Well, all the parts came in and I just got everything together and it's running well. Thanks to everyone who helped get me setup. And thanks to the forum as it's a wealth of invaluable information. In my attempt to give back to the forum, here's my consolidated story for others going down the same path.
Initial Assessment and Cleaning:
I picked up a stock 2000 SA50P with 2k miles. Previous owner had crashed it and replaced some parts, with some parts left with minor damage. There was an oil leak by the oil level sensor. It looked like the 3 o-rings making the seal there were a little sketch. I don't know if this is a common problem but a quick internet search showed at least a couple others who had the same issue. It might have been fixed by replacing the o-rings or oil tank (if it was the tank neck that was deformed). I didn't see any other leaks or places on gaskets where there might be an air leak. I drained the transmission oil but there wasn't much in there and it was pretty black. I'm guessing the original owner never really did any maintenance to the bike. Upon breaking her down for cleaning, I saw that the airbox foam filter was missing. That was a bummer as I was expecting the worse in the carb bowl. Opened up the carb and a decent amount of debris was in there but nothing overly alarming. I'm a little surprised it ran as it did in that form. Cleaned the carb very well in and out with carb cleaner and compressed air. Dishwashing soap and concentrated Simple Green was great in cleaning up all the oily goo on everything and it's easy on the skin. For the tougher caked on stuff I used strong solvents. I baked out the exhaust (don't forget to remove the rubber kick stand stop!) but it didn't smoke out too bad. As it heated up, I shot some compressed air in there to assist in the smoke flushing. Next I took a wire brush on the angle grinder to work on the exterior of the exhaust. Worked all the rust off then painted it with Rustoleum 2000 deg engine flat black. I put a few medium coats on it. When I pulled the carb I found out the petcock wasn't working. Cleaned it out and did suck tests and it appeared to be working, but I just put fuel back in it for the first time and looks like it's still broke. I'll deal with that one later - it'll still flow fine for now.
Modifications:
I wanted power without the noise and my usage is very limited (basically I'm just using it around the hilly property). I thought a stock 94-01 SA50 variant with a couple power upgrades would do the trick fine. Since I wanted it quiet I would stick with the the stock carb/airbox. My plan was to just derestrict my SA50P, put in a 72cc+ BBK, and make a couple of transmission mods to round it out. I also decided (after some back and forth) to remove the oil tank, put in a plug, and do straight premix. So, I ordered:
- Malossi Cast Iron BBK 72cc
- Oil delete plug
- Polini center/contra springs (+7% and +15%)
- Koso 1k RPM clutch springs
- #88 main jet (Honda OEM) and a #74-#96 main jet kit (Naraku, for Gurtner carbs)
While the parts were being shipped I went ahead and derestricted the bike: (1) dremel out exhaust washer, (2) removed speed restrictor in front pulley, (3) cut out the extra tab inside airbox, (4) replace main jet with something bigger and since I was going BBK started with a #88. Dremeling out the exahaust washer took a lot longer than I remembered! I used a cylinderical red stone bit at max speed. I wore gloves, hearing, eye, and inhalation protection. Removing the speed restrictor plate is pretty straight forward so long as you can get that front nut off the drive face. I used a big vise grip to grab the drive face plate, wedged a screwdriver to hold the vise grip to the engine case, and was able to work the nut free. Inspect the rollers for flatness before putting it back together. Be careful putting the belt and drive face nut back on so you don't strip the splines on shaft. I found the best way put it back on was to get the belt on the rear clutch pulley, then pull the belt plate (not sure what you call it) toward you to compress the center/contra spring letting the belt hold it in the compressed state. This gives the belt a lot of slack for you to work with as you put the front pulley together. With all that slack you should be able to put the drive face on easy and can feel when it's sitting properly in it's shaft splines/grooves.
