2013 Agility 50-72cc

[zombie]

Put in the 100MJ, and the 7gr weights. The needle may have to stay up because the main might still be lean but you can also compensate with a larger pilot.

I use torch tip cleaners to open up small jets. Use the files to slowly open them up. If you go too big you can fill the jet with solder, and begin again.
It really helps dial in the size you need.

[JayAre]

Thanks Zombie! Here's my theory. The stock 16mm carb was just too small. CV carb's have to be sized to the displacement & air flow/volumetric efficiency, of the motor. So I'm guessing the increased motor displacement/air flow, was opening the slide too fast causing a lean miss. I tried opening the pilot screw/it bleeds air into the slide diaphragm to slow it's opening. But like I said, the 16mm carb is just too small. I can recreate the miss with the 20mm carb by turning pilot screw out only 1 turn.
So, last night I drilled a jet with #60 drill bit which is .040"/1.01mm reset the needle at the 3rd slot & have the pilot screw set at 2 1/2 turns. I picked up 2mph up hill, 29 on Hewitt st my test bed, & 2-3 mph top end 45. So I think I'm happy where it's at. I'm sure I could probably tune a little more out of it but I'm going to stay with the 8 gram weights to hold the rpm's down & maybe make the motor last a little longer. It's running stronger than our 2 stroke Zuma's on hills, & maybe 1-2 mph faster top end.
Here's my final set up. 72cc/47mm Big Bore with the big valve head from Enviromoto/E-Bay. Derestricted variator boss with 8 gram rollers. Bone stock on the rear/driven clutch, with the stock belt. Stock CDI. 20mm Keihin CVK carb 101 main jet, Stock pilot jet/has no markings, & 2 1/2 turns out on the pilot screw, 3rd slot down on the needle. The float level I couldn't find a factory spec so I set it 13mm to the seam in the middle of the float from 1 of the 2 small oval pads next to the outside edge of the floats/hope that makes sense I did ALOT of derestricting/grinding & cutting holes, in the air intake system/pre carb, with the stock pleated air cleaner. I'm at 600' to 1500' altitude. Opened the exhaust outlet to 1"/cut the old 5/8" pipe off drilled 1" hole & welded an exhaust tip back on. I set the valves cold at .004". It doesn't tick so I'm sure it's fine, too tight would hang the valves open & drop compression once it warms up. Betting & losing had mentioned most other models were set at .004" so I'm going with it. Thanks everyone! It's been a learning experience for me. I hope this will save somebody some headache's if they Big Bore one of these!

[JayAre]

Here's a great web page on carb theory I found, www.slideshare.net/J.T.A.JONES/carburetor-theory       
I had never messed with CV carbs before this just round & flat slides I found it very helpful, how the pilot screw controls the slide speed. There's some great info on all types of carbs. Personally I wish everything I owned was fuel injected. With the new block learn systems they've come out with, just enter some basic data/displacement, compression... & the fuel maps will tune themselves! Now if they could do that for clutches 

[zombie]

That sounds great Jay. Finding what works for you is the goal in all of this tuning business.
Thanks for the link on the carbs too. That alone is worth the price of admission.

The only point I differ on is the rollers. If you get the correct weight, it will not increase wot rpm. All they will do is hold a lower gear longer. Not by much but enough to help on hills or coming off corners.
It's a balancing act between the rear control spring, and roller weight.

No biggie at all, as long as you are happy w/ your bike.

7 years, and I'm still changing my 2t around. Odd thing is I have been more than happy with it in a dozen incarnations. I just always know I can get more from it.
My 4t??? Set it, and forget it. Done once, and happy.

[JayAre]

My clutch theory comes from racing snowmobiles. I started years back by reading a book, Clutch Tuning Handbook by Olav Aaen. It's about tuning CVT's/constant variable transmissions, belt drives or whatever you want to call them. The big difference between snomo's & bikes are, the primary/drive clutch uses ramps & rollers/weight to create force to over come a compression spring & it controls the max rpm's. The secondary/driven clutch uses a helix/ramp angles, with a spring that applies either torsional or compression or both to control the shift on my snomo's. So I tune to the motors max HP/target rpm, on the drive clutch & the upshift/backshift on the secondary/driven clutch. The rpms can vary if the 2nd is shifting too fast/pull down rpm's, or too slow/over rev. But like you said it's a balancing act. Find what works for you & go with it!

[zombie]

It is similar in these but much simpler.

Max RPM is dictated by the rollers, and the rear spring in combination. The biggest difference here is rider weight is the variable. Our CVT's have to be tuned to let a low HP engine deal w/ a high weight rider.

[JayAre]

10-4 My last race sled was the opposite, 700 lbs with me on it & 140 HP stock class. The biggest trick was hooking up. When I had good traction 0-60 was less than 3 seconds. Once the clutches were set up you'd pin the throttle to the bar & it would hold 8300 rpm's start to finish. Carb tuning was on the lean side because they always run the best just before it burns down. $$$ LOL

[zombie]

100% on the same page!

I considered 1/8 mile drag scooters for a little minute... That went out the door for speed record builds.
I'd rather blow a crank every few runs, than a piston every run. I have a box full of pistons from my ZX. Maybe 20 of them. several lasted less than a mile total.

One thing I never tried is sleds. Just never found the time. I'd build dirt bikes all winter, and wait for spring.

[JayAre]

I don't remember the guys name from Enviromoto who I bought the BB kit from I believe he's the head, or some part of the Midwest Scooter Racing Association. Or maybe it was The Great Lakes Scooter... I thing for sure, these bikes are a lot cheaper to build than my sleds.
My wife has been running the bike for a few days now, plus I've done a couple of 20 mile "tests". She's got about 100 miles on the BB. Time to change the oil & clean the screen. I'm going to wait to put synthetic oil in it though. I'm thinking somewhere around 500 miles to make sure everything is good & broke in.
 It's great to be able to pull 25-30 mph up hills so she's not getting passed. It seems to get better mpg's than the 2T Zuma also. Not ripping on the Yamaha they build great machines! That's what I used to race for 15yrs. Then Yamaha gave up building 2 stroke snowmobiles & I gave up racing. Now I just play in the backwoods, boondocking & getting stuck on a Skidoo.

[JayAre]

Just an update on the bike. We have 700 miles on the BB & it's running great! The variator/clutch started to engage a little hard so I pulled the cover & cleaned everything up with some brake cleaner & a Scotchbrite pad. The engagement is nice & smooth again. So much for the thought of upgrading the Malossi variator, if it works don't fix it.

[JayAre]

5 years & 3500 miles on this 50 - 72cc Big Bore. New belt at 2500 miles. The only problem was at 2800 miles it became hard starting but a new plug solved that.