I got a l28 that had dual Webber’s on it I fully rebuilt the Webber’s and got a colortune. I was having issues with the front 3 cylinders running lean no matter were the mixture screw was did compression test all within 2 psi at 150 I swapped in a rebuilt pair of su with good throttle shafts with no leaks new floats and set at .55 balanced carbs with flow meter. I had n54 needles in it I could get the back 3 cylinders to go rich at idle with the mixture screw the front still no luck. I readjusted the front carb float level higher to try and get it richer with same results. Then swapped in sm needles and I’m still can’t  get the colortune to show yellow. I checked for vacuum leaks everything looks good any ideas? Last time I checked timing was around 8. New full pump filter and rubber hoses. Could it be timing or the valves?. The engine does run pretty good besides the fact I just can’t get it the cylinders to run rich so I can adjust it to lean. 

Edited December 5, 20213 yr by Daluvian

So... Front 3 cylinders run lean with two different carb setups. 

the lean condition is probably not down to the carbs. 

what else do all 3 front cylinders have in common? 

did both carb sets use the same manifold? look for hole in the manifold, or warped mounting face. 

Ya same Manifold different balance tube 

look for the air leak on the engine side of the balance tube. 

maybe.... 

Edited December 5, 20213 yr by jonbill

Ya I did it had a small leak I tightened down more checked with brake cleaner and confirmed that it was sealed. Same issue still. Maybe the Manifold is leaking somewhere I can’t get a good spray on? 

have you looked at the underside of the manifold? 

Here's one way to check for leaks.

1. Build a smoke machine. Start with the link below and work your way down the thread. I included a link to the YouTube video I used as a basis of design. 
2. Take off the valve cover. Rotate the engine so the intake valve on one of the front 3 cylinders is open. The cam lob for the intake valve will be down, and you can tell which are the intake valves because they line up with runners on the intake manifold.
3. Remove the spark plug for that cylinder and plug in the smoke machine.
4. Have someone push smoke through the cylinder while you're on the other side looking for leaks. If you have to do this solo, figure out how to set up your phone to take video of the manifold while you push smoke through.

You may have to do this for each of the front three cylinders.

Cool I have access to smoke machine

No luck with the smoke machine I’m gonna try and change all the gaskets see what happens if anyone has any other ideas

Wont hurt to check the flatness of the manifold while its off.