I have recently started working on 2 stroke scooters and a bike. I have been upgrading the 50cc engines to 72cc and most times things go well. I hope this provides some help and insight to others.
I literally just did a PGO Buddy 50 scooter that the crank bearings were rust seized and had to be replaced. This is a record of the problems I encountered and how I overcame them.
I chose to remove the engine, separate the case halves,inspect and clean everything up. I removed all the CVT components, exhaust, cooling fan, shrouds, cylinder head, jug, piston, flywheel, stator and crank sensor. Then I removed the flywheel "Wood ruff key" and placed it in a safe easily remembered location.
Next I looked at the crank seals from the outside and they were viton, which meant IF I were careful they were still usable. Next I removed the center stand, autolube pump, carburetor, intake, reed valves and starter.
Then I carefully looked over the engine for the bolts holding the case together and removed them. Corrosion still held the case halves together and I gently tapped them apart using a brass mallet and a small pry bar. But even being as careful as possible the case sustained some minor gouges in the seal / gasket area.
The crank stayed in the bearing on the cvt side, and needed to be tapped out with the brass mallet. The bearings came out easily using a small long punch that would easily go through the seal opening without damaging them.
Inspected the crank, rod and bearing revealed only some slight corrosion at the bearing positions on the crank. No other damage. Lightly polishing that with a ScotchBrite corrected the corrosion.
The correct crank bearings were sourced from Scooter Assassins and arrived in a few days from Taiwan. After cleaning everything and lubricating the seals with 2 stroke oil, I installed the bearings, then lubricated the crank and set it in place on the cvt half. I chose to make my case gaskets from photo copying the right case half onto construction paper. After cutting and punching the new gaskets out, I applied a thin coating of RTV sealant to the surfaces, installed the gasket, and assembled the halves by sequentially tightening the bolts to pull things together straight.
I rebuilt the autolube pump and pre-oiled it, inspected the reed valves, flipped one reed valve, and reinstalled them and the intake manifold.
Since the crank bearings had sized and someone might have tried to crank it and damaged the starter, I decided to open the starter up and check it. The commutator was OK, just normal brush deposits that were easily removed. The front armature bushing was a little stiff so I put a thin coating of silicone grease on the armature shaft for both bushings. Then bench tested and installed the starter.
I installed a Taiwan made BBK, reinstalled stator, flywheel, crank sensor, cooling shrouds and fan. Inspected and reinstalled cvt components, and exhaust.
Reinstalled the powertrain into the chassis and connected everything up, I replaced the fuel and vacuum lines with American made Tygon and installed a filter.
I had previously disassembled the Keihn carb and had it soaking. I removed it, flushed the cleaner with water, and blew through all ports with compressed air. Checked the jets for size and flow. The main jet was an 85. Reassembled, check needle and seat operation, then reinstalled.
I had previously flushed the tank, and installed 30:1 premix. Connected the fuel line, energized the fuel control valve and filled the carb with premixed fuel.
Flushed and filled the oil tank with Lucas 2 stroke semi synthetic high temp racing oil.
I started it up and had it revving a little while I adjusted the a/f mixture screw.
All of a sudden, it did a soft seize which I had never experienced. This soft seize is an indication of overheating.
Some causes of this are;
- air leaks into the crankcase (charge area)
- bad reed valves
- non working cooling system
- too high heat range plug
- clogged carburetor
- main jet too small
- air cleaner changes from stock.
I am sure some others can name a few more possibilities.
Using a cylinder leakage tester I applied 15 psi to the combustion chamber at TDC and it held, then I removed the carb, exhaust, and cooling shrouds. Next I mixed up some soapy water, reinstalled the exhaust and plugged it. Then plugged the intake port and applied 5 psi to the engine. Then sprayed the soapy water around everywhere it could leak.
A leak was coming from the autolube pump seal as I had forgotten to install a tensioning bracket. Installed and retested. No leaks at 5 psi, tested at 15 psi, no leaks but the reed valves were rattling from the reverse flow. I only did this long enough to quickly look.
Put everything back together, start it up and another soft seize. I checked the plug and it was dry, so I went one heat range colder. Restart, another soft seize.
I pulled the carb off, checked the inside diameter of the main jet and tried a 100 main jet for a CVK type carb. Still soft seized, but not as quickly. Using the 100 CVK main jet, I drilled it open to a 105 equivalent. Restart, no more soft seize.
Keep in mind that this soft seize testing is in the shop, in the field it may prove a need for further jet size increases.
NOTES: I had a hard starting problem on one 2 stroke that had a slight exhaust leak at the engine.
Compression could easily be increased on these by substituting copy paper made gaskets for construction paper type gaskets or the OEM gaskets.There is more than adequate clearance to do this, however it might increase the heat more and require main jet up-sizing or increased oil.
I have found the difference between some of horizontal Minarelli engines is the 'clock position' of the exhaust studs. On the more common type they are at 10 and 4 o'clock, on the less common type (read as premium scooters) they are at the 2 and 8 o'clock position.