Annoying high RPM

My '70/'72 Fastback is now running again after a long storage and a few unnecessary mishaps with the electrical system. It started first kick and warmed up nicely. As I revved the wonderfully mechanical song of the vertical twin it decided to stay at a higher pitch even after the thottle was back at it's no go position. The single Mikuni carb was completely disassembled and cleaned a few weeks prior to remedy the storage gunk and the slide is moving freely with a very clean and snappy throttle and cable. If I blip the throttle when it is stuck at the higher RPM it will come back to idle but is very tempermental about staying at idle and sometimes just dies.
Because it starts up very easily I don't think it's anything electrical (any posibilities being electrical?) The pre '71 electrical system with the Boyer is very clean and tidy. If it is carburation, after a completely scrutinized cleaning, I'm at a loss... I need the best Commando minds for some insight...What do you think?

Holeshot said:

As I revved the wonderfully mechanical song of the vertical twin it decided to stay at a higher pitch even after the thottle was back at it's no go position. The single Mikuni carb was completely disassembled and cleaned a few weeks prior to remedy the storage gunk and the slide is moving freely with a very clean and snappy throttle and cable. If I blip the throttle when it is stuck at the higher RPM it will come back to idle

Click to expand...

Some Mikuni throttle slide return springs tend to be a bit weak apparently, and the cure is to fit an extra Amal spring inside the Mikuni spring.

Holeshot said:

Because it starts up very easily I don't think it's anything electrical (any posibilities being electrical?) The pre '71 electrical system with the Boyer is very clean and tidy.

Click to expand...

Are the pre-'71 12V (17M12) coils still fitted (and wired in series for the Boyer ignition)? If so, then they really should be changed to either two 6V coils in series, or a 12V dual output coil (with a primary resistance of more than 3 Ohms).

I'll try the Amal spring and I DO have the 12v dual output on the bike...

Are you sure the goo you cleaned out of the carb was from storage? If you have a glass tank and have not lined it the goo could be from corn gas breaking down the fibreglass resin. It happened too me and it did not take very long for the carb, intake manifold and head to be coated in the brown goo.

Have you checked the rubber manifold connector? They tend to crack when old and that leaks air casuing a lean mixture enough to rev up the idle.

Mick

The extra sping did the trick...a little more wrist fatique on the long trips but worth having the slide cooporate!
As far as the goo that boz mentioned, I do have the tank sealed (a green sealer, can't recall the brand) and it has been OK for many years BUT it seems to be getting a bit soft... I think I'm going to reseal it with the suggested brand from Jean (Bill Hirsch products).
Thanks for the great input!
Tom