Tuning Your 50cc 4 Stroke GY6, QMB139

[streido]

Was looking for some info on carb tuning earlier and i stumbled upon this site with some good basic info for beginers/newbies.

It covers roller weights, variator, clutch springs, carb jetting, etc and is specifically for 50cc 4 Stroke GY6 QMB139 engines such as on the Agility50, but some of it is useful for all 50cc restricted scooters. Some good basic info and pointers for those who arej ust starting to think of tuning or de-restricting. Not all points are general and Kymco models vary slightly for some precedures , but generallly its pretty informative for those learning the basics.


http://www.towingdelivery.co.uk/ScooterManual/TuningOverview.aspx

[KymcoManc]

What mods would you do/have you done/recommend in ways of ease, cost effectiveness, best gains, etc etc.

Now it looks like i'm on the way to sorting my Agility 50cc (with the help of people on here :-) ) i'll next be interested in tuning it (but must be cost effective).

Thanks

[streido]

After derestriction, i fitted a race variator - £20, lighter 5.5g rollers - £5, and a 1500rpm torque spring - under £10.

This gave me a couple of mph top end but more importantly helped going uphills, and its all cheap things to do.

Next would be an open air filter prob or a 72cc kit.

Prob my best felt improvement was from the race variator after derestricting.

[KymcoManc]

Brilliant.
When you say derestriction, mine is bog standard, so how do I go about derestricting? & is there a good benefit in this alone without the other mods?

What kinds performance gains have you made on yours?

Thanks

[streido]

Theres loads on here if you search "derestrict" in the a50 section.

Basically you need to remove the variator and swap the boss to a smooth one, change your carb main jet from #80 to a #82 and depending what year your scoot is notch the cdi to remove the rev limiter, its all on this forum, even a few vids there too, dont have it to hand right now cos im at work.

This should get the speed up to around 42mph on flat roads, maybe 44-45 downhill.

I average between 44-48mph most days, had a 52mph downhill which was great.

Set up i use is derestricted, race variator, 5.5g rollers, 1500 clutch fly spring, 1500 contra/torque spring, never really drops below 30mph uphill unless its a real steep hill or i lose my revs by having to brake.

Im in the middle of doing a few other bits and pieces, just messing about experimenting in reality, but thats what it takes is trial and error.

All parts for a derestrict should be readily available from any Kymco dealer/seller or online.

[KymcoManc]

Theres loads on here if you search "derestrict" in the a50 section.

Basically you need to remove the variator and swap the boss to a smooth one, change your carb main jet from #80 to a #82 and depending what year your scoot is notch the cdi to remove the rev limiter, its all on this forum, even a few vids there too, dont have it to hand right now cos im at work.

This should get the speed up to around 42mph on flat roads, maybe 44-45 downhill.

I average between 44-48mph most days, had a 52mph downhill which was great.

Set up i use is derestricted, race variator, 5.5g rollers, 1500 clutch fly spring, 1500 contra/torque spring, never really drops below 30mph uphill unless its a real steep hill or i lose my revs by having to brake.

Im in the middle of doing a few other bits and pieces, just messing about experimenting in reality, but thats what it takes is trial and error.

All parts for a derestrict should be readily available from any Kymco dealer/seller or online.

Fantastic & how i'd like those kinda figures. In costs, how much would you say it'll be to get to yours?

The vids would be great if/when you have time to find. I take it your in the UK somewhere?

[streido]

Variator was £18-£20 i think, rollers under £10, torque/contra spring under or around £10, clutch fly springs under £10, main jet £2-£4, i also got new Bridgestone tyres £55/wheel after i came off twice in the wet on stock tyres, and been trying different belts and air filter etc but that can wait for you for now.

Our local Kymco/Yamaha bike shop charges around £60-£65 to derestrict an agility if your not confident doing it yourself but its pretty easy if your mechanically minded or ok at basic maintenance etc on cars or bikes.

If i get time later, and if i remember, i'll try see if i still bave those links saved in my bookmarks on my home pc if nobidy else pist them today.

The basics are pretty cheap and easy to do, things only get expensive once you want keep it moving over 50mph really.

[thebatman]

I average between 44-48mph most days

That's mine as well...
CAN hit 50 on flats...weather permitting
55 i know i've seen downhill. Have touched the needle to the corner of the odometer but she was revving out at like 9300, so i let off some. She was screaming.....
(but of course i have a 1000 spring set-up w/5.1g malossi rollers..)..Maybe a longer belt... I have seen that mod work...   
Vid is here from The WORD MAN himself.......

