What could be causing my acceleration issues?

by Kevin
(Portland, Oregon)

Changed my drive belt after it wore out and snapped. My new belt performance is crap. I use my scooter for everyday commuting in Portland, OR. I know I have the correct belt for my scooter.

I even went to a dealer and bought a different brand belt and tried that one to no avail. I have a 139 QMB Honda rip-off engine with 10 inch wheels so I use the 669-18-30 belt.

I have a Jonway 50CC, 4 stroke Chinese scooter. Despite their lack of quality (I already know this, helpful comments please), my bike had more low end acceleration and a faster higher RPM climb to 30 mph with an old worn out belt than the new one does.

So here is what is specifically different. With my new belt on, my bike pulls away from a stop with the same decent acceleration as it did minutes before it broke. The RPM's climb at the same rate as they always did.

Then when my bike is at around 15-20 mph, the RPM's drop and/or begin to climb very slowly. My scooter now moves extremely slow from 20-30 mph. (this can take like 25 seconds on a flat) Also, my bike's top speed has changed.

On the same stretches of flat road where my bike easily did 40 mph with a specific higher RPM sound, it now goes 30 with a lower engine RPM. Climbing hills that I used to take easily at 30mph, I now do at 20 mph.

My bike sat for only a week from the time the belt broke to the time I put the new one on. My bike has nothing modified since the belt broke (except new roller weights which had no noticeable change) and the performance went out the window.

Any ideas? I put my first 1800 miles on my scooter before the belt broke while scooting. I figured I would just change the belt at like 3000 miles since they can last up to 8000. I learned my lesson.

I guess I ride pretty hard, the hills I climb probably also help to wear out the belt sooner.

What I've already done to try to fix this problem:
  • I have taken the entire CVT transmission apart piece by piece and cleaned it all up. There were belt fragments wedged in the pulleys and belt powder everywhere.
  • I tried putting some new roller weights in which are 6.5 grams. No change. My old rollers were not that bad, around same weight (worn) with no flat spots and could still roll fine in the variator.
  • I have thoroughly cleaned the carb out and the airbox.
  • I've tightened and checked to makes sure all my hoses are in good shape and seated properly.
  • My spark plug looked great. I use an iridium NGK plug. The gap meets the recommended spec of between 0.6 and 0.7 mm.
  • Tire inflation is at normal and always has been kept there with no change.

I recently took apart my centrifugal clutch (the driven pulley) to make sure it was clean and functioning properly. The large spring is fine as well as the small springs that hold the pivoting weights back. I cleaned this thoroughly.

I did notice that the pulley rotates and slides out as you pull it against the large main spring, which would allow the drive belt to ride lower at where you achieve your higher speeds. (My problems are in obtaining higher speeds and dealing with a power drop at 20 mph and up so I think this could be something...)

When I pull on the pulley, it slides out but seems stiff, even without the spring and doesn't seem to want to open up all the way, sometimes. There is no way for me to take this apart more so I just put some white lithium on the shaft, opposite of where the belt rides, and worked it in.

The pulley seems to move better, but still gets hung up about 3/4 of the way out in an in consistent fashion. I never took this piece off before my problem so I don't know if this could be the cause. The shaft where the belt rides on the driven pulley seems to be getting slightly greasy as if the rubber bushings at the bottom notches on the pulley may be failing?

I have noticed some minimal slippage while riding and can easily differentiate this slip from the overall lack of performance.

Are they related issues?

If any one has an ideas or knows of a few more simple things to troubleshoot, please pass it along. I'll give them a try and get back to you.

Thanks for any help in advance.


Comments for What could be causing my acceleration issues?

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Nov 11, 2011
Scooter Stall and Drive Wheel issue
by: Anonymous

I was thinking of another possible solution to your bike drive wheel and stalling issue. My bike was having a similar issue shortly after I bought it, within 4-5 months where it would idle erratically whenever I came to a stop while out riding. Sometimes it would try to pull me forward and other times my bike would stall. I found my issue. The carburetor had an air leak. The carburetor has two big openings. One is air in and the other is air out. Connecting to these openings are tubes. The air coming in comes from your air filter box and the air going out goes into your engine. If the tubes that are connected to your carburetor are not connected tightly or have any cracks or holes in them then your engine will erratically idle. Check the hose clamps connecting these tubes to the carburetor. One of my hose clamps was broken. I got a new one for a dollar and my bike runs great now.

Nov 10, 2011
This might be your scooter issue
by: Kevin

It sounds like your idle is too high. When your idle is too high the drive wheel will tend to want to roll. I can explain more if you want. Just try to turn your idle screw down. The screw is on the carburetor. it is usually a little gold screw. Before you try adjusting it put your bike up on the kickstand and start it up. when you adjust the screw inward the engine will idle higher (this is like giving it gas with the throttle.) When you move the screw outward the engine idle will slow and your drive wheel will stop spinning. You need to slow your idle. Just don't unscrew the idle screw too much or the bike will stall. The idle screw it a fine tune adjustment.

Nov 06, 2011
My scooter drive wheele is always going.
by: Anonymous

I have a chines scooter and its only about 2 mouths old and after about a mounth the drive wheel keeps going when I dont have my hand on the throtale and now its started to stal when i dont give it any gas what could this be from?

