Posted under 'very puzzling moped behaviour' about my moped's high speed engine cutout. This has me baffled and not even any the good advice from Ron Brown has led me to an answer though it had narrowed it down quite a bit. From most recently, here is everything I've done:
-sprayed carb cleaner into carb during miss - engine dies completely. Repeated this several times - even the slightest shot of carb cleaner kills or almost kills the engine.
-Checked float bowl vents - found that they were clear
-took float bowl off while running, level looks perfect
-did screwdriver spark test while running on stand - spark still jumps during miss
-changed plug, coil, condenser, wire - no effect
-adjusted timing and needle settings - this affects the speed and throttle level where it will cut out
-changed points and plug - no effect
-tried different float levels - no effect
-cleaned the carburetor - no effect
-changed carb with other moped - misses a whole lot less but slower than other moped
-checked valves + cylinder head, cleaned off all the carbon - no effect
-tried putting choke on during miss - makes the engine miss more
I would conclude:
-the engine isn't losing spark
-fuel flow isn't the cause of the problem
-carburetor is most likely causing it
-the miss would be caused by too much gas rather than too little gas.
By adjusting the needle, I can make it lean enough that it will run at almost full throttle without cutting out, and go pretty fast, but it is undrivable at this state because it hesitates so much you can hardly get it moving. It appears to me that it gets too much fuel at high speed / throttle and and not enough at low speed. The moped was previously running great with the same jet and needle in it.
For the record, compression is excellent (oversize piston / rings, rebore), valves reground, ports cleaned., and yes I did put a new spark plug in, though it wasn't by any means the first thing I tried!