Max Performance - Yamaha QT50/SG50

Christoffer Janner /

Re: Max Performance - Yamaha QT50/SG50

Dirty30 Dillon /

In my opinion the QT50 is a great platform, but I don't think you'll see 55+ with that mild setup. The biggest limiting factor in the QT50 is final drive ratio, which I believe can be swapped out for a pw50 gearset for more speed, but you'll need to setup the clutch for proper engagement and such. Also you haven't factored in airbox solution, replacement cables etc.

Re: Max Performance - Yamaha QT50/SG50

Mine currently goes 41, faster down hill, this is not a platform for even higher speed. The bike only weighs 95 lbs.

The brakes, wheels, frame, suspension, etc. can't handle it. It's very loose at speed. I'm scared to take a hand off the bar. My brake drums are brown from heat.

You don't want to thump into a pothole at speed.

Re: Max Performance - Yamaha QT50/SG50

i had a stockish QT50 going 40+ with different final drive gears. you get them from PW50 forums sometimes.

my stock-ish MJ50 (more like an SG50) would get up over 45mph. isnt SG50 also a 2 speed?

Re: Max Performance - Yamaha QT50/SG50

I hate to be that guy, but if you have to ask on a forum about whether something will go 55 plus, not only can't you do it, but you probably shouldn't. Nopeds in particular have terrible dynamics over 45.

Build the best bike you can and be happy with how fast it goes. Probably 45 for the parts you listed.

If you really want to go 55 on a QT, your best bet is to read this thread and learn it.

this thread should be a sticky

Re: Max Performance - Yamaha QT50/SG50

Jack Rutherford /
qt16.jpg

I think a qt could do 55. You have to go to the pw gears and I laced up a 16" rear tire for it. This particular pipe is more low to mid range IMO but I could be wrong on that. That's just what it seems to me. So anyways, I've got the gearing to do those kind of speeds now. Now it's just a matter of getting the engine to do it.

Actually this thing has pretty good acceleration which is a combo of the pipe and the HPI system and I'm using the HPI CDI with the more aggressive curve that advances timing in the low to mid rpm range and then retards it slightly in the higher rpm range. You can look up description on treatland's site as to how it exactly works - I'm using my somewhat faulty memory.

I'm at just under 51 mph. But I think just raising the exhaust port a little could give me a tiny bump in top speed. Switching to a different pipe would help too and I could play with different timing settings on the HPI. But after taking several nasty falls learning to snowboard, I'm a little hesitant of experiencing similar falls on asphalt. Despite it's small stature, the qt does feel pretty stable at higher speeds though (I've added a crossbar which helps). Obviously the stock brakes were not meant for this.

Short answer - yeah it think it could be done - 55mph on a qt.

Re: Max Performance - Yamaha QT50/SG50

Jack Rutherford /

Which reminds me of a question I had. This qt50 has the Mikuni TM24 carb on it. It's a popular opinion to claim that the TM24 is too much carb for this engine.

But it idles fine, accelerates really well, and I've jetted it right on the top end (with HPI system installed and tweaked a tad along with jetting, it runs in the 360s-370s). I rarely ride it so when I yank it out of storage, it can be a hard starter.

So my question is - what would you look for in terms of starting, idling, accelerating and performance to determine that a carb is just too big for a bike? (edited)

Re: Max Performance - Yamaha QT50/SG50

Keegan Wilhelm /

> Jack Rutherford wrote:

> -------------------------------------------------------

> Which reminds me of a question I had. This qt50 has the Mikuni TM24

> carb on it. It's a popular opinion to claim that the TM24 is too much

> carb for this engine.

>

> But it idles fine, accelerates really well, and I've jetted it right on

> the top end (with HPI system installed and tweaked a tad along with

> jetting, it runs in the 360s-370s). I rarely ride it so when I yank it

> out of storage, it can be a hard starter.

>

> So my question is - what would you look for in terms of starting,

> idling, accelerating and performance to determine that a carb is just

> too big for a bike?

this is from my thread. i was asking about putting a vm20 on a 50cc motor.

