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Kehin CVK Carburettors

Kehin CVK Carburettors

Kehin CVK Carburettors

Trying to tune an inline 400cc 4cyl using 32.5mm Kehin CVK carbs. Tune is ok but I am constantly getting a lean spot between 12500-13500 rpm at wot. Using an AFR of 11.5:1 from 7000 to 12500 then leans drmatically to 13:1 and comes back to 12.5:1 by redline at 14500rpm. The carb is well onto the main jet by this stage, chaning mains only moves the entire curve up and down accordingly. Can a change in emulsion tube or float jet change this????

RE: Kehin CVK Carburettors

It sounds like the needle profile is too thick at a specific throttle opening.
Try raising the main jet needles a notch, or if that makes things too rich, add shim washers under the circlips.

This works on a large number of recent Japanese engines, which are set lean to pass emissions tests. When you move the needle positions it seems you put them back to where they were before tweaking to pass the tests.
One point to note is that emissions tests are usually not carried out at the engine speeds you are working with.


RE: Kehin CVK Carburettors


The needle isn't involved in this situation (WOT), and the fact that his mixture returns to normal above the lean point indicates some kind of flow harmonics, pulsations, etc.

RE: Kehin CVK Carburettors

Not needles tried many different needles plus as Metalguy said the motor is at WOT. Have tried different shaped velocity stacks plus heads with different port shapes. Unfortunatly stuck with stock carbs as per class rules. Only thing that has helped was blocking up air main air jet. This richened up base line AFR however the lean peak is still too strong, this is what made me think that a change in float needle jet or emulsion tube may help.

RE: Kehin CVK Carburettors

Confused me by writing "WOT" as "wot", so I thought you might have meant "what".
I have to agree with Metalguy about harmonics. Have you altered the exhaust system from standard regarding length?

RE: Kehin CVK Carburettors

If you have the knowledge to have got this far, and have the equipment to know that you really are getting those ratios under full load, then i guess you probably have more ability  than most of us reading your thread.

Are you using a Dyno to get readings at WOT? If so then the Dyno operator probably has more knowledge and also carb spares to try, than anyone else. If not then I don't see how you could carry the measururing kit, take the readings and ride the bike all at the same time.

Suck it and see - try some alternative carb parts and see what happens - just don't go too lean too quickly, or it'll all melt.

RE: Kehin CVK Carburettors

You have CV carbs,having the throttle plates wide open doesn't mean the slides are at the top of their travel,they will move up as engine speeds rise - although you are rather high in the rpm range here,it may be pulling from the mains rather than the needles,could be the transition between.You have removed the airbox,some Japanese bikes don't like that at all,sounds like it's a Honda - is it a desized 600 or a 400 only,if it's desized I've found they don't like being opened up in the breathing dept.

RE: Kehin CVK Carburettors

LadaTrouble, the bike is a Kawasaki ZXR400, it has a fully enclosed airbox. I think it is the transition from the needle to the mains that is the problem area. VTR1000, the bike is being tuned on a dynojet inertia dyno. I have tried plenty of main & needle combinations without success. I am thinking that this problem is a characteristic of this carburettor, however class regulations say that stock carbs must be used. Has anyone tried changing the float jets, emulsion tube, or float heights to change this part of the fuel circuit.

RE: Kehin CVK Carburettors

Hi, I think you will have trouble getting any tuning parts for those OE carbs. (except main/pilot jets.)
I suggest experimenting by blocking some or all of the bleed holes on the emulsion tube.
Maybe you could make some new needle jets in primary style? (As opposed to bleed type.) These bleed air into the emulsion tube after the restrictive point instead of before. That is the system used on the aftermarked flatslide carbs so I suggest it may be better for control of the mixture at large slide openings?

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