While I had the transmission apart I pulled the clutch springs and center spring out. I don't have a clutch tool to hold the clutch bell so I used the vise grips to grab between two of the face holes, than wedges it to the ground with a big pipe/wrench/whatever. Now you should be able to work that nut off. Once the bell is off, you got another challenge getting the big (39mm?) nut off the clutch assembly. Since I didn't have the socket, I once again did it the "whatever you got" style. The nut was really stuck on there. I put some cloth down on the ground and laid the clutch down on it. Then standing on it with both feet on the edges, used a flat head screwdriver to catch the nut edge and hammered it loose. You might have to notch it in a little bit to give it a better bite location. I marked the nut/clutch with a sharpie line so I knew if I was making any progress getting it loose. Once you get it moving, remain standing on the edges of the clutch and work the nut free. Be aware that it's under the center spring pressure so keep standing on it till you're ready to release it! Then I reached down and put weight on it with my hands so I could remove my feet and release it. Next, to get the clutch springs out I used a vise grip again (love that thing) to grab the edges of the spring (on the more open side) and carefully pull it away to unhook the spring. Obviously you'll need a decent amount of force so be careful it doesn't break loose and you elbow the garage wall. I put the new springs in the same way. Since they were only 1k RPM they were relatively soft. When I put in stiffer ones, I had to use a flathead screwdriver to fulcrum them into place, but I don't like that way since I tended to scratch the clutch a little bit. While I was in there, I scuffed up the clutch pads a little bit with coarse sand paper in a cross directional motion (putting small scratches perpendicular to the direction the pads normally engage). I put everything back together basically in the reverse order.
Once all the parts came in I installed the BBK. The kit additionally includes a carbon fiber reed (didn't use), main jet for the Dio (unable to use), an exhaust gasket, and two exhaust mount bolts (but no nuts). I pulled the old head/cylinder off and scraped off any remnants of cylinder gasket material from engine case. I put in some shop paper towel in crank area to catch debris then shop vac'd out any little bits. Getting the piston clips off to remove the old piston isn't too bad with needle noise pliers. You only need to remove one side then you can slide the piston pin/shaft out of the open side to remove piston (careful the piston pin bearing may fall out of the connecting rod so take note).
Now is a good time to install the exhaust studs into the new BBK cylinder. Use two M6 nuts put in back to back to drive the studs in. The short side of the studs goes into the cylinder. You spin one nut on the stud almost to the end, then put a second nut right behind it. Together these act as a locked on single nut you can use to drive the stud into cylinder. I put high temp anti-seize on the studs before going in. After those are on, I washed the new BBK down with dishwashing soap and a toothbrush. Blasted it with compressed air and left it to dry. After dry I coated the bore, piston, rings, anything in physical contact with stuff with 2-stroke oil. Put the piston rings in carefully trying not to scratch the piston. After that, get one of the piston pin clips installed. I found it easier to work on the right side (exhaust side) of the bike as the carb isn't in the way, so I installed the first pin clip on the opposite side while the piston isn't mounted yet. Make sure you have the "EX" stamp oriented the right way toward the exhaust when you're doing this. With the clip in, the piston should go in without drama (make sure the piston pin bearing is still there!). Now the hard part... Getting in the second piston clip while the piston is installed. I used thin needle nose pliers to grab the clip close to it's edge/opening. Then try to guesstimate how much the clip will compress as I twist, so that it will just clear the edge of the piston and let the nose of the plier drop into that little slot in the piston. Careful not to over bend the clip as it needs to be tight in it's slot or it'll come free and ruin your piston/cylinder/day. Now's a good time to put in the cylinder gasket. After that, the second hardest part of the install. Getting the cylinder over the piston. You got to find the notches in the piston ring grooves by rotating it so that as the rings are compressed everything is flush. Take mental note of where these notches are. I prefer to install the cylinder one ring at a time. First I compress the top ring making sure the notch/ring groove is correct and while holding it with one had, drop in the cylinder with the other. You might need to twist a tiny bit to get it to fit in there. Once that top ring makes it into the cylinder, repeat for the second ring. Go slow as you don't want to unduly scratch the rings/cylinder. Buttoning up the rest is pretty straight forward. As the spark plug angle/location on the Malossi head is slightly different than the OEM head, you'll need to either remove the rubber spark plug cap (that sealed the air gap) or you can cut out a little bit of the cover to fit with the rubber spark plug cap installed. I opted to cut the fan cover with a dremel. With the spark plug cap installed, there will still be a bit of a gap. I haven't decided if I want to do something about that or leave it as is.
