22mm carb general question

[Adamlam83]

I have recently built an rt195, clubman and remote bgm filter. I have Scootopia variable ignition set to 21 btdc as recommended by the dealer I got it from. I have a 22mm Scootopia carb and I'm struggling with it. When I first built it, at around 1/2 revs it felt 'crackly' but cleared and smoothed out as I increased the revs. I took it for Dyno to make it safe enough for running in and I was told my jetting was lean but is now a touch rich for purposes of running in. The crackly un smooth spot around 1/2 revs is still there. It's like it can't decide whether to go on to main jet or not until I increase the revs and decide for it. It's obviously far too rich as plugs are black wet and petrolly.

Current jets, 48 pilot, 5 atomiser and 138 main. If they can be made to run smoothly throughout the rev range I'll be happy but if they always have this weird spot I might have to change for a different carb.

My questions are-
Has anyone managed to jet these carbs so it revs smoothly from tickover to high revs or is it just the nature of these simplistic carbs?
Which jets affect the 1/2 rev on these?
Why would my plug be wet dark? Is that petrol oil mix or main jet too large?

Any help appreciated.

Thanks

[hullygully]

138 SEAMS RATHER LARGE BUD, COME DOWN 3, TO 135 & DO A PLUG CHOP, THEN COME DOWN TO 130 & DO ANOTHER CHOP, ITS NOT A MAGIC ART, JUST KEEP AN EYE ON YOUR PLUG

[martyn dwane]

Didn`t the Dyno guy pick this up ?

[Adamlam83]

He said it was lean but is now a bit rich but when it's run in further to take it back and he'll adjust accordingly. My concern around this carb is that the grizzly mid rev range was there when lean and rich and I wondered if folk had successfully smoothed that mid range out or if I should expect that with this carb.
As has been mentioned he has advised dropping the main a step or two and possibly the pilot if mixture screw doesn't help but will the main affect 1/2 rev or is that just atomiser related?
Thanks

[Martin s]

I ran a st4 200 with 22mm on, and swapped the atomiser to a 5899-4 as used on gp 150. It made the mid range a lot better.
It has been discussed on here a couple of times if you do a search.

[Mikedale]

why do people set up carburation rich to run in, rather than correct?

[Warkton Tornado No.1]

why do people set up carburation rich to run in, rather than correct?

The two stroke should always be set to be just slightly richer - if anything - rather than lean. I just wanted that to be established prior to trying to answer your question…..

Running in is really a process of ‘wearing in’ the piston to barrel in the instance of the top end. As Lambrettas are OEM fan cooled they rely upon revs to cool the top end. So any tendency to labour the engine rather than spinning it will not help to shred any heat.

Adding extra oil will not help the running in process. That will not only slow up the process but also weaken the mixture.

Richening up above the ‘safe-just-rich’ will also delay the process & although it may be the only wise safeguard compared to too much oil & too less revs, it will affect plug chop readings.

Running in the top end means working the engine, but not to extremes. Using the gearbox by changing down & up again for no particular reason will help. Just don’t roll off the throttle from higher revs as can be cautiously done with a fully bedded in engine, as can be the practice when going downhill. The two stroke engine need it’s supply of oil via the fuel tank, so feed it by not letting the throttle snap shut. Unless - possibly - a Cometa

I believe the correct process of running in a fresh top end involves a deal of empathy for the engine that involves feel, listening & even smell.

I hope my attempt at explanation helps

[Fast n Furious]

What exhaust are you using here Adam?