Issue 850 MKII with motor cut-off after 3 km driving

A friendly Hello to the Norton enthusiasts.
My name is Lorenz and as a Norton novice I bought a 850 MKII from 1974 last month.
Unfortunately I have some issues and hopefully I can be supported by your great experiences.
I'm not a native speaker please excuse if I use some uncommon words for explanation of issue.
Setup of my bike.
850 motor.
Single Mikuni Carb (VM32wrong) is it 34mm I guess with main jet 290 (rest so far unknown)
Mikuni has assembled a open velocity stack; no filter
Tri-Spark ignition with standard coil; assembled as written in Manual
Iridium NGK sparks assembled.

General:
Cold Motor Start after first / second Kick runs in idle approx. 1200 rpm.
The starter can released after 30 second - 1 minute
Motor runs approx. 800 rpm idle
After motor is warm acceleration is good without cut-off.
BUT:
After approx. 2/ 3 km in different rev the motor do cut-off by popping and it sounds like misfire.
Motor stops.
No way to restart without using starter at the carb.
After use the start, 1 kick, Motor runs for next 500 m - 1 km. And then again the cutt-off.
Later if I do recognize the popping I push the cold starter at carb and motor runs still, but very bumpy. After I release the starter again motor runs good for next 1 km and so on.
Strange that this problem appears after short time of driving.
Also the sparks are black and full of soot.
It seems the motor runs too rich.
But how can this be due to motor is only able to restart hot motor by richer setting using the cold starter.
Sorry; I don't know how better to describe.
Hope you get me.
I read a lot of the point of setting the carb by different values for jets and needles.
But I assume there is something other wrong beside the carb setting.
Thanks for your attention.

Lorenz

try with fuel tank cap open. Might be airtight
Kev

Thank you for reply.
Will check immediately this evening
Lorenz

Also the sparks are black and full of soot.

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your spark plugs are no good anymore, they are fouled

immediately throw them away and install new NGK BP7ES

1up3down said:

Also the sparks are black and full of soot.

Click to expand...

your spark plugs are no good anymore, they are fouled

immediately throw them away and install new NGK BP7ES

Click to expand...

They are new... drove less 5 km!
But sure I will try the NGK BP7ES. Already ordered. The NGK Eridium was a replacement of Champion NY7CC

ludwig said:

Ditch the Trispark .

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:shock:
NO!
I have throw away the BOYER after the kicker has bitten me 3 or 4 times.
Something I really hate! :twisted:
With the tri-spark the Commando start so easily.

When you say starter I think you mean choke. This is not a choke on a Mikuni but an enrichment device. Your initial running with the enrichment device at 1200 rpm is good and after at 800 is also good. What I suspect is lack of fuel to the carb. Check the flow of fuel through the fuel taps, clean any fuel filters including those fitted to the fuel taps, check the fuel pipes are not coming apart inside and blocking the flow, then remove the plug in the bottom of the float bowl and turn the fuel on see if you get a decent flow. let us presume 50 mpg at 50 MPH (roughly) so you need to flow one gallon (UK figures) in one hour, half a gallon in 30 mins, 1/4 gallon in 15 mins, 1/4 gallon is 20 ounces in UK so 7 ounces in 5 minutes etc.
Let us know what you find

gripper said:

Let us know what you find

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Tonight I will clean the carb and will check needle and jets.
And Yes: I mean choke. Also in German is choke a choke.... :?
Once I have cleaned everything I will check the points including a test drive with open tank cap.
I assume also this is the issue for cut-off ( is this the correct word for the issue?).
But nevertheless: The motor runs to rich. Black sparks look not good!

Thank you all for the kind support! :wink:

Lorenz said:

ludwig said:

Ditch the Trispark .

Click to expand...

:shock:
NO!
I have throw away the BOYER after the kicker has bitten me 3 or 4 times.
Something I really hate! :twisted:
With the tri-spark the Commando start so easily.

Click to expand...

Check for spark as soon as it quits running. Take a wire with good clamps on both ends so that you can easily ground the plug to the engine or frame and make sure it has spark . Sometimes when they fail, they go through a period in which they work when cooled off for a little while.

