Mikuni Carb Settings

[Duccout]

Managed to get in a run today (don't tell anyone). The engine is still reluctant to run without the cold-start when cold, so I had to juggle the throttle between tick-over and half throttle to keep it running, as it keeps spitting back and dying, but gradually as the engine warms up it runs OK. Once warm, it runs beautifully, so smooth and tractable, and the plug was a perfect colour when I got back. I will try raising the needle, and I may need a richer needle jet again, but the settings cannot be far out.

[Duccout]

To those with Mikuni carbs: advice needed please!

My 250 runs perfectly with its 28mm Mikuni and I'm really pleased with it, but starting from cold is an issue. I have read that when using the Mikuni cold-start jet, that the throttle must be closed; OK, no problem, bike starts first kick, then I turn off the cold-start lever and the engine ticks over, but will not take any throttle without dying. To keep it running I have to keep the cold-start lever depressed and hold the engine at about 2000 RPM until it has warmed up, then turn off the cold-start lever and then struggle to keep the engine running until it warms up enough for me to ride away. Is this normal, and if not, any thoughts?

[themoudie]

Aye Colin,

See Tassie Graeme's comments earlier in this thread and DanC's comments as well.

Graeme in Tassie will get the cooler weather of the two, as DanC is in inland NSW that gets a bit droughty!

As for holes in ports, see installment after Nigel's and afore yours at bottom of page 16 of the "450 Desmo carb" thread.

Good health, Bill

[graeme]

Hello Collin,

I can only report on a VM 34 Mikuni fitted to a Mk3 Commando

Same as you say,

No throttle, choke/inricher lever down, kick and it fires straight away.
It will run but very rich,
Slowly lift the lever while slowly opening the throttle until the lever is off.
Keep the throttle at about 1500 to 2000 until the cylinders have some warmth, then it will idle.


Graeme

[Duccout]

Thanks Graeme,

My engine seems to run fine with the cold-start lever down, and lifting it results in the engine dying. I'm guessing that the cold-start should basically flood the engine? So what do I need to do - fit a richer needle, or a larger cold-start jet? I will raise the needle to the top notch and see if that helps.

Colin

[George]

Is this at tickover. If so needle has little or no influence below quarter throttle . Only ticking over with choke indicates weak idle, first move is bigger idle jet.

[Duccout]

Hi George,

No, tick-over is fine, it is when opening the throttle that the engine wants to die. Possibly the slide cutaway needs to be smaller, but it is already as small as Ducatis normally use, and Motocarb who supplied the Mikuni said that it is definitely the needle. BUT, the engine runs perfectly once it has warmed up, making me think that the cold-start jet is too small.

Colin

[George]

No experience with Mikuni. Sounds like transition from idle. There should be, as throttle starts to open, a means of increasing air flow to pick up fuel before slide opens. On Amal there is an airway further along throttle intake that comes into play with change in vacuum. I have noticed that on mine it takes longer to release choke and warm up than the Moto Guzzi.

[George]

Reading my post I should add that the transition period is still controlled by idle jet, and engine stalling on small throttle opening indicates idle jet to small

[Dan C]

Hi Colin,

Firstly apologies, I haven't logged on for a while, my feeble excuse is too busy riding motorbikes....

My experience is different to yours. I put the VM 30 on the 450 back in August 2020 which can be fairly cold by Aus standards (-6C is fairly common overnight and some days less than 10C) and our altitude is 1000m asl (3300 feet). Looking at my notes I noted at the time it was starting with no choke.

My usual routine when cold is to depress the cold start lever and with no throttle give it a kick. In one or two kicks it will usually fire but not run. Then I lift the cold start lever up and again with no throttle give it a kick. If that doesn't start it I'll just crack the throttle a fraction and it will start. If that doesn't work, I say swear words and have a sook. Then it works.....

When warm, its no throttle and one kick. Very reliably.

So I really can't make any suggestions except have a look at the jetting settings I posted earlier in this thread (I think its this thread....) and see what the differences are bearing in mind my settings are for 350 and 450 not 250.

My plug readings are a bit rich at idle and the "non choke" starting indicates that's probably the case but the bike runs fine without surging etc at around town speeds so I'm not touching anything!

Cheers
Dan