Broken ring lodged in exhaust?

[SML Boomer]

When a broken ring and piston was replaced in my 1988 100 HP Evinrude the pieces were not found in the cylinder or powerhead. A 1" piece of the piston between the two rings broke off as well as part of the lower ring. I heard it happen at idle and cut it off immediately. The engine was only spun after that to check compression for diagnosis. There was very little scratching by the exhaust port. The engine was repaired but I am down 400-500 rps at WOT. Also the plugs stay carboned up. We have checked almost everything. Is it possible that those parts blew out the exhaust port and are lodged somewhere causing backpressure at higher RPM's? If so, where would the restriction be to catch parts that small?

 

[Dhadley]

Re: Broken ring lodged in exhaust?

Not likely the pieces are there now. I'd start at the beginning to look for the cause. Basic compression test first. Did you bore the block and install new pistons?

What's the timing set at? What plugs are you using? Carbs clean? Cooling system all there and working properly?

 

[SML Boomer]

Re: Broken ring lodged in exhaust?

Thanks for quick response. Block was checked at marine machine shop and only needed honed. Piston failure looked like a bad casting with slag line in break. With my old non screw in guage I now get readings of 115 on three and 105 on the repaired cylinder. ( Marina screw in guage showed 125 on other three when diagnosed so my guage is probably reading low. )Don't know if compression will rise when ring wears in on new piston. Link and sync good, timing set at 28. New water pump with 14 lbs pressure at wot. 50/1 gas mix 87 octane. I get a stator ohm reading of 450 across the brown, brown/yellow wire. One manual said should be 550-705. Post on here said 500. If I am reading it right it looks like about 180 volts from stator to rectifier at 2,000 rpm's. Don't know if any of this is causing weak spark. I will pick up spark tester this afternoon. I dropped a prop pitch which had little effect. I have read enough here to know that I don't want to lug the engine at 4900 rpm's.

 

[Dhadley]

Re: Broken ring lodged in exhaust?

What spark plugs are you using? When you say they stay carboned up what do you mean? How does the motor idle?

 

[SML Boomer]

Re: Broken ring lodged in exhaust?

Installed new QL77JC4 gapped at .030 this week. Idles okay down to about 800. Still a little rough and tach bounces just a little, maybe 50 rpms. Pulling plug wires has the same effect on all four, rougher and quits. Old JC4's were tried gapped at .035. The plug color is black with crusting on the plugs at the end of threads. It will scrape off and may be slightly oily. It is not powder dry.

 

[Dhadley]

Re: Broken ring lodged in exhaust?

Good choice on plugs. I have a hard time believing that a 20 year old motor didn't need boring. Especially after it was honed (accelerated wear). The compression numbers kind of indicate that. Seems logical that they honed the bad one a bit more and hence the lower compression. Of all four it should have the highest compression, if everything was in spec, since it seems 3 used pistons were reinstalled and probably one new one in the bad cylinder.

But lets start with a silly question - are you sure the pistons are in correctly?

 

[SML Boomer]

Re: Broken ring lodged in exhaust?

Motor was rebuilt May of 2007 so the other three pistons and rings are fairly new. The block was sent to a popular machine shop in Tenn. that does nothing but marine engines. They determined that it needed no boring and should go back standard. It ran like a dream until the piston failed. The local mechanic who did the rebuild also did the repair so I would like to think that the pistons are in right. My question to him was if the damaged cylinder may have been out of round. He could not confirm that it had been checked, only honed. I'm not satisfied with the repair but don't want to be unreasonable with the mechanic either. He is getting more difficult to contact so that is why I have been scouring this board. A shop in the next town has an experienced certified OMC tech with a water brake. That probably should be my next step. I really appreciate your comments.

 

[SML Boomer]

Re: Broken ring lodged in exhaust?

Spark looked good at 7/16, blue with a good crack. I re did the link and sync. There is not a timer base stop screw with a rubber cap on mine that I can see, just a rod from the timer base to the throttle assembly. There is an eccentric bolt/roller that rides in a slot in the throttle assembly to advance the timing. The rubber coating has a flat on one side. How is the eccentric roller supposed to be positioned? Also is it supposed to roll? Mine does not. With the worn roller and the slack in both ends of the rod, there is a good bit of play ( maybe 1/8" ). Is that normal?