1998 40hp Yamaha 2 stroke, 1cyl at 50psi

[SaltH2OHokie]

Worth attempting to figure out or just buy a rebuild kit and figure on redoing the whole powerhead and call it done?

Stuck ring or cracked cylinder are my guesses based on 2 strokes on land...those valid guesses on a 2 stroke boat motor?

[fitz73222]

Pull off the cylinder head and look at the bore and piston. Make your decision from there. Look for a leakage path in the headgasket to another cylinder or water passage if cylinder damage isn't apparent.

[SaltH2OHokie]

Motor is off the truck and in the garage.  Have company Christmas party tomorrow night, but I'll hopefully start to dig into it Saturday.  I'll post pictures if I find something gnarly.

[SaltH2OHokie]

So first, some background: traded for this motor (for an item that cost me $0).  Motor is a 1998, 3cyl, 2stroke 40hp Yamaha with oil injection.  Got her from a mechanic who took the motor from a customer as partial payment.  It has a new starter, impeller, starter relay, 3 coils, 3 spark plugs, and battery cables.  Mechanic said after he replaced all that, it would idle rough, but would run...compression was tested at 110, 110, 50 (or so he says, I've not tested it, nor performed a leak-down test).

Tonight, I put her in a tub of water on the stand, and immediately realized that I have no Yamaha keys  

Used a small key from work the get the switch to "on" but not "start" and just jumped the starter terminals from there.

As advertised, she lit right off, but ran rough.  We pulled the plug wire on the cylinder advertised as being low and nothing much changed (if anything it got slightly smoother).  Replacing it and pulling either of the other two killed the motor unless you throttled up.  Next, we switched the plug on the bad cylinder with the one above it...2 of us witnessed the results or I'd think I was confused...pulling the cap on the cylinder that was advertised as low acted like it wanted to kill the motor...BUT so did pulling the cap on the one above...this time, pulling the cap on the plug we didn't touch was the only thing that didn't markedly affect the motor's attitude.

Other notes: Red oil(?) light was on on the front of the motor in spite of adding 2stroke oil. Revving in neutral acted like it was hitting a limiter far earlier than I'd think it should have and it only peed for a brief moment in the middle of all of this messing.

Any revelations based on all this info?  I'm hoping to limp it around and duck hunt while there's still some season left, and then tear into it...so maybe run some Seafoam through it and carry a paddle for the next month?  

Looking into the spark plug hole on the "low" cylinder showed an oil (carbon?) covered piston.  Definitely sparking (shocked the dog poo out of you if you grabbed with bare, wet hands.)

[fitz73222]

So first, some background: traded for this motor (for an item that cost me $0).  Motor is a 1998, 3cyl, 2stroke 40hp Yamaha with oil injection.  Got her from a mechanic who took the motor from a customer as partial payment.  It has a new starter, impeller, starter relay, 3 coils, 3 spark plugs, and battery cables.  Mechanic said after he replaced all that, it would idle rough, but would run...compression was tested at 110, 110, 50 (or so he says, I've not tested it, nor performed a leak-down test).

Tonight, I put her in a tub of water on the stand, and immediately realized that I have no Yamaha keys  

Used a small key from work the get the switch to "on" but not "start" and just jumped the starter terminals from there.

As advertised, she lit right off, but ran rough.  We pulled the plug wire on the cylinder advertised as being low and nothing much changed (if anything it got slightly smoother).  Replacing it and pulling either of the other two killed the motor unless you throttled up.  Next, we switched the plug on the bad cylinder with the one above it...2 of us witnessed the results or I'd think I was confused...pulling the cap on the cylinder that was advertised as low acted like it wanted to kill the motor...BUT so did pulling the cap on the one above...this time, pulling the cap on the plug we didn't touch was the only thing that didn't markedly affect the motor's attitude.

Other notes: Red oil(?) light was on on the front of the motor in spite of adding 2stroke oil. Revving in neutral acted like it was hitting a limiter far earlier than I'd think it should have and it only peed for a brief moment in the middle of all of this messing.

Any revelations based on all this info?  I'm hoping to limp it around and duck hunt while there's still some season left, and then tear into it...so maybe run some Seafoam through it and carry a paddle for the next month?  

Looking into the spark plug hole on the "low" cylinder showed an oil (carbon?) covered piston.  Definitely sparking (shocked the dog poo out of you if you grabbed with bare, wet hands.)

