139qmb gear swap problem

[chico357]

I changed my secondary gearset from 52/15 to 49/17 hoping to get about 5-10mph more top speed. I'm unfortunately getting less 🙁

Before the gear swap, my average top speed is at about 53mph and max RPMs usually around 8200.  After the swap I'm only getting 46-47 (50mph down hill). I had 7g sliders so put 9g rollers in to see if the top speed would change but it didn't. Max RPM is about 7100 rpm with 9g, 7500 with 7g. It also "seems"  that it needs more gas, as if it were a bit lean.

While I was doing the swap and spinning the gears by hand, it seemed to move freely; A slight bit rough but figured the bearings need replacement. I didn't replace the bearings as I was just going to do a test run first to see if I like the swap.

After I put the gearbox back together, I spun the tire by hand. It spins freely most of the way then all the sudden stops tight. I can turn it by hand without too much force but it's a bit tight, then it moves easily and spins freely again until it gets to that same spot. You can also hear a grinding sound around the same spot. I did inspect the teeth on the gears and didn't notice anything wrong with them.

The bearings do need replacement for sure but I'm not sure if they could cause this type of an issue. I think the issue could be caused by the bearings, but, I didn't have this issue before, so, I also think it could be an issue with the new gears somewhere.

So I have some questions:
1. Do you think replacing the bearings will resolve the issue?

2. After doing a gear swap, does one normally need to re-tune the carb with a larger main jet?

3. Other than damage to the teeth on the gears, could something else be wrong with them? What else should I inspect on the gears?

Here's my current setup:
52mm NCY bbk with 52mm head, 20mm carb with 91 main 32 pilot, 1000k torque spring, 1500k (normally 1k) clutch springs, performance exhaust, UNI foam filter, Dr pulley variator w usually 6 or 7 sliders before gear swap.

Any other thoughts on what the issue could be?

[jeff84]

no you shouldn't need to retune the carb after an up gear. you might want to retune your cvt though. you need to get your rpms back up into the power band. I would start with lighter roller/sliders. I would try some 5 or 5.5 gram. the gearing will not gain you more top speed unless you turn them at the same rpms you were before. turning them at a lower rpm should yield the same top speed though. and the tight spot could be where the new gears are rubbing against each other or the gear case.

[chico357]

Gotcha! Thanks. I'll check it out later today and then provide an update.

[humanshield]

1). If you didn't have the issue with the bearings before, you shouldn't suddenly now.
2). As Jeff said...No
3). Always possible. If they aren't machined perfectly concentric this can happen.

It's also possible the new gears will "wear" in after a few hundred miles. There could be a slight machining defect that has to wear away.

[dexameth]

I got a question for ya, what did you use to press the gears on, or did a shop do it for ya? I've been wanting to grab a set of gears for myself, but I know this will be the biggest problem I have: the press on/off.

For your issue, I'd try lower weights like Jeff said. Get the rpms high, then slowly up-weigh until you find the best power band. It's time consuming but make it fun!

[chico357]

I had a motorcycle repair shop do it for me for free and refused to accept payment.

[humanshield]

Looking forward to hearing updates Chico. At some point I may do a gear change but probably not until I get the airsal piston installed and the 23mm BVH.

[acvw74]

FYI...I had a set of gears that had a "rough spot" in them. I even thought I had smoothed them out, but they kept shedding filings into the gear oil.
I hope your wears in...but keep an eye on them just in case.

[chico357]

Thanks for all of your responses. I did put 5g sliders in and they did bring my RPMs back up to 82-8500. I'm still not getting past 50 mph down a hill though which is a bummer. I too was also thinking that the gears would need to be worn-in and have left them in for about 4 days now. I usually drive about 30 miles a day. The tight spot is not as tight anymore, but, the grinding noise is still there. I'm still driving around with them now.

I have another 139qmb engine that I was going to use to swap the counter-shaft out of and go back to 51/15 but noticed that it had different gears (52/14). I also noticed that the counter-shaft was bigger and had 53 teeth instead and knew it wouldn't fit. I also had to literally drill through the input shaft because the clutch bell nut was stuck on it real good. It was actually a bit disappointing but rather interesting and a bit exciting all at the same time.

