70cc Mxs Very bad takeoff

[booster]

I can't find the problem with my cvt tuning. The takeoff is very sluggish and I dont know why. I have tried 2.5g 3g 3.5g 3.8g 4g rollers, different clutch springs (red and orange) with different tensions, different torque springs, no shims and up to 4 shims. I have already spent several days trying to get the takeoff like it should be but with no luck and im starting to lose hope. It maybe gets better with every shim that i add but 4 sounds like a lot already for me. They arent the thinnest shims.

My setup is:
Mxs 70cc Racing transfers and exhaust ported
Roost MV70 exhaust
2win Vulkanflow reeds
Mxs highflow intake
28 Pwk
Mvt millenium
Stage6 oversize cvt with stage6 oversize belt
Stage6 Torque control clutch
Motoforce clutch bell

I have a full throttle acceleration video from standstill with my cvt setup. In the video I had 3.5g and 3.8g rollers, Stage6 Medium torque spring, orange clutch springs with 3/4 tension and 4 shims. The rollers are maybe a bit light but that shouldnt impact the takeoff like that.

youtu.be/nd4ElYoIusk

Every advice is appreciated!

[Zino]

I could not see your rpms from video
what rpms are you targeting
For take off ?
When you slowly roll on the throttle slowly what rpm does the scoot start moving ?
This dials in the clutch springs

For shifting ?
What rpm does the rpm jump too and hold as you accelerate to top speed
This is for dialing in variator weights

When you move to stiffer torque springs you need to have heavy enough weights to overcome them to get the same shift points.
With your set up you may need clutch springs that start you moving in the 8k range and weights that have you shifting above 10 k rpms

Your carb is pretty large and can slow down throttle response it will help with top speed as a trade off

[booster]

I don't know what rpms I should target for take off. I'm just trying to get the takeoff better and no bog.

With orange springs 3/4 tensioned the clutch starts to grip at around 6k and it starts to move at closer to 7k. After the takeoff when the bog stops and it takes off like it should the rpms are around 10k. When im accelerating towards top speed the rpms just keep rising until at around 14,6k.

The carb is large but I have high flowing reeds that are from am6 engine and high flow intake with them. The guy who ported my cylinder told me that the ports should have enough flowing with 28mm carb if the intake and exhaust is good enough.

If I understood correctly should I try stiffer torque spring and heavier weights and stiffer clutch springs?

[bigmatthew86]

Does it pull well to that high of rpm? If so with that & a big carb you’re going to need with a lot of rpm to get near the power band & not bog.

[aragorn]

For me, it sounds like its a bit overreving, like its max RPMs are out of power range... By the way, what is your ignition timing?

[booster]

Yes after the bog it has no problem pulling in high rpms. After some cvt adjusting the rpms are now ~1k lower. The bog is still there but now it doesnt rev so high and it seems to pull overall better. Ignition timing is stock it cant be adjusted on mvt millenium outer rotor ignition.

[rtnifty]

I'm just spitballing here, but it almost seems like you are outside of the pipe on the RPM range at low end. According to the 2T exhaust Database, that Roost should be around 13k. 49ccscoot.proboards.com/thread/1391/stroke-exhaust-database. That also seems to check out with Roost's catalog entry for the MV70 - Maximum power 13.000 - 13.500 RPM www.roost.si/pictures/Catalogue_2012.pdf

If you're taking off at 6-7K, you're really below the power range for that pipe. That is likely why you're sluggish until you start to spin up around the 10K mark and start to get into the pipe. You may want to go with something really stiff that kicks around the 8 - 9K mark like the the Stage/6 Purple springs. Or some kind of hyper-race springs that engage around 10-13K.

Edit:

Found this doc. from MXS.  If you look at the power curve for that cylinder, it's around 12,500.  blog.mxsracing.com/wp-content/uploads/2016/07/mxs-racing-ndm-cylindre-booster-racing-en.pdf

What I gathered is that thing wants to spin around 10-13k.  I think if you can get closer to that, things might be a bit better.

