73 850 Commando Misfiring badly

You are taking the choke off after start up?

(cable under tension).

Start the bike and while it is ticking over steadily-warmed up. Wiggle the pick up wires at the pickup, any issues the bike will stop instantly. When trying to drive, switch the lights on-any change? disconnect the rect/reg or rectifier output, (ride bike on battery) any change? Disconnect the H/Bar kill switch (under the tank)........continue with carbs......report back.

Hello Eddie, I would very carefully check the wires in the Boyer stator (in the points housing) - it's not uncommon for them to break-down inside the insulation or where they are soldered to the stator (on the rear). Before you remove the stator to check them, mark the stator position (felt-tip pen or similar) such that you retain the (presumably correct) ignition timing. These wire faults are well known for causing mis-firing etc.

I had a Boyer fitted Commando that would only run badly with one cylinder cutting in and out, changed Boyer boxes, rotors. coils HT leads etc none of which worked. What did work was running a dedicated earth wire from head to battery positive terminal. Also Boyers can suffer from the feed voltage being too low which can be caused by an iffy kill switch on the handlebars and also a bad earth, so I run the Boyer earth to the same bolt on the head the dedicated engine earth from head to battery.

Eddie

Grant Tiller is your man for wiring diagrams...

https://granttiller.com/norton-commando-wiring-diagrams

Andy

The very common 'sticky tape' syndrome. The lump that has the WHT/PPL wire is very suspect as this is ignition!! Sticky tape is to be avoided at all costs, it eventually comes unstuck and just hides a hoast of ailments/bodges.

The RED crimps used on the other side are to be avoided, they look horrible and unless fitted with a decent ratchet crimp tool are subject to falling off, but the coils look well fitted. Although the insulation is not needed on new coils. If such insulation is needed then the coils need to be replaced as they are shorting to earth inside.

The item above about the pickup is true but I doubt a problem in this case as the bike starts fine. Taking the cover off  and wiggling the wires will soon show up a problem if there is one. The idea about the Earth from cylinder head back to frame/battery is worth examining.

I had a similar fault, blamed the Trispark ignition, changed it to Pazon, no change eventually found it was the ignition switch where the push on connection had come loose, causing a intermittent connection.

I do like these threads but they are hard to answer with limited understanding of the symptoms. When does the misfiring and popping start, all the time or after a bit of a run? Does it misfire at all revs?

Could be fuel starvation, low float heights, blocked fuel tap filters or tank cap breather blocked.

Have you tried replacing the spark plugs? Check HT lead connections for corrosion at the contact points.

Also worth checking contacts at the kill switch. Bypass the switch to see if it makes any difference.

I would also run an extra earth from the cylinder head steady to one of the nuts on the coil mount bracket.

On a separate note, I have recently fitted an oil pressure switch to mine and have been monitoring oil pressures. Oil pressure can be surprisingly low at tick over when the oil is hot. In my opinion, 500 rpm is too low and an idle nearer 1000 is better for the engine.

Please keep us up to date with your findings!

Steve

Read somewhere that the 2MC cap has to be mounted vertically. Used to think it was to keep water out, but  apparently not so.

I have had an old harness suddenly develop internal breaks inside the wrapping, impossible to find and even if fixed when is the next break going to happen. If you are going to unwrap and remove the redundant Interpol wires and those made redundant by the Boyer I would either start with a new harness or buy a DIY harness kit off Alan Osborn and make a bespoke one. 

http://www.aoservices.co.uk/info/WIRING_KIT.pdf

But, have you made sure that the carbs don't have any water in the floatbowls, blocked passages etc, etc? - I do know that my Morini is a bugger for low speed misfires if any water gets in the floatbowls. Blocked passages or jets might make it run too weak 'just off idle' and cause the misfires too.

Remember the old phrase, if it seems like an ignition problem it is the carbs, and if it seems like a fuel problem, it is the ignition!

Looking at the pictures you have posted however, I agree it does look like an electrical issue you have,

As regards a new loom,  I find it quite therapeutic doing the wiring. Just make sure that you plan for any 'upgrades', like modern voltage regulator, front brake light switch (if not fitted), better fusebox with each circuit having its own fuse, 'better' switchgear etc before you start. otherwise you will tearing it apart soon after to put the extra wires in/blank wires off.

Al O's wiring kits get a good write up from members as being good quality and value. A decent pair of crimping pliers need not cost a fortune either. If it were mine, I would do all the wiring in one go as that will eliminate any questions about its integrity.

Keep going - You'll get there.

George.

Reading through this post I kept saying to myself, stop messing with this old wiring-rewire-my kit. Then all you chaps sell it for me!! Seriously the effort to strip off all the old tape and uncover the wires and attempt to reconnect every thing is just not worth it. In fact take note all, old wiring on British bikes (and cars) when you came to reconnect you find the copper under the insulation is going black-BLACK DEATH it is called. You will not make any decent connections to this tarnishing. IF you are stuck on the road with this problem and trying to remake a connection then scraping the wire with your trusty Swiss Army should do the trick. But please consider rewiring properly as soon as you can.

The idea of a small relay for the engine kill? Yes it would work but you are introducing another load of connections and contacts with the relay, something else to fail in the ignition circuit. Why do you need an ignition kill anyway?

From George-'Remember the old phrase, if it seems like an ignition problem it is the carbs, and if it seems like a fuel problem, it is the ignition! ' Exactly, after 55 yrs of fixing engines etc I still have to remind myself.

It sounds like the spark plugs were the main culprit and if it runs OK now I would say get some miles under your belt so you can enjoy the bike and discover if anything else is not quite right THEN RENEW THE WIRING!

There are various sources for new looms (all have unneccessary wires) but I'm just in the middle of re-wiring mine from scratch using AO Services superb kit and a huge amount of sensible advice from Grant Tiller (check his website). It's not as scary as you might think but you do need time and some decent crimp tools. Personally, I've found Vehicle Wiring Products MP71 and D1 tools very good for 'spade' terminals (Al O calls these 'Luca' terminals) and do get the essential 0400 Closing tool for bullets.

I had big problems with solder bullets (intermittant dry joints = terrible misfire) and personally would avoid these. I prefer hex crimps for the bullets and Auto Sparks sell a superb AST1 tool (not cheap but well worth it). AO provides W crimp bullets as an alternative and the MP71 & D1 will do these. Leave some extra length of wire (safely) where you can so that you can upgrade other things later if you decide e.g. ditch the Zener diode + rectifier and go for 1 box regulator/rectifier.

A quick word on that kill switch: yes it's nice to have one but this is the only "press to break" switch on the bike and it relies on a relatively weak spring contact to maintain feed to your ignition. The good news is that if you DO experience problems it's relatively easy to disconnect the white feed into the h/bar switch and feed it directly to the Boyer (or whatever ignition unit you have by then).

But the main thing is keep going Eddie because once you have sorted it out, the Commando is a wonderful bike that makes you grin every time you use it. I've covered thousands of miles on mine riding all over Europe and NOC rallies are great fun.