Totally stumped on my 2002 Johnson 175 hp

[Olyfishin]

HELP!

I bought a new to me boat and was taking the motor out for a test... about the third time I'd had it out since I bought it. Ran about an hour this way and that way, and suddenly one of the six cylinders quits working. I motor back and think that I will start simple, so I change the spark plugs. Seems a good place to start, right?

Well, the next time out, it seems just fine. Yeah! I fixed it and it wasn't anything bad.

However, the next time out, I notice an irregular miss at high RPM and then turn around and head back and notice that it's running quite poorly by the time I get all the way back. I pull the plugs again and two of the plugs now have no gap in them. It's like the piston pushed it shut?

There isn't any vibration, the compression is fine. The ignition seems to be working just fine. What the heck can cause the spark plugs to do that, without having any other impact on the engine? I'm afraid to take it out again until I figure this out. I don't want to just "hope" it won't happen again, which is what my local mechanic suggested.

Thanks for any help!!!!

 

[boobie]

Re: Totally stumped on my 2002 Johnson 175 hp

What plugs are you using ??

 

[Olyfishin]

Re: Totally stumped on my 2002 Johnson 175 hp

What plugs are you using ??

Champion QL78YC

They are the recommended plug in the shop manual.

I didn't think I should change the kind of plug... seems like that's not fixing the underlying problem.

 

[multimech]

Re: Totally stumped on my 2002 Johnson 175 hp

If they are shut, something in the cylinder hit them, or you are looking at the wrong manual and got the reach wrong. Although the reach is pretty much the same for those type of engines. Lastly, maybe you dropped them and did not notice you had closed the gap when installing them or they could have been dropped prior to you getting them.

 

[seahorse5]

Re: Totally stumped on my 2002 Johnson 175 hp

The wrist pin holes in the pistons that hit the sparkplug are probably elongated allowing the piston to move up higher in the combustion chamber. That is often caused from detonation from a lower quantity and/or quality of engine oil at some time combined with lugging the engine from over-propping. Yamaha had that problem in the early '90's.

If it were a rod bearing, you would have the accompanying clunk while running.

 

[Joe Reeves]

Re: Totally stumped on my 2002 Johnson 175 hp

(Piston Play Check)
(J. Reeves)

This procedure will tell you if there is any wear or play in the Wrist Pin, Wrist Pin Bearing, or Connecting Rod to Crankshaft area.

Remove the spark plugs. Insert a screwdriver, dowel, punch, or some object of your choice through the s/plug hole and into the cylinder/piston area. Hold the tool lightly against the top of the piston. Turn the flywheel by hand, bringing the piston up to top dead center, then continue turning the flywheel to pull the piston down about a inch..... all the while holding the tool lightly against the piston top.

Now, at the point whereas the piston has been dropped down about a inch, grab the flywheel and hold it firmly. Now, push that tool solidly and firmly against the piston top.

If the piston moves without moving the flywheel, there is a problem (wear, play, clearance, etc) somewhere with the items mention in the first sentence. Be sure to check all of the pistons/cylinders.

 

[Olyfishin]

Re: Totally stumped on my 2002 Johnson 175 hp

Those are GREAT tips! thank you!!!

I know 100% that they are the right plugs (I just noticed there's also a sticker on the engine), and I'm also 100% that they were gapped exactly when I put them in. I'm not the best mechanic, but there are a few of things that I'm pretty anal about and gapping each plug out of the box so that it's correct is one of those simple things I can do well.

I will diagnose this a little bit further with your tips. I'm encouraged about it maybe NOT being a bearing. The fact is the engine seems real tight... sounds real good at high RPMs and has amazing power. I'm just nervous because it's new to me and it sat for a number of years before I got it... and I knew the lack of use could contribute to premature motor death.

I'm open to any and all suggestions.. thanks in advance!
Dave