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only in first and second. it's geared very very tall. the front and rear sprockets are almost the same size. the rear hub has to be machine to get the next sized down sprocket which i will need but, again this is all another. story. when i pick that project up again, i'll try to cross post my on going build log from planet minis forums over here.

Any ways if i source some springs or find any thing cool about crank balancing out i guess i'll post back. i found some cool information over OMB forums about crank balancing.

juicemotoparts.com/crankbalancing

www.neme-s.org/Model_Engineer_Files/2924-Balancing%20Single%20Cylinder%20Engines.pdf

i'll try to get some specs on my springs and post back for what it's worth. If i don't forget and fall off the earth.

Displacement is being increased. I've though about balancing. The stroker cranks are balanced fairly well according to the seller. That doesn't make it so of course. At least they do have a price to reflect that though. And they and maybe one other seller are the only ones offer such a crank, i might be inclined to believe that some since it's isn't some run of the mill fake titanium coated part.


All sports bikes are dual over head came these days. Too bad they don't make a dual overhead came 139qmb head. lol


On displacement... This is somewhat mild build but, these are Chinese engines. I learned all about too much breathing. One of the first things i bought was a150cc carb with manifold. I was very quickly educated on stupid that is my Dad and friends. Kind of hard to increase performance if you have no vacuum to draw in the fuel. So eventually the intake was ported, valve guided measure and cut from the intake port/runner. (plenty of length.) a second set of valve shims added and the exhaust port totally left un touched friend said not enough displacement and compression to do any thing with exhaust side. and the gob of chicken booger looking weld bead ground out of the exhaust pipe flange and the carb adjusted.

Results were and are QUITE positive but, as we all know if it's not broke your not trying hard enough. So picked a 60cc jug kit with all the need bits for about 35 bucks. I went with 60cc for a few reasons. First and foremost my buddy used to be a deal and i got to ride of those jonways that was really a 60cc and not a 50cc. Even with my port job the old Jonways still had a way of getting up to speed faster and still had a bit more top end.

Yeah the old jonway had likely a different CVT and small tires but, we can't deny the extra 10cc didn't help. The OTHER reason is these engines don't like BBKs how many times we get a help me tread because some crappy BBK kit or job went sour? These 139QMBs were not designed for that AND they are Chinese. And 3rdly I keep the same carb and exhaust, i don't want loud pipe or LOOK WHAT I DID vibe/look.

I plan to keep the original head, port the exhaust ans deck it. IF the exhaust guide is long enough it can be kissed by by from the exhaust port just like the intake. Will probably swirl polish the valves too. I am also looking at the A9 cam for a bit more top end. STG has one for 20 shipped. It has more lift and duration. Not sure how the springs would like that. I know these springs are somewhat common size for small OHV engines, so the quest to find some seemed interesting.

The only other weak point would be to have the crank balanced. On the upside new piston and pin shouldn't weight much more than the stock piston. Currently it's seen red line a few times. one of the biggest imbalances i believe to the be the rear RIM and probably Tire to. It dances on the stand when revved and rear hums quite notable 49.9 mph (GPS) which is also the top speed and also red line. IF only it didn't have to be steep down hill with a tail wind. Also why i'm on the fence with the A9 cam but, for 20 bucks we could just try it out compare with the stock cam and look duration and lift changes.

HOWEVER if you deck the head you have to account for lift that could mean if switching back to the stock cam having to get the head deck AGAIN. On the other hand that's all a part of the experience. The best part about it is that it's all super cheap! the A9 Cam and jet kits is no more than $40 bucks. The scooter was free and so is tinkering in the shop with friends on the weekend. :-)


EDIT: the 139qmb stroker crank is more than i want to spend but, would help displacement and isn't totally off the table yet. i think that would take it to some where about 66cc. the 60cc kit is actually 62cc from what i calculated.

They flat out stated they haven't sourced/found any thing yet. At least knowing the size is a start. I think it might be possible to find a quality set maybe NOT from China.

Your absolutely right. However All those finite details they look over is one more tweak i can make for a bit more reliability or power. I'm not going fora 120mph TT 50cc class here or any thing but, i am having a bit of fun tinkering and modifying.

Cyborg I'll try to take that in to consideration and learn what i can about that.

Any one know the 139QMB Valve spring size? any by any chance know how many LBS the springs are or how to measure that?

Some kits probably already come with a honed cylinder. Honing makes a surface that helps "seat" or "break in" the rings but, if the rings aren't gaped it's probably all for nothing because then the rings are very possibly to big in around to properly seat and possibly score the cylinder wall.

However if it's a new, not used kit and the cylinder is already crosshatched i'm not too sure why one would need to hone it.

bang on it with hammer, not too hard but, hard enough that if there was serious rust, that it would knock that rust loose, which you would hear.

BANG! BANG! *sounds of rust falling and sliding down tubes.*

one guy put a 150cc in his mini trial 70 and went half way across the united states to Sturgis. he had to find shops along the way that would let him change his oil since 150s don't usually have enough oil for such a long trip.

His first engine blew then he got the same engine i have in my mini trail 50 and the last i heard it was still going.

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