piston damage

[bugmud777]

Engine is 35 horse Johnson mod# J35ELCNC. Just had a new head gasket replaced. Tech told him it might have ignition problems he wasn't sure.<br />Owner took it out on the water about 200 yards into bay when it started running rough. Pulled the head off and found the top piston had a hole blown thru under the spark plug. What might cause this. <br />Enjoy "da Mud Bugs"

 

[DJ]

Re: piston damage

bugmud,<br /><br />About the only thing that will cause a piston to "hole" is detenation. This could be caused by two things.<br /><br />1. Lean fuel mixture to that cylinder.<br /><br />2. Improper timing.

 

[Waterbugtoo]

Re: piston damage

Power pack could also be "double firing".

 

[bugmud777]

Re: piston damage

Most of my exper. is in small engine and aircraft. These would be the same reasons they would have suffered detonation also which confirms my suspicion. <br /><br /> When I pulled the flywheel the top of the crankshaft looked like it had been polished. I had to look real hard to find the key slot. The remaining key material in the flywheel was in my opinion, not even a quarter of that which should be and there was no debris. If I understand right this is the only timming there is, as the rest of the system is electronic. Is this correct? and how would one check the fuel system?<br />Explain please "double firing"<br />Thanks for your help<br />Bill

 

[DJ]

Re: piston damage

bugmud,<br /><br />Flywheel key shearing is not uncommon. Yes, that will throw your timing way off, probably advanced.<br /><br />Double Firing is when the spark plug fires twice for every revolution of the crank, instead of once.<br /><br />I believe you have a single carb. They ususally feed both cylinders equally. About the only thing that will cause one cyl. to go lean is a bad reed block. If that is the case, you will see other signs of a lean condition in that cyl.-such as: ring sticking, cyl. wall scoring.<br /><br />I think you found your problem, with the flywheel.

 

[bugmud777]

Re: piston damage

djohns19<br /><br />Yes it does have a single carb. I saw no problems with the reed block. However there was one broken wire for the ignition module. As far as the cylinder walls, they are in great shape. Good ol' "Detriot Iron". For the pistons, the one as I said has the hole in the middle with most of the slag staying on top around the hole.<br /><br />Both pistons however have a serious "BAD" case of ring stick. Also, have you heard anything about Johnson doubling the head gaskets to account for the cheaper fuels now used? (heard that somewhere)<br />Once again your help is greatly appreciated.<br /><br />Bill

 

[Hooty]

Re: piston damage

If the flywheel moved on the crankshaft causing one piston to hole, I'd expect some damage to the other piston . They're both gonna be out of time. The only thing the flywheel key does is keep it aligned with the crankshaft while the nut is tightened. The taper is what keeps the two of them together. When replacing the headgasket,use the skinney one if two are offered. DO NOT use two gaskets.<br /><br />c/6<br /><br />Hooty

 

[BillP]

Re: piston damage

It isn't always timing that burns holes in pistons. A cold running engine can let carbon crap build up on the piston crown. The build up causes a hot spot which can weaken and eventually blow or burn a hole in the crown. Make sure you decarbonize and make sure the t-stat is operating correctly. This happened to me on a 140 Johnson.<br /><br />Johnson commercial outboards had a different cyl head that lowers compression. This lets them run down to 67 octane. If there was a commercial version of your engine one of those heads will fit. A 35 (domestic) like yours would probably be sold as a 30 commercial. They lose hp with the low compression head (70hp domestic = 65hp commercial). The head looks identical to a domestic head but is thicker.

 

[bugmud777]

Re: piston damage

No the other piston looks good. I'll check the thermostat. As far as hot spots, there could have been, as he just had the head gasket changed so I've no idea what it looked like before. There was one thing, spark plugs were mentioned earlier along with a cold running engine. My book calls for a Champ QL77J4, I found Champ L77JC4's installed. As I am not that versed in the mystical theory of plug interchange, is this acceptable?<br /><br />Once again thanks all!!!<br /><br />Bill