New carb! Hlp understanding air screw?
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180ss with moded engine
Joined: 29 Mar 2009
Posts: 34
Location: Montreal
Mon May 03, 2021 10:05 am quote
Hi everyone, recently i decided to get s new carb. I picked up a new Polini pwk 28mm carb. It came with a 45 pilot and 115 main.
I drove a bit and could tell it was running a little lean. I upjetted the pilot to a 50, and think it is a little rich ( at about 1/2 throttle, it runs flat. I can try to go in the middle at 48.
I understand the adjustment of the air mix, in that the engine should hit peak revs between 1-2 turns out, correct. My thing is..once i see the pilot is in range, what/how is the adjustment made for the air screw?
Ie: get engine to highest rev, then lower the idle back down? I've always been stumped on this.
Thanks
Fab
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BV 350
Joined: 22 Nov 2016
Posts: 892
Location: Nebraska
Tue May 04, 2021 11:21 am quote
Usually
Usually when tuning for idle mix, you adjust the air or fuel screw until the revs are at their highest, adjust idle speed to normal, fiddle the air/fuel screw to see if further changes improve idle speed, etc.

Once that's done, then you see if it needs to be changed to improve transient (off-idle) response. Sometimes the perfect idle mixture needs to be richened up so the engine doesn't hesitate when you open the throttle.

You didn't say if the 1/2 throttle response was steady-state or just passing through to full-throttle. Generally by the time you get to half-throttle the influence of the pilot jet is waning, although there's some interplay between that and the midrange circuit. If you are convinced the half-throttle condition is rich, might try dropping the needle one notch (assuming your carb has a needle and jet) to lean out the midrange. Mostly, you want the idle mix to be as rich as the ignition can stand, to improve cold-weather starting and throttle transient response. So if it idles fine with the 50 and a warm engine, I'd leave it alone and adjust elsewhere for the midrange.
Member
180ss with moded engine
Joined: 29 Mar 2009
Posts: 34
Location: Montreal
Tue May 04, 2021 12:49 pm quote
Jimding , the Flat spot is passing throught to full throttle.
Okay, so i'll raise idle (via idle screw, count the turns out on air/mix screw. When engine revs at its highest point, i'll lower the idle till it's smooth...correct?
As for the needles, I believe it came with JJG. I also have a JJH and some other needles available.
Thanks
Fab
Addicted
BV 350
Joined: 22 Nov 2016
Posts: 892
Location: Nebraska
Fri May 07, 2021 1:28 am quote
Usually
You want the smoothest idle at the actual idle speed, with a bias towards rich. So the final step, after adjusting the idle speed to normal is to touch up the mixture screw a bit towards rich.

Certainly not impossible that it is going rich at midrange, but I've seen a 'stumble' from being lean far more times. Lowering the needle should tell you.

If the float level is adjustable, I assume you have verified that it is correct. A low float level will cause lean running at mid and high throttle. On the other hand, a very high float level won't affect idle much, but will show up when the needle is being pulled out of the jet.

Most bikes are relatively tolerant of rich conditions, unless it is absurdly rich (like leaving a choke on). Assuming your ignition system is up to spec, with a fresh sparkplug, I'd expect any hesitations to come from lean, not rich.
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