Now the oil delete plug. I removed all the oil tank paraphernalia including the mount that holds the oil pump in place. To properly install the oil delete plug you additionally need (I think it should come with the kit as purchased) a shorter M6'ish bolt/nut (old bolt is kinda long) and a large fender type washer. You need this to hold the delete plug down into it's new location. If you can't find a perfect length bolt, you can install it with the bolt head down with it's threads facing up. For the washer I found a big one but had to dremel out the hole to fit an M6 bolt.
Engine Tuning:
So I just put everything together and got it buttoned up and did a first start. Started right away and idled for a bit, blipped the throttle a little and it dies. Restarted and let it sit at idle, it goes for a little bit then dies. Restarted and quick WOT and it hesitates but picks right up after 1/2 throttle, let it idle and again and it eventually dies. Any 0-1/2 throttle and it fades to death. My first thought is dangit clogged pilot, which I had cleaned well. Since there was all that other junk in the bowl (since no air foam filter) maybe I pushed something into the pilot holes. Pulled the carb, did a light test and everything seems fine. Cleaned it by blowing from inside the throttle body outward, hoping whatever is stuck on there gets blown outward. Put it back in and the same thing. Dang... I know it's in the pilot fuel circuit and not the main because of the throttle response. I'm thinking it feels like it's starved of fuel but I cleaned the pilot and can't change it out to something bigger. The only thing left is the air mixture screw which I had set for the standard 1-3/4 turn out. For giggles I close it down all the way and back out 1/4 turn. Boom, holds idle and actually has decent response. At the high end, 3/4 throttle and up feels/sounds pretty good. This is with a #88 main jet. I'm going to run this for a little while until the other jet kit arrives (and it sounds like it'll be awhile). I would like to try some larger jetting to see what she'll be happiest with, but #88 doesn't sound like she's lean. I also want to generally run a little richer because of my specific usage, which is basically start her up, WOT up a steep big hill, then turn off. I'm sure that's not the best usage for longevity. I premix 32:1 Maxima Scooter Pro 2-Stroke oil (synthetic). So there is it guys, get the air mix screw in there deep and #88 or up on the main.
I was never a big subscriber to all the different break-in procedures. Maybe I'm just ignorant. I suppose I do something that is in between a soft and hard break-in. My rules are: run it at the correct jetting or slightly richer, give it some idle warmup time, ride it but not too long and hard trying to keep the RPM reasonable, and definitely don't let it overheat. WOT is ok so long as it's short enough not to overheat and it can rest afterward. I'm ready for people to tell me I'm doing it wrong now
Transmission Tuning:
I installed Koso 1k RPM clutch springs and they feel just like the stock OEMs. Now that I think about it, I think the stocks are 1k RPM haha. Oh well. The engagement is very early but for my usage it'll be ok. If I could do it again I'd put in 1.5k or 2k RPM. I'm using the Polini +15% for the center/contra spring with the stock weights/ramp plate/pulley/gears/etc. With that combination the bike wants to lift the front tire. It's very torquey with a wide low-end power band before it up shifts. This works for me for now but my usage is very unusual. I'll mess more with the transmission after I'm more comfortable the bore and springs have settled in. If I was building this as a street bike I'd put in different weights/pulley/ramp plate/contra spring/gearing to even out the power band and provide some top end speed.
Now I get to dump untold amounts of money into making it look pretty and provide no performance usefulness whatsoever.