Russ

Suggested starting point for a 32mm Mikuni is:
25-30 pilot jet
3.5 slide
159 type needle jet size P-2, P-4
6DH2 needle
230 main jet

you say the plugs are new even though they are badly fouled

I have had a single Mikuni for many years, use the choke only for cold starting, if you leave it on it pours a rich mixture and can easily foul the plugs

turn the choke off after around 10 seconds after starting and use the throttle to warm it up enough to take off

Mikuni choke is so rich I would not even leave it on for 10 seconds , maybe 5.

... and I wouldn't recommend going for a test ride with the fuel filler cap open! If fuel is fouling the plugs, it's most unlikely that the filler cap is a problem anyway ...

Lorenz

I have sometimes the same issue, the bike runs for a few Kilometres then it starts to spitfire and bangs and finally it stops
it happens after wet outdoor conditions

Please check the power supply to your Ignition, my case it is the connector in the Headlamp shell, after treating it with WD40 it runs strong again.

With cold engine a can be Ok, after running a few kilometres it starts to behave strange and stops, because of the connections.


Regards Marcel

Marcelx said:

Lorenz

I have sometimes the same issue, the bike runs for a few Kilometres then it starts to spitfire and bangs and finally it stops
it happens after wet outdoor conditions

Please check the power supply to your Ignition, my case it is the connector in the Headlamp shell, after treating it with WD40 it runs strong again.

With cold engine a can be Ok, after running a few kilometres it starts to behave strange and stops, because of the connections.


Regards Marcel

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A better long term solution is to use dielectric grease on all connections to prevent corrosion and water problems.

triumph2 said:

Suggested starting point for a 32mm Mikuni is:
25-30 pilot jet
3.5 slide
159 type needle jet size P-2, P-4
6DH2 needle
230 main jet

Click to expand...

Thank you for suggestion.
It's a 34mm Mikuni
Measured yesterday evening diameter of motor side. I was all the time assuming it is 36mm but it is 34.

I know nothing about Mikuni. Is the choke operated with a cable like the Amals?
If this problem came up with the Amals, I recognize it. I was using the choke backwards, trying to ride with the choke on all the time, fouling plugs, far too rich.

NortonMKIIA850 said:

... and I wouldn't recommend going for a test ride with the fuel filler cap open! If fuel is fouling the plugs, it's most unlikely that the filler cap is a problem anyway ...

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But at my considering and pro and cons I guess a test with a bit lifted fuel filler cap will be the easiest way to check what happened.
I will aslo check the cap itself.

1 I ordered new spark plugs NGK and Champion NY7CC
2 will check the carb to see what is inside; Needle jets and so on
3 will make test drive by open and closed fuel filler cap

Once the motor runs constantly by lifted cap I will do check the set-up of carb.

Target is a smooth running bike, not a racer.
The short rides I have with the Commando was very impressing to me.
Hopefully I can finish my work until weekend. I let you know the outcome

Mr. Rick said:

I know nothing about Mikuni. Is the choke operated with a cable like the Amals?
If this problem came up with the Amals, I recognize it. I was using the choke backwards, trying to ride with the choke on all the time, fouling plugs, far too rich.

Click to expand...

No. My Mikuni have no cable. Chocke is on the carb direct by a lever.
This works fine. If you bring back in normal position it is fixed.
This was also something I checked.

1up3down said:

Also the sparks are black and full of soot.

Click to expand...

your spark plugs are no good anymore, they are fouled

immediately throw them away and install new NGK BP7ES

Click to expand...

Slightly odd advice here. I've often cleaned a fouled plug and used it again. Spark plugs do not suddenly die just because they have a bit of carbon on them. Once they get up to a running temp. the carbon fouling will burn off. The trick is to get the plug clean enough that it will fire. I've used a brass wire brush very successfully. I've also acquired a spark plug sand blaster, again very effective but you do have to take care to remove all traces of the blasting grit.

Iridium plugs in particular, which the original post was about, are expensive enough that one does not just chuck them over one's shoulder.

Many years ago when I was a trainee engineer at a car manufacturer, we did a test and ran a set of plugs for something like 300 000 km or more, just cleaning the plugs and filing the center electrode square again. We had to stop when we could not reach the electrode anymore, but the engine ran perfectly.