MNSBR: Jetting Help - MNSBR

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Jetting Help

#1 User is offline   SHAZZAM Icon

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Posted 24 August 2015 - 08:13 PM

Does anyone in the metro have a jet kit for a mikuni/china 18mm 50 carb. My tuning is off and id like to trade/barrow some keys to try and get it working right.

Looks like:

http://ecx.images-am...51uOzr80VuL.jpg
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#2 User is offline   Aprilian Icon

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Posted 24 August 2015 - 08:33 PM

I gave 2K a big bag of jets a couple years ago when I got out of riding. You will need to open yours up and see what style and size you have and then contact Kyle.
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Posted 25 August 2015 - 07:34 AM

What are your symptoms? Mine tends to clog the pilot and die on the bottom end and at idle.
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#4 User is offline   Jim Hubert Icon

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Posted 25 August 2015 - 07:38 AM

I've drilled many a jet in my lifetime. Not ideal, but may get you where you need to be (or at least nail down a few sizes to buy). I've even soldered the orifice shut and then re-drilled. After drilling, I always obliterate the jet marking so the next poor bastard does not assume.

Attached File  80 - 61 wire drills.JPG (216.85K)
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Additionally, a set of micro metric drills fills in sizes not covered by AWG drills.

Remember that flow is proportional to area, which is proportional to diameter squared.
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Posted 25 August 2015 - 08:01 AM

Lots of good ideas, mostly just trying to get this done tonight. I had a jet kit once, it just walked off with someone.

Symptomes mainly are the pilot size is off, air screw is all the way in (I think) and it doesnt quick idle plus stalled with quick throttle opening. I'll verify the setting/current jet sizing later.
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Posted 25 August 2015 - 08:08 AM

I'm pretty sure that carb uses a fuel screw. So all the way in is leaner.
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Posted 25 August 2015 - 12:32 PM

I couldn't remember if it was a fuel screw or air screw. (I believe my CR is an air screw). This would also make sense.

So my FUEL screw is turned all the way in, bike starts and runs under a little throttle, doesn't idle for more than a few seconds. I'm thinking the pilot jet is to large at this point.
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#8 User is offline   Jim Hubert Icon

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Posted 25 August 2015 - 12:41 PM

Almost all 4-strokes use a fuel screw. Almost all 2-strokes use an air screw. You can tell by which side of the slide the screw is located. Fuel screws are on the motor side.

It sounds lean to me. Personally, I'd clean the pilot jet properly (that does not mean spraying carb cleaner through it) and set the mixture screw at about 1.5 turns out and try again.
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Posted 25 August 2015 - 12:46 PM

I'll give it a shot once I get home.

Also if disassembling the jets and blasting them both directions with carb cleaner is not how you clean them what is? Or did you mean spraying carb cleaner into/though the carb assembled was not the right way?
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#10 User is offline   Jim Hubert Icon

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Posted 25 August 2015 - 12:52 PM

A jet is not really clean until the proper size gauge pin will go through it.

Attached File  pin gauges.JPG (211.28K)
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There are lots of different chemicals you can soak a jet in that will clean it, but just spraying carb cleaner through generally does not.
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Posted 25 August 2015 - 01:25 PM

Is the jet size # directly related to the gauge pin size? If you are drilling the hole how do you deal with out of roundness?
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#12 User is offline   Jim Hubert Icon

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Posted 25 August 2015 - 01:38 PM

Except for Mikuni hex jets (which are numbered by flow), jet sizes equate to millimeters. A #100 jet has an orifice approximately 1.00 millimeters in diameter. No it is not EXACT because manufacturers take into account other things like entrance and exit chamfers that have an effect on the flow coefficient. This is especially true for the smallest sizes (like pilot jets).

If you hand-drill a jet it will not be perfect. But it will get you in the ballpark -- especially on a single cylinder where you don't need to match another jet. Main jets are typically about 3% apart in terms of flow. You can calculate more closely than this by ratioing the squares of the diameters.

A partly clogged jet may be 10 to 20% low in terms of flow.
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Posted 25 August 2015 - 01:55 PM

I use the hack method of strands of finely stranded wire through the jet. Some of the pilot jets have TINY orifices.
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Posted 25 August 2015 - 08:09 PM

Pilot is super pluged. I tried soaking in carb cleaner, then .3mm copper wire (32 jet size) and then more carb cleaner, finally tired heating up and all the above. Nothing.

Also im not sure I have the right setup at all. The pilot is a 32 and main is 82. Its am 88 kit with china 18/19mm carb. Main is a k-branded pilot was not.
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#15 User is offline   Jim Hubert Icon

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Posted 26 August 2015 - 06:14 AM

What size wire can you get through it? 0.3mm may be too big for a #32 jet -- especially considering Chinese manufacturing tolerances.

A copper wire that small may just be too flexible to push through the buildup.
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