Li150 - PHBL Carb Tuning - Ruling Out Air Leaks, etc.

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kurtminus
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Posts: 33
Joined: Sat May 08, 2021 4:51 pm
Main scooter: Lambretta Li150
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Hello,

Ive got a 65' Li150 with an SIL ignition. I had to replace the indian jetex carb with a PHBL 24mm which involved dropping the eninge, replacing the intake, and also removing and refitting the exhaust with a new gasket, because I also had to replace the cooling shroud with one that would allow the carb to fit. Anyways, I've got it all buttoned up and starting and running well. (I sorted a hard starting issue I've posted about in the past... it was due to incorrect choke adjustment).

My question is about idle rpms. With the new PHBL carb it feels like the eninge takes longer to warm up... what I mean is when it's cold the idle speed is lower, and when it's fully warmed up the idle speed goes up more than I remember with the Jetex. The concerning sign is I also notice that when the engine is fully warmed up there is a bit of a slow return to idle speed. The engine is a little racy for a second or two, and then goes back down to idle speed. Could this be a tuning issue or a sign of an air leak?

I checked for air leaks at the intake and exhaust by spraying them with carb cleaner at idle. Seems all good. I don't have a leak-down set up but I can say when I cover the tail pipe the engine dies after a couple seconds, which I understand is good.

My jetting is as follows:
pilot - 60
idle- 53
needle - D29 at richest setting
main - 104
Air mix screw 1 full turn out.

Starting and acceleration is great, and you can even kind of feel the engine hit the too- rich wall at WOT and high rpms, I have a 106 main I can try just to find the 4-stroking point, so I know I'm in the correct range. Plug is a bit lean looking, but it's also brand new, so only a few miles on it. It's a champion N30, equivalent to an NGK B8ES.

Thoughts??
cgt75b
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Joined: Thu Sep 22, 2011 8:05 pm
Main scooter: lambretta
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Hi

Here’s my thoughts

I’d turn the mixture screw out to minimum 1.5 - 2 turns out.

I’d go 45/50 pilot jet

The needle clip I’d take up to the second clip/groove

Main jet I’d come down to 100

Good luck and let us know how you get on.

Plug chops may also be a good idea.

Regards

Cgt75b 😎
Daggs
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Joined: Wed Jan 07, 2009 1:23 pm
Location: Addington Kent
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The last time I responded on your issue, I suggested the problem was' jetting' You've reported back that it was a choke problem (unexplained) which vindicates my suggestion, as 'jetting' includes choke operation. IMHO
This time; the issue you describe is too lean (eg. too much air) at idle. Not necessarily an air leak.
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