yasuni C21 on a 54mm bore. thoughts?

[repherence2]

Looking for thoughts, opinions, wisdom.

Anyone have success using a C21 on a 50+mm bore?

I'm thinking about trying it but I'm thinking that I would have to stall the clutch even more and retune the weights. Is all of that clutch work worth it?

[190mech]

It depends on the port layout,,the bigger bores are transfer duct restricted due to engine design so max RPM's will likely be below the C21..The pipe could be lengthened,but many times its somewhat of a 'bandaid' fix..

[90GTVert]

Most of the 90cc based Minarelli builds end up stuck around 10,500RPM for peak power because of that transfer issue that 190mech mentioned. My understanding is that the C21 is pretty dead till 8,000RPM or later and pulls to about 14,000RPM. It may work, but with the pipe and porting to operate up there it's probably gonna do nothing till 8 or 9,000RPM so you will have to set the clutch and CVT up to get there quickly or it will struggle to accelerate or may not want to accelerate beyond stock speeds if it can't hit the pipe. You might be able to use some of the C21's capabilities if you over rev for max speed. I used to run one of my 103ccs up to 12,000RPM to hit my top speed before I got an overrange. Once I got an overrange, it wouldn't really pull past 10,500 with good results. Maybe timing could be advanced a little to make the higher revving pipe work better with the RPM the engine probably likes if you end up with it?

[repherence2]

190mech and 90GTVert thank you for your informative input. That's why i love this forum. You are appreciated.

Thanks for the breakdown and explanation about the transfer port issues, it was very easy for me to understand. I was brainstorming a way to increase my max RPM. The initial peak RPM gets to 9600 off the start but as the variator starts to transition, RPMs go down to the 8000's and then comes back up to about 9300 at max speed, wide open throttle, speedo needle burried. I guess I was just getting a little greedy.

I think I might just try it out as a "learning experience" before I finally decommission my current GTX. I think that it would be a great tuning challenge in preparation for the Motoforce mid-race build that I am currently working on. At least I will gain some experience in tuning a CVT for high RPM.

Hopefully I can share the experience with the forum. Lessons learned, win or lose.

[190mech]

For S&G I calculated the C21 tuned length with a 54X45 motor,came out to 12,450 RPM..Kinda hard to get there!Would still love to hear your test results..

[repherence2]

Feb 9, 2018 8:20:12 GMT -5 190mech said:

For S&G I calculated the C21 tuned length with a 54X45 motor,came out to 12,450 RPM..Kinda hard to get there!Would still love to hear your test results..

I'll put that 54mm Taida to the test lol. I only get ~42mm stroke though . I raised the exhaust port about 1-2mm a while back.

I'm thinking of installing the stock GTX gear set in trans. The gears that came with my 90 block seem pretty tall, plus the Malossi up gear set...I think it is geared somewhere around 7:1. I think the stock GTX ratio would allow it to rev up a little more. I'll reverify the current gear ratio this weekend.

[repherence2]

yasuni pipe installed. Stripped one of the screws for the transmission cover. Need to drill out that screw so I can adjust the clutch. It was trying to launch at 7000+ rpm but not enuff power. I need to stall the clutch a lot more, at least to 9000 I think.

[repherence2]

Several weeks ago I got the CVT cover off. I stalled my stage6 clutch to 9000 and I got the bike to drive. It had 4.3g rollers and NCY yellow medium contra spring. It could go, but it didn't have power. I stopped at that point and put the V8 sectional pipe (bws100). The V8 performed even better on takeoff with the clutch stalled to 9k.

I think that it was the roller weights being a little too heavy. I got 4.0g 3.5g and 3.3g rollers in my tuning box but I didn't get around to testing them.

I think I should test the Yasuni again with the lighter rollers but the 28mm OKO carb is super loud when the engine revs up to get going. One day I'll try it before I tear down the bike and decommission it.