[streido]

..Maybe a longer belt... I have seen that mod work...  Wink

Yeah ive been running a 736-17.5-30 Malossi Kevlar belt for a few months and it got me my highest speed downhill. Im in the middle of experimenting with a few different belt etc just now. I saw the Malossi was begining to fray at one part on the outside edge, after 3-4months??? Really , anyway i tried a 737-18-30 rubber belt for a few days but that extra .5 mm width made a difference, but not in a good way, so i just put in a new stock belt for now till i order up one or two others next week to try out.

Think i plan on trying a rubber 736/737-17.5-30 since that worked out pretty good before on the Malossi. Doubt i will ever buy Kevlar again cos even with the little bit of fraying i had dust and fibres in my cvt case, even got some into my variator and mustve been sticking a roller or two cos i only replaced then 2wks ago and a couple had flat spots already. New rollers needed now too i guess 

[KymcoManc]

Has anyone got any links for lighter weight variator weights, also the belts you mention. Whats this about removing the variator washer i see on youtube?

If my bike is still 100% fine in a few days i just want to do any mod that is either free or cheap to do. Funds arn't that good atm.

Is an after market air filter worth purchasing, if so what is recommended? I could remove the big air box I guess if I purchase one to go directly on the carb?

I've not looked as yet, but if my belt has been on from new & its on almost 10,000 mls, I guess it'll be worn & so be taking drive/power away?

What would happen if a drive belt actually snapped? Ouch!

[streido]

Ebay or any scooter/bike parts shop will have rollers. You want 16x13 ?g rollers (? Being what weight you want) 6.5g is stock weight, mayne go for 6 or 5.5g to try 1st.

Theres no washer on the agility you either have to swap to a smooth boss or if you have access to a lathe you can lathe the restricted boss so its smooth. If you have a kymco dealer locally he will have a smooth boss or again ebay or online has them, 38mm smooth variator biss suitable for a 50cc gy6 139qmb engine.

Also if you do the boss you should fit a bigger main jet, use a #82 instead of the factory #80 jet, look for a Keihin M5 or 5mm main jet in size 82.

Check the look of the belt, if its not slipping or is not cracked or worn down then it will still be ok. I used my original for about 14-15,000km and it was only then begining to get a couple of cracks in it. Unless you fit a race variator then leave it stock length, 729-17,7-30 belt.When the belt gets worn you tend to lose some top speed or get slippage when pulling away.

As for the airbox thing, you will need to fit an even bigger main jet if you do this. Currently im doing mine now and have been trying to get it jetted correctly, narrowed it down to between a 112-118 jet but its cost me about the price of 6 jets to get there, its all trial and error with jets and once you buy it you cant return them. Prob spent about £20 on jets so far and just order another couple tonight. Add the cost of the filter and you may not want to do this right now if your on a tight budget. No 2 scoots are the same, some only use 88main jets on the open filter, other need more, trial and error. And patience


If it was me i would do the smooth boss, 82 main jet and if its a pre 2010 a50 then notch the cdi, search wordslingers post on cdi derestrict for pictures.

[KymcoManc]

Thats all great & plenty to think about in now & the future, thank you.

What does the boss do so I can understand why a smooth one is beneficial to the stock item.

On the belt, I think mine could be worn because I've noticed today I am getting slippage when pulling away at lights. Maybe ive not noticed this before because the engine has never run so smooth before, in the time ive had it anyway. It now feels not only much smoother but also quieter. I'll be checking the belt over the next few days.

[OregonDad]

Streido, which race variator did you use?  Can you share a brand name or link?

[streido]

I bought a cheap chinese clone last year to try it out and i was/am impressed with it for the money. This is the one i got, http://www.ebay.co.uk/itm/RACE-VARIATOR-for-GY6-50-KYMCO-BAOTIAN-LIFAN-Chinese-/300540989339?pt=UK_Motorcycle_Parts&hash=item45f9a38f9b . Been on about a year now i think and done about 8-9,000km on it and its still going ok. It is now begining to get a few grooves starting from the belt but its still good for a few more km's yet.

Think i plan on getting a Malossi Multivar once it finally does wear down tho.

[OregonDad]

That plate looks like it has a steeper slope so it holds the CVT in a lower gear longer, correct?