Increasing Idle

[Mad Mart]

I've had this anomaly for years now, on my Track Sprint. The engine is fine if I'm in normal, flowing traffic and when I'm at full chat around a circuit. But when I get into start, stop traffic the engine revs steadily increase to 2k or more. I've rebuilt the engine several times but I still get this problem. I'm running Weber DCOE 45s by the way.

[xvivalve]

Leaky servo vacuum pipe? I had this once where the pipe had chafed on the underside of the bonnet...

[Dolly-Nut]

Having fitted many Weber/Dellorto setups now i can certainly say it's a characteristic of the Weber. My black Sprint always had this problem and to sort it i had to use a very strong return spring on the linkage (Rotax Max kart exhaust spring to be precise). I also removed the spindle bearing covers and lubricated everything possible. Mine used to idle happily at about 1k after that.

It might be worth popping the linkrod off the linkage and seeing if the idle drops incase it's binding.

An overly rich idle mix can also add to the problem and make the RPMs surge. Try screwing each cylinder in 1/2 turn and see what happens if all else fails.

[AndyJ]

Increasing air temp in the engine bay could be weakening the mixture, Webers running slightly weak at low rpm could be adding to it too. I had similar on an A series once and it was a weak return spring on the vacuum advance in the dizzy, it wasn't fully returning the timing when using low throttle (in traffic) but in normal driving it worked ok.
Andy.