18hp johnson damaged spark plug

[nathangormley]

What could cause the gap on my plug to go from .040 to zero after running?

[okelley77]

Play in the needle bearings on the connecting rod, letting the piston travel to far up the cylinder wall.

[lindy46]

Or wrong plug with too deep a reach.

[1946Zephyr]

Yes, I second that. If you use Champion J6C's then you won't have that problem.

[boobie]

What plugs are in it now? If you have J6C's, you have some internal probs.

[Cofe]

If the plugs are J6c's you might have excessive carbon built up on the piston. You can remove the spark plug and turn the motor over by hand, and use a flashlight to look at the top of the piston when it is on top dead center.

[F_R]

If you have the correct plugs, DO NOT run that motor till you find the cause of the problem. It probably is about to scatter its internals all over the place.

[wifisher]

If the plugs are J6c's you might have excessive carbon built up on the piston. You can remove the spark plug and turn the motor over by hand, and use a flashlight to look at the top of the piston when it is on top dead center.

Have you ever had this happen? While the engine is running, carbon will not gather in areas with interference. It will find a different area to grab onto.

[Cofe]

Have you ever had this happen? While the engine is running, carbon will not gather in areas with interference. It will find a different area to grab onto.

I have seen it happen on a motor with weak/intermittent spark on one cylinder.

[nathangormley]

Thanks for all the advice so far. I will check to see if they were the correct spark plugs after work/school. Lets assume for now they are the correct plugs and there is no way the gap could be flattened by over tightening.... What should start doing next to fix this problem? I will look at the top of the piston with a flashlight and post what I see tomorrow. Thanks in advance.

[F_R]

Well ok, getting ahead of ourselves, If it has the correct spark plugs, then stick a screwdriver into the plug hole and slowly turn the flywheel, following the piston up and down with the screwdriver. Just follow it, don't push.

When the piston is about half way down on the downstroke, stop turning. Then give a light push on the screwdriver. If it moves with any appreciable amount, and gives a clunk, you have serious trouble inside. In that case, the powerhead has to come apart for inspection and repair. There is no other way.

[nathangormley]

So I checked when I got home last night and sure enough....wrong plugs. I appreciate all the advice even assuming that it may have been the correct plug. I did not have time to do any further inspection so now the question is if there has been any damage done to the piston?

[kbait]

I've seen the same thing happen w/no piston damage. Motor ran fine w/correct, shorter plugs.

[kfa4303]

you should be fine. no major damage done.

[boobie]

Let us know how it runs with the correct plugs in it. By the way, what plugs did you have ??

[1946Zephyr]

I had this issue with the NJK's, but they soon fixed the issue by re-designing the plugs with a shorter thread shank.
If you were able to pull start the motor, before the pistons slammed the gaps shut, then you won't have to worry about damage to the pistons. Fortunately, they only hit hard enough to close the gap.

[nathangormley]

Let us know how it runs with the correct plugs in it. By the way, what plugs did you have ??

The plugs that were in it were champion 828. Which the champion JC6 is also CHA 823. Or atleast thats what the guy at O'rielys said.

[wifisher]

Never trust a saleaman's cross reference.

Just sayin'

[boobie]

I don't trust salesmen, period. Anyway, did you ever take the J6C and measure the length of it against the plugs that were in it???

[Steve A W]

You might want to do a compression check.

Steve A W

[nathangormley]

I have not had an opportunity to compare the correct plugs with the bad ones yet. Been way to swamped with work and night classes. I plan to spend some time on it tomorrow. When I took the bad plugs out I just put my thumb over the opening and and turned the flywheel and there was plenty of suction. I'm sure I would need some special tool to do a real compression check. This motor had been setting for two years when I bought it so yesterday I went ahead and ordered the carb rebuild kit, and Clymer manual. But I just read something that said the Clymer manual's aren't that good......

[kfa4303]

maybe not the greatest mauals, but they can walk you through the basic stuff. Check out this link too. It gives some great exploded diagrams and parts number info. Just enter your model year and hp rating.


http://www.marineengine.com/parts/parts.php#manuals