Yamaha 70hp Piston striking plug?

[philkelly]

Hey Guys, myself and my buddy are thinking of buying this engine in the hope we can get it fixed without to much expense. Its a yamaha 70hp 2 stroke from the mid 1990's. Apparently it got a rebuild about 2 years ago, was working fine, then the centre piston (3 cylinder) would hit the plug and squash it a bit so it no longer sparks, the engine would continue home on 2 pistons with significantly less power. This happened approx 3 times.
Apparently it only happens when you go to high RPM. The guy selling it wants a reliable engine for the sea. I need a cheap engine for lake use! Could it be something like if the engine timing requires fine tuning, since the rebuild? Could it work with a recessed plug? Or perhaps some kind of build up on the piston head, that could be removed if we pop the head off? He has it still on his sea boat, if I did a comression check on each cylinder and it looked good (at around 100 psi), its going cheap is it worth the gamble do ye think?

Thanks Guys, great forum

Philip

 

[Bifflefan]

Re: Yamaha 70hp Piston striking plug?

sould like a piston and or bearing problem.

 

[philkelly]

Re: Yamaha 70hp Piston striking plug?

I was thinking I might get away with a second washer on the spark plug or to decarbon the engine, or adjust timing? Is it only wishful thinking.....I'm only a novice but would like to give it a shot with a good manual....I have a another 75hp yamaha I was hoping to use for parts (its 10 years older though). You reckon it could be open heart surgery? any chance could be fixed with just popping off the head?. If there is good compression ~100 on each cylinder, is it worth taking a chance with it?
Cheers...

 

[sigma]

Re: Yamaha 70hp Piston striking plug?

I was thinking I might get away with a second washer on the spark plug or to decarbon the engine, or adjust timing? Is it only wishful thinking.....I'm only a novice but would like to give it a shot with a good manual....I have a another 75hp yamaha I was hoping to use for parts (its 10 years older though). You reckon it could be open heart surgery? any chance could be fixed with just popping off the head?. If there is good compression ~100 on each cylinder, is it worth taking a chance with it?
Cheers...

Your looking for a cheap fix and going to make an expensive problem. The motor needs to be tore down and fixed right. The only thing that could be cheap about this situation is if the cylinder your talking about has the wrong plug in it but it doesn't sound like it.

Take it from a guy that has learned the hard way.....Don't buy it if it ain't right

 

[noelm]

Re: Yamaha 70hp Piston striking plug?

kind of doubt that it has a timing issue, but I also doubt that the bearings are so worn as to hit the spark plug only when going fast, and there is not an easily noticable knock at any speed, you could chance it with an extra washer on the plug, but you never know how long it will last if it has a serious problem.

 

[robert graham]

Re: Yamaha 70hp Piston striking plug?

Maybe you've got the wrong plugs(long reach threads) in the motor. All the plugs have different lengths of the threaded part and if yours are too long the plugs are extending too far into combustion chamber. It's worth a look!
Good Luck!

 

[philkelly]

Re: Yamaha 70hp Piston striking plug?

Thanks guys for all the tips, I'm learning lots here

 

[rodbolt]

Re: Yamaha 70hp Piston striking plug?

either a rod bearing or a wristpin bearing is failing.
not much choice for plug reach on that motor. the b7 b8 and b9 all use the same reach.

 

[robert graham]

Re: Yamaha 70hp Piston striking plug?

If it's a rod or wrist pin bearing failing like rodbolt says then you better get it fixed before the motor self destructs. Like don't run it at all until it's fixed!

 

[stylesabu]

Re: Yamaha 70hp Piston striking plug?

WHY IS THIS ONLY HAPPENING IN 1 CYL? a dial indicator inserted in plug holes may give a little more insight. it will tell if there is play in only 1 cylinder. measure all 3 and turn engine slightly by hand while measuring. i cylinder in question will show more movement. if they all move the same then, you'll need to disassemble engine and measure everything to find out why. the dial; indiacater should give you insite as to whats happening. sure carbon build could cause this but,you show also see a differance in the spark plugs.keep us informed with the results.

 

[schinia]

Re: Yamaha 70hp Piston striking plug?

philkelly, turn arround and run as fast as you can. its probably not one cyl but all of them. i would guess pieces of the piston is breaking off, and hiting the spark plug. with a compression test , they should also do a leakdown , which should be 5% or less. like i said runnnnn.