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Interesting...

I already suspected my dead carbs to be damaged by turning the mix screw in too tight... The exact size of this hole, and whether it had possibly cracked open a bit or not, was the only thing in the whole carb that I didn't check well enough to be sure.... And with both carbs the malfunctions were in the idle zone, they ran OK at WOT, but flaky at idle and picking up, and the mix screw had too little effect on idle rpm...

Your experience with the DR350 confirms my suspicion that I should be more careful when turning in the mix screw... And thinking back, I realise that this could also be the reason that it only occurred with Kymco carbs so far, because I never used this trick on any carb until I read about it in the Kymco manual (my previous bikes were all 2T and my cars never needed adjustment...)



Perhaps this is the source of a lot of other people's carb trouble as well ...

Ive never seen a 10" wheeled agility but they must exist

Yep, they do...  I own one, but compared to my 12" it feels a bit smallish and unstable.

It also looks funny when parked next to a 12" version, the top half is exactly the same as the 12", the bottom half is different, with small wheels, shorter engine case, shorter wheelbase, the front fork is thinner and less wide, and overall height is 5cm less...


Anyway, on topic : I wouldn't buy a 139QMB engine for a Kymco..

With those generic Chinese engines you never know whether it's just as good as a Kymco one, or whether it's made on a Monday morning by the cheapest manufacturer of the planet....

I hear a lot of disturbing things about this generic Chinese stuff... I would rather fix whatever is wrong with my machine, even if it means a complete rebuild - heck, It's probably even safer to buy a second hand Kymco engine instead of a shiny new 139QMB...

"Mine goes to 11!"

 

A 4-5% increase in cylinder bore can increase CR by .2-.4 ratio

This could only be important for kits that come without a matching cylinder head - and even then, the shape of the piston crown is the most determining factor, not the bore size....

Heck, I own a 72cc kit with a hollow shaped piston, which, when measured, proved to have almost the standard compression ratio with the standard head, although the displacement was increased by a factor of 1.4 ...

Apart from that, more compression doesn't always mean a significant increase of power. The compression vs. power curve is almost flat near the knock limit, which means that if the compression for your setup is already quite high, the only thing you'll gain is an increased chance of detonation...

_{(Interesting read for example : http://rescomp.stanford.edu/~efroeh/papers/RDH_Engine_Performance.pdf )}

Its funny how the Chinese don't know the same math the rest of the world does. I know mine quite well.

The math is obvious and simple, even for the Chinese, but the reason nobody cares about exact numbers in lots of cases is because they don't matter that much most of the time. There's no use, for example, bothering with a 4..5% difference in displacement if other variables in the same system make much larger differences in both actual and perceived performance...

Manufacturers just choose the actual sizes of their products for convenient design and production instead of aiming for round numbers. They round up the marketed numbers to the nearest common sounding number, because using numbers like 81.248cc in the name of a vehicle or aftermarket part would sound rather ridiculous to most people...

This is not something that's limited to Chinese manufacturers or motorcycle business, it's standard industry practice - and I'm happy with it, because it increases the signal to noise ratio of this already far too chaotic world 

Often it's enough to just remove the brake pads, pump the brake carefully to extend the pistons a bit (not too far, or they will fall out and drain the system all over the place  ) and just clean everything with a dry cloth and a toothbrush (no solvents)...

BTW it isnt any more chinese than kymco is!

Lots of Kymco parts are made in China, but Kymco does a lot of quality control, and they use only a few selected well known manufacturers, whose brands are imprinted on those parts, together with the Kymco brand.

This is completely different from the generic no-brand GY6 and 139QMB parts out there, like those cheap BBKs, which can come from any source and without any sane quality check - and if you let those manufacturers decide for themselves how to manufacture cheap sh**, it will not only be cheap, but sh** indeed 

just as i thought. what makes of bbk do people recomend?

I'm very happy with my NCY 85cc BBK (including a cylinder head with larger valves) so far : quality and finish looks good to me, all fits well, runs well, sounds well, no excessive oil use... My engine runs like it was designed and manufactured this way 

When you get used to the extra power and you're looking for something extra, I can recommend the NCY T2 camshaft, which is a perfect companion for a 72cc BBK...

It will not only increase power above 6000rpm or so, but it will also extend the powerband of the engine well beyond 8500rpm - which is especially noticeable in acceleration above 50km/h - it will make you smile, I guarantee