What Went Wrong

[Polara Pat]

Alright, I'm past the point of anger and frustration and I've safely moved onto acceptance of my recent catastrophic (well not entirely) failure.

Here's the jist of my situation.
I purchased a '93 Merc 40 a few months back to replace my 55 OMC electric that was giving me problems and folks here generally warned me to stay away from. The guy I bought it from replaced a bad piston and new rings on that hole but didn't re-ring the rest of the pistons, he also added a new control box. We did a test fire on the muffs and it fired right up and gave just shy of 100 psi on all of the holes. I know it's a bit low but it's a used motor and everything was very even. I hooked it up to my boat as-is and went for a spin. First thing I noticed was that it was hard to start in the water, so with any back pressure. Then once I got out in the water it seemed weak on power, even for a 40. We limped it back to the dock and I started tuning a bit. First off, the timing tab that's bolted to the flywheel was clocked almost 180* out so I fixed that but it probably ran the first couple of times severely out of time. After that I did the usual carb clean and set, link and synch but it never really ran right again. This is an oil injected motor with the tank bolted onto the front of the block and there was a theory that it was over oiling. I've since eliminated this system in favor of mixing my own.
The other day I had a local OB guru to come over and try to help me and we found #2 hole to be around 35# Booo. We didn't check 3 and 4. I pulled the power head and through the exhaust cover(?) I could get a pretty good look inside the cylinders and nothing seemed particularly scored but #4 had a broken ring and what looked like molten aluminum welded to the top of the piston.
I brought the power head back to the guy I bought it from and he may or may not help me out. Safe to say I got screwed. I just really want to know where I went wrong so this doesn't happen again and maybe other folks can learn from my mistakes.

Sorry about the small novel, this is actually the shortened version.

 

[CharlieB]

Re: What Went Wrong

You didn't mention which cyl has the new piston. A new piston should have the fuel mixture doubled and break-in procedures followed.

Incorrect ignition timing, either too advanced or too retarded can cause heating and damage.

Idle mixture was too lean, has to be far richer than best idle on the hose as once launched the exhaust back pressure in the water changes everything.

Hope the PO decides to help, either way repost your progress as the motor must be dialed in and broken in correctly to remain a reliable performer.

 

[Polara Pat]

Re: What Went Wrong

You didn't mention which cyl has the new piston. A new piston should have the fuel mixture doubled and break-in procedures followed.

Incorrect ignition timing, either too advanced or too retarded can cause heating and damage.

Idle mixture was too lean, has to be far richer than best idle on the hose as once launched the exhaust back pressure in the water changes everything.

Hope the PO decides to help, either way repost your progress as the motor must be dialed in and broken in correctly to remain a reliable performer.

Interesting.

I doubt a proper break-in was done after the piston replacement and I'm fairly sure that the one replaced was #4. AKA the one that failed.

 

[Dukedog]

Re: What Went Wrong

Was tha repaired hole bored or did he just clean it up from previous damage and stick a new piston in it? That's what it sounds like ta me.......

 

[Polara Pat]

Re: What Went Wrong

Was tha repaired hole bored or did he just clean it up from previous damage and stick a new piston in it? That's what it sounds like ta me.......

I'm feeling like he maybe installed the rings incorrectly. He mentioned that he was having troubles installing them and maybe broke one so he is obviously trying to spiral them on. Now that I think about it I should probably just repo that powerhead from him and do it myself. I was just so pissed off at the time that I told him to fix it since he sold me a lemon.

I've also been offered an earlier 40hp Merc twin powerhead but I wasn't sure about the interchange with my leg.

 

[CharlieB]

Re: What Went Wrong

Re-reading the thread I see I failed to ask WHY the first piston failed?

And was that cause corrected?

#4 doesn't easily 'just' fail, there must be a cause and it usually is NOT a cooling issue as the top cyl overheats first.

Which leaves a fueling problem. a dirty carb leaning the mixture and melting the piston.

