When is an engine rebuild required (1981 XJ650 4KI)

I have been told the bike I purchased (In bits with engine separated from frame) has done 30,000km. The engine has been sitting for maybe 3 or more years without running. Are there any guides as to when a rebuild might be needed for the engine? Or does it require a dismantle and thorough check before replacement of pistons, valves, head skimming etc needs to be done?

 

You can do things like a compression test and/or leak down test with the engine out to determine your engine status with the engine out of the frame. In fact it is probably easier than in. These are very tough engines and unless it has been severely mistreated it should not need a rebuild at 30,ooo kms. Check your oil status, change it out and check your valve clearances and the compression test. If all are good, put it back together. The weak points are the cam chain tensioners, not strength but age, the rubber can/will break apart. If it the engine is out and you really want to do something to it those parts should be changed.

 

Agreed, do valve clearance check then run a compression and leak down test and go from there.

 

I feel that sometimes people rebuild engines for the experience and to bring the bike back to factory performance but not always because its "needed".
I have a friend that rebuilt an engine just cause it had an oil leak and he was going to replace the gasket and winter was coming up so it was a winter project.

 

Thanks for the tip!
I dont know if a compression test can be done without being able to turn the engine over though. Would the starter need to be hooked up? Also the carbs are not fitted either. Guessing a leak down would be the best approach????

 

Yeah I kinda wanted to do it for the learning oo. But I dont have the skills/experience to go that far and I really dont want to Break anything

 

You'll want to hook up the starter unless you can turn the engine over some other way. When doing a compression test you want the carbs "wide open" to let air into the engine, not having carbs mounted accomplishes the same thing.

If you want the experience of rebuilding an engine find an old lawn mower with a Briggs and Stratton engine in it, way cheaper and you'll get the same basic experience from it.

 

What LAB3 said. I doubt you would get any useful info turning the engine by hand for a compression test. So, yes put the starter on and use jumper cables to a battery.
Doing an XJ engine rebuild for the learning experience would be an expensive deal. Even just a gasket set and such aren't cheap.

 

You can watch @Mezzmo videos on his engine rebuild. It will give you an idea of what is involved without any expenditure.

 

If main and big end bearings were cheaply and plentiful I would say crack on, but sometimes what you find stops you dead - meaning the thing is in bits for a long time, maybe never gets rebuilt...
Then also you have to make decisions on measured wear and tear which you are not used to, again, it stops you dead...

 

Did a rebuild on a briggs single once. It had push rods for the valves. I got it going again but an XJ650 engine is a whole nuther level.

 

Is it hard to find bearings for this motor?
And yeah the whole run out for bearings and plastigauge measuring scares me.
But I think what I will do is.
1. do a compression and/or leak down test. - If all is good then Ill crack on with the carbs.
then valve shims.
and hopefully that is all that's required.
If I have to do a full rebuild then ill be ringing up Mezzmo

 

This series is brilliant. I have watched them a number of times. Particularly like the refit of a 900 rear wheel and all the machining to get it fitted and centered on the bike.

There's a guy called Jish on YouTube who also has a full build. Doesn't go into so much detail on the engine and carb stuff but uses some pretty good building techniques (Like 3d printing) to mod the bike.

 

Yes l like the mods. Watch Allen Millyard l like his machines especially the flying Millyard.

 

Amazing! Built his own engine. Top Tier bodging lol