1987 merc thunderbolt overhaeted

Question

first off how do i do a compression test have know idea.

second how pricy is a broken rod and how pricy is a melted piston if i sent it in and is it even worth to fix or can i do it myself

Answer 1

if it simply overheated, you may not have a problem - does it still start after cooling down? If so, a compression check is in order.

follow Jtexas advice about how to compression check - he's got it right.

I rebuilt my '87 XR2 twice and it really isn't that tough, butI've been around outboards and two strokes and engines all my life. I rebuilt my first motorcycle when I was 12, my first car engine when I was 16, and my boat engine when I was 36.

Honestly, it may be bnetter to just buy a powerhead from someone and bolt it on. see examples at the links below

Answer 2

If the spark plugs look ok (not damaged by piston impact and no aluminum transfer) then get yourself a compression gauge (about $25 at a car parts store).  

1. Warm the motor up to operating temp.

2. Remove spark plugs.

3. Disable spark by pulling the emergency lanyard, or else ground the plug leads to the engine block.

4. s***w the compression gauge into the first spark plug hole.

5. Turn the engine over with the key for a few seconds -- let it turn over 4 or 5 times.  Repeat.

6. Write down the reading on the gauge.

7. Reset the gauge and move on to the next cylinder.

There's some dispute about whether the throttles should be open, I think it doesn't matter on a 2-stroke.  Either way, the above procedure will tell ya what you need to know.

The difference between holes is more important than the raw numbers.  You want to see less than 5 or 7 percent difference between the lowest and highest.  You could probably live with a 10 or 12 percent variance.

If you have a couple low readings, do 'em again just to make sure.

It's ok to test a cold motor if it won't start, but warm engine is better.  Make sure the battery is fully charged, so you'll have plenty of cranking power.

There's probably no internal damage but if there is, you're looking at a complete teardown.  It can be fixed if the cylinder walls haven't been scored too deeply, but it's gonna be $2 or 3 thousand...way, way less if you can turn a wrench and follow instructions in a service manual.  A do-it-yourselfer with a good set of tools can rebuild an outboard but it takes a lot of patience.