Case matched Puch Polini, Hi-Comp Head, Techno Circuit, 20mm PHBG Uni Filter 4 Petal Mallosi Slightly Advanced Timing NGK B7HS 92 Octane 40:1 with Bel Ray Synthetic.

I'm struggling to understand how to tune a 20mm PHBG. From my understanding, there are three tunable items in the carb.

Main Jet - Effects over all performance, metering fuel in the top 2/3 of the throttle position,

Needle / Clip Position - Move the needle higher in the slide to richen your top end, and lower to lean it out

Idle Jet - Effects bottom 1/3 of jetting, metering fuel when the slide is mostly closed

Little Mixture Screw - Effects the idle circuit? I have no idea!

Am I way off in understanding how these effect the bike? Is it possible to plug chop at half speed and get an accurate reading to see if your mids are running lean? How about an idle plug chop? When performing a plug chop, do you need to hold the throttle open after you've killed the ignition until you come to a stop, or just killing the ignition, and then closing the throttle is fine? Can anyone teach me to plug chop / tune? I've read all kinds of things, and I just never feel like I'm doing it correctly. I usually just jet until I four stroke, and then bump it down until I don't, but now that I'm up to a PHBG I don't think that method works, now that I've got all these variables. I'm not looking for the worlds fastest bike, I just want a reasonable understanding so I can jet appropriately and safely for the weather and maximum speed with the least danger of seizing.


The little screw is your air mixture screw. It adjusts throttle response at idle & low RPM. Don't mess with it for rough tuning. Standard is all the way in and then 720 degrees out.

A plug chop is done after running a mile or so with a warm engine on wide open throttle (WOT). You kill the engine keeping WOT.

Upsizing till you 4-stroke, and then going down till you don't is a good method to tune in a carb. Just make sure to do a plug-chop after you've downsized to make sure you're not running lean.

Good luck!


yeah be carefull with the air mix screw i let mine out a bit to far i guess rode a couple blocks for plug chop got back home and my air mix screw rattled out somewhere and now my bike is out of commission


Yeah. I always go with all the way in, then about a quarter to half turn out. Mine worked its way out a bit, and then it wouldnt start. Took about 2 hours to figure out I needed to turn a little screw about 1 full turn. Summer bummer.

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