.25 over the line - new CB125 piston installed.

I found a spare hour to drop the new piston into the freshly bored CB125 engine.

I'm bored

Well that was fast. So much for the 6 week turn around time I was quoted. RB Designs had that cylinder bored out and shipped back to my door in under a week! That's exceptional by itself, and absolutely stunning when you consider that they're all the way on the west coast and I'm in VA. They must have had it bored and shipped out the same day they received it.

It looks good! I can't wait to try it out.

I owe  RB Designs  a huge thank you for knocking that out so quickly (and affordably). Also, bonus points for the fantastic packing job on everything. I feel downright embarrassed about my packing/shipping skills after seeing that.

CB125 Cylinder Leak

So even after finding a replacement dowel for the CB125 motor, it still is not up to par. In fact it seems worse now. I pulled and inspected the rings, reassembled and measured and they seemed fine and within specs but the cylinder is now showing a 12% (best reading) loss. That's about 3% worse than before. I can hear the hiss coming from the crankcase as the cylinder bleeds air. The only logical explanation is the piston and/or rings. Rather than screw around with it any longer I'm just going to have it bored .25mm over and go from there. I'm not even going reassemble it fully to do a compression test. On the upside the head seems fine and I guess I won't be needing those new valves I bought :-/

CB125 parts motor

I pulled the motor out of the CB125 parts bike yesterday. It wasn't pretty.

Those of you with weak constitutions might want to skip this post.

Yes it's as dirty and rusty as it looks.
And yes the floor is covered in PB Blaster.

A sign on things to come. The carb is filled with mud.

The bolts came off and motor dropped out without issue.

She's dirty but seems to be in fair shape.

A bit of Simple Green a scrub brush and a hose can work wonders. 

This used to be oil.

That sure is some corrosion. The piston was rusted in place.

The valves had seen better days too.

Lots of scraping, cleaning, PB, heating, and hammering
with a socket freed the piston from it's cylindrical prison.

Think it might just need a hone?

Yeah...

The dowel came out easily and has found a new home in the good CB125 motor. Funny thing though, this motor was also missing a dowel; just a different one than the good motor.

Finding the source of the CB125 compression issue

Well I knew I wouldn't be able to sleep a wink without knowing why the hell the CB125 motor won't hold compression. Seeing as how I can use all the beauty rest I can get (I am not a pretty man) I cracked into the motor as soon as I got home this afternoon.

It took about 20 minutes to get the motor off the bike, partly because I was slacking, and partly because I got distracted a bit. Once the motor was off, the hunt was on...

It took about another 20 minutes to get the motor apart because I completely forgot that I needed to remove the rear axle to pull the rotor again.

Axle / makeshift rotor puller removed.

5 minutes later this is how she looked.

Things seem good so fa-oh wait a minute...

One... Two... Shit.

Well I somehow missed that there are supposed to be dowels around three of the cylinder studs. Probably because that one was missing to begin with.
Do I think this is the cause of my problem? Probably not, at least not entirely. I'm gonna replace it though and see how that affects things before I start boring out cylinders and cutting valve seats.

Now here's the kicker... I don't really feel like waiting 2+ weeks to get a 10x14mm dowel from Honda, and I don't have to. However, solving this issue ASAP requires me to deal with this:

Freshly hosed off, this baby is about to get torn to pieces.

Don't get me wrong, I am TOTALLY appreciative that this bike is here for the picking (props again to Bill Becker for hooking me up with this bike.) Unfortunately, the motor is completely seized and there isn't a single thing on this bike that isn't rusty, muddy, bent out of shape or all of the above. Also the lack of a center stand should make things interesting.
Given my lack of frustration so far, it's all I can do to not hang the bike by a noose around it's head and chop the motor out with a sawsall and a mechanical laugh. Before that happens though, I'm going to float the bike in some PB Blaster and call it a night. Maybe tomorrow will hold some good luck for me.

Well now I'm just confused...

I'm taking another stab at working the CB125's issues out.
Since I couldn't keep the cylinder at TDC for a leakdown test, I removed the tappet adjusters so the valves wouldn't engage at any point during the stroke. With the cylinder at BDC of both compression and exhaust stroke I got this:

85 out of 90psi is about a 10% loss

I'm showing less than 10% loss. I know this is totally acceptable in larger bikes, but I'm not so sure if this is good for this particular bike. I'm going to remove the carburetor and exhaust entirely and re-do the compression test. We shall see how it goes from there. If that turns out okay I'm completely stumped and I suppose I should re-check my timing? It was spot on when I last looked, maybe all that kick-starting changed something though...