John's 72 Commando woes

Hi all,

I’m John, a long time lurker and new member. I’ve always wanted a Commando since I was a child as my grandfather used to race them on the isle of man and I have vague memories of sitting on those old Gus Kuhn Nortons.

Late last year the stars aligned and I finally pulled the trigger on a 1972 Combat Commando.

This leads me to my reason for my first post.. I am 33 years old and this is my first classic bike so please excuse my ignorance as I am attempting to slowly ascend a very steep learning curve LOL!

When I got the motorcycle it ran great, started first kick and had no issues at all. Since then it has developed an issue where under load, the bike will bog. It starts fine, pulls away no problem but when I try to accelerate it will bog until I let off.

So far, I have checked what I know, which isn’t much..

I checked the plugs, right side plug is consistently black left side is good. This led me to change the air filter, and lean the mixture slightly with no change.

Checked the battery and it was a little low, not sure if this is because I have had the bike idling more than moving before checking it (will check this again after charging).

The only other thing that I can think to add is that after this bogging occurs, sometimes but not all the time the bike will not start at all. If I leave it for approximately an hour it will start again (heat related?)

Anyway, again sorry for my ignorance! If this sounds familiar or anyone could point me in the right direction it would be greatly appreciated.

Many thanks!

John.

The orbital 2 stroke think old Detroit diesel on gasoline. Nothing new here.

John Rollins said:

my grandfather used to race them on the isle of man and I have vague memories of sitting on those old Gus Kuhn Nortons.

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That's pretty cool. Any pics?

As for the bike, sounds like a carb problem. Assume you have stock Amals. I run Mikunis so really can't help but you need to learn the full spectrum of tuning the carbs. It's not simply turning one screw (there's various jets and needle valves that affect different throttle positions). Once you get them dialed in, you'll be happy

Edit: The carbs should be balanced (i.e. set up the same). If they are and you're still rich/lean on one side you have other problems (e.g. gasket leak, overflowing float bowl...)

WTF?

Anyway, get that battery tested at a battery store. Replace it. Batteries aren't like a gas tank. Just because it gets pumped up
with a Battery Tender to a correct specified level doesn't mean it is fully charged and good to go. A load test is required.

Choke left on?

TomU said:

That's pretty cool. Any pics?

As for the bike, sounds like a carb problem. Assume you have stock Amals. I run Mikunis so really can't help but you need to learn the full spectrum of tuning the carbs. It's not simply turning one screw (there's various jets and needle valves that affect different throttle positions). Once you get them dialed in, you'll be happy

Edit: The carbs should be balanced (i.e. set up the same). If they are and you're still rich/lean on one side you have other problems (e.g. gasket leak, overflowing float bowl...)

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Hi TomU, his name was Charlie Sanby there are many pics from Isle of Man and other races out there. Wasn't old enough to ever see him race unfortunately. Yes I believe they are stock, Amal 932. I have a theoretical understanding of the Amal's but have never pulled them apart. I'll have a read up and watch some tutorials and see how we go. I just assumed that because they were fine before that they should still be mostly there other than some fine adjustment? Thank you for your input, it's appreciated.

lazyeye6 said:

WTF?

Anyway, get that battery tested at a battery store. Replace it. Batteries aren't like a gas tank. Just because it gets pumped up
with a Battery Tender to a correct specified level doesn't mean it is fully charged and good to go. A load test is required.

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Hi lazyete6, why WTF? I've been testing the battery with a tester and after it had been at idle it was reading 11.4V. This was after the bike had been at idle for multiple hours over a few days. So my question mark re the battery at the moment is.. has the battery been discharging whilst the bike has been at idle over that time or is there an issue with the battery (new battery). I cant ride the bike for any length of time so I can't check to see if the regulator/stator are good. Thank you for your input, it is appreciated.

batrider said:

Choke left on?

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Choke is off. Thank you for your input, it is appreciated.

You might check to be sure fuel tank vent is not clogged.

What type of ignition do you have? points? Boyer? other?

NickZ said:

You might check to be sure fuel tank vent is not clogged.

What type of ignition do you have? points? Boyer? other?

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I will check that for sure, thank you. I have the Wassell electronic ignition. Thanks for your input, it is appreciated.

