I'm in the process of jetting my two Zumas. I've never tuned by CHTs alone so I wanted to give it a try. I dug around a little and decided to use 350f as a target and 375f as a safe limit (both air cooled).
Scooter 1 (Pam's) is a 70cc athena sporting, stock carb, stock airbox with snorkel removed and a UNI stock replacement filter,and Tecnigas silent pro exhaust. I started with a 110 main and the cht peeked at about 280, it was noticeably rich and didn't want to accelerate after the cvt was fully engaged. I worked my way down to a 100 main jet before I got close to my target temperature (345f). It still felt rich and performance was down from previous jetting. It wasn't horrible, but it has much more in it IMO.
Scooter 2 (Allen's) is a 70cc stage6 mk2 sport, Yasuni C21, 21mm Polini cp carb, V force reeds, stock/modified airbox with uni stock replacement filter, overrange cvt, etc... I started with a 106 main and the CHTs were at 376f and climbing when I lifted. I switched to a 110 main and it wouldn't even come on the pipe it was so rich. Switched to a 108 and CHTs were 361f when I lifted. Performance was also down from previous jetting.
The fan and shroud are both in perfect condition on both scooters. Both have new crank, bearings, and seals and have been successfully pressure tested.
Is there actually a magic, safe temperature to tune for, or is that just a myth? Would tuning for best performance (rpm/speed) be a safe method? After, I could monitor CHTs and keep an eye out for any drastic changes. I'm trying to keep them as reliable as possible, but I also don't want to leave any power on the table. Any input, suggestions, or advise would be greatly appreciated.
I hoped some else would pitch in,but you seem to be on the right track,the magic number is 350 or below but that's just what I think from lots of forum browsing,air cooled two stroke witchcraft is another way of looking at it.just take little steps and keep notes.
The temps you selected are correct for our air cooled motors. Tuning without a CHT gauge is like driving blindfolded.
You always leave "power on the table" when you tune for reliability. Think of it like this, reliability is this guy > performance is the devil. As one side goes up the other must come down. One way is not as much fun, the other is a lot of fun for only a while.
Your total tune is a combination of the carb, and the CVT tunes converged. One will affect the other to some extent. Sometimes only a little, sometimes a lot.
You spoke of tuning the main jet, how is the rest of the throttle range? You can be lean down low, rich at the top. So the motor heats up until you go WOT to cool it down, then after it cools down you are suddenly too rich.
One thing to watch for, I have seen jets consistently grossly mismarked. You can go crazy putting in say a 110 jet you think is larger than 105 you are replacing.
I have spent WEEKS getting a tune just right, pulling my hair out along the way. Hats off to those that can get it done in an evening after work!
The are no magic tricks I know of, just patience, trial and error and sometimes dumb luck!
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Thanks for the info guys. On both scooters, up to 3/4ish throttle is pretty darn close. No flat spots, nice and responsive, and the CHTs don't get out of hand. On my scooter one jet size down from it not even making enough power to get on the pipe, it just hangs at 7500 and smokes like crazy, yet I drop one size and the temps are over 350. What would I do in that particular situation? I'd definitely choose reliability over performance, but it seems I don't have that choice. Thanks for the input guys!
On the jets, I did the exact same tuning process on two separate "jet kits" a Polini kit and a stage6 kit. It does seem like the Polini kit was slightly richer than the stage6 kit, but it was later in the day and the ambient temp had gone up about 6 degrees.
Something seems off with the jet sizes. Like said above, if you have not brought genuine jets from a reputable dealer, they might not be right. I would get a jet gauge, or try new jets of in doubt.
You posted after i started reading. It's still strange the two jet sizes can be so different. Normally you can run at least a few sizes either way.
Last Edit: Nov 15, 2020 13:15:19 GMT -5 by aeroxbud
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It wasn't a huge difference, I can't tell you seen exact temp difference because I lifted before the temperature peaked, it just seemed like the equivalent stage6 jet gave a faster rising temp, but lie I said, that also could be the change in temp. I ordered up an egt gauge, hoping that maybe it will give me a little better idea of what's going on. I have no idea what a "safe" EGT reading would be, but at least I will have more data.
Another though, on this lazy Sunday morning. Is it possible that the factory timing is too advanced for the added compression (on my scooter), causing higher CHTs? I have an Ignitech programmable ignition ready to install, I was just wanting to get everything as close as possible before I added another variable to the equation.
What revs are you getting? That Yasuni pipe is a high revving pipe. &500 it's just getting started and if your not hitting the pipe it will feel rich.
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When I had it leaner it seemed quite happy at 12,250-13,000. But the CHTs would quickly get out of hand. I just checked the factory timing and it was 14* spot on. I just finished installing the ignition and verified the timing again. I think I'm gonna pull a few degrees of timing across the board, and lean it to peak performance, assuming CHTs don't get out of hand. Any thoughts? Thanks everyone for the input!
Thank you Zino. Before I had put CHT on her scooter, I saw the exact same results as you. 60' and .1 mile times and speeds were best with jetting between 85-92, only varying by .2 seconds and 1 mph. I was happy with the 92, thinking it was on the safe side, until I measured the CHTs. Hers runs decent with the 100, I may leave it there for the time being. It's plenty fast for her and I want it to be dead reliable.
Got to mess around with my scooter a little more today. I installed an egt, expecting to see low egts due to the rich mixture. Took it for a quick blast and they were actually higher than what I expected. I saw a peak of around 1275F. I pulled the plug and it was dripping wet. I'm assuming that the excess fuel is lighting off in the pipe, when it comes in contact with more oxygen 🤔. I'm still running stock timing, so I didn't expect to see any change in cht. I'm waiting for some time where I can spend the entire day changing and logging timing and jet changes. Any thoughts on the high egt? Has anyone tried adjusting the ignition timing? In theory, with the added cylinder pressure, it should want much less timing than stock. Thoughts?
Senna1Rossi: Thanks renagade281!
May 23, 2016 14:59:12 GMT -5
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Jun 18, 2016 16:26:04 GMT -5
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