Carburation is crisper / sharper when engine is warm NOT hot

[johny brando]

I bet I'm not alone here !!!! I'm asking if any one has managed to nail this Curley little bugger that I have been putting up with & try as I may I cannot find the answer too ?
My 900 runs really Crisp & Sharp...brilliant, when it's just come off choke ....for the next 4-5 miles it's 100% but as it gets to normal operating temp it looses that CRISP edge and then when I blip it ....say changing down at say 4000rpm, it's now Mmmm, fluffy for want of a better description =( half a blip ..if you know where I'm coming from ? ) other than that fluffing on a blip ......well its fine ! ...... it pulls hard, no hunting or holding back - but what I'm chasing here & really want... is that Crispness all the time !
This little issue has always been there, bugging me . The motor if fully rebuilt & good it has 135psi on all Cylinders. MK2 Ignition & timing is good as is A/V unit, there's no vacuum leaks , running 29mm smooth Bores that I bought brand new a long time ago these were set specifically for Kawasaki 900 as oppose to a Suzuki Gs1000 ( now they have just 7000 miles of use on them...nothing is worn ) The internals i.e. Slide ,needle,needle jets ,mains ,air jets & pilots jets are all correct as is Float height & I've done the .7mm slide set up too + a vacuum balance to finish that off. I've tried every conceivable needle position & even made 12 thou shims , so I could try ...half needle clip positions too. Plug colour on the electrode & insulator is fine ...always have a slightly sooty outer edge... around the thread perimeter face only. If any one has input or idea's on how to get this Bug out ...I'd appreciate it . Cheers Boy's, Johny Brando.

[PAULJAC47]

What plugs and type you running?

[ADE]

I run 33CR carbs.That ran fine until i changed the cams.Even though the cams were bigger it seemed to be running rich low down.I dropped the needle one and turned the air screws in a 1/4 turn and all seems good.With blipping the throttle you may be sucking a little too much air?
Ade

[Hinckley]

Try going up one heat grade on the plugs, it sounds like the plugs are overheating as you get up to temperature.

[rickman CRR]

Valve clearances tight perhaps?

[Pigford]

Sounds like it might be a tad rich.

Maybe a weak ignition.... heat = more resistance (possible coil breaking down or naff plug?)

[johny brando]

Sounds like it might be a tad rich.

Maybe a weak ignition.... heat = more resistance (possible coil breaking down or naff plug?)

Yes I agree ...you'd have to say - running slightly rich ! but like I said plugs don't indicate that at all , if any thing just a whisker weak ! I've been through the Carbs & cannot seem to get them closer no matter what.I had ignition issues recently & I then checked thoroughly the Coils, CDI box & pickups & I found the Coil resistor was the route cause ...... All was well then ...but obviously it still retained this Fluff/momentary blip hesitation. Thanks for input.

[johny brando]

Valve clearances tight perhaps?

Just been fully rebuilt by me: New OEM guides, S/ Steel valves, genuine Seals & Seats cut to 1.0mm valves set at 4thou inlet & 6 thou exhaust .Problem has been there for ever .....can't find it but it's carbonation alright ! Cheers.

[johny brando]

What plugs and type you running?

Brand spanking new NGK BR8EiX Iridium's . Cheers Paul

[johny brando]

Try going up one heat grade on the plugs, it sounds like the plugs are overheating as you get up to temperature.

Thanks ! Now thats a fare point that I hav'nt considered ..well done . I could try running one grade cooler= set of B9ES to see if it improves . I personally don't think it is that... but without dipping your foot in the water & trying it, well there's can be no definitive answer . Thank you your suggestion much appreciated.

[Mr Bump]

Hi,

Bearing in mind I don't have any personal expirience of these carbs, if it's 'always' been like it, you're confident the motor itself is mechanically good and the ignition system isn't to blame, I'd suggest that the problem may be in yr pilot circuit set up, or maybe float height, and that it's running slightly rich. Pilots seemingly influence up to 1/8th throttle, which means that in traffic or cruising on a big bike we run on pilots a lot of the time. I think mikunis tend to have 37.5's (?) so maybe try 35's.

Could also try dropping the float level 1mm to see what happens (not at the same time as changing something else!).

If these changes make it worse then at least you know you it's not richening up but leaning out instead.

Another outside chance (and this one is a guess) are you running the std mechanical advance? It could have got a bit slack or the springs have weakened so it's not retarding quickly enough.

Olly

[Mr Bump]

Sorry, just re read yr first post - ignore the idea about the mechanical advance - you don't have one!

Doh.

Olly

[johny brando]

Hi,

Bearing in mind I don't have any personal expirience of these carbs, if it's 'always' been like it, you're confident the motor itself is mechanically good and the ignition system isn't to blame, I'd suggest that the problem may be in yr pilot circuit set up, or maybe float height, and that it's running slightly rich. Pilots seemingly influence up to 1/8th throttle, which means that in traffic or cruising on a big bike we run on pilots a lot of the time. I think mikunis tend to have 37.5's (?) so maybe try 35's.

Could also try dropping the float level 1mm to see what happens (not at the same time as changing something else!).

If these changes make it worse then at least you know you it's not richening up but leaning out instead.

Another outside chance (and this one is a guess) are you running the std mechanical advance? It could have got a bit slack or the springs have weakened so it's not retarding quickly enough.

Olly

Thanks for your input , it is appreciated . Although it appears little known.... the pilot system is actually operational all the time the engines is running but it is the only system running off choke up to about 1/8 throttle opening. Around a 20 Pilot jet is the normal pilot jet size for this type of VM Mikuni set up & thats what is in mine .This issue is actually occurring solely in the range of Slide Cutaway & Needle System . My float height is set at 23mm from the gasket face - with the gasket still attached.... this apparently is the recommended setting.
The inlet Inlet tracks were lightly cleaned up & the Inlet Rubbers have been matched to the Carb venturers. Just in case others are wondering what these 29mm S/Bore internals are running here is a brief list
: Mains 120 - Pilots 20 ( air screw 1.5 turn out ) - Emulsion Tube 261 0-6 - Air Jets 0.9 - Slide 1.5 - Needle 5DL 31 (clip:centre notch with 12 thou shim) & Individual Pod air filters .
The exhaust is a mix of an Alpha front with a Harris tail end.. and is straight through.
Your advance/retard point that you raised is a good call even though you later dismissed it , as the A/V is ..... whilst being a magnetic Pick-up system is still advanced & actuated via a Std type, sprung Bob-weight system..... but I did recently check the advance to Rpm curves and it's starting and finishing at the correct rev's with the correct amount of advance. Thanks again for your input.

[Mr Bump]

Fair enough. I'm out of my depth now - my thoughts were based on what's worked in the past for me on diaphragm carbs, so not quite the same thing.

I'm interested to know what the answer turns out to be.

Olly