My thoughts on Jets & Air box....
1. I tried going from the stock #
The jet that provides the most benefit is the pilot jet because from sitting the passage that it delivers gas too is very small and difficult to clean so often the stock pilot jet is not able to deliver.
My Notes: 04-15-2014
Installed and tested three pilot jets: #58, 42, 52. Stock is #38s. #42 still hesitated, #58 too rich - exhaust smelled nasty like gas at idle - made my pants smell, #52 - no hesitation (pilot screw set to 2 turns out). Then on the last test drive the Drain screw for the bowl fell out because I forgot to tighten it and the carb ran out of gas and stalled. I had to push the scooter home ~ 1/2 mile. Put epoxy in the carburetor bowl until the drain screw arrives.
Main Jet: 05-21-2014 Stock jet is Kehin #102
Scooter, changed the jets again. Experimented with #115 main - way, way too big. Changed to #48 for the pilot jet. Works great. #45 was lacking in power a little. #46 might work.
I also tried a #105 Main but didn't notice more power just reduced mpg.
I disagree that the stock air box is restrictive. The air filter is HUGE for a 250cc engine. It is close in size to the air filter on my '92 BMW K75S which was 750cc! Even if it has some dirt on it it would take a lot before air flow was reduce to where the motor was starving for air. For reference I read a test where a dyno shop took a stock k75s and ran it 3 ways: 1. no air filter 2. stock air filter 3. stock air filter 50% blocked with plastic. All three tests returned identical results across the rpm range! The reason by the way is the guy complained about a "flat spot" which showed up on the dyno. The solution? Remove the aftermarket exhaust and go back to stock exhaust problems solved.
I agree that on small scoots 50- 100-150cc jetting can be fun and cool and yield noticeable gains in power but with the Kymco 250cc the aftermarket options are more limited, there is more than enough power to blow-away any 200cc and down scoot no matter how much it is jetted.
Obviously, if the engine is modified with a "Big Bore Kit" and / or the exhaust & air box are modified to increase air flow - then the jets must be enlarged as the engine will run too lean. Still, for the time and effort and poor mpg that will result if you really want more power you are still way better off going to a 350 - 400 - 500 - 600cc maxi scoot. There is a place for the custom scoot with modified engine, exhaust etc but before putting the time, effort and $$ you should make sure you are doing it for the right reasons.
Also, before modifying.... always tune it perfect stock first. Ride it for a month then mod if that is what you really want. Most people take a carb with dirty jets or a scoot that needs a proper tune up and instead of tuning it properly (stock) they start modifying. My 2005 GV250 now has 20K miles and it runs perfect. I'm ready for more power mainly for the highway so I'll get something bigger like an BV350 or a 400cc scoot. I ruled out Burgman 650 because it is too big and bloated.
Read my post here:
http://www.kymcoforum.com/index.php?topic=11323.0
