I should point out that this was probably a Covid lockdown resuscitate an old model from the attic exercise with the main aim being to work with what I already had with minimal spend. So buying new carbs isn't really part of that plan. Having said that I do have an MDS 40 engine that I pulled the carb off. This has run before many years ago and I seem to remember it ran OK then. The venturi barrel hole is the same size as the GSM but I haven't check it'll actually fit in the GSM crank hole. A cheap Russian engine with dodgy bearings perhaps but the carb is OK. Also the carb in my GSM has the full width spray bar unlike most of the engines in this thread so maybe it is OK and just needs tuning ... least that's what I thought before the latest attempt to get it running.

I bought some fresh fuel (12%) and managed to get it running seemingly OK on a test stand with a 10 x 7 prop rather than in the Heli. Idles OK, transitions sort of OK to WOT but the HSN is still 2.5 turns out which is more than the manual.states and possibly it's too lean even then at WOT, just seems to get a bit hot. If you don't lean the HSN back though it 4 strokes at mid throttle and bogs so too rich which seems to the problem here. Idles OK mixture good but when the high end is set right just backed off from the slightly vague peak then the mid throttle is too rich to the point it barely runs. If you fix the mid throttle by leaning the HSN it's probably too lean at WOT but it does run like this just seems like there's minimal margin on the mixture through the rev range. You can't lean the LSN any more to fix the mid throttle because it won't idle, too lean.

Anyway since it was sort of running OK on the test stand I stuck it back in the Heli and had to tweak it some more because the Heli setup is different, engine vertical mounted, tank location lower. It was almost working right except for the transition from idle to mid throttle, just too rich at 1/3 throttle, it 4 stokes which you can clear by pinching the fuel tube briefly but it returns when you release it and possibly cuts, so very rich in fact. LSN is right on the limit of lean at idle, WOT produces plenty of smoke but it "seems" to get hot pretty quickly which may actually be normal, maybe I'm just being over wary of burning more plugs ?. IR thermometer shows 140C but I'm not sure I trust it on a shiny gold anodised head (Reads 60C on a shiny metallic boiling kettle, you'd expect closed to 100C). It will clear the 4 stroking at half throttle and run OK with head temperature of 110C (IR reading). All pretty inconsistent and seems like this full width spraybar carb really has the same problems as the other engines in this thread, the LSN needle and carb in general just can't meter the mixture reliably throughout the rev range, I have a feeling the taper on the needle is almost fully in the spraybar tube hole at idle with the full diameter of the needle essentially doing the metering which means you get a sudden relatively large increase in flow area when the throttle is opened slightly and the taper starts to meter the flow.

Having said that watching videos of 3D Helis flown by experienced pilots the transition from idle to hoover always seems pretty stuttery and smokey so maybe this isn't unusual ... so long as it keeps going to mid throttle you're OK.

So try the MDS carb perhaps, mess with increasing the idle speed to see it it can help improve the low throttle problems, try it with the exhaust tank pressurisation disconnected to see how that affects things .... or give up and stick it back in the attic.