Until you understand the low compression issue and why; you're wasting time. It may be just a bad head gasket as I mentioned earlier which is causing the other symtoms. Spend the 20 minutes to remove the cylinder head, buy a $20 head gasket and look at the cylinder walls and piston. Wishin and snake oil is not going get her running any better. My $.02

[SaltH2OHokie]

Voice of reason strikes again. You and my wife must hang out  

[SaltH2OHokie]

Head is stuck on the block.  :evil:  Mallet?  Heat?

[fitz73222]

Head is stuck on the block.  :evil:  Mallet?  Heat?

Salt,

First make sure all of the head bolts are out. (duh) but it happens! No need to take the smaller water cover bolts out. Give a wrap with a dead blow (lead shot) or rubber mallet for starters. You just have to break the seal; quite normal. Avoid prying with a screwdriver unless you can find and ear on the head to pry against that is not part of the mating surface between the block deck and machined head surface. You don't want to damage the mating surfaces. It will come off. Quite possibly never been off before.

[GoneFission]

Head is stuck on the block.  :evil:  Mallet?  Heat?

If you still have the starter on, put the plugs back in and turn it over a couple turns.  Often the cylinder compression will pop the head right off (of you have all the bolts out - see above).

[SaltH2OHokie]

Head's off.







See anything glaring/telltale?  This is the first 2 stroke outboard I've ever had apart.  2 stroke dirtbikes are my only 2 stroke experience...

[fitz73222]

Hey Salt,

Looks like a broken ring on #2 cylinder that has migrated into the combustion chamber. See the pock marks imbedded into the cylinder head. Also looks like #1 has ingested water as evidenced by how clean the combustion chamber in the cylinder head is. The water will actually steam clean under compression while attempting to fire. Take a picture of the number 2 cylinder wall with the piston at the bottom of the stroke and look for grooves and deep scratches in the cylinder walls. She needs a rebuild so you need to realize if its worth investing in or use as a parts motor for another one. The gearcase and power trim unit are worth about $550-$650 together if in relatively good working condition and not ate up with corrosion. If your attached to this engine; look for another 40-60hp similar vintage with a bad lower unit and good powerhead that you can buy cheap and swap the parts out. Bottom line, go find another engine. This one is not worth much except the sum of its parts and really isn't worth rebuilding.

[SaltH2OHokie]

Overbore pistons and a day at the machine shop won't cure what ails me?  We've had dirtbike motors literally blow chunks of piston everywhere and we just get it punched out and buy the corresponding piston/rings for the new overbore.

[fitz73222]

Overbore pistons and a day at the machine shop won't cure what ails me?  We've had dirtbike motors literally blow chunks of piston everywhere and we just get it punched out and buy the corresponding piston/rings for the new overbore.

Absolutely if you're so inclined,

The engine needs to be completely disassembled and inspected for bearing damage and any other anomalies that have occurred. Determine root cause of the failure and proceed accordingly.  Your Yamaha has a pressed together crankshaft similar to the motorcycle engines you have worked on. There are no removable connecting rod rod caps to inspect the crankpin surfaces and bearings so it needs to be pressed apart for inspection. Outboards are very finicky about having everything right because of the stresses induced in normal operation so cleanliness and measurement/ fit up are next to godliness for a successful rebuild. So if you want a nice little project, go for it but be prepared to drop a $1000 to get her back to perfect and reliable... That number all depends on what is salvageable and what needs to be replaced.

[SaltH2OHokie]

If nothing else I'll tear it down for the sake of seeing it down to its basic components.  From there I'll see what's in spec and what's not, and proceed as the budget allows.  Can one purchase new crank assemblies if its out of tolerance?

Oh, and thanks for the help.  

[fitz73222]

Parts are easily obtained. Since you have a pressed together crankshaft, the individual crankpins can be replaced separately without having to replace the entire crankshaft which is a big savings. You just have to find a shop that can press it back together and maintain the correct phase angle between the counterweight throws so she'll time correctly. I do not believe the crankpins are keyed for the correct firing angles to be maintained so this needs to be done in a jig. Additionally, if only the center bore is damaged; you only have to bore that cylinder and hone the other two. It is not necessary to rebore all the cylinders. The slight difference in bore size adds nothing to speak of from a performance or running quality concern. This is a roller bearing engine so you can only buy the original bearing sizes and replace the components accordingly to maintain original clearances. You should be able to bore in .015 or .030 oversize for the oversize piston/ring fit up.