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Morning all my lambretta series 3 is a li 125 special with a 175 iron head sh20 carburettor and 42 clubman exhaust runs well but very high geared seems to be screaming at 48mph whany tip on gearing cheers mick currently 15 drive sprocket. ..

I've got the same "pacemaker" gearbox in mine and I'm running 18-47 with an 82link chain, works perfect and tops out at about 75mph.

According to sticky's book the Li125s is an Italian Gp200 box running 15x46 which is 6.13:1
My Li125s in standard format felt like it needed a 16thooth sprocket anyway so upping to a 16 tooth front sprocket would work but may not give you the desired result
Most standard 150s are 5.65:1 and the Li 150 was down at 5.21
changing the front sprocket to 16 tooth will give you 5.75:1
Or
17 tooth gives 5.41.

Having not had any experience of this it may have to be a suck it and see.
You will have to change the chain as well

A factor for the gearing is where you intend riding it.........if you're just tootling round town then shorter gearing may be fine but if you're going to be doing motorway miles you would probably benefit for as tall as you can go.