Likely to be a sticking float sooting the plug up. Best to scrap the plug now. could also be a failing spark .dud plug cap etc. Join the NOC.

norton bob said:

Likely to be a sticking float sooting the plug up. Best to scrap the plug now. could also be a failing spark .dud plug cap etc. Join the NOC.

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Thank you, I will take the carbs apart and check it out. If the bike is good starting and holds a good idle can I rule out spark issues? And if not, from the experience forum members have, where is likely to be the chink in the chain? Leads, ignition coil etc? Why Norton Owners Club? This forum seems filled with knowledgable people and is very active, is there something I'm missing? Thank you for your input, its appreciated.

if carb related could be any number of issues, worn needle jet, incorrect needle position, incorrect slide etc..

Familiarize yourself with this: https://static1.squarespace.com/sta...c06d52/1481956877861/Amal+MK2+Carb+Manual.pdf

Double check the ignition timing as well.

Mark Savage said:

Could be lot's of things but I did have a similar problem when using NGK spark plugs which went away with new Champion ones.

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Hi Mark, yes I have tried the NGK but it didn't run quite the same so switched back to the Champion N7YC plugs also. Thanks for your input, it's appreciated.

acadian said:

if carb related could be any number of issues, worn needle jet, incorrect needle position, incorrect slide etc..

Familiarize yourself with this: https://static1.squarespace.com/sta...c06d52/1481956877861/Amal+MK2+Carb+Manual.pdf

Double check the ignition timing as well.

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Hi acadian, that is an invaluable guide, thank you very much. I will look into ignition timing also. Many thanks, that's very helpful.

Try swapping the spark plug leads and see if the problem changes to the other cylinder. I had a similar problem on another bike and it ended up the spark plug lead on the problem cylinder was faulty.
David

Welcome to the world of used Nortons.

Ran great for how long, and what did you try to fix that was not broken before it stopped running great?

Check the electronics stem to stern. If the battery was not new, get a new one to rule the battery out completely. Test everything.

Could have a pin hole in the right carburetor float, or a bad float needle. You find out if either of those are true when you take the carburetors apart.

If the ignition checks out and it is not a coil or plug wire, might be a blown head gasket on the right side. That is a little negative, but the symptoms are similar to dropping ignition on one cylinder. The guessing possibilities are endless. Consider it an opportunity for discovery.

You'll figure it out if it ran great when you got it. You have plenty of time to figure it out given your current age.

Welcome John. You have Norton in your blood, so no wonder you've been hankering after one. Your grandad was a regular name on race programs in the 60's 70's and 80's, and I remember it well. According to this article on GusKhun.net he narrowly missed out being in the JPN team after he and Peter Williams won the Thruxton 500 mile endurance race in 1970 on a Commando.


This is a great picture of him jumping Ballaugh Bridge on the IOM, this time Guzzi mounted..





A thought about your bike issue. Contrary to most other carbs the choke will be off when the when the choke lever is all the way counter clockwise ( i.e. cable under tension) so it may be that the choke cable for the right carb has snapped and allowed the choke plunger to drop.

Also check that the plug lead is fully pushed into the coil. I found mine was not, and was being held in place by the boot so the spark was having to jump 1/2" before it ever got to the plug.

Good suggestions.

I had several teething problems with mine, after I first bought it. They prefer to be ridden and don't like standing, waiting for a new owner! I went through a range of emotions in the first few months and came through it with a much improved understanding of how the bits I'd touched work (and fail) and improving confidence I'd be able to keep it on the road long term. Especially with the generous advice from forum members. Then I got the opportunity to ride it and these bikes are a joy to ride and still hold their own, 50 years on.

The byproduct of the initial torture is a healthy affection for my Commando and a sense of pride and achievement every ride. Priceless. Stick with it, it's worth it.

Check the easy stuff first.
I’ve had this happen on a few old bikes.
If the bike has inline fuel filters they can become full of gunk and restrict the fuel flow hence it bogs down under load. Same with the fuel tank petcocks, over time they gunk up and will restrict the fuel flow.
The first time I spent months checking carbs electrics etc before checking the fuel source and then it was wtf.
Good luck with the bike.