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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 laurentdom » Tue Sep 12, 2017 9:44 am

I meant "a test with your TANK CAP open".

Sorry for my mistake.

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1960 De Luxe Featherbed chassis + 1972 750 Commando engine
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby Biscuit » Tue Sep 12, 2017 10:18 am

laurentdom wrote: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.

.


True enough, if the carb drain fuel flow test does show a marked slow-down in flow, and opening the fill restores good flow, you will have your answer.

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

Postby MFB » Tue Sep 12, 2017 1:25 pm

Biscuit wrote: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 Lorenz » Tue Sep 12, 2017 11:42 pm

triumph2 wrote:
Lorenz wrote:
triumph2 wrote: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

Yes very sorry. Was a mistake. I measured the carb. It is 34 mm
Yesterday I check every needle and jet by a watchmaker glass:
35 pilot
6DH3 Needle
P5 Needle Jet
3 Slide
2 Air Jet
290 MAIN

Could be the size 2 air jet and Main 290 reason for the soot at the spark? :?
Ah by the way: What influence have the clip at the needle :?:
It was at position 3 from top.

And also I checked yesterday the aeration / vent hole of fuel cap .
Roughly 1 mm max 1.5mm.
I will drill a bigger vent hole into the fuel cap, something like 2 mm
Today I will reassemble everything. Weather in Germany is a disaster and it's dark outside when I'm ready with work.
So hopefully on weekend I can check.
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby NortonMKIIA850 » Wed Sep 13, 2017 2:46 am

Lorenz wrote:Weather in Germany is a disaster and it's dark outside when I'm ready with work.

Oh how I know that feeling! It's not been great here in the UK either, you have my sympathy ...
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby MFB » Wed Sep 13, 2017 1:28 pm

Lorenz wrote:

Could be the size 2 air jet and Main 290 reason for the soot at the spark? :?
Ah by the way: What influence have the clip at the needle :?:
It was at position 3 from top.



I can't answer those questions, especially as I am not familiar with Mikuni carbs.

But I will say this: always keep in mind that an engine NEVER requires a rich mixture in the combustion chamber, no matter how cold it is.
The choke/enrichener is used to compensate for condensational losses in the manifold and port when the engine is cold, so that the mixture is correct by the time it reaches the chamber.

By using the enrichener to re-start your engine when hot, you will be ensuring that the spark plugs will be sooty.
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Re: Issue 850 MKII with motor cut-off after 3 km driving

Postby Biscuit » Wed Sep 13, 2017 2:13 pm

MFB wrote:

But I will say this: always keep in mind that an engine NEVER requires a rich mixture in the combustion chamber, no matter how cold it is.
The choke/enrichener is used to compensate for condensational losses in the manifold and port when the engine is cold, so that the mixture is correct by the time it reaches the chamber.

By using the enrichener to re-start your engine when hot, you will be ensuring that the spark plugs will be sooty.


Wow! Absolutely true. Without getting into specific A/F ratios for various conditions, let's just say the "rich" mixture required for a cold start is really the CORRECT mixture for that cold start. Not too rich, not too lean Goldilocks.

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

Postby Lorenz » Sun Sep 17, 2017 12:14 am

Hi All.
First off all thank you for the input I got from you.
On Saturday I have done several tests.
Outcome
The gas flow out of the tank had an issue.
If I come under 5 liters gas in my tank the gas flow is not ok.
Over 5 Liter good flow.
I checked this by run out the gas and chewing if there is an issue by depression
Guess have to check the gas cock.

By reassemble the carb I set the needle leaner by moving the clip
Plugs are black still.
I ordered now some jets to make a new setup
By the way
I’m really happy with my tri-spark. No kick back, no fear!
Starts very good, cold even hot.
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