Mikuni carb help just wont balance

[megabob]

Hi All,

I'm running Mikuni PHH carbs on my AE86 and i've been trying to scync them up for some time but one is not inducting correctly in idle and low throttle (makes a sounds like it only runs on 3 cylinders) but as soon as you put the throttle down a little more it pulls great.
using a syncrometer i measured the air flow and from front to back on the engine the first 3 cylinders induct at the same rate (5) but the 4th at the back only inducts at 1.

i'll give you a list of things I've already tried:
- rebuild both carbs
- checked compression on all 4 cylinders
- checked for air leaks
- rebuild the throttle linkage

I did swap the carbs around to see if the issue followed the carb but when i did that all the syncrometer readings were off but only slightly by about 1-2.

I'm really scratching my head trying to figure this out.

[totta Crolla]

You could try running the engine under load for a while to "clean" the
plugs and then let it idle for a while.
Check the plug colours to see if there is a difference between the cylinder
that is causing the problem and the rest.

[allencr]

Compression & valve lash OK on that cyl?
If OK, then that carb's throttle shaft is twisted, check by drawing your thinnest feeler gauge or a piece of cellophane between the bore & throttle plate while holding down on its throttle stop. You'll see zero difference between the two if the carb is OK & big difference if it is twisted.
Just twist it back, either a wrench on each end of the throttle shaft or just pushing on the butterflys, then check again with the feeler gauge. You'll have to twist more then it's out because it'll want to spring back.
****
Also, some ass-whole may have played with the throttle plate screws and that's the cause of the problem. Loosen, then snug them up just a little bit, keep some pressure on the shaft while pushing on the butterflys to center & seat them in the bore, then tighten their screws. Check with feeler gauge & repeat if they aren't equal.