I stripped down the carburetor today. I was rather surprised to find that it is complete as they have a number of easily-dropped bits that can skitter off unseen. I've had one apart before, so was especially careful.
I'd assumed it was Mazac or similar, but no, it's aluminium & at some time a cack-handed fiddler has broken off one side of the flange & another more sensitive hand has gas welded it back on - pretty fair job too considering the thin sections involved.
For those unused to these curious direct lift carbs, let me take you through it. From the base of the carb proper, a long pipe descends into the cast aluminium fuel tank to which the engine is bolted. Both fuel pipe and engine base must make a good seal to their faces or the fuel will not rise as it should. At the foot of the pipe is a ball valve that stops the fuel being pushed back down the pipe by back pressure. It also makes this type of engine more easily started by hand.
The depression caused by the air flow raises the brass ball off its seat & allows the petroil to rise up the centre tube, across the bore and about another 1.5" to the top of the carb. At its top there is a needle with a fine thread that controls the mixture. (I screwed it right in then out 2.25 turns.) The needle allows the fuel to spill out of the tube which is itself a close fit in the carb body. The metered fuel now pours down the outside of the tube until it meets the bore where the rushing air pulls it into the engine. There is no means of regulating the air supply except for the choke lever. & there is no provision for an air filter.
Trapped between flange & crankcase, is a curious device that I can best describe as a toothless five spoked clockwheel with a centre spindle pointing away from the engine. It sits in the recess in the carb that is provided for it and appears in the parts list, so it is obviously supposed to be there. The parts list is unco-operative & just describes it as a "Spider" - very useful. Anyone like to speculate with me as to its function?
The engine is a constant-speed device & engine speed can be adjusted between
1,250 and 2,000 RPM. It is controlled by a brass segment in the points cam rising out of its slot under centrifugal force and holding the points open. It works very well actually and can be finely adjusted by changing the weight of the spring against which it acts. There is a coil spring under compression in a little tube attached to the body of the points box & it operates on the back of the moving point. It is adjustable, bringing more or less pressure to bear on the points arm. I can't quite figure out why, but I assume it is there to steady the arm, giving fine control to the points.
Once you have decided upon an engine speed, the engine is run under load until it has reached a decent working temperature. Final adjustments are made by taking off the chrome plated cone on top of the carb and finely adjusting the mixture until the engine two strokes properly and complete combustion is achieved.
All this will take some time & effort, but all two stroke owners expect that their enigmatic beasts will be cranky & need more maintenance than a desmo Ducati!
Regards,
Kim Siddorn