'75 MKlll Won't idle

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chasesa

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Looking for ideas, please.

My '75 Commando Mklll starts great, but won't idle for long. It will idle for a while, over and over, but not long, including when warmed up.

As it idles, it will gradually slow down, headlight will flicker as it does, then die.

Carbs and cables are clean and set properly. Timing is on. I think it has to be electrical issue (what's left?). I do have a Boyer installed.

Right now, I have run out of ideas, except to clean all the terminals and grounds. Gonna try petcock filters too.

Thanks!
Stephen
 
Lights dying indicated electrical issue for sure a wiring problem or short as the battery should be running the lights not the motor
Nothing else but as you say trace all the wiring grounds etc unless its your Alternator the fix should be fairly simple
A voltmeter is handy if you have one otherwise a process of elimination or an auto electrician

Good luck
 
chasesa said:
headlight will flicker as it does
Click to expand...

It wouldn't happen to have a Canadian model 06-6392 assimilator, by any chance?
 
L.A.B. said:
chasesa said:
headlight will flicker as it does
Click to expand...

It wouldn't happen to have a Canadian model 06-6392 assimilator, by any chance?
Click to expand...

Possibly, L.A.B., as it's likely a Canadian version. How do I locate that part on the bike? What is the suggested fix?
Stephen
 
chasesa said:
L.A.B. said:
chasesa said:
headlight will flicker as it does
Click to expand...

It wouldn't happen to have a Canadian model 06-6392 assimilator, by any chance?
Click to expand...

Possibly, L.A.B., as it's likely a Canadian version. How do I locate that part on the bike? What is the suggested fix?
Stephen
Click to expand...

The MkIII assimilator looks like this.....
'75 MKlll Won't idle

....and it is normally bolted to the top of the battery tray, underneath the black plastic airbox (if fitted).

The Canadian 06-3932 assimilator has an automatic 'lights on' function that switches on the lights immediately the engine has started if the key is only turned to the 'ignition only' switch position.
According to the factory manual, with the engine running at that switch position, the lights: "....are receiving direct alternator current only ..." so the lights are being powered directly from the alternator (presumably running AC current as the 'direct' output doesn't pass through the rectifier?) however, I have no idea if the apparent increase in power drain on alternator output at idle speed could somehow be affecting the operation of the Boyer ignition or not?
 
How do the plugs look. Just with normal running and not with a throttle chop routine? And how do they look after the engine idles until it quits?
 
Following-up on past advice from the Forum

Still cannot get the 850 running properly. I replaced the assimilator and battery, but no difference.

It starts fine, but won't stay running. It has the Boyer, so do zener diodes stilll represent a factor?
Cheers!
Stephen
 
Before you replace any more components blind and waste more money, WHAT is the electrical system doing?

You will need an accurate multimeter such as a Fluke or an old fashion AVO meter.

First check that ALL ground connections are good - normally headlamp shell, frame and cylinder head. Clean and clear of corrosion etc. Using the lowest resistance scale on your multimeter check values from the battery pos terminal (Pos Earth systems) and the frame, cylinder head and headlamp shell to verify; this should be virtually zero - check lead to lead value as this will add to the total measured value.

Take some voltage measurements. For example, connect between the battery pos and neg terminals and measure the terminal voltage:

1. Ignition switch OFF
2. Ignition switch ON engine stopped, lights OFF
3. Ignition switch ON engine running @ 1000 RPM, lights OFF
4. Ignition switch ON engine running @ 1000 RPM, lights ON
5. Ignition switch ON engine running @ 2000 RPM, lights OFF
6. Ignition switch ON engine running @ 2000 RPM, lights ON
7. Ignition switch ON engine running @ 3000 RPM, lights OFF
8. Ignition switch ON engine running @ 3000 RPM, lights ON
9. Ignition switch ON engine running @ 4000 RPM, lights OFF
10. Ignition switch ON engine running @ 4000 RPM, lights ON

Dependant on the above results then the Charging System is OK/Not OK.

Report back and I can suggest some more tests.

Regards,

John S
 
Update:
Checked my charging system and it's generating 12.5 to 14 volts across rpm speed.

Carbs settings are on spec. So is timing. Bike will now idle but, LH cyllinder is not running as well as RH.

Now need to check coils and Boyer.

Need ADVICE: could Boyer partially fail? ie. Could Boyer enable one cylinder to run OK and not the other? Or, is a bad coil a more likely suspect??
Cheers!
Stephen
 
chasesa said:
Update:
Checked my charging system and it's generating 12.5 to 14 volts across rpm speed.

Carbs settings are on spec. So is timing. Bike will now idle but, LH cyllinder is not running as well as RH.

Now need to check coils and Boyer.

Need ADVICE: could Boyer partially fail? ie. Could Boyer enable one cylinder to run OK and not the other? Or, is a bad coil a more likely suspect??
Cheers!
Stephen
Click to expand...
I'd mention worn out Amal's slides leaking air, imprecise idle. The flickering headlight (based on L.A.B's A/C explaination) is totally normal as an engine (equipped with A/C lighting) idles well below intended speed and just before stalling. Reference many off-road bikes/sleds/ATV's.
 
Re Boyer.

The Boyer can't partially fail as you suspect as coils are wired in series, therefore it either works or not at all. Swap the the HT leads over so RH coil fires LH cylinder and LH coil fires RH cylinder and see what happens (wasted spark system so will work this way). If the symptoms swap over then one of the coils is likely to be defective - occaisionally the secondary windings (hundreds of turns of fine wire) coil start to fail which reduces the HT output - this assumes that the leads, plug caps and plugs are good. Good replacement if you can get them NOS are the Lucas (red or blue coils - can't remember which colour are the 6 volt versions) or use TriSpark coils.

Regards,

John S
 
I posted this query in June thinking I had an electrical issue.

Someone wisely said that may "electrical" issues are really carb issues, which proved true in my case.

The tiny idle openings in the base of the Amal venturi were plugged, at least partially. A careful cleaning fixed the issue.

That said, I did switch temporarily to a known set of Amals to find out the truth.

Thanks to all who tried to help!! I'm now ready to put the bike away without having had much wear & tear in 2014.
Cheers!
Stephen C
 
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