showed .03 on the meter. I was read .02 through .04 is good. How do you test the plug wires on these coils? It looks like they are a seal unit and the wires are not removable from the coil side. I can't get any secondary readings at all. thanks for the help.

ckkoch09 said:

I have not checked running temps. I did check the resistance on the coil and it showed 3000 on the meter. I was told 3000-8000 is good. How do you test the plug wires on these coils? It looks like they are a seal unit and the wires are not removable from the coil side. Thanks for the help.

Click to expand...

Hmm....sounds like the coil and wires are replaced as a unit, therefore no way to test individually.

I read a page that told me how to test the secondary resistance through the coil but I can't get a reading. It most not have been for my coil.