73 Norton 850 Right hand Plug

Attached is a picture of my right hand plug. That cylinder pops quite a bit on deceleration. The plug has a shinny appearance , almost like varnish. I have nver seen that on a plug before.

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The bike has the twim Mikuni 34 round side carberators.
I believe the engine to be stock. I recently did a top end with new pistons and rings.
Compression is the same in each cylinder. Head bolts have been retorqued. I used the flame ring head gasket.
I was very careful with gaskets and such and do not belive I have any leaks in the intake system.
The left plug is the normal brownish color and matches the plugs in my other bikes.
Both the right and the left carb are set identically.
I took the right carb off and cleaned it well with carb cleaner. There were no loose jets inside.
The bike idles well and accelerates well. It runs strong at 4,000 rpm with plently of pickup.

Any thoughts or direction would be appreciated.

Bob

Do you still have contact breaker ignition? Perhaps one side is off-time or a coil is weak. If not, there could be a difference betwen the float levels or the float needle sealing from one c arb to the other.

seems electrical rather than anything else, good advice on checking points and timing
also would check the spark plug wire, including both end caps, cheap to just replace both with new

The bike has electronic ignition.

I swappd the plug wires from side to side and there was no change.

I think I am going to fit a new maifold and run just one carb.

I am curious to se the difference in performance with just one Mikuni.

Bob

Bobs850 said:

The bike has electronic ignition.

I swappd the plug wires from side to side and there was no change.

I think I am going to fit a new maifold and run just one carb.

I am curious to se the difference in performance with just one Mikuni.

Bob

Click to expand...

It will definately be smoother. But you'll be pulling smaller stumps.

Are these left and right carbs and can they be swapped? And if not, maybe you could swap slides, needles, jets and what ever else like float bowls, floats from one side to the other and troubleshoot.

Just a thought, but with a "shiny" plug and popping on overrun, could you be leaking some oil into the combustion chamber via bad rings or valve guides? That would put a sheen on your electrode (typically a shiny "porcelain-like" black, though) and dilute (lower the octane of) your mixture, possibly enough to cause pinging.

Grasping at straws here, but possible...?

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The ignition wire, speedometer cable , and the tack cable were all zip tied together on the frame rail by the previous owner.

Just goes to show what 37 years of vibration will do. The insulation on the black and white ignition wire was completely worn through to the bare wire. The ignition wire was the bottom wire against the frame rail. It was amazing that the bike even ran.
I will pull in new wire tonight. I just hope that the black and white lead was the lead that went to the right hand cylinder (as you are sitting on the bike).

Bob

Since you have dual mikuni carbs I would guess they are identical (no right and left like amals). Try swaping the carbs and see if the condition follows the carburetor swap. If so, you know where to turn your attention. By the way...I would be curious to see how you managed to fit those mikunis in. I bought two 32mm Mikuni VM round slides and gave up trying to mount them into my MKIII. They fit O.K. until you try to add an air filter. Finally ended up with a single 36mm VM Mikuni and MAP manifold.