Ok so I've been sprucing up this brat I bought and threw a brand new weber on it and can't pinpoint my issue, when I start the car I set the electric choke turn the key runs perfect idling at about 1200 to 1300 rpms kick off the choke runs great and purrs at 800 rpm. So the problem comes into effect when it hits operation temp the idle kicks down and sounds like it's running on three cylinders, almost like it has a loppy cam or something. It picks up fine when you give it gas usually,sometimes a hair of hesitation. So things I've changed thus far, new plugs, wires, distributor, alternator, fuel filters, oil filter, fuel reg all were oem stuff. Weber was tuned best lean idle and no vac leaks I could find, any suggestions on things to check into?

Have you checked for a bad plug? Which cylinder is it? How about a compression test?

Ed

Does it merely sound like its running on 3 or is it really?  Can you pull plug wires one at a time and see if nothing changes with one? 

Does it merely sound like its running on 3 or is it really?  Can you pull plug wires one at a time and see if nothing changes with one? 

This.

 

And check to see that your plug wires are hooked up in the correct firing order.  It is a simple mistake that bites us all, myself included.

I'm positive it's not the plugs just put a new set in even tho the others only had a couple hundred miles on them all looked good on color, 99% sure plugs are in right order and the timing is set to ten according to the usmb recommendations. Will pull wires tonight and see if one is getting cut out, it almost seems like it's either flooding out or leaning out at idle I have the regulator set a 2 as well will update after work tonight.

so finally got some time to tinker with the brat and pulled the wires while it was running and everything checks out that way so I'm thinking maybe order up a jetting kit from weber?  seems the only problem is when its hit operating temp and something changes right when it gets to temp, seems like mixture is off just on idle would an idle jet potentially fix this? also has some dieseling problems does anyone have a link to an anti dieseling part i can't find one? Thanks a bunch 

Fast idle on my Brat is more like 2-2.2k. I know you stated it was set for best lean condition but sounds like to me it needs more adjusting. I fiddled and fined tuned mine over a couple of weeks.