Probably not, the size difference is too large. I doubt the crank is long enough to fit the width of a 390 you would need to extend the crank 
.
I don't have a 120 but just comparing the size of a 390 case to a 200 case the difference is quite large. Then all the machine work that will be needed. Possibly lengthen the crank, weld and regrind journals to make a rod fit. You are most likely going to need a JR Race Car rod and I'm not sure they would fit a 120 journal.
Probably easier to just get a 390 crank worked over: weld, grind, balance, lighten, knife edge :thumbsup:
.I don't have a 120 but just comparing the size of a 390 case to a 200 case the difference is quite large. Then all the machine work that will be needed. Possibly lengthen the crank, weld and regrind journals to make a rod fit. You are most likely going to need a JR Race Car rod and I'm not sure they would fit a 120 journal.
Probably easier to just get a 390 crank worked over: weld, grind, balance, lighten, knife edge :thumbsup:
ahh found the picture in my album
Here is a top view of a 390 and 200
Message - Old Mini Bikes Photo Gallery
Here is a top view of a 390 and 200
Message - Old Mini Bikes Photo Gallery
As others have said, it won't work. The GX120 has a crank with even smaller journals than the 160/200 and won't even come close to fitting in a 390. The only real difference between a GX340 and 390 is the bore and it's the same with the 240 and 270, so there is nothing to be gained by swapping cranks. The 240/270 journals are smaller than those of the 350/390, so that won't work either.
In addition, the rod to stroke ratio in these engines is not the limiting factor; valvetrain is. There is no need to get R:S ratios over 1.8 unless you can come up with a way to control the valves beyond 11,000rpm.
In addition, the rod to stroke ratio in these engines is not the limiting factor; valvetrain is. There is no need to get R:S ratios over 1.8 unless you can come up with a way to control the valves beyond 11,000rpm.
I believe you. It's moreof a learning curve for me. That's why I'm not whoring too many new threads and just sticking to a few.
How long do these live at 10k rpm? I've been doing some homework and watching the GY6 scooter crowd more than minibikes. My build-up turned out physically much larger than a Doodlebug, closer to an actual scooter with a heavy front fork assembly. I figured too much RPM would kill the motor quickly in a heavier bike just trying to move it around.
There's plenty in the yard bike crowd doing 40mph+ on a TAV and a governor. What speed are they going sans governor? 55-60 is still really all I want it to do. It's a scooter with some scoot, not really a speed machine in the fullest sense.
If I had a TAV2, this is about how fast it would go. My bike looks a lot like this one, actually.
YouTube - Mini Chopper
Gotta love Porkchop, too!
YouTube - porkchopminichopper's Channel
How long do these live at 10k rpm? I've been doing some homework and watching the GY6 scooter crowd more than minibikes. My build-up turned out physically much larger than a Doodlebug, closer to an actual scooter with a heavy front fork assembly. I figured too much RPM would kill the motor quickly in a heavier bike just trying to move it around.
There's plenty in the yard bike crowd doing 40mph+ on a TAV and a governor. What speed are they going sans governor? 55-60 is still really all I want it to do. It's a scooter with some scoot, not really a speed machine in the fullest sense.
If I had a TAV2, this is about how fast it would go. My bike looks a lot like this one, actually.
YouTube - Mini Chopper
Gotta love Porkchop, too!
YouTube - porkchopminichopper's Channel
It depends partly on how much power the engine is making. MDB would know better than me, but I would guess an hour total at full power, maybe two hours tops. Others have said before that RPM Ruins Peoples' Motors and I tend to agree. They last a lot longer if you don't rev their guts out. That said, a short stroke/long rod engine can sustain higher rpm longer because the piston speed and stresses on the reciprocating assembly are lower. The valvetrain is still slapping for all it's worth to keep up, though, and will still suffer. ARC has a long rod kit for the GX200s, BTW.
I have done some research on this in the past and I guess if someone has nearly a $1000 burning a hole in their pocket, they could put a GX160 crank in a GX200, install an ARC GX200 long rod, a Wiseco 1982P120 piston, and buzz ~.090" off the block. That should get 163cc and a very high rpm-friendly 2.09:1 rod:stroke ratio. The problem is still the valvetrain, and I don't mean just the stuff in the head. Cams and lifters take a thrashing with big lift and RPM. I haven't verified that the above parts are the right ones, so try it at your own risk.
I have a Champion GX200 clone from Costco(from when they were going for $69.99 :biggrin and a Honda GX140 crank(same stroke as the 160) and I will probably try doing this sometime, but I have some ideas for better holding the valvetrain together.....and for a four valve head, but that's another story. :wink:
I have a Champion GX200 clone from Costco(from when they were going for $69.99 :biggrin
and a Honda GX140 crank(same stroke as the 160) and I will probably try doing this sometime, but I have some ideas for better holding the valvetrain together.....and for a four valve head, but that's another story. :wink: