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Discussion Starter #1 (Edited)
Next in the ongoing saga of my '77 Spider with OER Weber copies on an adapter manifold.

OK, first off, things keep getting better and the problems are getting smaller. I can get the engine to run smoothly with no apparent vacuum leaks. I've disconnected the brake booster and plugged its feed, blocked the hose attached to the front of the part of the SPICA manifold that's still being used, made sure the advance springs are tight, used starting fluid to verify that there are no air leaks at the carb bodies, set the fuel pressure, REMOVED THE RETURN LINE TO THE TANK, balanced the carbs,and set the idle mixture. Now, when it's warming up, I get a nice smooth idle at 700-900 RPM, and it will hold steady for a couple of minutes, then it will slowly rev up to 4500 RPM and hold there. I can loosen the idle speed stop screw and bring it back down to 900, but then it will stall and not start until I tighten that screw again.

Once it's warm, the idle stays at 900 and doesn't waver and I can blip the throttle lightly and it backs down. However, if I bring it up above 4000 RPM, it won't go below 4000 unless I back off the idle speed screw. It doesn't waver when it's at 4000.

I'm wondering if it's time to pull the carbs (again) and give them a thorough cleaning to see if there's a blocked low speed jet or something else along those lines. It sat for a few years with gas in the carbs, and I've run Seafoam through them, but I can't think of anything else that might be causing this.
 

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But Mad North-Northwest
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Well, that's a bit unusual.

I'm far from a Weber expert, but it's difficult to think of something on Webers that could cause that sort of behavior. I know the OERs have that externally adjustable float height feature...possible something is screwy with that and your float level is varying with time and/or running conditions? No idea how that system functions, though, so not sure if this is really a possibility.
 

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The approximately correct setting for the idle speed screw is as follows:
Disconnect the throttle linkage.
Back out the idle speed screw.
Back out the synchronization screw then turn it in until it just touches the arm that opens the second carb.
Adjust the idle speed screw until it just touches the arm that moves the throttle plate of the first carb. Then turn it in 1 turn. That should be close.
The engine should then run reasonably well. The next step is to accurately synch the carbs.
Do the throttle plates always return to the closed position when you release the throttle? There should be some free play in the gas pedal before the carbs open.
The only way for the engine to rev at 4000 rpm is for the throttles to be open more than they should be at idle unless you have late type Webers with throttle bypass screws that are not fully closed.
 

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Discussion Starter #4
The only way for the engine to rev at 4000 rpm is for the throttles to be open more than they should be at idle unless you have late type Webers with throttle bypass screws that are not fully closed.

Apparently not. The throttle plates are in the same place, whether it's idling at 900 of at 4000 and these don't have the bypass screws.
 
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