Help Troubleshooting 1970 Commando

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QuinlanPhotographic

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I am very new to Nortons, but I have worked on a few Honda's in my day.... I recently purchased a 1970 Norton Commando S which previously ran well. When I first started it this season, I noticed it backfiring, and the idle seemed low (had to keep the throttle turned slightly to keep it running). I rode it a few times, and most recently I took it for a quick 20 minute ride, and when I pulled back in to my driveway the motorcycle stalled, and was very difficult to restart. It would run for a couple of seconds and then stall. Now it won't start at all. When I kick the bike over, it really wants to kick back (it never did this before). I replaced the plugs (although the old ones looked fine), the timing looks to be at 28 degrees TDC, the plugs have good strong spark, the points seem to be opening and closing correctly (I even cleaned them a little), I also tried to put a little fuel down into the cylinder head but the bike will not start. Any ideas out there?
 
It sounds like blocked pilot jet(s) (the nasty inaccessible bush). If you search this forum (and any other which deals with Mk1 Concentrics) you'll find plenty of the same.
 
Check your battery voltage ?
If it has electronic ignition, these can be very sensitive to a low battery - ie if its not charging.

Bikes left standing can get fuel blockages in the idling circuit passages in the Amals, which can prevent it idling. Can take a lot of cleaning out.

Good Luck, let us know how it goes.
 
I should have mentioned that the battery is sitting at about 12.5 volts, which shouldn't be an issue. If it was an issue with the carb and the jets wouldn't the bike start if I put a little fuel in the cylinder head?
 
QuinlanPhotographic said:
I should have mentioned that the battery is sitting at about 12.5 volts, which shouldn't be an issue. If it was an issue with the carb and the jets wouldn't the bike start if I put a little fuel in the cylinder head?
Click to expand...

I don't know. I've never done it but the rest of the symptoms sound typical. A small difference in throttle opening can make a lot of difference on starting and if (partially) blocked jets are included, they become quite unpredictable. It's also one of the things which typically occurs when the bike's laid-up.
 
When you say the timing is set to 28 degrees BTDC - this should be the FULLY ADVANCED setting, at a few thousand revs at least.
If it tries to start like that it could well kick back, bigtime.

Checked your sparkplugs ?

Whenever my bikes behave like that, I know I probably need to dig into the floatbowls,
and tip out any dust or debris or water droplets that I bought in the fuel last time.
 
It may be a good time to remove the fuel tank and empty the contents into a container that can show if any rust, water, or other debris is present.
Clean out and completely dry before adding fresh fuel.
The petcock and fuel line filters can still allow fine particles to pass and foul the Amal idle passage.
 
Check the pilot jets first.The jet size is 0.016".
If you take out the mixture screws,you should be able to push a wire (0.016" diameter or less) at least 1-1/2" into the carb body.The wire inside a plastic bag tie normally measures 0.014",if you can't find anything else suitable.Just melt the plastic off it first.

A number 78 drill (0.016") or 0.4mm drill will clear it out,but you need to extend the drill by gluing it into a WD-40 tube or something.Some people us a guitar string to clear the jet.
 
Okay sounds pretty obvious that everyone is on the same page.... But one quick question. If it was a carb issue (which I'll clean today), wouldn't the bike try to start if I put a little fuel down the spark plug hole into the cylinder? Wouldn't that bypass the carbs all together and confirm the diagnosis?
 
Engines run on a vapourised mixture, not on a liquid. I'd be surprised if neat fuel was volatile enough. That's why those ether-based 'easy-start' compounds exist for engines with problems (this is not a recommendation to use that sort of product).
 
Well, I am absolutely stumped.... here is a quick update:

-Removed carbs, cleaned them (they already looked super clean), noticed primary jet was loose, tightened it.
-Cleaned K&N air filter
-New NGK spark plugs
-Double checked the plugs (definitely getting spark)
-Timing looks correct, 28 degrees TDC
-Points were cleaned and seems to function properly

The bike is still not starting, and it is still kicking back. I was reading more, and some people had problems with the automatic advance unit being stuck.... can someone point me in the direction of where to find instructions to correct this?
 
Timing is 28 deg. before TDC at 3000 rpm. I think you haven't looked at the AAU very closely. It has springs and weights and you need a special washer which can be made that will hold the weights out so it's acting like it's at full advance all the time. That's how you time it, with the AAU fully advanced and the gap needs to be correct. You better get hold of the workshop manual. It's all explained in there better than I could tell you. It's actually in the Rider's Handbook too. http://s251.photobucket.com/user/DogTi/ ... t=6&page=1 Wouldn't surprise me that's the problem, but I've been wrong many times.

Dave
69S
 
QuinlanPhotographic said:
Well, I am absolutely stumped.... here is a quick update:

-Removed carbs, cleaned them (they already looked super clean), noticed primary jet was loose, tightened it.
-Cleaned K&N air filter
-New NGK spark plugs
-Double checked the plugs (definitely getting spark)
-Timing looks correct, 28 degrees TDC
-Points were cleaned and seems to function properly

The bike is still not starting, and it is still kicking back. I was reading more, and some people had problems with the automatic advance unit being stuck.... can someone point me in the direction of where to find instructions to correct this?
Click to expand...

How did you check you timing if it won't start?

Remember, this is a mechanical device and is more hands on rather than hooking up a computer it.
If you can, pull the advance unit off and check the springs, slop in the action, and cam action on the backing plate spindle. Check for any binding or sloppiness and for basically a smooth operation. It almost sound like you are stuck in an excessive retarded mode. Hobot love these things and I believe jeandr also. You might PM both of them to see what they think.
 
Its not likely to kick back on starting if its stuck on retarded. ?
Kicking back is more likely to be too advanced when trying to start it.

But it will usually start, you just have to be wary of giving it a half-baked kick.
 
do you still have last years gas in the tank?
unleaded gas especially ethanol blends have a short shelf life .
what is your starting procedure ?
 
New Condensers. Pull and grease AAU.Check for wear in bobweight guide slots (grease or renew), Retime.Clean electrical contacts. You have cleaned out pilot jet and fresh gas.
 
My '71 auto advance unit actually separated and started to kick back on a kick start. Back in the '70's that seemed to be common with the 750's. Replaced with a later 850 unit and that worked until I recently tore it down. Do your plugs look wet when you pull them to indicate that fuel is making it's way into the cylinders? If you're pushing down on the float ticklers until gas runs out, you should be getting enough fuel into the cylinders. I still suspect carbs, sometimes you have to have wide open throttle to start with idle circuit clogged passageways. I never got a good clean up on Amals until I employed a short length of a G string guitar to poke out debris in the idle mixture port after removing the screw.
 
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