sooted sparks with MIKUNI VM34 and 260 Main Jet

Lorenz · May 2, 2018

Hello to everybody.
Here I am back after long cold winter to restart my Commando 850 MK2. But I have issues by sooted sparks.
May I gife a short overview:
Single VM34 / Slide 3 / Needle 6DH3 /Main Jet 260 / Air Jet 2.0 / Idle Jet 35 / Needle Jet 159 P-5, Issue: nevertheless which recommended Sparks I do use; they are sooted.
Means not oily, they are sooted. I sett the needle in leaner postion, bit this have no influence to the sooted sparks. No change. Avfriend of mine who drives behind me told me that on left side the exhaust makes a huge amount of smoke, but not blue, means typical oil burning.
Could have the Air Jet such influence to the sooting? Everybody writes 1.0 but my one is 2.0 Or does anybody see somthing in set up?
Motor is not tuned, Carb have no Filter, so far, but I will assemble one. Sparkes are from Champion and NGK even Iridium Plugs sooted.
Have anybody an idea where I can start my investigation?
Start so far good, when hot need two / three kicks, but by the end she starts. Tri Spark ignition is also used.
I look forward to hear your friendly comments. Thank you in advance and have a good time with yout bikes.
Regards from Germany Lorenz

Steves · May 2, 2018

I would check the ignition. Seriously sometimes you have to think laterally! As they say 90% of fuel problems are electric and 90% of electric problems are fuel.

Fullauto · May 2, 2018

I would try a smaller pilot jet. A 32.5 or a 30. I am currently running a 27.5. Always start at the low speed settings and move up as they affect the fuelling at greater throttle openings. The Mikuni kits always come jetted rich, usually excessively so, for safety reasons. The air jet has nothing to do with low speed running. It is for fine tuning the main jet, which seems a bit rich. I run a 1.0 air jet, which allows fine tuning either way. The 2.0 is the largest air jet, so you can't go leaner. Maybe try a 240 or 230 main jet, as that seems a bit rich and would account for the smoke as you accelerate. But remember, low speed settings first. I fitted an O2 sensor and air/fuel guage, which continually shows me that I knew nothing about tuning carburettors! I now realise that I know a little. It is amazing how the bike will run quite well over a wide range of jetting on all circuits but when you finally get it dialled in, you will realise what good running really is.

concours · May 2, 2018

Single Mikuni, If only one side smokes, continue to search for oil related problem. Not ALL oil smoke burns blue.
If both sides were affected, an inspection of the Mikuni's enrichener plunger seal would be the first check.

RoadScholar · May 2, 2018

Your main jet and needle jet are way too big, the air correction, the needle and the slide are fine. A 230, possible a 240 main jet are more like it; a 159 P-0, or possibly a 159 O-8, will solve the sooty plug problems, assuming you don't have other gremlins lurking...

If you have any trouble sourcing the correct parts, I sell them.

NorComCycles · May 2, 2018

Lorenz said:
Single VM34 / Slide 3 /

# 3 slide sounds fine.

Lorenz said:
/ Needle 6DH3

That is exactly the needle I would recommend/use.

Lorenz said:
/Main Jet 260

In my view, especially for a stock bike, too big. I do not think this is really part of your issue. nut nonetheless, either 230 or 240 would almost certainly be better.

Lorenz said:
Air Jet 2.0 /

Should work fine. This only has an effect on the main jet at larger throttle openings, and, as a result, when the engine is turning a little faster.

Lorenz said:
/ Idle Jet 35

30 is more likely to be quite adequate, possibly 32.5 at the most(but less likely).

Lorenz said:
Needle Jet 159 P-5,

Way to big. By a country mile. Use either O-8, or P-0. If using O-8, needle likely to be in middle groove. If using P-0, needle likely to be 2nd from top (as in, one groove leaner).

Lorenz said:
Issue: nevertheless which recommended Sparks I do use; they are sooted.

Of course. The bike is running way too rich.

Lorenz said:
Have anybody an idea where I can start my investigation?

Yes, you need to recalibrate your Mikuni.

Lorenz said:
Tri Spark ignition is also used.

In my experience, Tri-Spark does not cope well with a rich running environment.

Lorenz · May 3, 2018

Thanks to all of you.
I will follow the advise of NorComCycles.
@ concours --> Single Mikuni, If only one side smokes, continue to search for oil related problem. Not ALL oil smoke burns blue.
OK, but the coulor of the sparks are 100% similar. I guess if one side the motor burns oil the colour of Spark shoulkd be different?

