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Moped Riders Association • View topic - Yamaha QT60 (actually MJ60J) Jetting questions

Yamaha QT60 (actually MJ60J) Jetting questions

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Yamaha QT60 (actually MJ60J) Jetting questions

Postby TC-ST » Tue Feb 10, 2009 12:59 pm

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re: Yamaha QT60 (actually MJ60J) Jetting questions

Postby supercub » Tue Feb 10, 2009 1:39 pm

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Re: re: Yamaha QT60 (actually MJ60J) Jetting questions

Postby RebelMob » Tue Feb 10, 2009 5:46 pm

Processed on shared equipment that may have come in contact with nuts.
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re: Yamaha QT60 (actually MJ60J) Jetting questions

Postby xrongor » Wed Feb 11, 2009 6:42 am

"In this world there's two kinds of people... Those with loaded guns, and those who dig." -- Blondie to Tuco
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re: Yamaha QT60 (actually MJ60J) Jetting questions

Postby TC-ST » Wed Feb 11, 2009 10:19 am

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Re: re: Yamaha QT60 (actually MJ60J) Jetting questions

Postby RebelMob » Wed Feb 11, 2009 8:20 pm

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Processed on shared equipment that may have come in contact with nuts.
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re: Yamaha QT60 (actually MJ60J) Jetting questions

Postby xrongor » Wed Feb 11, 2009 9:08 pm

you may want to have a look at this thread:

http://mopedriders.org/viewtopic.php?t=9093

as mentioned the plug took 700 miles to get that way, and using a new plug on a short run yielded no result at all. imho the main thing to look for is dots on the porcelin which would suggest detonation. but i never did get any plug readings i felt all that confident in. the ring around the base was really all i had to judge.

as for why reading the tan works on some bikes but not on the QT60 i have no good answer.

ultimately i kept upjetting until it 4 stroked sometimes. then backed it off to the next smaller jet. 2000 miles later and i have no signs of problems based on the last teardown.

based on my research i see no reason to read plugs for 4 strokes any differently than 2 strokes. others may not agree. my advice, start googling and decide which charts you like best hehe. there are lots of ways to read plugs and i simply dont believe the tan on the porcelin gives the final word on your mix. there are other factors at work.

i agree, if you milled 30 off it, you probably dont want to use that head. maybe mill a little off the YT head, after you get it running decent.

as for part numbers, it could still very well be the same exhaust. ive noticed the same things on PW50 parts vs QT50 parts that i know are identical.

reading the plug at WOT tells you about your jetting. reading the plug at non WOT tells you if you have the needle set properly. and it is pointless to do non WOT chops until you have the proper jet. that said, maybe you should play with the needle a bit and see what happens.

im getting 40+ with the stock exhaust which is plenty fast enough for me. you can probably get better performance with a (properly tuned) custom exhaust, but at this point i would just put the stock exhaust back on because its a known working configuration and should give you a decent baseline to gauge other exhausts against.

just because the pipe came off a 50cc bike doesnt mean it works for all of them. port timing and size, bore, stroke, compression, all come into play.

anyway some things to think about. good luck!

randy
"In this world there's two kinds of people... Those with loaded guns, and those who dig." -- Blondie to Tuco
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re: Yamaha QT60 (actually MJ60J) Jetting questions

Postby TC-ST » Mon Feb 16, 2009 10:40 am

Update

OK, so we had some slightly warmer days here in PA, so I decided to play with the MJ60 a little... I pulled the 92.5 jet and put an 80 in there. I noticed an immediate improvement in power, acceleration and top speed. To speed was 40 on flat road, and 45 downhill. I may have been able to get a bit more out of it downhill, but the engine was screaming at that point and I didn't want to push it. I did several quasi-plug chops while I had it out, starting with a new plug, I did a mile long run at WOT, cut the ignition, pulled over and took the plug out and looked at it. I put the same plug back in it and repeated this process every time I had the opportunity to run flat out for a mile and have a place to pull over, which resulted in 4 or 5 stops... Each time the insulator of the plug remained nearly completely white, save for a trace amount of color. Even after my final chop, the base ring still only had 3/4 color, with the remaining amount being "plain steel".

Getting a little nervous, I upjetted to 82.5 yesterday afternoon, cleaned the base ring of the plug, and did a normal plug chop. The result was a bas ring with only 2/3 color...! Hmm... Granted, the outside temp was significantly lower, so that could have been a factor. Top speed was down, too, to 37 MPH on the flats.

One thing I thought of, could my running heavy oil in my mix (32:1) be affecting the buildup (or lack thereof) on the base ring? Maybe too much oil in the mix doesn't allow deposits to build up as quickly... I know it throws the mixture off a little.

As for exhaust, I still have my pocket bike pipe on there, but found a complete YT60 exhaust for a good price, and will try that out when it gets here. (I have heard of someone using one on a QT60 before with some minor modifications)

Again, thanks for reading this and offering any input. I am having a blast both tinkering with and riding this quirky-looking machine! I only wish my Tomos was as smooth at higher speeds...
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Re: Yamaha QT60 (actually MJ60J) Jetting questions

Postby churioz » Tue Oct 03, 2017 8:52 pm

I'll add some finality to this one (years later of course). That 92.5 jet was way too rich. I'm betting the 80 was too rich but closer. I would've tried a 77.5. Temp gauge would've been handy to help figure out the jetting as well. Anyhow, inspiring - I may try this on my towny.
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