Mikuni Quirk

My Mikuni carbi performs faultlessly, virtually a set and forget item. But one strange thing that occurs from time to time is application of the choke causing the engine not to fire. Every so often on a cold start with choke on (I have lever on carbi) the starter is spinning the engine over very well but it won't fire up. The first time this happened I didn't know the cause. There was a good spark from both plugs. Something made me cycle the choke lever up and down whilst operating the starter and the motor roared into life. Very occasionally this occurs and always cycling the choke lever allows the motor to start straight away. This is the second Mikuni I have had on a Commando but have never experienced this peculiar behaviour.

Anyone else experienced this or suggestions?

Starter 'Choke' jet too big?

L.A.B. said:

Starter 'Choke' jet too big?

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I doubt it Les. 99% of the time it functions just as it should, but every so often I get this strange occurrence but clears itself after I cycle the choke lever, but only whilst engine is turning over. Then all is normal as you would expect. Very odd.

Mikuni on my MK3 also.. Your starting problem is one reason why i kick start when cold. In my case 3-4 seconds on the starter will "flood" the motor when cold.. So 1-2 kicks full choke on cold,,, fires up every time ..... unless i havent turn on the key, let the fuel thu (and oil) for a minute or so prior to kicking.... Thats all assuming it runs well under normal circumstances..

Maybe there is a hiccup drawing fuel up now and again given the cycling of the choke lever while the engine is cranking fixes it.
The choke on the Mikuni is a plunger with a rubber seal on the end,when you use the lever the plunger rises allowing fuel to be drawn into the cavity below the now raised plunger and into a passage that exits in the floor of the carburetor in front of the slide.

olChris said:

Mikuni on my MK3 also.. Your starting problem is one reason why i kick start when cold. In my case 3-4 seconds on the starter will "flood" the motor when cold.. So 1-2 kicks full choke on cold,,, fires up every time ..... unless i havent turn on the key, let the fuel thu (and oil) for a minute or so prior to kicking.... Thats all assuming it runs well under normal circumstances..

Click to expand...

My VM Mikuni-carbed(x3) T160 exhibits a similar characteristic in that if it doesn't start on the first press of the button (or a after a couple of kicks) when cold, then the chokes must be set to off- otherwise further cranking with the chokes on quickly floods the engine. With the chokes then turned off, it normally fires up immediately.
If I attempt to start it up under the same conditions (cold engine) with the chokes off, it won't start, so it does need some initial enrichment-but obviously not too much.

olChris said:

Mikuni on my MK3 also.. Your starting problem is one reason why i kick start when cold. In my case 3-4 seconds on the starter will "flood" the motor when cold.. So 1-2 kicks full choke on cold,,, fires up every time ..... unless i havent turn on the key, let the fuel thu (and oil) for a minute or so prior to kicking.... Thats all assuming it runs well under normal circumstances..

Click to expand...

Thanks Chris
Just very odd that it does it so rarely. Every other time it bursts into life almost immediately. But when it does this odd thing of not firing, it certainly does when choke lever is moved out of full choke position then instant life, at this point choke can then be returned to full on to keep engine running until warm. So you might have hit the nail on the head. Just for some reason, not all the time. I can certainly live with it now I know how to resolve it. But it certainly had me puzzled.

Phil

Time Warp said:

Maybe there is a hiccup drawing fuel up now and again given the cycling of the choke lever while the engine is cranking fixes it.
The choke on the Mikuni is a plunger with a rubber seal on the end,when you use the lever the plunger rises allowing fuel to be drawn into the cavity below the now raised plunger and into a passage that exits in the floor of the carburetor in front of the slide.

Click to expand...

This would certainly explain the infrequency of the occurrence. And that is exactly how it seems, as though something has prevented fuel inflow. There is no splutter, cough or anything from the motor as though no fuel is getting to it at all. Then suddenly she comes alive instantly after choke cycle.

Thanks Time.

L.A.B. said:

olChris said:

Mikuni on my MK3 also.. Your starting problem is one reason why i kick start when cold. In my case 3-4 seconds on the starter will "flood" the motor when cold.. So 1-2 kicks full choke on cold,,, fires up every time ..... unless i havent turn on the key, let the fuel thu (and oil) for a minute or so prior to kicking.... Thats all assuming it runs well under normal circumstances..

Click to expand...

My VM Mikuni-carbed(x3) T160 exhibits a similar characteristic in that if it doesn't start on the first press of the button (or a after a couple of kicks) when cold, then the chokes must be set to off- otherwise further cranking with the chokes on quickly floods the engine. With the chokes then turned off, it normally fires up immediately.
If I attempt to start it up under the same conditions (cold engine) with the chokes off, it won't start, so it does need some initial enrichment-but obviously not too much.

