96 Kawasaki STS 750 Major Problem After Rebuild

[BigD46]

I rebuilt the engine after ordering a crank and top end kit from sbt. Rebuilt carbs, oil pump block off kit, and all new gaskets... (all ordered from sbt) after putting everything together and putting it in the ski, took it to the water. Fired up fine ran for 30-45 mins at 1/4-1/2 throttle it did great. Had plans so had to go and couldn't run it any longer...took it back out this Saturday and it was Very hard to start, after finally getting it running, I rode it for around 15 mins at 1/2 throttle it died! Never to start back! Brought it back to the shop and had no compression on the rear cyl, so pulled the head to find the piston scarred up BAD and rings collapsed with piston melted onto them!
My question is what could have went wrong? Any help would be greatly appreciated!

 

[Dr Honda]

Just some random thoughts.....


1) When you put the engine back together... did you verify the ignition timing?

2) did you verify the compression, and dome clearance? If it was tight... it would overheat the crown.

3) When you put it back together... did you do a leak-down to make sure there were no case leaks?

4) Did yo have the cyl's machined to the correct tolerance for the new pistons?

5) Did you check the ring end-gap?

6) Did you use good oil and fuel?

7) since you went pre-mix, did you put oil in the fuel?

8) Since you went pre-mix... did you block off the oil nipples on the engine?

9) Did you verify that you had water flowing though the engine?

10) The most important thing....... Did you figure out why the last engine gave up?


As you can see.... there are a lot of reasons why. Since the "Melt" is down the sides... I'm going to say it was a lean run, or over temp.

 

[BigD46]

Ok here goes... I see a some things I should have done..
Here's what I did do.
Sent the pistons with the cyls to the machine shop where they did all of the clearance work, and ring gapping. When I got it back from them it had each piston with rings on them in the correct cyl that it was bored for.

Compression was 150 on both cyl after build

Mixed gas 40:1 with quicksilver oil

Blocked off oilers

Water was coming from "pisser" on the side of the ski while it was running

And the last engine seized due to lack of oil!
(let someone else borrow for the week) guess its still my fault for not telling him to make sure it had oil...

And carbs were rebuilt 2 1/2 months before initial breakdown.

And thank you for your help!

 

[Dr Honda]

OK. Since it was run without oil... the crank seals could have been damaged. A leak-down would have been very important.

Was there any "Melt" directly under the spark plug?

 

[BigD46]

I put a new crank and seals in it when rebuilt.
And no the top of the piston was almost as smooth as a new one. After looking over the front cyl I noticed some minute scarring on that piston as well. Nothing on the cyl wall just the piston itself

I took pictures, but can't get them to upload from my phone.

Also after pulling the engine i noticed there was gas standing in the crank case... 1/4 of an inch maybe a little more, in the bottom... Could this have something to do with it? Is there supposed to be fuel there?

 

[Dr Honda]

Hummm. Sounds like it just may have been a piston failure.

Yes... there is normally some fuel sitting in the bottom.

Try to post this pics... that will help. You may have to do it from a PC.

 

[BigD46]

http://i1053.photobucket.com/albums...15B8784-2046-0000033DF5D348A5_zpsc3413a8a.jpg

maybe one of those will work

 

[BigD46]

 

[Dr Honda]

Nope... not a piston failure. With the color on the crown... and the damage to the side.... that was a simple lean run. If you look at your front piston crown... you have a "Wash" pattern in the carbon, on the crown. The cool, fresh fuel is a solvent... and it will keep the piston crown cleaned off, where it's exposed to the bypass ports. Reading piston wash is a way of tuning a 2-stroke.

Anyway... on your rear piston... there is no sign of "Washing". That tells me that you weren't getting a good, fresh fuel charge.

Either the carb on that side wasn't happy, or you had an air leak in the case.

The other thing it could be... is lack of oil... but your front piston looks good, and you were pre-mixed... so that's not really a possibility.

 

[Dr Honda]

One other thing comes to mind.... Since you have a reed engine... did you check the reed cage for leaks, or for high clearance? If the reeds are back flowing... it will cause a lean run, same as a case leak.

 

[BigD46]

Thank you for the help! I will definitely do a pressure check and check the reeds when I do rebuild it.
I will also go back through the carbs and check the pop off again.

Thanks again for taking the time to help me.