Flat Spot. At my wits end

[hal]

I have a bad flat spot off idle on my Z1B starts ok, runs ok, ticks over ok, as soon as you touch the throttle off idle the motor almost cuts out and then picks up, spark plugs are sooty I am at my wits end tried everything
New Dyna coils and ignition
New leads and caps
Denso plugs
New carb inlet rubbers
Engine, valves and shims all rebuilt
Standard air box and filter
4x4 exhaust
New wiring loom and switch gear rewired but still tried the full power coil mod
Carbs stripped and ultrasonic cleaned, all the airways jets and pilot drillings clear
And slides set up spot on, Standard main jets, pilot jets & needles, float heights set.
Pilot air screws set
Valve timing checked
Ignition strobed
Advance and retard checked
Still the same, things point to the pilot system but I have had the carbs on and off that many times I could do it blindfolded with one hand tied behind my back
I don’t know what to try next. Please help

[z1bman]

invest in a colour tune

[Charlie]

Coming off idle and onto first throttle is pilot jet territory. Have you changed the jets ? If so, did you use genuine Mikuni jets ? Have you cleaned out the jets if you haven't changed them ?

[hal]

That was my thoughts that it could be the pilot system, I have tried the standard mikuni 17.5 jets ultrasonic cleaned, the pilot airways and drillings are clear and new 17.5 pilot jets from Z1 Enterprises with no change.

[Pigford]

They're are old carbs - a common problem - their design isn't the best compared with modern carbs.

Modern unleaded fuel exascebates this phenomenum too! The vacuum over the venturi is not sufficient to draw fuel quick enough or atomise the fuel - a different "cut" slide may help, but TBH you may well be flogging a dead horse - or just feather the throttle from idle

I dare say a few hours tinkering may improve matters - worth raising the needle a notch to see if it helps?

[jphaynes669]

Check the internal condition of the emulsion tubes under a magnifier, the surface needs to be smooth, any pitting or erosion will hamper flow. If you need to remove them, go easy when knocking them out , Nrp up in Manchester will supply you replacements.
When re commissioning my mk2 I bought complete rebuild kits which contained everything, needles , tubes , seats , o rings the lot! Once carbs were set up and balanced it transformed the bike and I haven't had to touch it since.

[hal]

Thanks for your help I think my next step is Keyster carb kits from NRP and a colortune kit.

[ZedHead]

Do the "Wired George" mod if you have not already.

http://www.wgcarbs.com/index.php/using- ... s/89-coils

[Charlie]

Pattern carb bits are notoriously problematic when you are down to a tenth of a millimetre making a difference, quality control is not the same as OE. Some may be OK if you drop on lucky, many are not.

Stick to genuine Mikuni stuff. You can chase shadows for weeks using aftermarket carb bits and never be sure you fixed or found the issue.

Needle profiles differ, flow rates vary on pattern stuff.

[z1bman]

remove the pilot jets & drip some thin oil / three in one oil into hole where the jet screws in & check which if any drain away at a slower rate than any others

[njwmct]

As per jphaynes669 .

I had the exact same problem with my Z1000 MKII

Check the needle jets (a press fit into the carb bodies above the main jet / emulsion tubes)

If these are worn or pitted they let fuel past the needle giving an over rich mixture just off idle & a massive flat spot.

You can get replacement needle jets from Allens Performance.

[hal]

I have stripped the carbs down (again) and the needle emulsion tubes are pitted, the keyster kit at NRP includes emulsion tubes are they not good quality ?

[njwmct]

I have stripped the carbs down (again) and the needle emulsion tubes are pitted, the keyster kit at NRP includes emulsion tubes are they not good quality ?

My mate Jeff didn't have a problem with the NRP kits.
I got the Genuine Mikuni items from Allens Performance.
So either should do the trick.

I think that Kawasaki actually call the item that presses into the carb bodies a 'needle jet'

[DogsbolloxofZ1B]

Are you sure your chokes are shutting off. Best to remove carbs and blow through each choke port and make sure you've got an airtight seal.
Slightest leak and carbs will not play ball at all.

[hal]

Checked the chokes are shutting off they are ok, I removed one of the needle jets and it is badly pitted so i have orderd genuine Mikuni needles & needle jets from Allens Performance, I will let you know the results.
Thanks for all your help.