First I will come leaner by the advice of NorComCycles.

Steves · May 3, 2018

If you have a multimeter with a temp gauge , check cylinders are running at the same temp, also check impedance of coils. I know nothing about Mikunis, but you need to be sure ignition is good. As was pointed out to me on here a visual check of the spark while it can be useful is not a guarantee. I took that thought and as a consequence solved a problem that I had.

oldmikew · May 5, 2018

I would clean up the Champion plugs and put them back until the problem is resolved. Once NGK are fowled thats it.

1up3down · May 5, 2018

other than too rich main jet which is not responsible for your start up one side smoking, so doubtful the single carb is the problem

time for a compression and leak down test, somehow oil is getting into that smoking side's combustion chamber and being burned
either a ring sealing problem on that side or maybe a valve guide leaking oil on that side, but not the carb's problem as only one side smokes

seattle##gs · May 5, 2018

While you are trying to solve this problem use the much cheaper NGK BP7ES spark plugs instead of the iridium plugs. Save youself some money. Now, buy at least 4 PAIRS of spark plugs OR MORE and change often as needed.

seattle##gs · May 5, 2018

While you are at it, clean up ALL your grounds, make them good and shiny and the best connections you can possibly provide. For a quick test, you can string two wires directly to the battery from the Tri-Spark box and by-pass the harness all together.
I agree with fullauto...these motors can run well on a wide variety of carburetion but it is the optimum we are all seeking. I have run Norton motors on a 25 pilot jet and others up to 45 pilot jet so my guess is that they all have their individual quirks, you have to find them for yourself. In a perfect world with unlimited time and money you would have several dyno runs and many hours of test riding to find the absolute best carburetion for your particular norton. Then there is season and temperature changes to throw you off. That last 5% is very tough to get.
The above settings recommended by Norcom should be very good and get you at least 95% there. I like single 34 VM carburetors quite a bit.

Lorenz · May 6, 2018

Hi there.
Today I have reworked the carb.
My setup is now:
230 / 30 / 159 O-8
Bike runs really nice. But one question: For restart or start my bike cold, even hot I need the choke!
Means: Bike is hot, kicks over kicks, nothing, chocke, one kick... bähm, running!
Is that a normal behavior?
@ concours --> Single Mikuni, If only one side smokes, continue to search for oil related problem. Not ALL oil smoke burns blue.
It seems I have an issue on one side. Right side spark is very nice, brown, and clean. Other side is a bit darker but not clean as on the right side. There is oil consumption on left Zylinder. Now I have to check by Valve or Cylinder. Next time I will do a compression test.
By the way: The bike runs very good with NGK; but pops and didn't run well with Iridium.

NorComCycles · May 7, 2018

Lorenz said:
even hot I need the choke!

I would start with the idle air screw only 3/4 to 1 turn out, and adjust it for best off idle response. This may improve hot starting if you have the screw further out currently.

RoadScholar · May 7, 2018

Lorenz said:
Hi there.
Today I have reworked the carb.
My setup is now:
230 / 30 / 159 O-8
Bike runs really nice. But one question: For restart or start my bike cold, even hot I need the choke!
Means: Bike is hot, kicks over kicks, nothing, chocke, one kick... bähm, running!
Is that a normal behavior?

Your idle (pilot) circuit is still too lean. A 35 or 37.5, adjust the air screw until the kick-over is a tad rich (somewhere between 1.25 and 1.5 turns out from a gentle seat---maybe less, but likely no more than 1.5 turns). A plug check is iffy at best, kind like an ass-dyno, but to extract useful data you need to be under load, you need to keep the twist grip steady in the circuit you are testing, and you need to de-clutch and shut down QUICKLY to get anything meaningful from the exercise.

Lorenz · Jun 11, 2018

Hello. I follow your advises. Now I have a good set up for me.
I use:
Single VM34 / Slide 3 / Needle 6DH3 middel position /Main Jet 230 / Air Jet 2.0 / Idle Jet 32.5 / Needle Jet 159 P-5
That all togehter with the small air-cleaner box. I assmebled the box due to optical reason. I like that hamster cage more then the K&N Aircleaner
I discover the 159-O8 was notz a good choice for my bike. The sparks are a bit too white.
Now with that setup I have a brown colror on left side, and a dark brown color right side.
After I found a good setup and do my first longer ride, I discover the oil consumption is very huge! Roughly 200mL on 50 km!
Nobody drives behind me due to extremly smell of oil.

So it seems I be faced with the next huge issue.