If the carbs have NOT been cleaned and the floats set properly it iss doomed before you mounted the motor on your boat.

Clean any aluminum off the cyl walls with muratic acid, careful not to spill it anywhere and to thoroughly flush all surfaces with plenty of water, immediately dry and lightly coat with oil.

If the cyl wall is undamaged or only lightly scored then very lightly hone to give a decent surface to seat your new rings, assuming the bore is still within spec for a new piston. Bolt it back together and break it in correctly.

If the bore is worn you can bore just the one hole as piston weights are within tolerance for a fishing motor.

 

[Polara Pat]

Re: What Went Wrong

Re-reading the thread I see I failed to ask WHY the first piston failed?

And was that cause corrected?

#4 doesn't easily 'just' fail, there must be a cause and it usually is NOT a cooling issue as the top cyl overheats first.

Which leaves a fueling problem. a dirty carb leaning the mixture and melting the piston.

If the carbs have NOT been cleaned and the floats set properly it iss doomed before you mounted the motor on your boat.

Clean any aluminum off the cyl walls with muratic acid, careful not to spill it anywhere and to thoroughly flush all surfaces with plenty of water, immediately dry and lightly coat with oil.

If the cyl wall is undamaged or only lightly scored then very lightly hone to give a decent surface to seat your new rings, assuming the bore is still within spec for a new piston. Bolt it back together and break it in correctly.

If the bore is worn you can bore just the one hole as piston weights are within tolerance for a fishing motor.

To be honest I'm not sure what the original reason was for the #4 failure. I did at one point pull the carbs, clean and re-set the floats which were within spec. I still haven't heard back from the guy and I'll assume that he doesn't want to talk to me.

Just a thought. If there was a lean condition caused by a dirty or poorly adjusted carb then wouldn't it also affect #3 since it shares the same mix with #4?

 

[CharlieB]

Re: What Went Wrong

Good question. When two cyl's share a carb you tend to think that the upper would lean more than the lower, and that is true at lower AIR speeds found at lower RPM.

Ignition timing comes into plaly also, IF, (big letters) timing is advanced just a hair the earlier timing cyl will run significantly hotter than the other. If the trigger for #4 was a half degree advanced during construction of the flywheel then #4 would run hotter than #3 and could toast a piston.

This is another reason why Merc jets ALL motors rich. Manufacturers tolerance on parts. If/when all tolerances 'stack' up a motor could be pretty short lived if there wasn't a LOT of security dialed into the system.

 

[Polara Pat]

Re: What Went Wrong

Good question. When two cyl's share a carb you tend to think that the upper would lean more than the lower, and that is true at lower AIR speeds found at lower RPM.

Ignition timing comes into plaly also, IF, (big letters) timing is advanced just a hair the earlier timing cyl will run significantly hotter than the other. If the trigger for #4 was a half degree advanced during construction of the flywheel then #4 would run hotter than #3 and could toast a piston.


This is another reason why Merc jets ALL motors rich. Manufacturers tolerance on parts. If/when all tolerances 'stack' up a motor could be pretty short lived if there wasn't a LOT of security dialed into the system.

Back to the fact that the guy had clocked the timing mark pulley and the flywheel wrong so the timing was way out from the get go. This wouldn't explain the broken ring though. I'll chalk that up to poor assembly.

Still haven't heard back from him either. No surprise.

 

[CharlieB]

Re: What Went Wrong

Timing is a critical factor, too advance OR too retarded and the cyl will overheat.

You still have to make the decision whether to repair the motor or not.

That decision needs to factor the condition of the cyl, is it repairable, and at what expense?

Cleaning any aluminum smeared onto the cyl wall is the first step, without knowing the condition of the cyl you cannot make an informed decision.

If it cleans up well and mics within tolerance it may be a relatively simple matter of replacing the piston and rings.

Sounds easy, the details are everything must be done perfectly which takes time and attention.