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The main thing that needs to happen is the main jet must be bigger. I found the stock jet labeled 82 corresponded to a jet drill measuring .79mm. I'm up to a .93 without the snorkel and seems to be running well. I will go up until it runs less well and then back one. This with the 190 kit and SOHO cam FWIW.
The second thing that I did was drill the vacuum hole in the slide to 1/8" which is probably the maximum given it's proximity to the needle. This makes the slide go up and down faster, read punchier. Not so slow to respond to throttle changes. Thats it basically.

*note on main jets
I left the original 82 intact in case this project was a failure. I used the 5mm DellOrto jet for the sha/shb carbs(small frame and Lambretta). The thread is slightly different but being brass and only three threads deep they are readily forced into place(yeah, I said that.).

**pitfalls I encountered and how to avoid them.
Two things that caused me some wasted time. The Diaphraghm not seating in the grove and the spring getting bound up.
I found it useful to fill the grove the diaphragm lip seats in with Magic Lube PTFE sealant/lubricant(used in the swimming pool industry for pump o-rings). It basically acts as a paste to hold the lip in the grove as you assemble the spring and top cover.
After assembly do lift the slide up and down by reaching a finger in from the airbox side to ensure smooth and complete opening and closing of the slide.
*** if you happen to remove the atomizer tube that the main jet screws into , be sure not to lose the loose needle jet that fits under it. It would be easy to do.

I'll update with my final jet spec.
⚠️ Last edited by scootermarc69 on UTC; edited 2 times
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So all you have done is remove the little elbow or "snorkle" from the air box? How is part throttle acceleration? Don't feel a need to drop the needle? And Keihin jets are available for these, no need to drill.
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bluecloud wrote:
So all you have done is remove the little elbow or "snorkle" from the air box? How is part throttle acceleration? Don't feel a need to drop the needle? And Keihin jets are available for these, no need to drill.
The first thing I tried was to shim the needle up .5mm. It really didn't like that, but in retrospect perhaps the diaphragm wasn't sealed. After having success with the larger main I tried the needle shim again. Still really didn't like that, but again possible the diaphragm wasn't sealing. Seems good all around at this point. Basically upjetted, drilled the slide and removed the snorkel.
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Tried a .95 jet today and performance was marginally decreased. Maybe good for winter time. Before going back to .93 I will open up the air box inlet and see if there is something to be gained there.
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Yeah, want to be able to lower the needle not raise it. If you go big enough on the main part throttle will be too rich.

I've been considering trying the stock carb again as I miss the accelerator pump and electric cold start. Took a look at the slide and think you could machine some material off then stack washers to bring needle back to stock height. This would give the ability to lower the needle by removing washers.
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bluecloud wrote:
Yeah, want to be able to lower the needle not raise it. If you go big enough on the main part throttle will be too rich.

I've been considering trying the stock carb again as I miss the accelerator pump and electric cold start. Took a look at the slide and think you could machine some material off then stack washers to bring needle back to stock height. This would give the ability to lower the needle by removing washers.
Same
Accelerator pump good.
Electric cold start good.
Replacing throttle cable was my only expense and it gave me the push to repack steering bearings while I had the plastics off. Might make more sense to machine the needle if needed, but you might find it works well enough as is.
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Airbox inlet opened as pictured requires yet larger main jet. Went with .98 which is the biggest drill I have. Definate improvement. Tried a 104 jet I had, which theoretically is 1.04mm, and possibly not as good as .98 so will go back.
Running the 38 idle jet and I can't perceive any change when adjusting the screw until it is all the way in. I think that means I need a larger idle jet. Anyone?
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scootermarc69 wrote:
Tried a 104 jet I had, which theoretically is 1.04mm, and possibly not as good as .98 so will go back.
This is where the needle comes in. In order to be rich enough at WOT but not too rich at part throttle may need to drop the needle.

Idle mixture on the Kevin CV richens as screw is turned out. Should have a rise then drop in RPM within 4 turns using idle drop method. If it doesn't drop until screw bottoms out you're too rich and need to go down one size. I think you've arrived at same conclusion as I when tuning stock carb that 38 pilot isn't needed.
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