Splutter on Higher Revs

[Starwave]

Hello

Rebuilt my motor a few hundred miles ago: Rapido 225, 30mm PHBH with foam filter, 48mm Clubman, low compression head machined to suit piston crown, LTH reedvalve with black reeds fitted, holes drilled in piston, timing at about 17 or 18 degrees, squish (maybe a little high) at about 1.6mm. The furthest I've ridden the bike in one journey is only about 30 miles. It pulls well and starts 1st or 2nd kick. I have noticed, however, that around 50mph and above it seems to cough n splutter a bit but not bad enough to cause the scoot to slow or falter. Almost more of an occasional cough n hiss. When I rev the scoot on the stand it feels crisp and clean until it hits the rough spot and it starts to run rough and splutters.
Maybe it's ignition related (something electrical breaking down?) but in case it helps - the carb set up is idle jet 50, choke 70, atomiser AV264, needle X7 with clip 2nd from top, main jet 115. Did have a 120 jet until this morning but changing to the 115 made no difference to the problem. I did originally use a X13 with the clip 3rd from top but it was defo too rich. Plug is now a nice choccy brown colour.

Any help much appreciated. Off to Cleethorpes on it followed by a wedding in North Yorks so could do without a blow up. Obviously hoping for a quick n cheap fix.

Cheers.

[Starwave]

Nice one. Other than replacing it is there a way of finding out if it's faulty? I.e. getting the old multimeter on it?

Cheers.

[coaster]

Have you tried holding it in the stuttering range say around 55 mph for a couple of miles and THEN checked the plug colour? it's not impossible that you are running weak there

[Starwave]

Yeah, checked the plug after a suburban trundle at home and at work after a few miles down a dual carriageway at 50 to 60 mph. Looks good.
Removed the flywheel last night and it had been fouling slightly on the clip that holds the stator wires out of the way of the flywheel. Had started to wear through the cable sleeving too but hadn't damaged any insulation. Should have the mag housing machined so the stator sits further away from the flywheel, I know, but ran out of time (or couldn't be assed). Thought I'd checked clearances ok but the old vibration and thermal expansion will have put paid to any tiny clearances. Anyway, had 1mm taken off the back of the flywheel at work by a kindly chap, refitted it and it's still rough on higher revs of course. Reckon I'll be ordering a new stator tomorrow and will try that. Or any other ideas? Cheers.

[Starwave]

Know what your saying but most of my local scooter mates run Vespas. The one other Lamby rider is still on points......

[Starwave]

Nice one, I'm hassling folk right now.
Cheers.

[guygrrr]

go get yr multi meter. check resistance between the red and white and green and white wires. from memory r + w should give a reading between 90 and 140 ohms and g + w between 800 and 1100 ohms. red is the pick up, green is to the low tension coil, and white is ground. i've had a similar problem that was caused by an over heated cdi. after a good thrash the cdi would be hot to the touch, hotter even than the engine, turned out the cdi was poorly grounded.

[coaster]

go get yr multi meter. check resistance between the red and white and green and white wires. from memory r + w should give a reading between 90 and 140 ohms and g + w between 800 and 1100 ohms. red is the pick up, green is to the low tension coil, and white is ground..

I think that the LT coil should be around 450 - 500 ohms

[J1MS]

You could check the Readings with the engine hot as well as cold, to see if there's much difference, usually only shows up on the LT when it's breaking down as the copper coil gets hot and shorts between windings, or the earth wire looses a little continuity giving greater a impedence.

[Starwave]

Just checked the resistance. Between red & white is 95 ohms and between green & white is 460 ohms. White is providing a good earth to the engine. So all good there, but only measured on a cold engine. Maybe something's breaking down when hot (unable to run t scoot tonight) and with higher current flowing through it? Need to get it done asap but feel a bit of a t1t ordering a new stator earlier today. If that doesn't cure it I could try another CDI I think I have in t shed.