1980 GS250T Project

[Guest]

Here is a little background: My father-in-law inherited this bike from his dad and hasn’t used it since the late 80s, so he gave it to me and I am making it a project. I am new to motorcycle mechanics, and there have been several headaches already, but I have been enjoying it for the most part. The bike had low compression so I decided to tear down the top end to see how things looked/learn how everything worked inside (I have a service manual I am following religiously). I unfortunately noticed a vertical scratch on the wall of the cylinder (see photo) and am wondering if I risk putting it back together with the scratch. I am able to feel the scratch and it is in the lower half of the cylinder. I've read old posts about burning oil or lost compression, reboring and using over-sized rings, but I am a college student on a budget and have quickly met my "budget". Any advice would be greatly appreciated. Thanks in advance!!

Matt

Scratch.jpg

[fbody_mike]

Did you take sand paper to the cylinder walls? The horizontal marks don't look like the typical cross-hatch you get from honing.
How low was the compression? Engine cold when you did the reading?
On a budget you may just get away with getting a cylinder hone and cleaning up the bore a bit. How many miles on the odometer? If a lot maybe some rings too.

Its your first motorcycle project, so good on you for jumping in and not being shy about tearing things down. But....if it were me, I would first concentrate on making sure the tank is clean and rust free, then replace all seals on the carbs, set the valve clearance, new oil and battery, then see how the engine runs and shifts before getting into a top end rebuild.

[RichDesmond]

...Its your first motorcycle project, so good on you for jumping in and not being shy about tearing things down. But....if it were me, I would first concentrate on making sure the tank is clean and rust free, then replace all seals on the carbs, set the valve clearance, new oil and battery, then see how the engine runs and shifts before getting into a top end rebuild.

Much wisdom there...

[mjfinch]

I apologize for not including more in my initial post. I should have included that the bike doesn't run, I absolutely cleaned the tank, got a new petcock, checked fuel lines, went through the carbs more than once, got a new battery, checked the valves, new air filter, new spark plugs, made sure there was spark, and changed the oil and filter all before checking the compression and deciding to jump into tearing things down.

No sand paper was used inside the cylinder. I rented a hone from O'Reilly's and honed it for ~15 seconds in an inward and outward motion using lubrication (maybe I went at too high of a speed?). Compression was ~65 psi on each side, engine was cold. There are only 4600 miles on it but I did purchase some new rings anyway. Could I hone it a bit more to try to improve the cross-hatch/smooth out the scratch?

[fbody_mike]

Both cylinders being down that much equally means you should be able to find something similar on both sides. Your new at this, but even seasoned guys make this mistake from time time. Did you have the throttle held fully open as you were doing the compression check?

Were the original rings free to move around on the pistons when you pulled the cylinders off or were they stuck inside the piston's grooves. Sometimes on engines that sit the rings get "stuck" and will free themselves over time. But if they are stuck you will get poor compression.

As for the honing, I would try to get a better cross hatch going before a set of new rings go in, it will help them seal better. By the way, when I was a young punk, and our 2-strokes dirt bikes needed new rings we would use sand paper and oil to make a cross hatch. It was poor practice, but worked. Those cylinder was probably more of a hexagon than a cylinder too afterwards, but we all got to start somewhere, then again, or new head gaskets was the old gasket plus some spray paint.

[mjfinch]

Yes, the throttle was wide open. I did add just a bit of oil into each spark plug hole and that did bump it up some (can't remember the exact reading, but it was still below spec).

The original rings moved freely when I pulled the cylinders off pistons. I also forgot to mention (apologies again, noob) that when I removed the cylinder jug, the gasket on the bottom side of the cylinder was torn on the front side. There was a lot of oily/dusty crap on the fins and crank case, but it was in a dusty barn for a LONG time where they used to do mechanical work. Would that cause the compression issue or not since it is below where the piston sits in the cylinder barrel.

So for the honing should I just speed up the in and out motion? Less lubrication?

Thanks for the help!

[Rich82GS750TZ]

Honing

Disclaimer: Not a mechanic. This is just what worked for me. The one time I honed cylinders, about a month and a half ago.

I watched lots of videos. One of which was probably the one below. I ran the drill at roughly the same speed but only for about half the time as this guy.

I would not use less lube, she wouldn’t like it. The bike! We’re talking about the bike!

before:


after:


Personally, I think my hone job was decent. I did not check compression before, after, nor since. So no idea if it’s above or below where it was. Engine wasn’t running when I took it apart, but I know it had run. Bores looked glazed, so I honed them.

I do not know if any amount of honing will remove that vertical groove in the wall. I can only think that you had a problem with a ring that caused it, but I don’t have the experience to even qualify that guess.