Re: carb fuel circuit question
Well, Bob: If you want to invest some money in your 125, set one carb to only 1/2 turn out--lean. See how long the engine runs at full throttle before you need to replace at least one piston. My guess would be 15 to 30 seconds. Then you can drive up to me and I'll rebuild it.
This is why I always recommend putting a filter AFTER the pump and BEFORE the carbs: Pump diaphragms can shed particles and pump valves can shed brass pieces which will lodge in (usually) one carb inlet needle. the carb goes lean and the pistons serviced by that one are destroyed.
Same thing will happen if you set the low speed mix needle too lean. Take it from me: I learned the hard way when I was still wet behind the ears. Low speed circuit on these carbs is always at manifold vacuum and while they don't contribute as much fuel as the venturi does at full throttle, they still do contribute. Enough so that loss of this fuel will cause lean running and damage. Add to that the fact that because of design, one cylinder of a pair serviced by a carb runs just a bit leaner and you get one lean cylinder and one very lean cylinder. Guess which one will be damaged first.
Look at your carbs next time you take them off to clean them. Under the welsh plug and behind the venturi is (usually) three metered holes. The needle controls fuel to these holes and at idle only, the other two holes act as an air bleed because they are on the other side of the butterfly plate. (some carbs will have only one large hole--It is the engineer's design) This is the so called low speed circuit because its function is to supply fuel when there is not enough air flowing through the venturi to draw fuel up the large brass dip tube and atomize it. At off idle, as the butterfly opens, the other two metered holes are exposed to manifold vacuum sequentially and deliver the extra fuel required by the engine. They are metered--that is a specific size-- to match carb design and engine size. HOWEVER, because of their location, after about 1/4 throttle, all three are always at manifold vacuum and thus always delivering fuel.
On a hole shot, when you bang open the throttle, acceleration should be smooth. It is possible to set the needles too rich and the engine will stumble because of too much fuel, then clear itself out and pick up. Conversly, if set too lean, the engine will "sag" until RPM rises enough for the venturis to deliver, the engine will again recover and accelerate. Set properly, the low speed circuit will deliver enough fuel for the engine to accelerate until the venturis deliver fuel. Acceleration will be smooth with no hesitation or stumbling. This applies to any force engine, not just the 125.
Since the limiting factor in any internal combustion engine is the amount of air it can take in it, is always preferable to have a slightly rich mix so all available oxygen is "burned." Thus, when the engine stumbles due to a rich mix and then clears itself out, it is because the rich mix will not affect power too greatly. Indeed, the loss of power from a slightly rich mix is less than the loss due to a lean mix. In the old days, Automobile carbs had vacuum activated needles that opened under lowered vacuum of full throttle and richened the mix for full throttle operation while timing was retarded a couple of degrees. This had the effect of preventing damage at the maximum horsepower production at full throttle.
