SP1000 no spark it idle/low rpm on 1 cylinder

[1down5up]

Gents,

Having issues with an SP1000, I can't get it to run nicely at idle.

Ive figured out (using timing light) that the left cylinder isn't firing at low rpm/idle up to about 2000rpm,after that it's firing.

The right cylinder is firing at idle (again verified by timing light), I've also changed/swapped the coils over across the cylinders and the issue doesn't move so I'm fairly sure the coils/points/condensers are ok.

Spark plug fires fine out of the cylinder on the left (held against the valve cover, nice strong spark)

I've checked timing, points gap etc

Not sure what could cause this?

Bike has Phf36 but appart from that it's a stock motor/igniting system.

[Old Jock]

Could be quite a few things I suppose

I'd also swap over plugs and HT leads, I've had more than one issue like that where you'd get a spark out of the cylinder but still have running problems and it turned out to be either the HT leads or the plug

[Scout63]

Maybe pull the left side idle jet and see if it is clear.  They usually need to be physically cleaned out with a #78 drill bit or .016” guitar string - too small an orifice for chemical cleaning (spec for the stock VHB carbs).

Or do what I do and crab the frame, pull the engine and rebuild the whole thing. Then clean the jet.

[fotoguzzi]

Dumb question, How does a timing light show a plug is not firing?