"You can attempt to put a VM18 on this, but general rule of thumb is you want venturi size to be at most 50% of bore. So, on a 38mm~ cylinder you wan a ~18mm carb max (and that's theoretical maximum, so including a pipe and porting). Aka, a stock port and intake, and a 12-15mm is more than adequate and youll get better response from the smaller carb"

TM24 does seem a bit huge for a stocker 50cc. I run a tm24 on my nu/nx motor, but im also running a heavily modified h2o setup that spins the motor to ~10k rpm

Re: Max Performance - Yamaha QT50/SG50

Jack Rutherford /

This is a 45.5mm bore so I'm on the edge.

But I would think there has got to be some signs to look for in how it starts, idles, runs that would flat out make it clear that this carb is just too big.

I will also check out your thread. thanks. (edited)

Re: Max Performance - Yamaha QT50/SG50

Dirty30 Dillon /

The biggest problem with running a large carb on small cylinder (especially on an single speed, fixed ratio transmission) is that you're really never going to get AS accurate tuning as you could with a smaller, proportionate bore, because your intake pulse/draw is weakened. Generally speaking, in flow dynamics you lose fluid speed velocity the greater your volume is. Big bore carb means air travels through it slower and has a slower reaction to throttle input changes because of this. In addition, many modern carbs are HEAVILY reliant on a strong intake pulse to properly meter

This is really true of mikuni carbs, where EVERY VM20 bike I have ever built or worked on whether for friends or Second Stroke/NYC Mopeds build, ran decently with the factory jets the VM20 comes with.

Now, I can tune it in from there, but their pulse metering is so good, they typically just work, unlike a PHBG.

Again, this can be motivated through many ways to get decent results, but in the end I will typically err to smaller carb per build.

Re: Max Performance - Yamaha QT50/SG50

Ok, what about too small (maybe) 44mm bore, OEM intake, 15mm carb.?

Re: Max Performance - Yamaha QT50/SG50

Dirty30 Dillon /

15mm carb on a 44mm cylinder is not too small. That's pretty standard and healthy, imo.

I run a 12.12 SHA on my 46mm Pinasco kitted ciao and love it. I have no issues tuning and it runs cool and well.

And to answer the OP: look at what Jack mentioned a few replies down. All the the items in your buy list, PLUS HPI, relaced rim, new gears, and a bunch of know-how, and he's at 51mph.

Could you get 55+ put of a QT? Yes.

Will you get it out of the parts listed in the first post? No.

Re: Max Performance - Yamaha QT50/SG50

I'm thinking even a Honda MB5 has to work/be modded to go past 55 mph. It has a cluth and 5 gears.

If my goal is over 55 I'd start at a 125 Honda and preferably a 250.

Now I'm just as guilty, wanting more. By all means get all you mentioned + PW50 gears, have fun with assembly, get the best you can, but it won't be over 55 mph.

It will still be a blast!

Re: Max Performance - Yamaha QT50/SG50

Dirty30 Dillon /

The MB5 is also a weak comparison because it's about 100 lbs heavier and it's output is not even that high compared to it's contemporaries.

It is a great bike tho.

Re: Max Performance - Yamaha QT50/SG50

> Peter v. wrote:

> -------------------------------------------------------

> Mine currently goes 41, faster down hill, this is not a platform for

> even higher speed. The bike only weighs 95 lbs.

>

> The brakes, wheels, frame, suspension, etc. can't handle it. It's very

> loose at speed. I'm scared to take a hand off the bar. My brake drums

> are brown from heat.

>

> You don't want to thump into a pothole at speed.

SO MUCH THIS ^ The brakes SUCK on these bikes... be careful.

Re: Max Performance - Yamaha QT50/SG50

I put EBC brakes on front and rear, probably best out there, way better then stock or another off brand I had. They're still inadequate. Why are they so narrow? They're not even using all of the drum.

Re: Max Performance - Yamaha QT50/SG50

I’ve hit 55 by using the yt60 kit, and raising the exhaust to get like 25 degrees blowdown, prep the cylinder and case for good flow and get a good pipe (mlm should do). My set up Hits 11k rpm for now

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