787 Lost compression on rear cyl. Can't see anything wrong with heads off. Please Hel

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Bobby78

Bobby78

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787 Lost compression on rear cyl. Can't see anything wrong with heads off. Pls Help

Less than 1 week to the holiday weekend! ARGHHH!

Engine cut out yesterday. Checked compression today - 60 psi on rear. 150 on front.
Removed heads, cannot see anything wrong.
IMG_1382.JPG
IMG_1383.JPG

Just gunked up head gaskets?
 
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So I assume I need a standard size kit like this and then hone the cylinder? Or do I need the 0.5 mm over-sized if I hone?
787 top-end kit.JPG
 
Clean off top of the bad piston and look for a stamped in number. It will be on the side of piston near the RAVE valves. Clean it all off and inspect closely. You will see it.

81.89 is the stock size ( they call that STD. 82mm in the top end world of sales )

I'm going through similar process now with my ski.
 
It worked right, no worries!

Post the numbers on top of piston before you order anything.
 
Either lack of oil or running weak I would guess.

If it was running weak that would cause it to run hot and for the piston to swell and stick in the bore.

Andy
 
Also on re assembly take note now of the orientation of the piston. There is an arrow on it. Make sure it matches how it is now upon reinstallation or the rave valve will murder the side of the piston lol.


Rob
 
Any advice on Hone grit to use? I just ordered a 240 grit Brush Research FLEX-HONE, but they also had 180 and 120. I don't think my scoring is very bad, but I have nothing to compare it to.

P.S. I've got the SBT Top-End kit shown above on order with Overnight shipping. :D

IMG_1391.JPG
 
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:facepalm: And the unthinkable happened. All back together and the engine locked up on cranking. Pretty sure a thrust washer must have fallen into the crankcase!

Any advice other than pulling the engine out and tearing apart the bottom end?

Heading to Harbor Freight now to pick up an inspection camera and flexible magnetic pickup tool.
 
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Generally running lean and or overheating is what kills cylinders. Are the oil lines hard and brittle it soft and pliable.
 
I was running low on oil, but I don't think it ever went completely empty in the tank, let alone all the way down to the engine. Previously I was running the quicksilver synthetic blend API TC, but I couldn't find any this time so I went with the Quicksilver synthetic blend PWC that has TCW3 on the front, but says it's also good for API TC on the back. That's not the end of the world right?

Nothing seemed overly hot when it died. Cooling system seems fine on the hose; water out the telltale.

All four carbs have been at factory setting since last year, and probably before that, but I will recheck.

Planning on dumping some oil into the fuel tank at about 60:1 to be safe - is that ok?

P.S. I also put Sea Foam in the gas at 1oz/gal and in the oil reservoir at 1oz/quart. And I will bleed the oil pump before running today.
 
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:banghead: So much for trying to fix it myself. The head and new piston are now destroyed. Time for a whole new SBT engine I think. :angelsad:

IMG_1399.JPG
 
When was the last time you went thru the carbs? And in your last pic, is that the exhaust side of the piston? If so you're definitely having fuel issues. A 2 stroke needs the fuel to cool the exhaust side of the piston, without it you will melt the edge of the piston off. So, if you get a new engine you better go thru those carbs with a fine tooth comb or else you be right back here posting the same pics on your new engine.
 
racerxxx said:
When was the last time you went thru the carbs? And in your last pic, is that the exhaust side of the piston? If so you're definitely having fuel issues. A 2 stroke needs the fuel to cool the exhaust side of the piston, without it you will melt the edge of the piston off. So, if you get a new engine you better go thru those carbs with a fine tooth comb or else you be right back here posting the same pics on your new engine.
Click to expand...

To me it appears the piston is pretty equally pitted around the entire squish ring.

I was through the carbs last year and, while I'm no expert, everything looked like new. I made sure both mixture screws were just slightly on the rich side of factory setting before running with the new piston and rings. I'm thinking there was still something in the crankcase that made its way into the chamber causing some pitting, which then started causing detonation, which just got worse until it destroyed the top ring?

I've still got the other half of the top-end kit, so I'm thinking about pulling the engine so I can inspect and clean the bottom end, re-check the carb and maybe try again before dropping $$ on a new engine plus two-way shipping. Then I might just sell it and get a new Scarab. lol
 
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Yeah, I would probably just tear the engine down and give it a good once over.I just saw the seafoam post. Not sure I would run that in a 2 stroke. I know it says it's OK to use on a 2 stroke on their website, but we're all leery about a "fix in a can" around here. It's amazing how much is gone off that piston and the walls of the cylinder appear to be unhurt. If you do put it back together, hone that cylinder with a ball hone, go thru those carbs and verify the fuel lines are 100% correct, set the setting to stock setting--don't fatten it up. And run the gas in the tank at 50:1 and bleed the oil injection system and roll the dice.

Is that a Pit in the boat with you?
 
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