#2 Piston Gone, Lean or Detonation-Why?

[NJCinMN]

Background: 1969 merc 1000, running fine, new wires, plugs, rebuilt carbs, fuel pumps, lync and sync...waterskiing and developed lean cough. trying to diagnose so starting and shutting off, richened idol mix screws, started and clunk etc. only 3000 rpm wot. frustrated/knew it was bad & went home.

Finally got around to taking a look about 2 weeks after (avoiding the truth) and found problem quite easily. Pulled plugs and #2 is done. Grey mud on plug and melting in dead center of piston right where plug fires. Others are fine...why #2, lean or early detonation? Why would #2 lean out before #1? Why would #2 detonate?

 

[sschefer]

Re: #2 Piston Gone, Lean or Detonation-Why?

One of the pro's on these engines will probably nail it faster than me but my initial thought is bad reed(s). A bad reed that's stuck open will allow pressure to escape from the crankcase and blow fuel intended for the cylinder back out the carb. This is also referred to as spitting. You have three carbs each feeding two cylinders so a bad carb would effect both cylinders, not just one.

Your electrical system is pretty basic and I don't see it as a logical cause but I'll let the pro's decide that for you.

 

[Fuzzytbay]

Re: #2 Piston Gone, Lean or Detonation-Why?

Normally one would expect, #1 or # 3 to go. A long shot, but maybe 2's plug was gapped wrong, or part of the electrode flew off, hotter spark, weaker piston, previous lean conditions, already had melted the piston dome? A lot of maybes, thats for sure. I would think on the reed being bad, it would have affected #1 as well.
Its a shame. Are you going to try and rebuild it?

 

[sschefer]

Re: #2 Piston Gone, Lean or Detonation-Why?

I agree, both cylinders should have been affected and it's hard to say they weren't without pulling it down and having a look. It's gonna have to come apart anyway you look at it.

 

[Laddies]

Re: #2 Piston Gone, Lean or Detonation-Why?

When 2 cyls share a carb and the lower loses compression, the engine has swallowed something or broken a ring. Remove the intake port cover and look at the piston that should tell the tale.

 

[sschefer]

Re: #2 Piston Gone, Lean or Detonation-Why?

When 2 cyls share a carb and the lower loses compression, the engine has swallowed something or broken a ring. Remove the intake port cover and look at the piston that should tell the tale.

Absolutely Ladies, I kinda go for the "hey, it's gotta come apart so either get dirty or go Engine shopping".

 

[NJCinMN]

Re: #2 Piston Gone, Lean or Detonation-Why?

Thanks, for ideas. I guess it was just gonna happen. I think I'm going to do both "get dirty and go engine shopping" two is better than one right. If it's only rings and piston I think it's worth rebuilding. I can't think of anything I could've done to prevent this. Maybe overpropped for waterskiing but still had 4700-4800 rpm, low end of range and not ideal...but wouldn't guess it would melt down #2?? The broken ring theory seems quite possible because I had just done a compression check about a week before it happened (obsessed with tuning this thing) and all were fine so it had to be sudden. I'll let you know when I get around to opening it up.

 

[sschefer]

Re: #2 Piston Gone, Lean or Detonation-Why?

Thanks, for ideas. I guess it was just gonna happen. I think I'm going to do both "get dirty and go engine shopping" two is better than one right. If it's only rings and piston I think it's worth rebuilding. I can't think of anything I could've done to prevent this. Maybe overpropped for waterskiing but still had 4700-4800 rpm, low end of range and not ideal...but wouldn't guess it would melt down #2?? The broken ring theory seems quite possible because I had just done a compression check about a week before it happened (obsessed with tuning this thing) and all were fine so it had to be sudden. I'll let you know when I get around to opening it up.

Overpropped for water skiing? You should have under propped. Merc recommends dropping an inch for water skiing and then being carefull not to over rev when not pulling skiiers. That's straight from the owners manual. It's one of the reasons we have rev limiters now (well those of us who haven't removed them anyway).

 

[Fuzzytbay]

Re: #2 Piston Gone, Lean or Detonation-Why?

Rev limiters? I thought that was the banging and clunking sounds, just before they seize up
On a serious note, I hopes it not much.

 

[sschefer]

Re: #2 Piston Gone, Lean or Detonation-Why?

LOL... yep thats about it. If you can find one they aren't too bad. I think CDI has them.