Agree with Andy. The Mk25C can have one of 3 carbs but I believe the Bantam has the Villiers Lightweight Type 3/4 carb. A single cable is offset centre to the top of the carb. Earlier Lightweight carbs on Mk25C (late 40s into the 50s) had a lever as well as a cable in the top plate but I doubt it is that model. The Villiers carbs are quite advanced and clever and are ‘compensating’ design, the mixture self-adjusts with engine speed. View it as factory-set but obviously as things get very old and worn this can get less true. Be certain that the timing is correct first before trying to adjust (5/32″ before TDC – points 0.015″). Maybe also try to ensure carb internals are clean. I have a lot of Villiers engines and the two-strokes seem to be very fussy regards spark plugs. Some modern plugs can fire but run rubbish, if at all. I recently did a test on a Mar-Vil which I know runs well. I swapped out several 18mm plugs of age and make and half don’t even start the engine. Only certain plugs like the magneto ‘power’. I don’t want to encourage more competition for the original period plugs!, as the prices are high, but the best plug for this engine is likely to be the Lodge C3. Get one and see if that solves the problem if you have a different plug. I have been paying up to £30 for NOS of these. Champion 8COM is worth a try and my Mk25C has an AC83TS plug. Also, only use old-style 2-stroke oil, modern stuff is a disaster in these engines. I have tried and end up with blue coating on everything, easy to order original Castrol XL on-line. If timing is correct and still problems, a 2-stroke should run relatively lean and should 2-stroke i.e. fire every revolution with no load, if rich it may not 2-stroke. To adjust mixture, unscrew top ring on carb, remove the throttle slide with cable. Under the top plate is the needle adjuster. clockwise weakens (lowers the needle). Only fine adjustment, quarter to max half turn at a time. Manuals say to set from ‘lost setting’ position, start with the end of the screw 2 threads (1/16th”) from top of throttle slide. If you can find one on auction sites, the book by B E Browning ‘Villiers Engine for Industrial, Agricultural and Horticultural Use’ covers M25C and the carbs. It is a rarer book than his similar books on motorcycle engines, presumably down to numbers likely to have been sold. I found one in Holland!
It should be easy enough to find a section diagram of the carb if you search for lightweight or 3/4. It is very similar to the Junior carb as well. It is possible to get different needles I believe but if it is the original carb for the engine it should be correct. I think that to change a needle size isn’t just the needle change though.
Best wishes, Ian
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