Aug 31, 2011
"acceleration issues"
by: Cougar7

I read your troubleshooting steps and I don't see how you could have been more thorough.I also have a 50cc Jonway (Wasp), and I had the same problem.I bought the bike used and inherited the problem.I did pretty much all that you did. I also took the carb off, gave it a good cleaning and put it back together using Permatex on the contact surface of the intake manifold to eliminate the possibility of a leak during high RPM operation. After putting the carb back on, I put in a new plug and noticed some improvement. The real improvement came when I took the muffler off. I had noticed a lot of residue in the end of the pipe and suspected blockage.After double checking the the air/fuel mix I went for a ride. I could immediately run smoothly up to 35-40 on the flat and I'm now looking for a new muffler.

Mar 02, 2011
Great Work Kevin!
by: Rick Rudge

Hi Kevin,

Thank you for posting this and for the updates to what you've been doing. You sound like you know your stuff and were able to fix your problems yourself. Congratulations.

I'm going to be digging into my 2003 Yamaha Zuma CVT here very soon, and will no doubt experience the same things you did. I just hit my 6,000 mile mark and it's time to change the belt according to the scheduled maintenance.

I live near Portland, Oregon and if you would like to go on any group rides, I'm more than willing to come along. This year, I'm going to try to participate with the St. John's Scooter Club and the Portland RuckStars Scooter Clubs group rides and I hope that maybe you can trying going on some of those too. I'd love to see you there.

Take care and happy riding.

--Rick Rudge

Oct 11, 2010
try looking into the contra - main spring
by: Anonymous K Dawg

Hello Everyone, Kevin...it sounds like you have done most things to get the top gear/speed back - Check your contra/main spring in the clutch assembly, it is responsible for pushing the rear assembly into making the belt ride higher. With your new belt being 'to' spec ie wider (not worn, thinner), main spring now must push like new and cannot as it is worn and out of spec (wears with belt and from CV) - I would check/change this before moving forward - if I read right, you cleaned all the clutch parts - the main/contra is what was keeping the pressure against the main nut, when you took the clutch apart. Try replacing with stock one first before any hi performance stuff,if you go with a perforance type you'll often need to further modify your setup. Good Luck bro.

May 20, 2010
Thank you Motor Scooter Guide Help!
by: Kevin

My update is a good one. I just finished resetting my valve clearances and my bike is running awesome. The intake is supposed to be at .06mm and a .08mm feeler gauge fit right in, although a .1 mm couldn't. Probably was between .09 and .1 mm. The exhaust is supposed to be at .08mm and it could take up to a .1mm gauge easily. Not good. I set them firmly back to spec and the bike is running awesome. I am back to the acceleration and top end that I had when before my belt broke. I am also able to keep decent speed up hills now. This just tells me that I need to spend a little more time getting to know my bike, learning how to fix it, and keeping a good maintenance log. Thanks a ton guys. You've all been a great help.

May 19, 2010
Bike speed
by: Kevin

My bike will very slowly work up to a top speed of 35 on a flat. 40 on a slight downgrade. I am not trying to make my bike go any faster than 35, just recover some of my top end acceleration from 20-35. Right now it's really slow in that range. Way slower than it ever was. I am going to adjust my valves in the morning before work. I'll post tomorrow. Thanks.

May 18, 2010
Belt Stretch
by: Jim Zeiser

I replaced my Chinese OEM belt with a Gates aramid fiber belt. When I stretched the two side by side the old belt was a good two inches longer or more. If the belt you put in was better quality than the one that came out it will take awhile for the new belt to stretch and ride out further on your variator. If they made like 670mm, 672mm belts and more so you could adjust the "gearing" and you could recover your top end. How fast is it going now?

May 18, 2010
more info
by: Kevin

So I degreased the whole cvt system and it looks great. I lightly sanded the pulleys and put my old (still in good shape) roller weights back in. I noticed there is no longer any random slipping of the belt so that is solved. This alone seems to be getting me closer to the pickup and speed I once had. There is still a lack up top end though. Next I'll look into adjusting the valves. I'll post more when I have something else to add. Thanks again everyone.

May 18, 2010
Lose of power
by: Izzy

I had the same thing happen to me. My drive belt broke. I had to replace it. I had to reset the roller weights too. After I did all of that, I still lost a lot of top end speed. I adjusted the valves and now I have all the top speed I need.

May 18, 2010
Lose of power
by: Izzy

I had the same thing happen to me. My drive belt broke. I had to replace it. I had to reset the roller weights too. After I did all of that, I still lost a lot of top end speed. I adjusted the valves and now I have all the top speed I need.

May 18, 2010
Thanks for the advice
by: Kevin

I have not had my valve clearances checked since my bike is relatively new. I'm sure I don't have the tools for it, but there are tons of scooter mechanics out here in Portland. First I'm going to Take my belt off again and make sure everything is degreased, roughen up the variator pulleys lightly and see how that works. I know my new belt doesn't stretch because it's not broken in, but when I bought my bike new, with a new belt it didn't have this problem.

Thanks for the advice guys.

May 18, 2010
by: scooterchick

My first blush at this was old belt particles in the housing. It sounds like you've checked about everything.
Is the fuel filter okay?
My only other question is, when was the last time you had the valve clearances checked? This is a common problem that gets overlooked a lot.
Good luck!

May 18, 2010
Clean it again
by: Jim Zeiser

I would take it apart again, make sure all the grease is off the everything, rough up the variator face with some light sandpaper and put the old weights back in and see if you get your top speed back. Also a new belt is not as stretched out and doesn't ride out as far out on the variator and will lower your top speed until it breaks in. Patience.

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