misfiring on the left cylinder - help

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jnash

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Hi,
Have a 1973 Mark II that ive restored. It is misfiring on the left cylinder after running very smoothly for a couple of hundred miles.
I stripped down the carbs, they seem OK
Compression is almost identical for both cylinders
Swapped the leads and plugs over
Checked the tank filter for blockages of fuel

It idles OK but the left exhaust gas out the back is colder than the right side, and when I ride it and open the throttle it missing often.
Other than swap the coils I am stumped at what to check next to diagnose the problem

Does anyone have any ideas on what to test next? I am still not sure if its a fuel problem or a spark problem.
It has an electronic ignition.

thanks in advance for any help
 
Have you tried strobe timing it ?
If the strobe light misses blinking its light, in sympathy with the misfire, then its electrical.

If the strobe lights every spark, then its carb or air leaks or a bad valve, etc.

Have you tried pulling up on just the one throttle cable, in turn - if all is good, it should rev up on just the one cylinder.
 
If plug and HT lead swaping didn't change sides then suspect maniffold or exhaust heater leaks or maybe elec. ignition trigger wires fault.
 
I'd swap coils.
If problem shifts you know why, if not at least you know it's not coils.
You didn't say what type of ignition.
If you have points maybe in there but if electronic ignition like Pazon that uses wasted spark (both plugs spark together) then it wouldn't be a spark problem since you've already swapped leads and plugs and confirmed coils ok.
Confirm throttle slides open exactly the same on both carbs. Since it ran fine for a short while sounds like something may have shifted with vibration.
Did the problem seem to be sudden or gradual since it will hint at the source.
 
Yes to brand new sparkplugs and swapping the plug high tension leads. Your battery could be getting old and weak like the future for most of us. Electronic igns. like strong power and a good clean ground point to frame so check wiring all over including fuse area. Should be an easy figure-out.
 
Single side miss firing is not an electronic ignition issue as it fires both at once so both jug should have same missfire not just one. All electronic ignitions for Norton are dual fire wasted spark type unless ya spend a lot more for a single fire PowerArch which the maker says is a waste of money on a 360' twin. Never a bad move to try new plugs but he wrote swaping plugs didn't change sides so coil sway next logical test. Always worth while to take a look in darkness.
 
I've had an electronic ignition with wasted spark completely stop sparking on one side (not Norton) - stranded me out in the middle of nowhere, one cylinder is not going to get you home from afar. Lucky still had old points and cam, never did trust those new fangled things. Later found out one power tranny had died, famous for it. All potted in epoxy, so essentially non-repairable (as if I'd trust it again).

So don't assume anything.
ESPECIALLY when someone tells you it will still keep going on the wasted spark.
 
I had the same as Rohan, boyer only sparking on one side, a dedicated earth lead from head back to battery cured it.
 
If that electronic ignition is a Boyer, I'd check the pickup leads on the plate of the Boyer before I did anything else. Those leads have an often common tendency to break. The break is inside the insulation and will not be visible. Check it out with an ohm meter. Wiggle the wire while the ohm meter is connected.
 
Still unable to solve this misfire and certainly appreciate all your help so far. Here is a review and the symptom, and things I have tried. Any help on how to proceed to fix this problem would be appreciated.

73 MkII Norton Commando wIth 20,000 miles – suspected original.

Last year rebuilt 500 miles ago, with new rings, valves and guides etc, rebuilt carbs, Pazon ignition fitted, and gently ridden etc. Engine had been running fine until after a 100 mile ride.

Problem: Misfires on the left cylinder on idle and when running. On full load, sometimes “kicks-in” on both cylinders, but not constant, or exactly repeatable.

At idle left exhaust is never more than warm with some black soot/oil on palm of hand at exhaust tip (assumed to be un-burnt fuel).

Actions taken:

Fuel taps cleaned – some dirt but not blocked.

New plugs gapped correctly – no difference

Wires and coils swapped with right cylinder – no difference

Ignition checked – both sparks firing and look good. Signal continuity and ground wires ohm’d and good.

Carbs disassembled and rebuilt – no problems found, no difference

Compression Test on cold engine: right 120 psi, left 104 psi, added oil to both, left came up to 120psi

Valve clearances Cold: Left In .007” , Ex .010” Right In .006”, Ex .005”

Even though the left compression is down due to rings possibly, I have been suspecting a sticky valve since engine has kicked-in while opening up the throttle riding under load as mentioned above.

Things I have thought to do next are:

1. Swap carbs, or install 2-1 manifold and use just one carb, or upgrade to new carbs (illuminate a fuel problem)

2. Remove the head and have it serviced.

What are your thoughs?
 
