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

Classic Norton Commando Motorcycles.

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

Postby triumph2 » Mon Sep 11, 2017 4:45 pm

Marcelx wrote: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

A better long term solution is to use dielectric grease on all connections to prevent corrosion and water problems.
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby Lorenz » Mon Sep 11, 2017 11:54 pm

triumph2 wrote: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


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

Postby Mr. Rick » Tue Sep 12, 2017 12:09 am

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

Postby Lorenz » Tue Sep 12, 2017 12:09 am

NortonMKIIA850 wrote:... 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 ...

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

Postby Lorenz » Tue Sep 12, 2017 12:14 am

Mr. Rick wrote: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.


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

Postby SteveBorland » Tue Sep 12, 2017 12:54 am

1up3down wrote:
Also the sparks are black and full of soot.


your spark plugs are no good anymore, they are fouled

immediately throw them away and install new NGK BP7ES


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.

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

Postby NortonMKIIA850 » Tue Sep 12, 2017 2:24 am

SteveBorland wrote: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.


I wish there were a 'Like' button for things like this, thanks for sharing!
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby gripper » Tue Sep 12, 2017 3:22 am

Is it soot or burnt oil?

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

Postby robs ss » Tue Sep 12, 2017 3:53 am

Although not directly related to this original post I thought I might add this...
On my Norton 500 single 11.3:1 CR ( originally model 50 but now with BMW R80 barrel & piston...)
It used to kill plugs frequently (every month) for some reason (NGK B8SE) to the point I always kept a spare with me - they looked good but wouldn't spark.
I contacted NGK and sent them a "failed" plug but never heard from them again.
I changed to NGK Iridium B8EIS and have not had a failure since (4 years) - go figure!
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby Lorenz » Tue Sep 12, 2017 5:10 am

gripper wrote:Is it soot or burnt oil?


I guess soot. Plugs are not covered with oily film. Also no blue dust from exhaust. A body was checking by driving behind me.
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby triumph2 » Tue Sep 12, 2017 7:23 am

Lorenz wrote:
triumph2 wrote: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


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.

In your iriginal post you stated VM32 carb. For a 34mm it should be:
30-35 pilot
3.0 slide
P4-P6 needle jet
6DH3,4 needle
260 MAIN
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby triumph2 » Tue Sep 12, 2017 7:23 am

Lorenz wrote:
triumph2 wrote: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


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.

In your iriginal post you stated VM32 carb. For a 34mm it should be:
30-35 pilot
3.0 slide
P4-P6 needle jet
6DH3,4 needle
260 MAIN
'73 Norton Commando 850
'69 Norton Mercury 650
'65 Norton Atlas 750
'63 Matchless G80CS 500
'05 Triumph Rocket III
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby Biscuit » Tue Sep 12, 2017 9:00 am

I think you may have two things happening or may be not. First off I think it's smart to do just one thing at a time. Open the fuel tap(s) and wait a bit. Take the bowl plug out. The bowl will drain quickly and then slow down but the flow should not diminish to just a trickle or drips. If it does this would tell you the flow is restricted either by an inline filter or the fuel tap screen (I am not sure if a Mikuni has it's own screen as an Amal does). This would mean you are simply running the engine out of gas. The time it takes to coast to the side and re-start allows some fuel back in and the choke (enrichener) picks it up from the bottom of the bowl so the engine starts and runs for a short while again. Repeating this a number of times means the engine is spending a lot of time "on the choke" and this could be the sole cause of the dark plugs. I would not go into the carburetor in an attempt to get the plugs looking "right" until the engine can be dependably run. Think about this: If richness and plug fouling was the cause of the engine quitting, it would not re-start with more choke.

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

Postby frankdamp » Tue Sep 12, 2017 9:11 am

It sounds to me as though the fuel tank is not venting quickly enough and the resulting low pressure in the tank is impeding the flow of the gas to the carburator. I think the vent is part of the filler cap. I remember having a similar issue with my old BSA A7.
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby laurentdom » Tue Sep 12, 2017 9:40 am

Fully agreing with Biscuit in saying that you should first check that fuel is correctly flowing from tank to carbs.

If not :
- No air entering the tank (reason why some of us advise you to do a test with your fuel tap open), creating a sort of air lock.
- Obstruction of the flow between tank and carbs: check (and clean if needed) every component (including the filter at the bottom of the carbs, and the jets).

or a combination of those both reasons.

The reason why your sparkplugs look too rich may come from that you have to use and re-use the enricher.

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