My bike runs great! ...

[volks28]

Finally got a good run out of Suzy... I've been batteling a hesitation issue in mid RPM's. This morning, I left the choke on 1/8 and it runs like new. It's quite pleasant to drive a bike that runs well!

However, the problem still exist and need a few pointers. Here's what I changed so far:

New plugs (old ones were tan, except for #4 which was really sooty black, after running with no choke over 2000km)
New caps
New intake o rings
New oil
New petcock
Cleaned the gas air passage (made a big difference!)

compression is around 140 on all 4's

I sprayed carb cleaner and WD40 around the intake and there is no change in RPMs.

Since #4 was pretty sooty, I swapped 1 & 4 wire and caps, it followed the plug / cyl

I swapped plugs between 1 & 4 and it followed 4. Pulled the spark plug out and grounded it. It fires.

If I pull #4 at idle, it makes no difference in rpm.

Carbs are sync'd, idle screws are 2 out

Air filter (K&N) has been cleaned and lightly oiled 500 km ago

Any suggestions for the next step?

[nabrams]

You say:

If I pull #4 at idle, it makes no difference in rpm.

I'm assuming you mean without any choke applied. You said the bike runs better with a bit of choke.
Do the same with choke applied and see if there's any difference in RPM when you pull the plug wire off of #4. If the RPM decreases then you've got a situation where #4 fires with choke but not without choke. That would indicate one of two things:
1. A clogged passage in the carb.
2. An air leak for something related to #4 - this could be an exhaust leak at the point where the #4 pipe joins the header, or it could be a leak at the boot that attaches the air box to the #4 carb, or a leak at the boot that attaches the #4 carb to the engine. The WD-40 test at idle is not always reliable if the leak is relatively small.

[volks28]

I pulled the spark plug cap/wire while idleing and choke on. It does change the RPM's so I followed the suggestions below.

I'm not sure how to check for an exhaust leak? All cyl have new exh gaskets
I know for sure that the boots at the airbox are not sitting "dead on". they seem to sag (because of the weight of the box) no matter what I do. I don't think it causes a leak though.
I know that cyl #4 is firing. the pipes get hot (although #4 gets hot a bit less quickly than the others). If I spray water on them after a 10 min ride, water evaporates in a flash.

Any clues on how I can check for intake/exhaust leaks?

That would indicate one of two things:
1. A clogged passage in the carb.
2. An air leak for something related to #4 - this could be an exhaust leak at the point where the #4 pipe joins the header, or it could be a leak at the boot that attaches the air box to the #4 carb, or a leak at the boot that attaches the #4 carb to the engine. The WD-40 test at idle is not always reliable if the leak is relatively small.

[nabrams]

When I had to fix a leak problem on my friend's bike I ended up putting tons of silicone on ALL the rubber boots attaching to the carb - especially at the junctures between boots-carb, boots-engine and boots-airbox. I don't know of any fool-proof test for checking these boots - they're rubber and may have tiny cracks. That made things better, but it was still not perfect (still some hesitation). The exhaust was leaking slightly in various places. You can hold your hand very near the point where the pipe joins the header without touching it and feel if there's any fumes escaping. Leaks at ANY point in th exhaust can effect performance (improper back-pressure WILL affect CV carbs), but the closer the leaks are to the cylinder head, the more dramatic the effect.
So I fixed the exhaust leaks and things got better, but still there was some very minor hesitation. So I re-cleaned the carbs and found that a couple of jets were not TOTALLY clear. After getting those jets absolutely spotless the last bit of hesitation went away.
In your case a bit of choke (which restricts air flow) makes cylinder 4 come alive, so it is clearly a mixture issue - which can only be affected by either too much/little air or too much/little fuel. Keep at it. You'll get it eventually.

[volks28]

In your case a bit of choke (which restricts air flow) makes cylinder 4 come alive, so it is clearly a mixture issue - which can only be affected by either too much/little air or too much/little fuel. Keep at it. You'll get it eventually.

If I understand correctly, If I can't find any intake/exhaust leak(s), the enxt step should be to crack the carbs open to hopefully find a blocked fuel passage?

Many thanks