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In top gear at say 45mph, wang the throttle wide open. If it four-strokes it is too rich and you should come down a jet size. Conversely, you can keep on increasing main jet size until four-stroking happens because nothing is worse for a two-stroke than a weak mixture, which causes over-heating, seizures, etc.. Petrol is cheaper than pistons and rebores…. With modern fuel in a DPY/DPZ/PY/PZ long-stroke engine the old standard number 3 slide can be improved on by fitting a 3.5 slide, originally made by Amal specially for Triumph. Mid-range pick-up is usually better. Something else to consider is your exhaust system. If you are running a Burgess type silencer they can be very detrimental if the mineral-wool lagging gets oil-clogged. The exhaust will become noisy and it begins to act like a long straight-through pipe, sucking mixture out of the engine before it has had chance to fully combust, and increasing fuel consumption. A cheap and very easy experiment is to put a Volkswagen Kharmann-Ghia tailpipe ‘muffler’ up your exhaust tailpipe. That can give a surprising improvement to the running of your engine, and I assume that it is because it has given a little back-pressure to the system.