1972 Kawasaki G4TR Trail Boss

A Co-worker approaches me a few months ago, asking if I'll "fiddle" around with his dirt bike and see if I can get it running. Now I maintain/repair my own bikes but am new to the 2 smoker realm.

First off, I unscrew the fuel cap & find this



Found some of the Tennessee River bank in there



After much cleaning, the gas tank is squared away.

Next up, the carburetor...



While all of this cleaning was taking place, I discovered it had a faulty coil. Ordered a new one from an Ebay seller in Malaysia. After a long wait, the coil arrived & I mounted it. A new NGK plug later & we've got a fat blue spark.

Now to the crux of the situation. Upon a compression test, my gauge reads 60psi. A look at the service manual shows that almost 140 psi is the new/rebuilt spec and the service limit is around 98. With very little 2 stroke knowledge, I ask....

What's my next move? Find parts & quote the guy a top end job? Tear it down and inspect for possible re-ring?

Apologies if my assumptions are off base & thanks in advance for any knowledge/guidance

This

Tear it down and inspect for possible re-ring?

Awesome, thanks!

per him, true one owner bike

I remember those.....I, myself had a Suzuki TS90, with the rotary valve carb. A friend had a Kwaker, like that one.....FUN TIMES!
On the front of the motorhome, from Ct. and into Mexico for 6 weeks!
No license or registration.
HUNDREDS of miles away from my folks......Mexico and NO CELL PHONES, etc........Danger around every turn.
15 years old and having the time of my life.
The Mexicans LOVED to watch the young Gringo kid swallow Tequila and PARTY!
Lucky I am still alive and even made it into high school.....60, this year.

Long overdue update. After scraping/cleaning the nasty muck from around the exhaust hardware/cylinder, I pulled the cylinder head and barrel and stuck them in a 50/50 simple green & water mix in the ultrasonic cleaner. They're marinating currently..



Some nicks here, should I try and sand these out?


Yuck




Cylinder looks pretty good to me


Measurements to come....

Cylinder measurements are right on the money at every spot. While taking off the rings, the bottom ring snapped. I don't believe I had anywhere near the tension on it as I had on the top ring during removal. Is this common? At any rate, new rings are needed, as are a proper pair of ring pliers. Any suggestions with that tool?

I went ahead and measure the ring gap with the ring in the cylinder and the free gap. While the piston ring gap was right at the tolerance limit (0.28mm) per the factory svc manual, the free gap was over the spec (4.5mm spec vs just shy of 5mm actual).

Would the consensus be to order a new set of standard rings, cylinder head gasket and base gasket? Possibly a new exhaust gasket as well.

Any more measurements I need to take or components I need to check?

If the bore measures good, re-ring it & enjoy. Did you remember to hold the throttle wide open when checking compression?
Don't get hung up on the compression numbers. Be sure to verify (new) ring end gap prior to assembly.

yes sir, 60lbs of compression with the throttle open.

All new NOS parts delivered and of course I've got a question. Per the service manual, "the piston contains two different rings. The top ring is chrome plated and has more gloss than the second ring." Also "the second ring......its' surface is parkerized for smoother sliding contact with the cylinder.

Now to my eyes, both rings look to be finished similarly. The only marking shown on either is a "N" near the split. This is shown in the pictures below. Now I know the "N" side faces up when installing but can anyone discern which is the top ring and which is the bottom or should it not matter?

Pictures of the ring faces and both edges at faces?
Believe manual's likely describing the face material/finish.

I sure hope one of these is what you're asking for.

I got nothin'

10-4, I appreciate you looking. Gonna run by the local Kawi dealer tomorrow and pick their mechanics brain. He's an old MX'er that seems fairly knowledgeable. Sure hope I didn't get 2 #1 rings or 2 #2s, but can definitely see how that'd happen with NOS stuff.

(Gather 'round for a funny, well rather I can laugh at myself and so should others)

So after visiting the local Kawi mechanic I learned that these rings are interchangeable. As long as the markings on the rings are facing up you're good to go. So with this info I dive in....

When I'm readying to slide the cylinder back down over the newly re-ringed piston I decide to rub at some of the carbon on top of the piston and start to uncover what I thought was just the orientation arrow for the piston (dumb, dumb boy). Long story short, once the reassembly was complete I checked the compression........still only a hair above 60lbs. My mind races back to the marking on the piston and what it REALLY was......



meaning.....yep, I'd installed standard rings on an oversized piston


I guess that explains the absence of compression ::exhale loudly::

Oops

Props for humility and sharing.
Bummer for the wasted time.
Do you see vertical traces in the cylinder hone-marks that align with ring-ends? Probably no damage was done.

I don't know about any markings on the rings, but the piston has pins to prevent the rings from rotating (and catching in a port), the rings are notched so they go over the pins, the rings won't go on upside down.

That's probably not nicks in the combustion chamber, it looks like pitting to me. It could be from detonation, head gasket issues, maybe even preignition. The old piston will also tell more of the story.

That pitting looks pretty moderate... you can probably find another good head pretty cheap... I'm not sure I would run it as is. It can cause hot spots. As deep as the pitting is, it would probably need welded and milled, if you sand it enough to remove the pits, you will be removing a lot of material and changing the compression ratio and combustion chamber characteristics.

Just my 2 cents, of course. I'm not a real mechanic... I just play one on TV.

OK I'm confused. The piston and rings didn't come as a set? How is it the "Cylinder measurements are right on the money at every spot." yet you have a 50 over piston that fits, and you have STD rings?

If the bore is 50 over (most likely the last size OEM up) the nicks on the head are from a catastrophic failure (what SpaceX Elon Musk calls a RUD aka: Rapid Unscheduled Disassembly) a bore job cleaned up a damaged barrel. The damage seen in the head is not a big deal. Run it! You won't notice any difference between a new head and this one. It was not from a copper head gasket, preignition or detonation.

Never seen such thing as a ring tool to install or remove rings, just be careful and put one end of the ring in the grove from the op and walk it around and in the top grove. After you get it there, put one end of the ring in the bottom grove and work it around.

well thanks. I always find it better to own up to my dumbassery, ask questions and learn something. It's less painful than the other route. Haven't pulled it apart but will check & report back.

Don't fret, I'm confused too I am an absolute novice and teaching myself as I go. First time using a micrometer, measuring cylinder bore, ring gap, free gap, etc. Allow me to explain more detailed to see how I got to this point.

I measured the cylinder bore and compared it to the service manual spec. Somehow I deemed I was in the clear and still within stock bore dimensions (Obviously I was wrong but bear with me). Someone posted earlier that if my measurements checked out then I should just install new rings and it should be good to go. I ordered up NOS stock rings, base gasket, head gasket & away I went. The standard rings fit around the oversized pistons without fuss. There was, however, some slack between the ends of the rings & the locating pins on the piston itself. Like both ends of the piston rings didn't reach to the pins. A more experienced hand may have known something was fishy at that point, but I digress....

I believe the "50" on the head stands for 0.50mm (or 0.20in) overbore, with 1.0mm (0.40in) being the largest possible.

I shall now start disassembly again and try to learn more..