Falls flat at low RPMs and hanging idle

[b_v2000]

After installing a Kerker 4-1 header and the Factory Pro 1.0 Carb Kit I’ve been experiencing the bike falling flat on its face after coming down from around 4000rpm and above. Most noticeably when coming to an exit from off the freeway or coming to a stop light after being in the same rpm range. Bike runs just fine at RPMs above 4000ish rpm.
Followed all of the setting recommendations that came with the carb kit and have tried several small, incremental tweaks in both directions to no avail. Went up a size for the jet pilots (#38) and have tried numerous settings (again, in small increments anywhere from 1 turn in to 3 out) and the problem still exists.
I’m fairly certain there are no intake leaks. Have had the carbs off twice to inspect for any linkage binding, torn/pinholes in the diaphragms, floats are level, throttle cables move freely, etc.. Ran some Seafoam, synced carbs to the spec in the manual, replaced the fuel petcock, and OEM air filter is clean. Emission system components have been yanked but everything is that needs to be capped is capped with new rubber.
As mentioned before, bike runs good in the middle-to-upper rpm ranges. Still have the stock exhaust so I may throw that back in just to seee. Any ideas? I’m all for checking the usual suspects associated with a hanging idle or poor performance in lower rpms AGAIN but…geesh…this bike is finicky.
Ramble complete. Appreciate any help you gents may have.

Bryan
1993 ZR1100

[Stereordinary]

Had a similar problem once. It was running rich. You said you already tried adjusting in both directions by a small amount, but try adjusting in a whole full turn.

[b_v2000]

Full turn in on the air screws? Feel like I’ve tried that but I’ll take a look at that again.

[Freddy]

After installing a Kerker 4-1 header and the Factory Pro 1.0 Carb Kit I’ve been experiencing the bike falling flat on its face after coming down from around 4000rpm and above.

My thoughts. Stick EVERYTHING back to stock. Look up an online carburettor parts schematic for your model, and find what main jet size was the largest that was used. I expect you'll find a whole range of sizes depending on the county the bike was to be sold in. Identify the largest used (assuming you're at sea level).
Then source some genuine main carby jets of that size from a motorcycle carby jet specialist and fit them.

Logic behind this .....A 4-1 exhaust is likely to give you a big mid range hole in performance, there is a good reason modern race bikes (and cars) only use 4-2-1 exhaust systems. The large 'collector' where the 4 pipes join creates a massive backpressure wave than can push right out into the intake manifold at the point of valve overlap. Especially a problem with carbies as fuel will be fed out with air moving in either direction. In my opinion your way better of using genuine jets than what is supplied in some after market kit. The only aftermarket thing I would ever use in a carbie is the gaskets.

[dolphinberserk]

My C1 was tuned to stock with stock exhaust. After fitting a yoshimura 4to1 Stainless Cyclone headers + silencer and a K&N filter I knew my mix would be lean (!). I rejetted and used washers on the needles and the top end was glorious (and I chairs colour suggested it was no longer too lean although I did not get gases tested) but the low end was poor. I had harder starting in cold conditions, weaker torque and less drivability at low revs. I had an inkling that overall it was still too lean, as I could hear some pinking/knocking if I gunned it at low gear under load. I used one of those Gunsons Colortunes that showed my idle as WAY lean across all carbs. I turned the air-mix screw over a full turn to richen the mixture and it was still blue flame with no sign of orange (still fairly rich). It started and ran much better at low revs, with no change at high revs. No pinking/knocking at all now. If you’ve added more airflow, I would suspect it is too lean, but you really need to test it. BTW the Colortune viewing parts are ABS plastic which melted from the radiant heat off the head. Plastic…

[b_v2000]

Update to everyone involved. After reading the replies and talking with the folks over at Ivan’s Performance I was recommended to look into the opposite of what I thought was a lean mixture issue. With the exhaust and jet kit mentioned above installed I was told that FP kits “never really have a problem with being lean” and I could go as far out as five turns of the air screws. I was also recommended to put the #35 pilot jets back in instead of the #38s that I had installed. This all seemed extreme and felt like I was going backwards.

Today, I made the changes and after getting the bike up to temp there I noticed a huge improvement. Idle was way smoother and didn’t hang at all. There was no sputtering or popping when given quick throttle all the way through 7K-8K rpm. And, the bike didn’t stall out when coming down from higher rpms one time. This was all while on the stand so once I take care of a small head cover oil leak I’ll do some real world testing. I’ll still look into coming in a quarter or half turn at a time on the air screws as I went from 3 1/2 at most before to 5 turns out now. May also have to play with needle position if I find it’s running rich on the higher side of things but at this moment I’m more than satisfied with how the things went.

Thanks all for the input! Very helpful.

Bryan