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1999 GP1200 Non-PV rebuild or New engine. HELP :(

187 Views 3 Replies 2 Participants Last post by  wmazz
HI super new here !!!! I recently bought a 1999 yamaha gp1200 non power valve for cheap. I was knew engine was bad and was looking for a fun project anyways. So I pulled the motor and found a load of issues. 1. Case was previously welded from a center rod failure. Had it looked at by a professional and said it was solid. But then found scoring on the inside wall of the bottom case closest to the PTO coupler. 2. that the crank has 3 bearings that are busted. 3. The cylinder head cover has pitting on 2 cylinder domes. My question is... Should I just drop a rebuilt engine in it ? honestly having a really hard time finding one though... Tried contacting but he has been SUPER flakey and doesn't seem to be interested in making some money. SBT is boom or bust so don't wanna take a chance with that. And Ebay is just a big risk I feel. Where should I go from here? Should I source used parts? what are your recommendations?


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If you sand off the sharp edges on the combustion chamber, the
damage will not cause a problem. But don't scratch or sand the
head gasket surface.

The crankcase is what worries me. Sometimes when a rod fails
it beats the cases outwards and lowers the center gasket surface.
If the center gasket surface has a dip, it is possible for the case
sealer to be ejected.

Most likely the piston fell apart, not the rod bearing. Should test
the cases on a flat plate (pane glass) with 220 or 320gr emery
cloth lightly glued with 3m spray 77.

Bill M.

That engine also had hard starting problems and sometimes a
damaged fuel pump check valve.
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To: Bill M.

The top end was apparently replaced under 30 hours ago. Also How can I Test if I Should use this case? can you explain the glass test in detail? You seem very knowledgeable and your help would be much appreciated.

In regards to sanding the combustion chamber... wouldn't that change the squish?

Also I found a reed completely missing? The cylinder jugs looked good no scoring.

Thank you !!!!
You need to perform 2 tests. (1) Test the centercase gasket surface.
(2) Pressure test the crankcase or just the center cylinder.

(1) To test test gasket surface, you need a flat surface, and glass may
be more continent than a granite block or a machined flat surface.
Try an Ace Hardware for a piece of 3/16" glass (thicker if possible,
it is good enough). Use a piece of shelving particle board as a base
for the glass. 3 pieces of 320gr emery cloth glued end to end ought
to be long enough. Use 3M spray 77. Spray a light coat on the sandpaper
and glass, then let it dry for a minute or two. Carefully lay the sandpaper
onto the glass. No overlapping edges.

Then tape or adhere the glass to the particle board.

Lightly sand each gasket surface. Use water to lube the sand paper.

You are looking for a low spot (that did not touch the sandpaper) on
the center cylinder.

(2) To pressure you either need to assemble the long block and block
off the carbs, exhaust, pulse line fittings. Or fabricate several block-off
plates from aluminum or plexiglass and 2 universal freeze plugs for the

If you assemble the the engine, keep the pressure low ~10psi. If you are
testing the engine cases and maybe the cylinder and head, try more
pressure. The limit (if you have a good set-up) would be an air compressors
line pressure 10-0 to 125 psi.

Spray soapy water on the welded area to test for leaks.

Bill M.

Good Luck.
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