I have a rebuild my 258 with a clifford combination package ( 264 cam, intake, duel headers) back in april along with a weber 38, 4.0 head w/larger valves and DUI. The only problem i had was that my engine would not idle with the speed screw turn in only a half turn. I tried all the available idle jets and mixture screw settings with no luck. I got so fed up with it, i just put mc2100 on.
I decided to take apart the weber to blow air through all the passages and study the carb to understand how everything worked. I could not find any problems or clogs at all. But one thing i did notice was when the with the idle speed screw was turned in a 1/2 turn, only one hole was barley exposed while the transitioning hole by the other butterfly valve was completly covered up.
So i thought maybe the two gears in the front were not synced up.
But i didnt now how to sync them up. In the weber set up instructions it does not say anything about it.
I even called redline weber and clifford performance many times and they gave the same "make sure your float is set correctly" or use differenct idle jets. Not once did they say anything about syncing the butterfly valves.
So when i was studing the carb i saw the screw on the throttle linkage and loosened it. That screw allows you so ensure that the butterflies are in sync. After loosening that screw, i made sure both transitioning holes were a barley exposed (cresent moon shape) and tightened down that screw.
Maybe my carb was dropped during shipping and that knocked the linkage out of wack.
Now after messing with this carb since april, I have finally got it to idle with the idle speed screw correct and no super rich exhaust! Hopefully i will now get better that 10 MPG.
Im posting this because i hear of people giving up on this carb. And i dont want anyone to go through the same crappy experience i went through. The carb is awesome and I can feel a difference between the weber 38 and the mc2100.
Thank you to all the guys here that have helped me and answered my questions.
Im running 55 idle jets, 170 air correctors, f-7 emulsion tubes, and my mixture screws are 1 1/8 turns out, i have zero vacuum at my ported vacuum port and my float is set at 17.5 mm from the top edge of the float to the top of the carb with the gasket on. ( i cant remember the main jet size.
Before i put the DUI in.