Mikuni "choke"

Or fuel enrichment as they prefer.Anyway,my new build has a starting problem.Its gone from a one kick starter, to 10 kicks if im lucky,especially when warm.
Pretty sure something electrical is breaking down,as its intermittent.
Anyway,Id like to discount one possible cause.Ive noticed when the "choke" is off ie in down position a tiny amount of play exists.It snaps down but a few mm of play exists in the lever.
This is probably just the design yet obviously if its not it maybe causing too much enrichment when warm.Its been a non starter on last two outings.I had to bump it twice to get home.(which it started easily)
Can a fellow owner who runs a mikuni please offer some advice?

I have a Mikuni carburettor on my Norton 750 and it is impossible to start without starter, for that I have an adjustement (choke lever) on the handlebar with mainjet of 240

Lever down is open as it lifts the needle out of the enrichener jet. Lever up is closed.

We have a problem of ups and downs here. There are different forms of choke operation on Mikunis. I have one with a button you pull up to lift the seal off the fuel enrinchment orifice for starting, and others that you push the lever down, it has a pivot and this action causes a fork to lift the seal and allow extra fuel in for starting! Cable operation is also possible. All of these parts fit onto a standard Mikuni body.

Mikuni's do not provide a partial 'choke' like an Amal MK I Concentric with it's choke slide that reduces the airflow. The effect of the Mikuni lever is normally, extra fuel, or no extra fuel. The exception is when the rubber seal
is worn it never shuts off completely. But in general these seals last longer than the equivalent seal used on a MK II Amal!

So first let's work out exactly what we mean.

When you have decided, please remember that if the fuel enrichment is providing additional fuel, even just a little, and you try a warm start, you will probably get the symptoms you have!

Blimey confused! Its standard mikuni from matt @ CNW

coomo said:

Blimey confused! Its standard mikuni from matt @ CNW

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The side the lever is operated (LH side) is up for choke OFF so the plunger side is then down.

coomo said:

Ive noticed when the "choke" is off ie in down position a tiny amount of play exists.It snaps down but a few mm of play exists in the lever.

Click to expand...

Completely slack "play" or is there light spring pressure?

There's normally some play before the lever begins to lift the plunger but the leaf spring on the lever assembly should force the plunger end of the lever down against the plunger keeping it closed when the 'choke' is OFF.

L.A.B. said:

The side the lever is operated (LH side) is up for choke OFF so the plunger side is then down.




Completely slack "play" or is there light spring pressure?

There's normally some play before the lever begins to lift the plunger but the leaf spring on the lever assembly should force the plunger end of the lever down against the plunger keeping it closed when the 'choke' is OFF.

Click to expand...

Really it just light pressure.ive another new one here for my comet.its exactly the same.something else is wrong.

coomo said:

Blimey confused! Its standard mikuni from matt @ CNW

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That’s because, (“standard” to you, is not standard across the WIDE range of Mik carbs) the enrichener plunger moves UP, off the seat to enrich. The LEVER, which has a pivot, reverses that motion, so you push it DOWN to enrich. Not all Miks use the lever. Some use a cable to pull the plunger.
That lever you have also incorporates a spring detent to hold the enrichener “ON”, against the plunger spring which, during normal running, holds the plunger down/off. That play you observe is normal.
Best to talk plunger motion on VM Miks, for clarity.

coomo said:

something else is wrong.

Click to expand...

Perhaps check the 'rubber' seal on the plunger isn't worn/damaged and that the plunger is seating correctly to fully block off the enrichener in the OFF position.

On my bike you only use the enrichener if it’s cold (ambient, not engine). On a hot day with a cold (hasn’t ran) it won’t start with the enrichener. So below 75F.

Thought for sure you would be a twin carb man ..... I have used both twin VM32 and the single VM34 mikuni , my start procedure from cold for the single setup is turn tap on , push enricher lever down kick through twice , pull enricher lever up , with hand on throttle kick through fires right up ... once it has been running for day never need to touch lever .... when putting away for night let gas in carb burn off and that shuts down Bike ....

Thanks guys.Spent 4 hours with my electrics guy tutoring me on the finer points of Norton electrical systems.Quite a challenge, as "electrickery" is not my strong point.Finally traced to a rogue spark plug.Fired first kick after replacement.I remember 30 years ago, spark plugs lasted forever.Now of course, I need to test ride to see if something is causing the plugs to fail.Stay tuned..

coomo said:

Thanks guys.Spent 4 hours with my electrics guy tutoring me on the finer points of Norton electrical systems.Quite a challenge, as "electrickery" is not my strong point.Finally traced to a rogue spark plug.Fired first kick after replacement.I remember 30 years ago, spark plugs lasted forever.Now of course, I need to test ride to see if something is causing the plugs to fail.Stay tuned..

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Lucky you, I recall 40 years ago going through a box to get 4 I could rely on!. It happens.

Sorry too confuse about the fuel enrichment, but...there are 3 ways to lift the plunger on a Mikuni! Seems I have one none of the others posting here have experienced! It is a right hander in a pair!

Craig said:

Thought for sure you would be a twin carb man ..... I have used both twin VM32 and the single VM34 mikuni , my start procedure from cold for the single setup is turn tap on , push enricher lever down kick through twice , pull enricher lever up , with hand on throttle kick through fires right up ... once it has been running for day never need to touch lever .... when putting away for night let gas in carb burn off and that shuts down Bike ....

Click to expand...

I have nearly the same procedure in starting my 85MK II with VM34. After a while full of frustrating about starting behavior, I had to learn that my strat sequences were nonsense!
When it is warm outside (20-25°) I open the Enricher, kick two times without ignition, CLOSE enricher, start ignition, kick 1 time / 2 times with throttle open 25%

Once Motor is hot, 1 kick. Tricky is the area when the motor getting cold. Sometimes I have to use enricher, sometimes it doesn't. Then it is a kind of gamble!
BUT: be careful that you have: the right timing for ignition point, correct valve clearance, and NO issues by the electric.
Hope you will solve the issue. I know exactly what does it mean to have a lost and frustrating day with trying to kick start a Norton.

Regards Lorenz

Yup, I’ve been bitten by the duff plug scenario recently too.

Swapping plugs is now my first experiment whenever I have mysterious running issues. Surprising how many times it solves things...