2 stroke bike on a budget

[dr4g00n]

Got one of those "80"cc 2 stroke bike kit's off ebay back in November to keep me occupied over the winter. Pretty much just using it as a test bed to learn how to tune a 2 stroke properly so when I get a real bike I don't blow it up. 
Surprisingly it's still holding up pretty well for a $100 motor, it needed some work done to it like cleaning up the ports and transfers and removing casting flash but otherwise it's seems decent out of the box.

Of course I didn't leave it stock because that would be kind of boring. 

Did a few things like,
Lapping the head and cylinder.
0.11mm brass head gasket to raise compression (stock is 0.72mm). 
Port matched intake and exhaust. (Still need to case match the transfers).
Honed and added a radius to the mouth of the carb. (It's a bit small at 14mm but if I'm to upgrade it I might as well get reeds also and redo the entire top end).

And most importantly, I hand fabricated a simple expansion chamber and silencer to give it a bit more power. Pipe is spec'ed for a wide powerband to make it more useful off road. Though it's still a small 2 stroke so the low end is absolutely gutless but if you don't let it bog down it actually pulls alright.

Stock rear sprocket is a 44t which is what's on there now but I ordered up a much bigger 56t sprocket to try out, should help keep it in the power at low speeds and maybe do some wheelies too if I'm lucky. 

[zila1]

That is so cool.

[Macsbeach98]

Looks like a lot of fun can you ride it n the road legally there?

[dr4g00n]

Apr 21, 2020 17:55:37 GMT -5 Macsbeach98 said:

Looks like a lot of fun can you ride it n the road legally there?

Sort of, technically it needs to be 49cc or under and keep it below 25km/h to not require anything at all. This one's a 70cc but as long as I keep to that speed limit I doubt anyone would care.

[sphincter]

Sweet!!    

I've had thoughts for mine like that, but i need the exercise.

[dr4g00n]

Apr 22, 2020 22:01:14 GMT -5 sphincter said:

Sweet!!    

I've had thoughts for mine like that, but i need the exercise.

Still pretty exhausting to ride around on trail's, no suspension means I'm standing up a lot more than I'm on the seat. I've considered getting forks for it but a brand new bike from Canadian Tire is only a bit more (a cheapy obviously but has suspension forks and a shorter frame).


In other news, the 56t sprocket showed up yesterday. Got it installed and it makes the bike much nicer to ride on rough ground, not having to slip the clutch a bunch to keep it from bogging.

Edit: Took it for a quick rip around the yard to actually give it a test. Pops wheelies with ease if I snap the throttle open when just under the powerband.

[dr4g00n]

Did a couple of tweaks after the silencer started falling off again for the third time.

Changed the pipe from a rear-exit to a belly-exit design, this made it much quieter and shouldn't impact the performance any since it doesn't rev very high with this pipe and the stock CDI. The weld looks horrible but the stinger is stainless so it didn't want to weld nicely.
Wish I could get the engine itself to quiet down, sounds like a ratting bucket of bolts.

Also made up a better intake/filter for better flow and somewhat less dirt in the engine, but now I need to angle the carb off to the side a bit. As it is the rear tire slings water & mud straight into the filter which is of course not the best, although hardly anything makes it through.

[ShrimpBrime]

That's pretty fucking cool.

[sphincter]

Looks like your using a wire-feed... you would have to get a spool of 309 mix n match wire to really make that work. theres a chance you could just pile up the material more and itll hold forever.

I welded a downpipe up for my buddy's old VW hatchback. I had used a stainless piece somewhere because thats what I had layin around. I welded the hell out of it (Tig using 309 Rod).
He never told me the car needed a flex coupler and I didn't have one layin around(this was an overnight project)  he said he "had all the parts" 
So anyway I welded it up it lasted a long time, but when it broke, the weld was just fine.

It cracked his cast manifold!  

[dr4g00n]

May 4, 2020 21:09:49 GMT -5 sphincter said:

Looks like your using a wire-feed... you would have to get a spool of 309 mix n match wire to really make that work. theres a chance you could just pile up the material more and itll hold forever.

I welded a downpipe up for my buddy's old VW hatchback. I had used a stainless piece somewhere because thats what I had layin around. I welded the hell out of it (Tig using 309 Rod).
He never told me the car needed a flex coupler and I didn't have one layin around(this was an overnight project)  he said he "had all the parts" 
So anyway I welded it up it lasted a long time, but when it broke, the weld was just fine.

It cracked his cast manifold!  

Yeah, I ended up doing the latter. Doesn't help that I'm sticking a 2.5mm wall stainless pipe to 1mm mild steel.

May end up filling it in with bronze rod and whittling it down with a file to smooth it out.

[dr4g00n]

Scavenged cheap springer forks from an old bike I have around although they were for a 24"er so I had to weld on 1" extensions for the brakes to line up properly (I put the brakes back on after I took this pic). Works pretty well to dampen the small bumps, I can go much faster before it starts getting sketchy now. They only have 1" of travel though so they're still a bit crap, any decent sized rock will bottom them out easy.

I'm liking the side-bleed pipe design too I can get the engine rpm pretty low without it bogging for crawling over rough stuff but still revs up quick when I give it the beans. 30-35km/h is about the top speed with this sprocket though and the engine has no top end.

 

[dr4g00n]

Spent some more time tweaking and tuning the bike. Went the next size up on the main jet to .65mm rather than .60mm which improved the top end, having a plug in the right heat range helped a lot. The original M14 threads stripped so I tapped it for M18 plugs, though because the plugs are larger I had to dig around for one in a pretty hot heat range.

I also noticed an air leak at the carb clamp so I just glued the carb permanently to the intake, can't take it off without removing the intake because of the air filter anyway. It actually idles smooth now and doesn't die while coasting down hills.

Did some head work too. The stock head is sized and shaped for the 40mm bore 49cc versions so it was impossible to set the squish gap for the 47mm piston. Just used the die grinder with sanding drums to reshape it, brought the squish band to the full bore size and reprofiled it to match the dome of the piston.
Now the squish can be set to the proper 0.8mm-0.6mm of clearance.

Though I now have a problem with the head gasket sealing, seems the head is too thin and so when it's torqued down it creates a slight bow on the sealing surface behind the plug. The leak is only very small though so it doesn't seem to effect it any.
Probably just going to replace it with a much larger PUCH Airsal 47mm head later on, they're only about $50 usd.