What is the ideal max RPM ? is 8500 RPM too high ?

[tocoo]

My chinese scooter is all stock.

I am adjusting the main jets on my stock carburetor and I would like to know what is the ideal max RPM at WOT for the usual 4 stroke 139qmb chinese scooter ?

The stock carburetor came with main jet of 78 and idle jet of 30. I tried main jet=80, idle jet=31, but I easily reach 8400 RPM at WOT with those and the mixture screw is 3 turns away in order to get the proper mixture.

I am currently with mainjet=85 and idle jet=33 and max RPM is around 8300. at 3/4 of WOT, I am at 7800 RPM.

[Happypancake!]

Dont know much about those motors but the Honda GET will run 10,000 rpm before valve float.

[90GTVert]

You should control RPM with the CVT tune; NOT the main jet. If RPM picks up too much with a main jet swap, try a little heavier rollers or sliders and then retry some jets. If you're trying to get the right RPM by selecting a main jet, you may be restricting your scoot in a way and not getting it's full potential because of an improper mixture.

[139qmb]

I just swapped to a 90 main jet on a stock carb 139qmb. Stock exhaust, UNI air filter. Before the jet change I could get 7300-7700 RPM with 5g NCY rollers, and the stock air box derestricted. After 90 jet with stock air box I dropped 5mph and RPM was at 7100. With UNI and 90 jet I regained the 5mph and RPM is up to 7300. My stock had no number to indicate size, how did you determine your stock size?

[90GTVert]

If it doesn't have a number you'd have to use some sort of gauge or just don't worry about it and use a selection of numbered jets and find what works.

Speed and RPM changes are telling you that you're either headed in the right or wrong direction, but if you get a main jet working well enough that you are outrunning the RPM you want to be at you need to use heavier weights.

[tocoo]

For safety of the mechanical parts, Is there a RPM that must not be topped on the 139qmb?
I do not try to fix a max RPM, but I worry more about breaking things by having to much max RPM.

[diynuke]

Well just try to get as fast as possible with as less rpm as possible. while still having decent acceleration.

And it's always better to not drive it in high rpm's 50cc does tolerate allot but its of course not preferred.

And watch out with the mixture if its really rich it could foul your head and then you will have 0 compression if the deposits break loose and get between the valve's






I kind of look for 7200rpm for acceleration and 8500 at max speed.     at 9000 my dashboard starts to flicker. so that can't be good xD

[jackrides]

Mine redlines at 8000, I figure there's a reason. How about some experiences of what fails first when enjoying excess rpm? (Of course, then we can explore stronger parts for the weak ones.)

[mrkswthwrth]

My rpm range is kinda like this with a 260lb gorilla sitting on it with gear.

Idle at 1800rpm
Clutch engages at 4200rpm
Wot on flat peaks at 9000rpm
Levels back down to 8500rpm
Downhill wot up to 10500rpm
Uphill wot at 15mph or less around 9200

This is stock drivetrain stock bore, just a slightly larger main jet.

[139qmb]

49ccscoot.proboards.com/thread/2953/displacement-torque-tables-gy6-139qmb

Link above lists peak HP at 8000, and peak torque at 6000. Would there be any benefit to operating above 8000 if that is the RPM at which the engine produces peak HP?

Also what weight rollers or slider are you using to get 9k and 10K RPMs? I weight 160pds and with 5g rollers only turn mid 7k RPM.

[mrkswthwrth]

This is entirely a stock setup, not sure about weights but ill guess its around 5g. Ill try getting the variator apart tonightx but i dont have na impact handy.

[dexameth]

After the extensive testing I've done, I've learned with the QMB139, they tend to have a power band in the 7k range. I tend to lose power after 8200, no matter the setup, just on average this is where my power drops. I feel like it starts to pull around 6900, anything less and it's a dog.

One night, messing around with belt sizes, I popped a longer belt on at a friend's house. I wanted to test it to see if it would fly off the variator, so I put the scoot on the center stand and went WOT. My tach read 10,800 rpms when I let off. I let it drop to 9k, then hammered back on it. Nice throttle response with no load at that RPM haha! Only a couple pulls, no belt flew off, all was well.

My setup:
clutch engages near 4k
take off gets up to 7200 quickly
55mph @ 8000
35mph cruising at 6600

[thedumbguy]

I may sound like an idiot but if it's all stock the motor will run out of torque and the revs won't be able to climb unless the load is relieved for example going down hill. I'm no expert nor do I try to be one so if I'm wrong by all means correct me.

[PirateLabs]

Apr 28, 2018 22:17:50 GMT -5 thedumbguy said:

I may sound like an idiot but if it's all stock the motor will run out of torque and the revs won't be able to climb unless the load is relieved for example going down hill. I'm no expert nor do I try to be one so if I'm wrong by all means correct me.

That all depends on your CVT tune.  The variator weights work along with your belt length, torque spring in the clutch and groove angles/profile on the torque sheave.  I run 4.5 gram Dr. Pulley sliders and, when my clutch is working well, it will accelerate at 7,800 rpm and hit 8,000 W.O.T.  As my belt wears thinner, the rpms climb and I usually have to adjust to a little heavier slider like maybe 4.75 gram.  I have modified intake and exhaust and it seems to make good power at about 7,800-8,000 rpm so I try to tune it to stay there.  My experience is that with these Chinese scoots you can get a perfectly tuned cvt and it will be great, and then it will start to drift.  I believe Brent mentioned this in one of his videos.  Maybe the torque spring gets a little fatigued from the heat, (which is why I added cooling ports to my cover) as mentioned, belts wear and get thinner and ride at different heights on the pulleys, etc.  It seems like I am chasing my tail sometimes, ha ha.  Like right now, I am working on finding out why I suddenly drop down to 5,88 rpm after taking off at 8,200.  I am building a new clutch that hopefully will fix this.


Bill

[humanshield]

My stock final drive was 14/52 (3.71)
With my 50mm BBK, real 20mm carb, big valve head, cam.....it would pull to 40-45 pretty good but I was running 8000-8600RPM and I could tell the gearing was just run out.

Right now I've got the rear end apart installing a 17/49 (2.88) set of gears. For every day use, I found the stock gearing was a nuisance because I don't like wringing out my motor so I had to settle for crusising around at about 35mph - 37mph to keep the RPM's under or right at 7,000RPM. I could still accelerate fairly well at 37 up to 44-45mph but the revs bothered me.

What I'm hoping for is that the 2.88 set will let me cruise at about 40 - 45 without killing acceleration or hill climbing performance. But, it's a trade off so to gain a little top end I know I'll be sacrificing acceleration. I can deal with that as long as the motor can pull these gears all the way out without struggling too hard. May have to do a bit more work to add just a bit more power to the motor. Will see.

btw...the acceleration with the original gears was good.  I could launch from a light and jump ahead of cars....up to about 35 that is then they'd go right on past me.   If the new gearing lets me accelerate on par withthe average traffic, but keep accelerating to a more comfortable top cruising speed I'd be really happy.