Rebuilding a 2-stroke Yamaha?

[Degnol]

This is in a 1979 G1 golf cart. The cart has been run hard with little/no maintainence. Kinda deep into it now with the rest of the cart...new upholstery, bodywork, paint, wheels and tires, now the motor, which has run strong (incredibly) has decided to make funny bottom end noises. Took the head off, piston and cylinder look fine. Have not split the case yet (need puller for clutch and stator cover)
OK, now the question.....Do you overbore? Sleeve? The main bearings are ball bearings, that seems simple enough. The upper end of the conrod is needle and replaceable. But, I've never done a 2-stroke. Should I junk it? Is there a compatabile M/C engine I can substitute?

He'p me out here if you can.
Thanks

 

[Dutchie]

Check the big end by measuring the play in axial direction on the small end. If you can wiggle the smallend left/right too much, the bigend's shot.

And overbore, sleeve and that kind of thing depends on how the engine's built, if overbore pistons are available and maybe it's still within tolerances and just needs a fresh piston, rings, wristpin and small end ?

Bottom noises could be main bearings too, besides big end, so you might want to replace them while you open it up.

Any manuals of this engine ??

 

[Degnol]

There is a CD on ebay that is a pdf file of the factory service manual for like $12.95. I think that would be a good investment.

Thanks for the reply.

 

[D'Animal]

2 cycle engines are pretty easy to rebuild.

Piston slap is often resonated through the engine and a person would think it is a lower rod bearing noise.

It is a little late but you will hear it the best while wearing a set of ear muff style hearing protection and right after you let off the throttle. The piston will "slap" more pronaounced in the bore on deceleration.

Now that you have it apart, you can slide the piston into the cylinder and see how much you can move the skirt of the piston in the bore.

Measure you ring end gap as well.

 

[Degnol]

Thanks Dan. I just ordered the FSM on CD. The crank bearings....do you think I have to go to Yamaha to get them or just take the old bearings to the Autoparts store and have them get them?
got any good sources online for parts?

 

[D'Animal]

Check with a Yamaha dealer and if the price is crazy, go to a bearing supplier.

Keep and eye out for a change in manufactures. If there is a Fafner, Fag, Koyo bearings in it, you want to go with the same manufacturer. The biggest differences are in the cages and the diameter of the balls themselves. I have seen many bearing places cross over a bearing but the "new" bearing has a polymer cage, others have cages that are riveted together. Sometimes the outer race or the inner race is thicker and the balls are smaller. There are more to bearings than the ID, OD and width. If you go to an Napa type parts supplier, they will measure the OD, ID and Width and will take you part number and cross it over to what they carry. A bearing supplier will generally spec the exact bearing.

Some bearings are designed or axial force and some for radial forces.

This may be overly cautious or bad info, but I deal with 2 cycles in the 15,000 RPM range with heavy loads so it is more critial.

 

[Degnol]

Good info, as usual. Thanks!

Ed