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Post by nicolascagefan on May 30, 2023 9:33:16 GMT -5
2003 Zuma, just put in a Stage-6 MKII crank,Polini sport 70cc (cast), and running the Polini Speed exhaust I previously had on the scoot. Stock carb and stock airbox, NGK BR8ES for sparkplug.
I started out with a 90 main jet (and dropping the needle clip down one spot), going off the ~10% increase for a 50cc to 70cc notion. It seemed to run great, very responsive, etc. Did a plug chop, and the ring at the base of the plug was pretty dark, and also only about 1-1.5mm. Figured a 92 jet would help, but it ran noticably worse, and the plug looked super lean. Tried with a 96, still didn't feel quite as smooth, and plug still looked very lean. I ended up throwing a 98 in, seemed to run a little smoother, and the ring at the base of the plug was a very light brown, about 1mm tall, so it seems like I need to go higher than that. Spark plugs also all had a bit of tan, ashy color on the electrodes, which seems to indicate a lean condition to me.
I guess my question is, should the jetting really need to be that high? I guess I forgot to consider that I'm running a "full circle" crank now, but does that actually effect jetting that much? I also checked the temperature at the spark plug bung after every run and it's consistently right around 200 degrees F, which seems a bit cold from what I have read, making me more confused.
To be fair though, I don't know how accurate my chop results are, considering it's a brand new setup, I'm only running it slightly above 3/4 throttle, which I'm assuming should be enough where needle position isnt effecting my results
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Post by aeroxbud on May 30, 2023 14:37:12 GMT -5
That does seem quite high, and the temperature is quite low. Is it possible the jet size is so large the fuel is washing the colour from the plug? Possibly giving you a false reading.
Does it make a really deep sound when you open the throttle?
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Post by nicolascagefan on May 30, 2023 15:19:19 GMT -5
Hmm I'd say it sounds pretty normal honestly, and the spark plugs have never looked wet while pulling them out. The exhaust (polini speed) is very mild, so from my experience I'd say it doesn't have a very distinct "correct" sound like a pipe with an expansion chamber would. Not sure if it would make any difference (I don't think it would) but I haven't tuned the cvt yet, so it's catching around 5,5k rpm. It takes off and rides exactly how it did before the bbk, except quicker and you can feel the extra oomph. (I ordered the wrong sized clutch springs, so no point of touching the cvt right now lol)
I have noticed the exhaust smells a bit "sharp", and really burns your eyes and nose if you take a good whiff... Does that mean excess fuel in the exhaust??
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Post by Zino on May 30, 2023 18:01:55 GMT -5
Do you have pictures of the plug ?
You are running a lower rpm set up with the polini sport and polini speed .
Your temps are real low if you were pushing 350 degrees I would be looking to upjet
You may have been able to get away with stock jetting .
On similar Zuma setups as yours
Stock jet with needle adjustment or A 84 yamaha jet in the stock carb with a needle adjustmeent was enough .
As i went to higher compression and higher rpm set ups the 84 jet would have a harder time keeping temps in check .
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Post by nicolascagefan on May 30, 2023 19:10:12 GMT -5
If it's hard to read, from left to right is 90, 92, 96, 98, and the last one is also with the 98 main, but moved the needle clip to the bottom most notch (did not help, moved it back)   |
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Post by Zino on May 30, 2023 20:11:40 GMT -5
use the 90 that is a pencil lead sized smoke ring .
You may find you can drop down to 88 or 86 from there
I tune off of feel responsiveness,sound and temps first and only do plug chops when I am not sure .
I set the main first Finding the jet that gives me the best top speed at wide open if 2 feel the same I take the richer one.
I set the needle by how on off the throttle it responds from 1/4 to 3/4 throttle
If 2 positions feel the same I take the richer one .
Then Just move the idle mixture screw so I get the best reponsiveness at 0-1/4 throttle
This how I set my Daily rider I run wide open for about 5 miles straight every day to and from work its a 20 mile round trip .
Monitor your temps she runs real well in the low 300 's
If on Long wide open runs your jetting has your temps level off below 350 degrees you are in good shape .
the fuel mixture provides about 80 % of cooling the fan does the rest
The polini is a great sport cylinder You will get a ton of miles from that beast with good oil
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Post by nicolascagefan on May 30, 2023 20:42:04 GMT -5
Awesome, thank you for all the great advice, about to try an 88 and go from there. It definitely felt better with the 90, but I was afraid of cooking the cylinder since I thought it was lean.
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Post by Zino on May 30, 2023 21:16:17 GMT -5
You will find at different speeds the temps will just level off if you are jetted right.
At higher speeds it will find a comfotrable spot.
What you dont want to see is the temp just keep rising above 350 degrees is what alot guys use for the casrt iron bores or spikes in temps at any throttle position.
My Vinos peak out about 325-335 depending on the humidity of the day on long runs.
Dont be surprised after a few tanks of gas it runs cooler by a few degrees after its starts to get broken in
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Post by nicolascagefan on May 30, 2023 21:29:29 GMT -5
Ok awesome, I don't have a temp sensor, so I've been checking it as soon as I kill the engine with a digital cooking thermometer  . Just about to take it for a spin with an 88 in it, I'll post back with some results. Also, curious as to why going all the way up to a 98 main wouldn't foul out the plug or even make it seem wet? Im not the most well versed in 2ts, but shouldn't the electrode on the plug look wettish and dark when it's running rich? |
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Post by nicolascagefan on May 30, 2023 22:54:19 GMT -5
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Post by bigmatthew86 on May 31, 2023 7:27:19 GMT -5
It takes a while for a plug to change colors with todays fuel. A temp gauge makes tuning so much easier. Get one & aim to peak out around 350 & you’re good. You’re going to have to buy stock in NGK with how many plugs you’re chopping up.
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Post by nicolascagefan on May 31, 2023 8:41:32 GMT -5
Lmao I know it. Already went through eight I believe. Good thing they're only $4 a piece. I looked at my piston head last night too and it definitely looks like it's been running hot though, it's a bronzish color (looks like it's zinc plated in the center, silver on the outer edges still). I just don't understand how I'm still seemingly so far off lol. Even with the 98 main jet and needle clip all the way down, it still seemed lean, even though I hear of plenty of similar builds running just over stock jetting...
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Post by bigmatthew86 on May 31, 2023 12:36:46 GMT -5
Different brand of jets don’t mean the same thing with their numbers. My last build I ended up with a way larger main than everything similar I had read. Turns out 98 in what I had was like an 85 in mikuni or round about. So I ended up at a 102 or 104 I believe. You still need to just get a trail tech temp guage. Cheap insurance & makes this so much easier. I’ve done it both ways. Hours of swapping vs like 30 minutes.
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Post by nicolascagefan on May 31, 2023 13:40:48 GMT -5
Oh damn that's good to know, I thought it was a standard measuring system. I've been using a polini set
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Post by bigmatthew86 on May 31, 2023 15:30:05 GMT -5
 Here is a conversion chart. Some use just a number system & some use the actual size of the hole. Might be your whole issue. |
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. Just about to take it for a spin with an 88 in it, I'll post back with some results. 
