Where is the speed control ?? QT50 Yamaha

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Where is the speed control ?? QT50 Yamaha

Postby QT50 » Thu May 25, 2006 3:37 pm

I have read all the posts on the QT50 and don't find an answer

I am convinced there is a speed limiter somewhere, it gets to 23-25mph then it cuts the power back, drop under that and back to full power, going downhill it will pick up more speed, but the engine sounds the same as when limiter is affecting it, also engine won't high rev sitting on stand, it will get so high and thats it, and you see some oil smoke when trying this

Carb is supper clean, tank is not pluged, pipe don't seem pluged, new spark plug and coil, has a new battery

Now I have a part of a QT50J service manual, and it shows a speed control unit that limits the revs to 2300rpm when switch is in start, only problem is its a 1980 and don't have this, I am a electronic tech and curriousness had me cut open the cdi unit to see if they built the speed control in it, not that I can tell, tryed changeing the cap in the gate circut of the scr no help, don't seem to be any zener diodes that are triggering at a certian voltage, its a pretty simple circut and I don't find anything in there that would trigger and cut the coil power off, unless that diode in gate circut is a zener higher than 22volts, thats what I checked it up to

Either way I still feel something is holding it back, it just won't rev up like a 50cc dirt bike, I am begining to think there is maybe a mech. gov., maybe it pushes the reeds open, or maybe revs are just limited by the port timeing

Thoughts anybody??
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re: Where is the speed control ?? QT50 Yamaha

Postby TowerTower » Thu May 25, 2006 6:31 pm

Port timing is a big part of the answer, as is the too-long exhaust pipe which produces a steep power drop off after peak horsepower rpm range, too small and short ports, too small carb, and restricted airbox. Hence, even with a small load the QT50 engine is just not powerful enough to push a rider past 25mph.

Also consider, as the air filter begins to clog, the fuel:air mix enrichens, and that also has a slight dampening effect at the top end.

Raise the jug with a thicker gasket. Shave the heads to match, plus a little more for a slightly higher compression (only takes .010 to make a big difference). Shorten the exhaust pipe and lengthen the divergent cone. Use a K&N filter and larger carb. Result will be another 1000 rpm rev-ability. Add a larger gear to the front and/or smaller sprocket to the back and you'll be up at 30mph.
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re: Where is the speed control ?? QT50 Yamaha

Postby tumbleweed » Thu May 25, 2006 10:33 pm

i dont think they have a limiter!my 83 dosent and its just a matter of low state of tune and its too restricted.get a yt cylinder and piston and really enjoy it!
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re: Where is the speed control ?? QT50 Yamaha

Postby QT50 » Thu May 25, 2006 11:27 pm

What it is that makes me think there is one is that if you creep up to 25mph on a slight downhill its just like a switch, right at that speed it acts like ignition is cutting down the spark, back off the speed slightly and just like a switch power comes back on, just don't act like a restricted flow problem because its so sudden at that specific speed, I was playing with the cdi somemore and if you disconnect one of the diodes that rectify the ac from the magnito, engine hardly idles and won't rev at all, just like it acts when it hits 25mph, so I will play with it somemore

Does yours act like that tumbleweed, a pronunced cut back in power at 25mph, or a slow loss of power ??

Has anybody split the case on one of these and see if there is some sort of govenor in there ??

And where do I get a YT cylinder and piston ??

Sorry for all the questions, I'am new at this, I troubleshoot problems for a living and this just bugs me !!!

Ken
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re: Where is the speed control ?? QT50 Yamaha

Postby kex » Fri May 26, 2006 12:44 am

That's you getting out of the powerband, and running out of gas. The tiny carb bowl can only hold so much gas, and at wide open throttle you're burning the bowl too quickly, and changing the fuel/air ratio, hence the slight surge of power at slightly less than WOT. It's not a governor. It is a combination of the things already mentioned: poor port timing, a restrictive carb/intake and exhaust.

YT parts can probably be found on ebay. I dount they are sold new any more.
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1982 honda urban express (nu50)-65cc metra kit, proma low boy exhaust
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re: Where is the speed control ?? QT50 Yamaha

Postby silverfox » Fri May 26, 2006 10:42 am

I think that he is suffering the typical 2-stroke, four stroking syndrome. It runs up to speed and then because the main jet is too big the mixture becomes over rich and the bike starts 4 stroking. The bike will then slow down until the air/fuel requirement drops and "cleans out" and starts to 2-stroke again.
Very typical 2-stroke operation. The factories put too large main jets in th bikes to keep boneheads like me from siezing the engines. You can try running it (temporarily) with no air filter to see if it runs cleaner at max speed.
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re: Where is the speed control ?? QT50 Yamaha

Postby xrongor » Fri May 26, 2006 1:37 pm

i used to think there was a rev limiter too. until i started tuning the carb and cleaned out the exhaust.

my QT50 with stock gears, stock exhaust, stock head, stock stock stock, properly jetted carb, premixed 50:1 with amsoil interceptor and a few holes in the airbox will hit 38mph downhill. down even a slight hill its up over 30 for sure. cruise the flats at 28-29 all day long and i weigh 185. if i tuck i can get a little more.

i have a bike computer calibrated as close as i can get it so these arent bogus speeds im posting. if anything you're probably going slower than 25. when i first got my QT it would only do about 22-23 and suffered from all the symptoms you are having.

there is NO 'limiter' in the sense that at a certain RPM the spark cuts out completely or gas flow suddenly cant keep up. you are either improperly jetted, or your carb cant get enough gas to it because something is clogged. when everything is working right the bowl can easily fill fast enough to keep up with 38mph.

