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

Hello all:

I have been working on my Yamaha MJ50J Towny for a while now, and am down to one last thing: sorting out the jetting. Just to bring everyone up to speed, (for those that don't know) the MJ50J has an engine very similar to the QT50 (2-speed trans instead of a single speed), and I have added a YT60 top end (Including the head), drilled the airbox, and added a custom exhaust using a pocket bike expansion chamber and muffler. I HAVE searched through the posts here and elsewhere, and it seems that most people who have done the YT60 swap end up jetting their carbs in the 70-75 range. Now, the reason I did the swap to begin with is that I ended up seizing the stock top end due to running lean, and I really don't want to do that again! (yeah, I did upjet after adding the exhaust & stuff, but apparently not enough...) Looking at the Yamaha service manual, the stock jetting for a YT60 is, get this, 92.5 ! Not wanting to risk anything, I installed a 92.5 jet in there and have put about 300 miles on it since. Performance has been ho-hum. It runs like it did stock, and gets terrible gas mileage. It will 4-stroke a little when cold, but once it warms up it doesn't. It "seems to be" running rich, but the plug that is in there is only starting to get color on it now so...

Plug chops on this thing, both before and after the swap, have not yielded any usable results. I have done plug chops before on my Tomos, and they worked perfect (ie: perfect smoke ring at the base of the insulator). Whenever I have done a plug chop on the Yamaha, though, the insulator is pretty much as white as it was when I first installed the plug, not giving a usable reading...

Now for my questions: Does anyone have any idea why Yamaha would use such a large main jet in the YT60?
(BTW, the YT60 uses a VM12 carb, just like the MJ50J) Any jetting recommendations here?

Plug chops - any idea why they "work" in my Tomos, but not in the Yamaha?

Any suggestions on any of this would be appreciated!

I know that I am just getting through the break in period on it, but I feel that there should be more power than there is... I would really like to get this sorted out because spring will be here before you know it, and with it-riding season!

Sorry that this post is so long, but I wanted to provide as much info as possible.

Thanks is advance!

TC
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re: Yamaha QT60 (actually MJ60J) Jetting questions

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

It is hard to read a new plug, I have found it is better to use the plug at various speeds and get some color in it and then wire brush it to try. Check thr timing, use stock setting. With performance mods you need to gear up to lower the rpm. At higer rpm's youre going to cook that gas no matter how rich you jetted. Go along at 20 mph for a while and pull the plug. Then 30. Maybee 20. Get a good range of checks in. Use the best oil.
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Re: re: Yamaha QT60 (actually MJ60J) Jetting questions

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

Play with one thing at a time. Even when someone posts a lot of possibilities to try, this way you can see what helps and what does not make a difference.
I am thinking that if you are burning a clean plug then you might need a little wetter fuel so up your oil a little, and if it is injection then just premix the difference to bring you up one percentage point. A lot of people feel that less oil is a good thing but the oil makes the seal between the ring and the cylinder work and helps transfer the heat away from combustion without seizing. Did you just get any pipe or did you do some calculations first and then find one that matched where you needed the sweet spot?
I would also suggest to set your timing a little more retard then stock if you have put in some performance mods, this will get your expansion chamber singing sooner and put the heat of combustion into the pipe rather than the head where you are blistering the plug. Retard the timing until you feel power loss then advance it just a little from that point.
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re: Yamaha QT60 (actually MJ60J) Jetting questions

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

i would suggest you give this a read. particularly the part about not looking at porcelain to determine jetting:

http://www.dragstuff.com/techarticles/h ... plugs.html

mj50's already have the same gear as a PW50 so it is regeared.

the YT60 likely used a large jet because its exhaust was much less restrictive than a QT50 is.

the timing is not (without serious modification) adjustable. the flywheel is keyed to the crank.

when all is said and done, if you want my advice: dump the fancy exhaust, put the stock 50 head on the 60cc cylinder, put a 75 jet in it, and go. time tested performance.

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 » Wed Feb 11, 2009 10:19 am

First of all, thanks for the replies!!

Timing - As Randy said (and I suspected), it is not adjustable.

Oil - I already am running around 30:1 (for break-in purposes), so I don't think that increasing the oil ratio is going to be beneficial... I am running "Yamalube" in it now, which is 50/50 dino/synthetic. I used straight dino oil for break in, and will switch to full synthetic when I use up the Yamalube (the Yamalube came with the bike)

The link to the site about reading plugs is great! One question, though.. Does that info carry over into the 2-stroke world? I am not saying I think it doesn't, it was just a thought that popped into my mind.

Randy - as far as using the stock cylinder head... I used the YT60 head because I had the MJ50 head milled .030 and didn't want to risk using it on the YT60 cylinder...

Exhaust: is the stock one that good? Mine was damaged (but could be repaired...), so that is why I went with the custom setup. It is a pretty straightforward expansion chamber, with a silencer on the end. It was designed for a 50cc pocket bike. I did experiment with different headpipe lengths until I figured out the proper length for maximum speed. While on the subject, I have noticed that, although they appear to be identical, the stock exhaust for the QT50 has a different part number than the stock exhaust for the MJ50. Any idea what the difference is?

Thanks again to everyone for their input!!
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Re: re: Yamaha QT60 (actually MJ60J) Jetting questions

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

4 stroke plug charts are for 4 stroke engines. Google 2 stroke charts, there are a kazillion of them out there, just make sure you use one for air cooled and not liquid cooled engines, there is a difference there also. I attached what to look for on a plug and a sample picture of how a good plug should read.

The pipe might not be giving problems but it might be making problems. A tuned pipe is tuned to the characteristics of the engine model it came from, unless it is the same engine it might not be an improvement. Stock exhause is a compromise so people can get a long range of rpm and idle. If you are going for performance then get a pipe suited to how you want the engine to perform.
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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
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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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