Compression test says I need new rings - While I have the engine apart...

1982 XJ750 - 39,000 miles
Bike was sitting for years - bought it from someone who bought it as a project - replaced the brakes and got bored. I picked it up for $750 and got a few things sorted out - However i am now getting down to figuring out everything i need to order and fix to have a fun bike...

The compression test says I need to rip the engine apart and do the rings - Since I don't mind doing this one... would REALLY hate doing this again soon...

Valves are within spec except for one (supposed to be .11-.16 and it's .10) but i wouldn't think it would affect it this drastically, and obviously across all cylinders.

• Anything else recommended to replace ?
• anything i should do to the head while it's off? (probably will do the valve stem seals while it's out
• Since it's stock, i should just need to get the standard rings - shouldn't need oversized as i'm not boring anything else, right?
• Can i re-use all the bolts/nuts? or is anything a stretch bolt?
• As for the starter clutch - can i just do a basic rebuild on it? (just the springs and those little rod dudes) - if i'm in there, might as well!

Compression test

 

As far as the piston ring size goes... Should I get a bore gauge and stay stock if it's within gauge? Does the piston size grow after just normal wear and tear?

 

Primary chain guide.

First thing you should do is check the head mating surface for straightness, in the method outlined in the service manual. Warped heads are THE main cause of head-gasket failure.

Assuming it is straight (or corrected by shaving to be straight), then a valve lapping is a good idea while it's apart.

You should measure the cylinder bore (using the proper gauges) for bore size and "ovalicity" as per manual specs, as well as comparing the cylinder bore ID to the piston OD (for each hole) to make sure there is proper (and not excessive) clearance.

Well, every bolt is a "stretch" bolt; the process of applying torque to a fastener stretches and distorts it by design. Some bolts are more critical than others. Yamaha recommends replacing rod cap bolts (and piston pin circlips) anytime they come apart.

Yes, check and make sure the central hub is not showing any cracks (look carefully, hairline cracks develop into larger ones!).

 

So from what I've been reading, head gasket failures aren't uncommon, and get leaky esp on air-cooled bikes... I've been wondering if this might be the cause of this leak? It's no where on the head like a valve cover leak would be.... just under it

 

Did you check the compression "wet" or "dry". Add a few squirts of oil into the cylinders to do a wet test. You might have a ring or 2 hanging up.

 

Just check the leak is not from the valve cover in the spark plug area and running out at the cylinder head to cylinder joint. Valve cover leaks are much more common than head gasket failures. You can put a little acetone and ATF 50/50 mix in the spark plug holes and leave it to soak a bit to free up gummed rings, and do other compression tests.

 

Sorry, I'm wondering if the photos aren't showing up in the first post.
Here is the compression test, uploaded instead of linked from Google photos.

@Franz @StahlMaster @chacal

 

How do I redo the dry compression test after I put oil in the cylinder to test the acetone / atf mix?
Thebike doesn't run, so I can't burn it off

 

If the engine turns over you can turn it over with the plugs out and get some of the ATF mix out. You only need a spoonful down each spark plug hole.

I don't think you should have any more than 14psi between cylinders on the compression tests. Others will chime in with the correct compression figures for your bike.

 

Now I'm getting different but steady readings... I wonder if something is loosening up?
1 180
2 165
3 120
4 175

 

Stuck piston rings especially on machines like yours that have not been used for a long time. That is the most likely cause. I would not be stripping the top end of the engine. 39,000 miles is not a lot for an XJ. Compression figures increasing are a good sign it is not the head gasket.

 

If the bike sat it can loosen up running , you should do a leak down test this is more accurate to figure out if you have bad rings or bent valves as this puts pressure into the cylinder. On another note every compression gauge will be different, you should have rented another compression gauge and see what reading you get .

 

I think for right now, I'm going to go down my same plan of rebuilding the brakes, Forks, carbs and keep cranking the engine periodically and see what happens.
I would love to do a leak down test but I don't have an air compressor, but may need to get one soon fkr stuff like this.
Just wish I had working carbs so I could let this thing run and idle for a bit and see what loosens up and see where everything is leaking from.
Thank both you guys! This bike has been fun but damn frustrating at times!

 

Would you still suggest the atf / acetone?

 

Yes just a little cannot do any harm. Or alternatively you can use a little starter fluid from an aerosol in the carburettors and crank the engine to see if it fires.

 

Oh, it fires...
I spritzed some starter fluid in the cylinders, maybe about an 1/8th of a second to see if it would stumble... Oth the carbs and the exhaust are off.
It stumbled like it was trying to catch and then went off like a rocket... That was a very deafening experience!

 

Please don't be offended, but this was a damn fool thing to do. Without carbs, the thing is unthrottled....
This asside, I'm also not a fan of pulling the head at this time. Get it running on carbs, put it through some heat cycles, then retest compression.

 

Noah,
I think you are on the right path, just take care of all the other things the bike needs and let them rings sit. I have had a few bikes along the way (v65 Magna and a BMW 1100GS) that both had stuck rings. The two bikes I mentioned had been sitting for a long time and were both out in the elements for that time. With the ATF and acetone does work. It might take up t a week to do it but it does work.

If you have a air compressor you can help the process. I used to add the ATF/acetone and let it sit, hand roll the engine a few times and then take a rag and place that next to the air noels down inside the spark plug hole. Hit it with air and the mix with any rust or dirt will come up with the fluid.

The V65 all came free on its own. The BMW freed up when I was riding it. Was WOT and the rings let lose..good thing I had a good hold of the bars...the bike woke right up...lol

If you know anyone with a bore scope you can have a look inside the cylinders. That would tell you if there is any damage or bad scoring of the cylinder walls.

Bottom line, done pull the head until you have to. When all bikes sit the rings tend to stick. As you have said the engine is free so just work with it and get them rings freed up as well.

 

Thank you for the info on that - i've picked up project cars numerous times that had sat for years and hadn't had anything like this before, so i was convinced the piston rings were shot. No air compressor here, but I will focus my energy elsewhere and pick up some ATF soon, make a mix and dump some down the cylinders.
Time to make an order from XJ4ever

No offence taken... quickly realized that afterwards - never once have i used starting fluid before and didn't realize how little it takes.

 

I think the cam cover gasket and the doughnuts on the cam cover bolts are the cause of oil at the head cylinder joint. Replace them and don't overtighten the bolts.