Mikuni Starter Jets

Has anyone tried fitting one of the starter jets for the enricher circuit on a VM or TM Mikuni? I've never seen them ship a VM with any starter jet. My 850 8-cycles after about 20 seconds, but it's not warmed up enough to idle so I end up raising the slide with the idle screw, then gradually backing it off. I've heard of the 1.0, 1.5, and 2.0 sizes. Maybe a regulated flow would mean I could leave it on long enough to get the bike warmed up.

http://www.carbparts.com/mikuni/mikuni_tuning.htm

Do you use the chokes? If your bike is jetted right, you should need to use them until the motor is warm. If you take them off too soon there should be spitting back from the carbs due to the mixture being too lean. You need to know whether your idle screws adjust air or petrol, and set the mixture when the motor is warm getting it as lean as possible without missing or spitting back. The stop screws should be set to give a quick idle when you are adjusting the mixture screws. You can then slow down the idle by lowering the stop screws and getting an even beat from the motor.

VM carburetors do not have starter jets, they have an enrichening circuits commonly referred to as chokes. The enrichening circuit on a VM carb does not have any interchangeable calibrated parts; it operates more like a primer.

Amal owners are used to opening the throttle at the bottom of the kick, if you do that with a Mikuni VM it will take a long time to start; the enrichenimg circuit, or choke, on a Mikuni needs a maximum vacuum signal to put fuel through the enrichener.

rpatton said:

Has anyone tried fitting one of the starter jets for the enricher circuit on a VM or TM Mikuni? I've never seen them ship a VM with any starter jet. My 850 8-cycles after about 20 seconds, but it's not warmed up enough to idle

Click to expand...

I'm guessing your Mikuni has the carb mounted "choke" (enrichener) lever or plunger knob so it is either "ON" or "OFF"?

Fitting a cable conversion should allow finer control, so the choke can be closed more gradually as the engine warms up.

http://www.power-barn.com/servlet/the-1 ... tor/Detail

RoadScholar said:

VM carburetors do not have starter jets, they have an enrichening circuits commonly referred to as chokes. The enrichening circuit on a VM carb does not have any interchangeable calibrated parts; it operates more like a primer.

Click to expand...

Check out part number VM17/1002. It's about 2/3 the way down the page. They are listed for the TM but it seems they could be used on a VM for the same reason they are used on the TM, because unregulated they put out too much fuel. I came across a post on a dirtbike forum where someone was going to tap threads in the bowl on a VM to give it a try, but there was no followup. Maybe it's because it can't be done.

L.A.B. said:

I'm guessing your Mikuni has the carb mounted "choke" (enrichener) lever or plunger knob so it is either "ON" or "OFF"?

Fitting a cable conversion should allow finer control, so the choke can be closed more gradually as the engine warms up.

http://www.power-barn.com/servlet/the-1 ... tor/Detail

Click to expand...

Les, I'm using the cable version. I've not been able to feather it back so that it acts like a conventional choke, maybe I gave up on it too soon. It seemed as if the proportions of the fuel/air mix didn't scale down together as I dropped the plunger, dies most of the time, maybe with some practice.

RoadScholar said:

Amal owners are used to opening the throttle at the bottom of the kick, if you do that with a Mikuni VM it will take a long time to start; the enrichenimg circuit, or choke, on a Mikuni needs a maximum vacuum signal to put fuel through the enrichener.

Click to expand...

Wiser words were never spoken. I learned this lesson the hard way. It's is a one kick proposition every time if you leave the throttle alone.

Hi Bob,
I know exactly what you are talking about as I run a TM Mikuni flatslide and I have the same problem that you do
IE the enrichener circuit runs too rich. With every cold start I have to keep my hand on the lever and carefully
position it on/middle/off for at least a minute during warm up.
What I have done is press out the starter jet (found at the bottom of the hole in the front edge of the float bowl).
I did this by drilling a small hole (don't remember what size, probably 1/8 inch) in the bottom of float bowl (in that little flat area where the jet is).
Then I used a small punch inserted through the hole to press the jet out from the bottom. I tried to reduce the size of the jet by soldering it closed
and redrilling with a smaller bit. After that I pressed it back in from the top, then plugged the hole by tapping it and fitting with a small set screw (grub screw). I was
not able to get the results I desired, probably because the solder and drill method is not too accurate. However, now
you have provided a website from which I can get different starter jets I will try again to fine tune my starter circuit.

Thank you,
GB

rpatton said:

I've never seen them ship a VM with any starter jet.

Click to expand...

The VM starter jet is usually fixed in the float bowl.




I would take a wild guess the number stamped on the underside of the bowl directly below the jet is the jet size.

The starter jet on the VMs is often too big. Too big is good for starting but it won't run. Often people solder them up and then drill them with the smallest drill in from a set to tip cleaners. I have one bike where I have placed thin wires in the jets to lean it out enough so that the engine will run very roughly with the enrichener turned on.

Really the tickler on Amals is a really good system. It gives a really rich mixture for starting and then quickly goes to a more normal mixture. It leans out over about a minute as the float bowl goes down to the normal level.

rpatton said:

Has anyone tried fitting one of the starter jets for the enricher circuit on a VM or TM Mikuni? I've never seen them ship a VM with any starter jet. My 850 8-cycles after about 20 seconds, but it's not warmed up enough to idle so I end up raising the slide with the idle screw, then gradually backing it off. I've heard of the 1.0, 1.5, and 2.0 sizes. Maybe a regulated flow would mean I could leave it on long enough to get the bike warmed up.

http://www.carbparts.com/mikuni/mikuni_tuning.htm

Click to expand...

On snowmobiles equipped with VM's, we use a three-position enrichener cable actuator to allow VERY rich to start, then slightly rich to warm up. Carb mounted levers with dual detents were used as well (non-OEM application)

https://www.denniskirk.com/motion-pro/c ... 198186.sku

need this too: https://www.denniskirk.com/mikuni/choke ... d/1926.sku

concours said:

On snowmobiles equipped with VM's, we use a three-position enrichener cable actuator to allow VERY rich to start, then slightly rich to warm up. Carb mounted levers with dual detents were used as well (non-OEM application)

Click to expand...

That pretty much proves it can be done, I've been rushing it with trying to feather it off too fast. I've got the plunger hooked up to the stock choke lever on the handlebar. The friction holds it at any setting so I'll try backing it off in a few steps. If I can't make a go of it, I can fall back to a jet change and using the lever in steps.
Thanks