I've had this bike for years, tinkered on and off with it, but never really dug into it until recently. For the longest time, it would just not start, not even a puff with starting fluid. And that was with 125psi compression and good spark. Tracked down my woes to bad crank seals, swapped those out, (re)cleaned out the pilot jet, primed the oil pump, and cleaned the points and finally got it running the other week! It's fairly low miles (2880km or 1790mi. It's a JDM bike.) and always been in either a shed or garage so internals and all that are fairly clean. Put about 70mi on it riding around (and learning to ride, it's my first bike) before running into trouble the other day.

I had just bumped up the pilot jet one size and had the bike idling to test, it was a big improvement on throttle response from idle, definetly what the bike wanted. Anyway, I was making the last few adjustments and trying to get the brake light switch working correctly when the engine make an awful screech noise and seized. Couldn't even kick it over with the kick starter after it cooled. I didn't think it had overheated, it wasn't even that hot to the touch, must have seized the piston cause the oil pump died (it had good flow out the bleed screw, but no pumping action with the throttle held wide open and turning the pump by hand).

I pull the head off, all looks ok, gently tap the piston with a brass rod and a 't the piston. There's a tiny gall spot and some scoring down below the ports, but nothing that looks too bad. Used some fine sand paper to remove the aluminum from the bottom of the cylinder and stoned the high spots on the piston per the manual, rings look a bit wonkey in that spot, but who knows if that's new or old? Engine is still locked up hard though, must be crank bearings. So I pull the engine out of the bike and start tearing it down, pull the flywheel off annnnnnnd one of the magneto screws had backed out into the back side of the flywheel. Whelp, that's an easy fix, do both magneto screws up with loctite this time and put everything back together and put the engine back in the bike and fill the tank with pre-mix (cause the pump is dead, I will rebuild that over the winter).

Here is where the fun starts, it made about two puffs while kicking and that was it, nothing after that, even with starting fluid. Ok, back to this mess, check spark, got spark, have fuel (starting fluid), nothing. Check compression, 115psi, not bad. Maybe timing got messed up, check that, oh, that sharpy mark on the flywheel is 1.8mm BTDC, good job past me. Clean the points with a super thin fine file and then set the timing, it was off. Probably from the magneto getting out of place. Anyway, now I have nice strong spark set at the correct time again, more starting fluid and

Ok, recheck compression and it's 90psi now. The heck? Spin it over fast with a drill, yep, 90psi. Put half a CC of 10w30 in there cause maybe the oil from when I put it together gave me my first good compression number? Yep, back up to

TLDR: Took the top end off cause I thought I had sized it, turned out to be magneto screw backed out into the flywheel. Put everything back together and now compression is only 90psi and I cannot get the bike started when it was running great just the other day. What are the odds rings will fix my problem or should I look elsewhere?

Also, what material are the rings made of for these? If they are chrome I need to deglaze, if they are cast iron, should be able to just slap them in and go.

Leaving this open ended cause I am sure there will be several more problems I run into on this bike (I already know the blinkers aren't working right and the front fork seals are shot. It will be fun seeing what other issues I find as I resurect this bike)

My guess is flywheel key gone walk abouts while flywheel was off. Compressions a bit low but not the cause inmy opinion. Had a lot of fun on my HT1 in recent years. Barn find--well--outside hidden by overgrown wet weeds n stuff.

I wish it were as simple as the key being out of place. The keyway and the key are perfect, the flywheel itself was still on tight, it was one of the little countersunk screws that hold the magneto plate that had backed out into the flywheel.

I am going to try a hail mary today and anneal the head gasket and reseat the head, but I have low hopes of that working as that oil on the rings really changes how much suck I get through the carb. I think it just isn't moving the air from the crank case up into the cylinder well enough.

Annealed the headgasket, compression came back , whatever, it happens.

Bike was still not running though, had spark, but not as strong as yesterday, so I triple checked the timing and filed the points lightly again. Still no dice, changed the spark plug to the old one 's running fantastic. Through the next size richer main jet in there cause I switched to premix. It had a factory 75, apparently the Japanese bikes are jetted different? I had tried an 85 before but that was too rich and made it unhappy, so I went back to the 75. Anyway, popped an 80 in there for safety cause I have 2R mixed at 32:1 (3.125%) so that would be like a 78 jet for the same fuel.

Already have a slightly richer pilot in there, idle air screw lands perfectly at 1.75 turns out per the FSM, and I had moved the needle one clip richer before when I was tuning it up (It wasn't perfectly happy at partial throttle and now it is). About to ride out to the bike shop to pick up the battery they happen to have in stock, we will see what kind of fit this bike throws next though and I will come back here to ask about it, lol.

 "changed the spark plug to the old one 's running fantastic."

Welcome to the world of counterfeit/crap repro parts! 

Anyway, well done on your work so far. 

"Welcome to the world of counterfeit/crap repro parts! "

OR possibly just "standard" NGK or Champion reliability out of the box?  

Back in the 80's and 90's I had a lot of trouble with brand new from the dealer plugs failing immediately or quickly.  I started "pre-running" every plug and NEVER starting a race with a brand new plug.  

I ran Champion stickers because they paid contingency and I could get free "gold palladium" (N2G) plugs from the rep at the races.  They failed so often I switched to NGK.  With NGK roughly 1 out of 10 would fail brand new.  

Switched to Autolite and I can honestly say I've never had a bad one yet.  Still using them today.

Not sure about Champion, but NGK seems better today than they did then.

I started using Autolite 4092 per MarkT on my Ht1 runs better I get mine from Amazon Thanks MarkT. You do need to change your brake shoes 50 yr old shoes can delaminate very dangerous 

So these are (were) NOS NGKs of the correct type. It was sparking just fine and was the one I had been using to try and dial in the pilot jet. I think it had just been in and out so many times it wasn't sealing anymore? Who knows.

I will look into the Autolites! and I have some NOS brake shoes I bought forever ago (more than 15 ), but that's good to know about delam! I will get some in this winter when I plan on doing a bunch of work.

Also, you know how these don't have voltage regulators and like to kill bulbs if you don't have a battery? Well I only learned that yesterday (and now have a working battery finally! The local cycle shop actually had an old interstate in stock that they were more than happy to sell me for $15 cause it had been in their inventory for more than 10 years at this point. They put some acid in and charged it up and it seems to be working). Anyway, long story short, one of the headlight filaments is dead now and these suckers are expensive.

I don't think a 5.75" sealed beam will fit? The trim ring is only a little over 5-3/8 at it's widest, were the JDM and USDM headlight buckets different?

The 125 and smaller bikes had smaller headlamps.  

USA LT and the HT had the same (6v 25w) sealed beam.  JT and GT had the same size but lower wattage (6v 15w).  AT is the same size but 12v in the US.

I think I have a brand new and rare "replaceable bulb" Yamaha headlamp, bulb, and bulb holder someplace if you can't find something.

P.S.  Have you removed the headlight yet?  JDM might have had the replaceable bulb lens/reflector?  If so, bulb shouldn't be hard to find.  It was the US that was a stickler for "sealed beams".

Yeah, I have the headlight out now and it's a small "sealed meam". It's really a slight bulb inside a housing, but it's all sealed up and soldered in.

I am anoyed I blew one of the filaments cause this one was working great earlier last week. Gonna have to come up with some sort of solution for it though.