Postmortem on First Engine Build

I currently have the third motor in the NC50.I have the original motor, stock, still runs. The first motor I built was pretty mild.It used a Treatland "Shocko" cylinder, stock cylinder head, MLM pipe, oil pump plugged, and initially a 15mm Delorto carb. Later I went to a 20mm Mukuni VM, because I was frustrated with the idle tuning by filing the slide. From 80s and 90s motocross, I was familiar with tuning that era's Mukuni's and even though I thought the carb was pretty big for the ports, especially the exhaust port. This motor would pretty consistently run a top speed of about 39 MPH. It started getting harder to start and I swapped it for the motor that is in there now.

I did a couple quick things when I pulled the motor.

• I pulled the carb apart and there was some water and dirt in the bowl. At that point, I had been battling with water leaking down the On/Off valve on the gas cap. I run a separate external fuel filter, so I don't have a good reason for why the bowl had some dirt in it.
• I pulled off the stator cover and there was oil. I was worried about that possibility. The crank seals are not very substantial, this motor spent most of its life at 10,000 RPM where the stock motor wouldn't rev much beyond the designed 7500 RPM and I had a slightly larger and heavier piston. My 80s Honda TRX250R quad, that I would race back in the stone age, I had making power beyond 10,000 RPM and it would loose a stator side crank seal multiple times a year. I used to have a habit of pulling the stator cover every weekend to look for oil.
• I lost the output shaft seal on the engine and it started leaking oil out the output shaft and down onto the rear tire. (And I don't really understand this yet, since the seal looks OK.)

I am finally going to take a look in the motor to see if anything else failed.

The compression guage I have, has a long hose on it and this is a small cylinder. I will do two measurements. The first is just wind the starter up and record the pressure, which is about 60 psi. Then, probably the more meaningful measurement and just keep cranking the motor over until it reaches a peak number.

It peaks about 160 psi, which sounds pretty encouraging. The ring seal was holding.

Now I need to do a leak down test, but the square exhaust port will be somewhat problematic.I will probably need to seal it with Compsimold.

Leak down test, completely failed. It sounds like it is coming out of the stator side. I will pull the flywheel to look.

The stator side seal is shredded. You can see flakes of rubber laying down near the seal. It certainly shredded that seal.

Single crank of the motor gives about 60 psi. I am not sure this tells me anything, but I will keep the number.

Cranking the motor until the pressure peaks, I get 155 psi. Which tells me the rings probably survived.

No good way to block that exhaust port off since its a weird square shape. I will just fill it full of Composimold.

Heat up some Composimold and pour it in. Wait for it to cool.

Leak down test is a total failure, I can hear the air coming out behind the stator. It's important to use a low pressure so you don't blow out the seals. My old rule that I learned from my mentors is the two stroke bottom end should hold 5 lbs for 5 minutes.

Pull the flywheel with the a generic moped flywheel holder from Ebay.

You can't see it well in the picture, but that seal is really shredded. The left side, L-side, oil side, seal looks good. This seal is the reason it was hard to start. There are a couple possibilities and it make be a combination:

• I have removed and blocked off the oil pump. This seal may get a lot more oil in a stock motor since the oil pump feeds that side of the engine.
• This engine would run about 10K RPM, which is higher than the stock engine. It may be that the little seal just can't take the RPMs forever. I used to race ATV motocross back in the 80s and 90s. I ran TRX250Rs with a stock peak HP around 7500 RPM and I would set the engine up to build HP through 10K. I used to the stator side seal frequently, to the point I would pull the flywheel off a couple times a month to check the seal. This might just be RPMs.
• I didn't have a seal driver for this little seal when I put this motor together. I know I put the seal in deeper than recommended and not totally straight. This is my best guess on the failure. I now have a seal driver that I am using to put these in.

Seal came out in three pieces. The seal spring was separated from the seal.