So I melted a piston in my 750 zxi

[CaptaimBosco]

So I have been having some issues with my 750, pretty much since I bought it last year. It ran great the first day I had it then the next morning it overheated. Turns out the head gasket went. So after replacing it she ran fine again, until late the next day. Everything was fine until all the sudden she just bogged out, died and wouldn't restart. Once I got back to shore I noticed the pulse line had blown off of the manifold so I replaced it and zip tied it back on. Got her running and noticed that I could see a spark shooting up the pulse line. So seeing that I replaced the reed valves. The valves didn't look bad but since I had it apart I decided to replace them with boyesens. Once they were in I was having issues getting it to run at all. This led me to rebuild both carbs. Once the carbs were done I finally had her idling. Today I go to start her up and nothing. I run a compression test and nothing on cyl 1. I pull the head off and the piston is melted. So I will be rebuilding in the next few days. Is there anything I should watch out for when I'm reassembling? What would have caused this? I don't want to rebuild and melt it again. I have an aftermarket head on it. I just want this thing to be reliable and so far it's been nothing but a headache. What are the torque specs for the jugs? Any help would be appreciated. Thanks guys.

[nmpeter]

Is there anything I should watch out for when I'm reassembling?

yes

air leaks. They can kill a motor pretty quick.

You must take steps to assure the engine is airtight.

and bravo, you best figure you what melted that piston, or you will be doing this again, real soon.

hint-Use Premium fuel only

[CaptaimBosco]

Is there anything I should watch out for when I'm reassembling?

yes

air leaks. They can kill a motor pretty quick.

You must take steps to assure the engine is airtight.

and bravo, you best figure you what melted that piston, or you will be doing this again, real soon.

hint-Use Premium fuel only

i only run 93 premix in it.
How do you pressure test the crank case? I read somewhere that it should hold 10psi but how do I preassurize it?

[nmpeter]

you need to be able to seal off the intake and exhaust and have a small source of compressed air you can use to check that the crankcase has no air leaks.

with all due respect, if you are not familiar with this procedure, you should find somebody that is.

Sealing a crankcase on a rebuild takes some experience and sometimes even experienced builders run into trouble.

do some searches on pressure test and leakdown test. lots of good advice here.

[jetski jerry]

yep what he said. air leak

[CaptaimBosco]

It was the head. The kid I had bought if from put a pjs head on it. Kept blowing head gaskets. Sent the jugs and head out had the head milled and the jugs decked and now she's been running alright

[steve45]

I guess you don't have a service manual...

Have you checked piston/cylinder clearance? Piston ring gap clearance?

Cylinder base torque: 25 ft-lbs, cylinder head torque: 22 ft-lbs.