missing left cylinder

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KevinS

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Apr 2, 2015
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After days of trying to figure my mysterious left cylinder miss I figured I'd bring my issue to the Norton gods... I have a 1974 mk2 that has around 30,000 miles and has never been apart that I purchased from the original owner a few months ago. The bike had a mikuni 34 carb and a boyer when I got her. The bike ran beyond smooth, had lots of get up and go and always started with one kick. I rode the bike a few thousand miles this summer without issue until I had a wire break at the boyer pick up. At that point I decided to do some preventative maintenance. I replaced the wire harness with a new lucas harness, I installed a podtronics regulator, and I replaced the boyer with a tri spark. After all of the new parts were installed the bike fired to life perfectly and ran just as good as before if not a bit better. A few days later I was out for a ride and the smoothness of the motor changed it had a slight vibration I had never felt and had a shutter or a miss when I would excelerate felt mostly in mid range 3000-4000 rpms. You can also hear it in the idle the bike doesn't start as easy it now takes a few kicks to get it started and I get some kick back once and awhile. Thinking this was fuel related I pulled the mikuni and installed the original amal setup and it had the same issue only with the two carbs you could hear the miss more popping from the left muffler. I re-installed the mikuni and checked the ohms on both coils both read 2.3 ohms but my meter doesn't quite zero out so I'm assuming they would rear about 1.9 actual ohms. I then replaced the spark plugs and plug wires. Still the same issue...rechecked all my wiring to the tri spark and coils and everything looks right I am getting the same voltage as the battery at the tri spark 12.8 volts+/-... I have ridden the bike with this issue about 20 miles to see if it would get worse or hopefully better, no change. So to recap the bike will start , idle and run it just has a vibration or roughness to what once was a buttery smooth Norton and definately has a miss or a shutter in the midrange. Thank you, Kevin
 
A compression test, or better yet, a leakdown test may be in order at that mileage.
 
Check the valve tappet clearance just to make sure allis right there. On your 850 clearance is .008” exhaust and .006” intake. When you have the valve cover off look for any loose parts inside such as valve spring parts or a lost collet keeper. Wiggle the valve stems..should be no movement side to side even if only slight. The leak down test mentioned above is a very good idea to compare right and left side if you don’t find anything obvious. Doesn’t seem likely this is fuel related if only left side with a single carb. I had a left side misfire...turned out the plug wire wasn’t pushed in all the way and the high tension was arcing to one of the low tension poles. Very visible spark when the engine was running.
 
illf8ed said:
Check the valve tappet clearance just to make sure allis right there. On your 850 clearance is .008” exhaust and .006” intake. When you have the valve cover off look for any loose parts inside such as valve spring parts or a lost collet keeper. Wiggle the valve stems..should be no movement side to side even if only slight. The leak down test mentioned above is a very good idea to compare right and left side if you don’t find anything obvious. Doesn’t seem likely this is fuel related if only left side with a single carb. I had a left side misfire...turned out the plug wire wasn’t pushed in all the way and the high tension was arcing to one of the low tension poles. Very visible spark when the engine was running.
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Switch coil, wire and plug to opposite side and see if popping switches to other side.
 
Air leak at the manifold to head flange? Head bolts torqued?

Exhaust roses tight and their seals good? If your headers have thecross balance pipe check carefully for cracks at the weld joint.
 
I guess I'm just a simple guy...my punch list:
1. mechanical integrity/compression check & leak down test
2. then ignition
3. then fuel/air
If good then go for a ride
If bad then fix
:)
 
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Well, if that’s the 1 thru 4 list then number 0 must be...

Check / replace spark plugs. Yes, even if they’re new (in fact, ESPECIALLY if they’re new) !
 
Bodger said:
Switch coil, wire and plug to opposite side and see if popping switches to other side.
Click to expand...
I'd do this first and if problem switches to other side I'd suspect the coil.
Problem doesn't switch to other cylinder I'd suspect the issue is the Trispark/pod known problem mentioned above or a wire connection.
 
Coil, HT wire & plug are the only parts unique to one cylinder. Noisy whatever would affect both cylinders.

edit: and intake manifold
 
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