A Boyer EI can and will fail causing a misfire on one cylinder. Usually, the cause is a broken wire at the pickup coils. The break will be inside the insulation, so it's not visual. Check with an ohmmeter. The misfire is usually below 3000 RPM, mysteriously clearing up above 3000. I suspect the reason being the shaking motor at the lower RPMs will cause the break in the wire to part.
 
After you investigate the boyer side of things, and fix, give the throttle cable to the left side a tug, and confirm that the left cylinder will rev up. A damaged or unravelling throttle cable can cause some mysterious symptoms. So can an air leak around the carb or manifold, or down the valve guide....

It does sound like those pistons/rings are getting a bit worn, those compression numbers are a bit low if they were tested hot with the throttle wide open...
 
i just started my rebuild for the first time today and having the same issue (same side too) mine seems to be fuel related, when the left side starts missing i hold down the tickler on that carb and you can hear it surge and the miss go away. try that to narrow down spark vs fuel
 
I came across this looking for a solution to my BMW R1100RT misfire.

The only thing i would point out is that no one has mentioned about doing a carb balance...it sounds very much as this may be the issue if the exhaust on one side is colder than the other it may very well be the Air/Fuel Ratio is incorrect for one cylinder - imbalance = carb balance? R1100 are extremely sensative to this despite being fuel injected.

Hope this helped.
 
jnash said:
Still unable to solve this misfire and certainly appreciate all your help so far. Here is a review and the symptom, and things I have tried. Any help on how to proceed to fix this problem would be appreciated.

73 MkII Norton Commando wIth 20,000 miles – suspected original.

Last year rebuilt 500 miles ago, with new rings, valves and guides etc, rebuilt carbs, Pazon ignition fitted, and gently ridden etc. Engine had been running fine until after a 100 mile ride.

Problem: Misfires on the left cylinder on idle and when running. On full load, sometimes “kicks-in” on both cylinders, but not constant, or exactly repeatable.

At idle left exhaust is never more than warm with some black soot/oil on palm of hand at exhaust tip (assumed to be un-burnt fuel).

Actions taken:

Fuel taps cleaned – some dirt but not blocked.

New plugs gapped correctly – no difference

Wires and coils swapped with right cylinder – no difference

Ignition checked – both sparks firing and look good. Signal continuity and ground wires ohm’d and good.

Carbs disassembled and rebuilt – no problems found, no difference

Compression Test on cold engine: right 120 psi, left 104 psi, added oil to both, left came up to 120psi

Valve clearances Cold: Left In .007” , Ex .010” Right In .006”, Ex .005”

Even though the left compression is down due to rings possibly, I have been suspecting a sticky valve since engine has kicked-in while opening up the throttle riding under load as mentioned above.

Things I have thought to do next are:

1. Swap carbs, or install 2-1 manifold and use just one carb, or upgrade to new carbs (illuminate a fuel problem)

2. Remove the head and have it serviced.

What are your thoughs?
Click to expand...

I had exactly the same symptoms a few years ago. Anyway to cut a long story short, I was in Spain on the bike,( misfiring on the left side ) and I glanced down to see the HT lead moving in and out of the coil slightly. Easily sorted then , I think you should change your coils round and see if the problem moves to the other side.
sam

Sorry , just re read your post and you have switched coils and HT leads around
 
Valve Clearances should be set exactly , per Cam type .

points ignition One Loose strand on a wire , a dirty / corroded connection , OR a Wire thats been Pinched & semi broken would be enough to cause it .

AS WOULD a CONNECTOR that Is Not firm on its Spade Terminal , cleaning and adjusting the ' Crimp ' of the Spade Terminals so they are a firm fit
will eliminate many a poor circuit .

easyist way to ISOLATE the TROUBLE would be to DIRECT WIRE the Ignition - so it bypasses all the wireing & ignition switch .
If its direct from the battery , AND youve checked any appropriate earths ( Grounds - Contact to frame - paint wont ).
and the B F D STILL DOES IT , youll know its ot the circuits you isolated . :( :lol:

Though Therefore you now know ( we hope ) WHERE the Fault is originateing . perhaps . :x Chipped Valve guide ??
No great dramas whipping the head off , if you throw a pile of paper on the floor to collect the debri , or dropped pieces .

backing of ( and rocking rockers to test freedom of movement ) rocker screws & resetting Accurately , should be a priority .
Crikey , a quater thou Out is OUT . 100 % out ISNT Far Out , its Way out . :shock:
 
FWIW, I had a similar problem, except the misfire started at about 3500-4000 rpm and the bike wouldn't rev above that. Turned out to be a slightly distorted bore in the left sided carburettor.
 
Thanks everyone. I cleared the problem by going to a new single carb set-up. I really like it. The engine fires up on a single kick, the idle is consistent and reliable, have lost a little top-end power and gained some low-end fun loving torque.
Those old Amal 1973 carbs were just worn out.
 
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