Rising idle! Twin Weber 2.0
 

Interesting situation, looking for advice.

Regular idle approx 600 rpm.
This morning it was approx 1200 on drive to gym.
On drive home idle at stop lights went up to 2000, then 3000 at next light then 4000 at next light (idle mind you!)

Pulled into home Depot (no Pelican Parts stores near by) fiddled with linkages, screws, etc.

In the end bought and attached an 8lbs pull spring to the throttle plate actuated by the idle screw to 'pull' the throttle down to 800.

Fun stuff! Any ideas where to start?

Sounds like something is hanging up if a heavy spring will pull the throttle closed. Time for testing the pedal for free movement, cable for free movement etc.

Could just be a sticky cable.

Sounds like sticky or rubbing throttle linkage. You’ll need a helper so you can watch the throttle movement and make sure it’s free.

Make sure your distributor hold down nut is tight.

Worn distributor advance springs can cause a high idle when coming to a stop. Usually around 1500rpm.

Set your idle @ 950 when hot. You may need to balance the carbs side to side if the stop screws are out of whack.

What type of linkage are you using?

Bad bushing on a crossbar support? Loose droplink jam nut? Loose droplink attachment screw? Loose manifold base screws?

Stock idle is 900-950. Balanced carbs at idle mean nothing. Any slop at all in the linkage throws things out of wack when the idle adjustment screws are off the stops. When warm, Use ONE idle speed adjustment screw to raise the idle (I use the driver’s side). Way up. Now measure the flow on the front barrels of the carbs. Still the same? Use the single screw, find the idle speed you want and then adj the pass side ISAS to just make contact. A long used, worn and unserviceable linkage makes tuning the carbs near impossible. What are the air by-pass screws set at? Thomlinson’s Weber manual is excellent. aircooled.net has excellent carb info too.

Throttle shaft could be binding or the throttle plates sticking, or both.