[booster]

That could be the problem because I had no problem with takeoff on my old exhaust which was yasuni c16. With yasuni the problem was lacking high end rpm range. Yasuni only rev to 12,8k. Do you know if they sell those springs in EU. It's pain to order something from USA because of import fees and etc.

[aeroxbud]

I definitely think you need stiffer clutch springs. At 6K take off, you are in the power range of a sports kit/pipe. Not a racing kit.

[rtnifty]

booster I don't know if they fit, but Malossi makes a set of black MHR springs for their fly and delta clutches, but they hit are supposed to hit in 8-10k range. With the ability to pre-load with that clutch, you should be able to get into the right RPM range. Here's the model number 29 8747. www.malossistore.com/en/us/mhr-spring-set-for-fly-clutch-and-delta-clutches-29_8747-P

The ones you want are the black springs 2,2 - I think you can buy them individually here, but double check with the guys at Malossi. www.malossistore.com/en/us/3-black-racing-clutch-springs-22-for-delta-and-fly-clutch-29_8745b-P

If I recall the delta springs and the Stage 6 should be fairly close.

Edit-

I did a bit more digging and what's odd is that the reds from Stage 6 should be able to hold off up to 13,500. www.maxiscoot.com/en/product/clutch-springs-stage6-torque-control-47040.

Have you tried getting close to max pre-load for the those springs? The only other thing I can think is that your belt is shifting before the clutch grabs which would put you at a take off disadvantage as well.  Not trying to overthink it, that's just the other possibility.

Are you kick starting only or do you have electric? Here's my thought.  Put bike on stand and run without CVT cover.  See where clutch engages.  Use that to get the clutch closer to rpm band.  On the other hand, watch to see if your CVT is shifting before the clutch grabs.  If it does, then that's a roller issue too.  Want that clutch to hit before the CVT starts to shift, and the CVT to shift just after clutch engages. If you can sort that balancing act somewhere closer to the 10K mark, I think you might get there.

Anyway, I hope all that help.  Everyone else, if I'm off the mark, please correct me.

[repherence2]

i just watched your video. initially from only reading your posts, i thought it might be the 28 PWK carb. however, after watching your video, i think what you are trying to achieve is how my friend's Honda Elite was set up long ago. it was a 90cc set up, but his clutch was set super high. when he let me borrow the bike for a day, he warned me that the bike had to rev up high, and then it would "bite", so he warned me to be careful with the throttle.

from what you describe, it seems to me that you are after that same style of lauch/take-off. the guy that built the bike was a tuner at Qwik Fix Cycles and he weighed 120 pounds. he had it set up so that he could wheelie it and have some speed too (hence the 90cc setup). i have read that the guys here in hawaii like to use Polini Blue springs for wheelie setups, but there are so much choices for gearing for DIO that it is easy to build a wheelie machine.

after running my stage6 MKII clutch, i am thinking of switching over to a Polini 3G Evolution clutch or a Malossi Delta clutch. i was running the Stage6 clutch earlier with race springs, but i tensioned it to the last tick mark on the adjuster and it still felt like the RPM engagement was not high enough. i guess i could tension it more, but my last Stage6 clutch failed at the clutch shoe eyelet where the spring connects to the shoe. the eyelet on the one of the clutch shoes broke.

the strange thing is, i just swapped out my Stage6 clutch with an NCY racing/lightened clutch with red NCY springs that i had in one of my parts bins. it actually feels better with the NCY right now. however, this is currently on a stock 49cc chinarelli with a YMS V8.

so back to the topic at hand, i think you need a clutch that can engage at a higher RPM. i have a Stage6 clutch, but the way i see it, to have the Stage6 set at high RPM, you run the risk of breaking the eyelet on the clutch shoe. i do not know how my friend with the Elite had his clutch set up, but you had to rev it up high in order for the clutch to engage. and when the clutch engaged, it kicked hard and took off. knowing him, it was some honda trickery