Click to expand...

Well both points by you and Chris are valid possibilities. Time's thoughts equally so. Given the rarity of it happening, I'm thinking Time's explanation more likely. Normally hot or cold, starting is an instant matter. But it would be too with twin Amals on I'm sure. It's the electronic ignition that has made life so much better for us in every sense.

I think from memory, Chris does not have an upgraded starter motor, so kick starting would be your only option whether you press the green button at the same time or not. In my case, I could never cycle the choke lever whilst kick starting at the same time without being gifted with ape arms. When performing this strange quirk, the motor will not fire in choke off or on position. Only when cycling the lever whilst engine is turning over. It doesn't worry me a great deal, only that one day it will not start at all despite cycling. You know how these little things can manifest themselves. An occasional occurrence can become a regular thing and finally catch you out all together. Usually just where and when you don't want it to. So early correction is best.

Phil

The Carb that you are using in your diagram doesn't even resemble the typical VM 30,32,or 34 mm carb that normally finds it's way onto a Commando engine .What is it off of?.........Skip

looks like that carb is off a two stroke, air screw is well forward, if that is the manifold next to it?

maybe off an ATV or snowmobile or something?

Its a Mikuni pumper.

The picture was a add on after making the post.
It was all I could find on Google,even looking again just now.
Given the hour of the night here it was good enough I think to show the starter tube layout in the float bowl area.
The starter system is fairly standard over the Mikuni range.
The plunger is either vertical or horizontal but works the same.
Phil has posted he gets the bike serviced at a shop.
If there was some ongoing fault or it escalated I am sure he would drop it in for attention.

Time Warp said:

Its a Mikuni pumper.

The picture was a add on after making the post.
It was all I could find on Google,even looking again just now.
Given the hour of the night here it was good enough I think to show the starter tube layout in the float bowl area.
The starter system is fairly standard over the Mikuni range.
The plunger is either vertical or horizontal but works the same.
Phil has posted he gets the bike serviced at a shop.
If there was some ongoing fault or it escalated I am sure he would drop it in for attention.

Click to expand...

Yes, the boys in the workshop will certainly know the ins and outs of a Mikuni. I'll have it looked at maybe when I pull the forks off to replace stanchion dust caps, both of which have splits in them. That will be decision time for gaiters to be fitted or not, but I'm thinking I'll keep the ceriani look. Didn't like it at all once on a fastback. But have come to like them.

The start pattern I find that works for my Mikuni VM when cold is:
Full choke and three kicks. Typically starts by second kick.
If no fire by third kick take choke off and kick with quarter throttle. Usually starts then. If not starting after two kicks go back to full choke and run cycle again.
If three more kicks doesn't work start to confirm that I've got spark and fuel.

With my current setup including Pazon Sure Fire it always fires and typically 2-3 kicks.

rx7171 said:

The start pattern I find that works for my Mikuni VM when cold is:
Full choke and three kicks. Typically starts by second kick.
If no fire by third kick take choke off and kick with quarter throttle. Usually starts then. If not starting after two kicks go back to full choke and run cycle again.
If three more kicks doesn't work start to confirm that I've got spark and fuel.

With my current setup including Pazon Sure Fire it always fires and typically 2-3 kicks.

Click to expand...

Thanks rx
My technique is:
Cold, choke full on.
Hot, choke full off.
Press button and bang, instant engine noise. I mean very instant.

But on rare occasions cold, no noise at all. So problem is very intermittent. With a fast spinning electric starter, no varying kick start procedures are required.

Thanks
Phil
ps love your bike and colour.

olChris said:

Mikuni on my MK3 also.. Your starting problem is one reason why i kick start when cold. In my case 3-4 seconds on the starter will "flood" the motor when cold.. So 1-2 kicks full choke on cold,,, fires up every time ..... unless i havent turn on the key, let the fuel thu (and oil) for a minute or so prior to kicking.... Thats all assuming it runs well under normal circumstances..

Click to expand...

Just as well I wrote these instructions on a piece of paper and sticky taped it onto the tank because this morning the starter let me down, well the battery did. I was suffering the rare choke problem and turning over the motor when the battery died before any engine start. The MkIII Riders Manual refers to an emergency start lever on the right side of the bike and says push down to start engine.
Just like my Hillman Minx had a crank handle. Well, I found the lever but no way can you push it down with your hand, and what position do I now have the choke lever in?

Chris' instructions saved the day and with boot on lever, one swing fired up the motor straight away. Very impressive indeed for an emergency start technique.

I will get this choke lever matter looked at sooner rather than later as rare occurrences of the affliction are creeping upwards, just as I feared they would.