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120 mainjets

wileyE

Tricycle
When I cleaned my carbs (storage got the PJ's) I re-sized the stock 125 mains to 120  (why is another story).  In short it was too lean and goes flat at ~8000,  not so much in the lower gears but more under more load in the higher gears.  I  duct taped off 1 intake horn to "fix" it which it did, not in any  hurry to get back into carbs.  I notice from reading posts here some are more or less doing the same thing with foam, on purpose?    My bike runs fine like this for now. a tad different, maybe not better or worse. Think it's more the reduction in the airbox honk , sound can sometimes fool ya. Seems like I'm turning the throttle a hair more to get the same thing, but that may just be to get the same "honk".  Anyway found it interesting there were others tuning what I did as a quick fix,  My bike is a 05, stk needles, 35 pj, baffles holed, clean airfilter.  mpg is usually 45-47 but I run tall 1500 goldwing tires which corrects the speedo and make the gearing taller.
 
If the 120's were lean on the top, you have another problem. My original kit used 117.5's and  there were no issues. I built the kit with a WBO2 sensor and also dyno runs, and it wasn't lean on the top. My guess is that either your fuel levels are low or you have an inline restriction like a fuel filter? BTw, I actually found the mechanism for the "2 minute mod" when I first had my bike on the dyno after I built the 1109 block and my mains were to small. I started taping and making runs. with 1/4 of one side block I made 7 hp more on top, but  also 8# more TQ at 5000, which is ALOT. I figured that when I brought the main jet back up I'd get all the same power, but I was wrong. I could replicate the peak power, but not the torque- and I wanted the torque more. To confirm my suspicions, I posted up an experiment by having guys block part of thier intakes ( the 2 minute mod - cus it takes 2 minutes to block the intake). they all confirmed more torque, though of course top end is off as is fuel economy if the stock jets are retained. I set about developing the dedicated jetting, and the 2 minute mod jet kit was born. HTH, steve
 
thanks for your reply Steve, value your experience.  you may be right about my (steel mesh) filter but I never had that problem with the 125's. Just so were on the same page my 120's ran out flat with a stock un restricted airbox, but were fine when I corked it.    Confused about the 117.5 since that is a mikuni sizing which looks just like a small round keihin,  will fit (though the threads squeek a little, but don't flow near the same?   

 
 
Could the fact you've holed your exhaust cans (less restrictive) have something to do with your leanness on top with the 120"s?
 
OK, many dyno runs gone by, the bafflectomy isn't affecting the jetting. The exhaust is really not an issue on these bikes, not as restrictive as one may imagine. The 117.5 is an aftermarket / keihin jet. It measures .001" less than the 120. If 120's are lean on top, you have either a fuel line restriction (filter, et) or floats set to low. HTH, Steve
 
OK, I will leave the 120's in for now and check those other things,    If the filt or fuel level were the problem would not the bike starve on top with my airbox 1/2 masked off too? 
 
I see your point, but not nessessarily - there's a better signal to the carbs and it'll pull fuel more efficiently. Of course if the bowls go really low it'll be lean again. also, is this a cali model bike? though I've checked the carbs 10 ways to sunday and can't find any differences, the cali bikes use 130 mains and some don't like 49 state 2 mm jetting, so I bump up the jetting on those cali carbs. Steve
 
About four tanks now have been using 120s, stock everything else, and now .020 shim under needle as graciously recommended by Steve. 120s had pretty raw feeling of power and lumpy idle. With shims toned down and smoothed idle with much smoother power delivery. Acceleration became linear for lack of better word.

Had a spare air box with a foam plug so tried plugged for 120 miles so far. Seemed to tone down engine sound immensely, brought more chocolate color to plug tips, and brought MPG to 40 from 43.

Saying all this subjective stuff here as I am surprised at the differences such small changes make,. Do wish I had a dyno. Ha!

I am not satisfied with my fuel bowl levels as I have all set to float bowl seam. Thinking might be better served with that different. Seems off.

Also have #2 running about 40F cooler than rest measured by IR measure. Not sure what that is about yet.

Planning on the real 2 min mod in winter maybe.

Have to get a few more miles this riding season. Not looking forward to pulling carbs again!
 
Found this interesting observation on my parts bike when scavenging carb parts. All floats were set evenly at 22mm!
 
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