you are probably suffering from a little of both. do you have a fuel filter in the gas line? if not, i doubt the carb is super clean any more...

as for the CDI, you didnt need to cut it open. the schematic for the CDI is floating around on this site. use search feature. there is an RC timing circuit that controls the advance a bit, but its not a cutout by any means.

as for YT/YF parts, ive posted everything you need to know about the 60cc upgrade including the cheapest source for them (unless you want to gamble with used parts off ebay), along with lots of other tips on the QT at the end of this thread:

http://www.mopedriders.org/viewtopic.php?t=5599

so before you do anything drastic, reclean the entire fuel system including the vents in the gas cap, install a fuel filter, burn out the exhaust and check the exhaust port for carbon when the pipe is off, and drill holes in the airbox as described in the above link. if it were me, i wouldnt spend any money on the 60cc upgrade until i knew i could tune it with 50 cc's to match my above stated performance just so you know the engine is good.

hey karl, you got a PW gear running yet??

randy
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re: Where is the speed control ?? QT50 Yamaha

Postby xrongor » Fri May 26, 2006 1:52 pm

oops posted this to wrong thread. oh well, might as well keep posting it so it stays in circulation <g>

i also wanted to mention i own 2 QT50's and had them both running at the performance level stated above before taking one to QT60. one has a cdi from an '80 and the other from an '84. so you have a long way to go until you start in with modifying the RC circuit in the diagram to alter the timing.

personally i think if you want to go down that path i would just use a PIC or something and make my own timing curve that you could modify ad infinitum.

cdi schematic for QT50:

randy
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re: Where is the speed control ?? QT50 Yamaha

Postby QT50 » Sat May 27, 2006 12:40 pm

Yeah Silverfox sounds exactly like you said

Randy--you definitly have that little engine figured out !!

On the cdi that r-c filter in the gate circut I just figured was just that a noise filter, but you're right it would alter the gate trigger time, allthough bypassing the filter didn't seem to change anything, thanks for the schematic, although I got it the hard way sooner !!!!!, and the diodes are just regular (not fast) 200v and 400v- the red banded are 200v and yellow 400v

I did put a filter on gas line, ist thing I do on any bike, and i did discover the pin holes in gas cap, inside and out were pluged along with fuel stop, so I'll go after that pipe and exhaust port, carb jetting is stock and clean, and I didn't notice if needle had adjustment grooves or not, did you change your main jet size ?? and how fast does the 60cc go ??

Thanks everybody for your help--Ken
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re: Where is the speed control ?? QT50 Yamaha

Postby xrongor » Sat May 27, 2006 1:36 pm

i posted all my tuning info on the link listed earlier in this thread. but i have a couple things to add now that im running synthetic and the engine is well broken in (i think i got about 600 miles on it now....)

after i switched to synth it seemed to drop the temp enough that a BP6HS is optimum. i didnt need a colder plug. i attribute some of this to better lubrication, and most of it to a richer mix since 50:1 premix will run richer than 32:1 premix all other things being equal.

i 'might' get better performance with a 65 jet (my QT50 is running a 65 jet in the summer. the QT60 is running 70 jet, which is stock in QT50's) but its good enough and i dont want to push my luck.

now that its all broken in i can cruise at 35-40 under most conditions. dirt roads rob a couple mph (unless they are in really bad shape and its more about how much balls you have than how fast your bike goes <g>). a headwind + a hill might drop me down near 30 and a monster hill on a dirt road all bets are off.

the absolute fastest ive had it is 48.5 (as per the max speed on the bike computer). man i wish i could hit 50 but its just scary up that fast lol and i dont want to blow the engine up so i dont go for max speed runs often any more.

ive got the needle in the top notch (lowest position) on the 50 and had it in the second highest on the other one when it was a 50. i dont use a very scientific method to set the needle. i start in the middle and keep lowering it until i find the peak performance. since the QT has such low compression youll probably find no position is perfect, but one will be better than the others. just keep in mind that needle settings affect midrange not full throttle. when you get done, do a 3/4 throttle plug chop just to make sure you arent too lean at that throttle setting.

dont forget to drill the airbox as per the other post. stock wont ever let enough air in for decent performance.

man we got rain for a few days. this sucks!!!

randy
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re: Where is the speed control ?? QT50 Yamaha

Postby tumbleweed » Mon May 29, 2006 10:58 am

no gears yet,ive gotten sidetracked randy,i finally got a digital speedo and as close as i can figger i can run 30mph, with slight down hill and get 35 to 40 wfo.not saying for sure but as near as i can get it calibrated.i also checked it with those how fast are you going things cops use.like you i can cruise at 28-30 all day,im using torco synthetic in the oil tank and preimium gas.
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re: Where is the speed control ?? QT50 Yamaha

Postby xrongor » Mon May 29, 2006 6:59 pm

am i reading you right? your numbers match the performance numbers for my stock 50. all day at 28-30 and 38 max. thats all your getting with the 60 kit? you really need those gears <g>

forget about trying to calibrate it by the speedometer. you'd do as well to just measure the tire and call it close. find two mile markers and calibrate the odometer so when you go one mile it reads one mile. if you have a bunch in a row calibrate it over 3-4 miles. thats the most accurate way.

randy
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re: Where is the speed control ?? QT50 Yamaha

Postby tumbleweed » Mon May 29, 2006 8:37 pm

yes the 60 dosent go too much faster with the stock gears but it gets there alot! faster.
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