The engine I'm riding is technically a 2013 Tao Tao 1p39qmb and came with 52/16 primary gears, and 51/15 secondary gears for a final ration of 11.05. The other engine is a 2011 Kaitong 139qmb and has 53/15 primary gears, 52/14 secondary gears for a final ratio of 13.12. The TaoTao always felt like it was a bit faster but it also came with a 1mm larger stock carb and intake than the 2011.

So with that being said, I plan to swap out the gears again and use the 52/14 setup which is what the Naraku gear ratio is. I have another 52T counter-shaft on order and will have it pressed when it gets here. To be honest, I may just buy a press instead as I'm getting more involved and have already found a few other uses. I'm also tempted to buy another 49/17 set just to rule out the gears themselves not being an issue. When I open the gear box up again, I'm also going to inspect the inside of the case to see if its rubbing up on anything. I'll let you guys know when It's been done.

I plan on doing another post about the process and steps I've taken for this project. It's a little bit more involved than just swapping gears and does take some proper preparation and pre-planning to ensure everything goes smoothly and easily. Hopefully it will help out some folks who may have been putting this on hold for a while.

[jeff84]

are you getting full belt travel with the 5g sliders?

[chico357]

Dec 27, 2016 23:17:19 GMT -5 jeff84 said:

are you getting full belt travel with the 5g sliders?

I'll have to check but I don't think so. My belt usually rides a bit higher than the variator's pulley and sometimes hits the bendix and leaves a line around the belt. I have noticed that the line isn't as pronounced as it used to be. I figured that was due to the gear ratio change though. I'll mark a line on the variator's pulleys and rear pulley too just to see how far the belt travels on both.

I've also been reading up about cvt speed ratios. There's some formula that you can use the data collected from the belt travel to figure out what range your in. Should be some geeky fun :cool:

[FrankenMech]

Dec 28, 2016 11:06:38 GMT -5 chico357 said:

I'll have to check but I don't think so. My belt usually rides a bit higher than the variator's pulley and sometimes hits the bendix and leaves a line around the belt. I have noticed that the line isn't as pronounced as it used to be. I figured that was due to the gear ratio change though. I'll mark a line on the variator's pulleys and rear pulley too just to see how far the belt travels on both.

If the belt travels higher than the variator or rear pulley the belt is too long. Belt wear will seat a belt lower but will not compensate for a belt that is too long.

[jeff84]

Dec 30, 2016 3:03:28 GMT -5 FrankenMech said:

Dec 28, 2016 11:06:38 GMT -5 chico357 said:

I'll have to check but I don't think so. My belt usually rides a bit higher than the variator's pulley and sometimes hits the bendix and leaves a line around the belt. I have noticed that the line isn't as pronounced as it used to be. I figured that was due to the gear ratio change though. I'll mark a line on the variator's pulleys and rear pulley too just to see how far the belt travels on both.

If the belt travels higher than the variator or rear pulley the belt is too long. Belt wear will seat a belt lower but will not compensate for a belt that is too long.

yes that is the struggle.  I have that issue myself.  cant find a belt the proper length.  the one I have now is a bit too long but not too bad, just barely touches the Bendix at top speed.  if I use the next shorter belt available I don't get full belt travel and top speed suffers.  you would think piaggio would offer a proper size belt but they don't.  (well if I were to use rollers instead of sliders then the stock belt would be ideal)  I just suffer through the shortened belt life for the extra mph.  belts are cheap enough that 2-3k miles is reasonable to replace them.  really by then they are loosing width anyway.

[FrankenMech]

You are correct about belt life. Maybe try other belt manufacturers. I have used Gates belts with no belt related problems. Different belt widths can also be used but they must not be narrow enough to touch the ID of either pulley even if worn a little or that magic smoke will be released.

[chico357]

It's good to know that I'm not the only one having an issue with the belt hitting the bendix. I've been trying to figure out why for months. I wont worry about it so much now that I know the cause of it is. Thank yall!

I used the bando Kevlar belt and the Gates. The Gates belt seems to take the abuse a bit better for some reason. I also wrapped the bendix with this hi-temp masking tape used for powder coating and rated for 500°. Seems to add a little bit more life to the belt. I'll make another post about it with pictures of it.

I haven't had a chance to mark off the pulleys yet. I will probably do it this weekend. I'm also going to try 3g or 4g weights to play around with. Top speed has increased slightly to 53mph without needing a hill . I'm going to keep riding it for a few hundred more miles in hopes of breaking it in more.