Totally different sparkplug color

Hi there, I am new to this forum, I didn't really find any info using the search option so please forgive me for starting this thread.

Well... I am a new owner of a Commando Mk3. The bike starts and runs great but when i looked at the spark plugs one was brown (almost perfect) and the other one was "dry" black. Is this normal? or should a well trimmed commando have similar spark plug colors? What is most likely to cause this (where should I look?)?

Valve gap?
Timing
Sync of carburettor
or
Carburettor difference (jetting)?

By the way, checked the compression, and the two cylinders have very very similar compression (checked by a compression gauge and by using the E-start)

All engines aim for even combustion in all cylinders but often miss the mark.
Oil tends to settle on side stand side but thinks carbs off one side on anther as don't appear to be oil fouling issue. New gas tends to leave plugs ligher color than lead gas would show at proper mixture. Light tan on procelin and some dark soot on exposed rim is about right. Also proper hot tune will mean you need to tickle or choke on initial cold starts but other wise just switch on and step on and go each and every amazing time after time, is the normal to expect.

Check float levels, and look for leaking float. Check that the clips on the needles are in the same grooves on both.

Mileage ?

I had an 850 (Mk1) that was dry black on one plug.
After much investigating, turned out the valve guide seal on one inlet valve had 'shrivelled up and fallen off'.

Needs the head removing to do of course, unless you are confident of the indian rope trick.

Ran perfectly otherwise.
If someone rides behind you, they may be able to detect it smelling oily.
Or even see smoke out one pipe, especially on the overrun or startup.
A sure giveaway..
A worn or broken oil ring may produce the same, but it may tend to smoke all the time....

Definitely check your ignition and float heights etc first though.

Hi again,

Thanks for the responses. I'll start by checking valve gap and timing. How do you check float heights can it be done without removing the carbs?

I sure don't hope its an valve guide seal. I'm pretty good at servicing and adjusting but I really don't feel like removing the head.

You can easily remove the float bowls to check the float height. You can even build a little jig to test them in action if you like, it checks the needle seat action too which is good to know it's not leaky. Make sure to read here, where all is explained. http://www.jba.bc.ca/Bushmans%20Carb%20 ... html#FLOAT

Dave
69S

Its simple to check float level by just noting where pilot screw ends up for best idle. Ideally should be 1.5 turns out but as long as engine is response to screw adjustments then the float level is quite good enough not to bother mixture to be seen on plugs or felt in operation idle to redline. Air Screwed out less than 1.5 implies float on low side and visa versa. My factory Combat got by gosh and by golly float levels bumped in haste yet runs perfect with one screw .9 turns and another .7 turns out. Implies these float levels on hi side but the low down grunt and step once starts

It is possible for a float to leak, and sink a bit. If you submerge then in hot water, you will see bubbles if they are leaking. Also if they are brass and you shake them, you can sometimes hear if there is petrol inside them. The leakage of air and oil past the valve guide is a strong possibility. Somebody riding behind you might be able to see a puff of smoke from one pipe after you've backed off for a corner, and wound the throttle back on. The port vacuum is greatest when the throttle is shut on the over run (change down a gear).
(You could imagine you are flying a spitfire and going into dive when a big cloud of smoke comes out due to the change in float levels.)

Just to end the thread, I just want to say that I've actually solved this, only problem is that I did two things, so I don't know what did the trick I adjusted the ignition and found the ignition was too advanced and a little difference between the cylinders. Got both cylinders pretty close to 28° @ 3000rpm (yes I run stock contacts and mechanical advance unit)

Then I applied my Morgan Carbtune and noticed a slight difference @ rpms around 2000-3000. Adjusted a little and the plugs are pretty equal now after a good test-run.

At first I thought it was oil consumption since oil was low, but I have now found that the oil level in the tank can change quite a lot from time to time, very strange? My Norton tends to make oil (very strange???)

A lot of oil ends up in the sump after sitting a while, each Norton is different in that respect. Then when you start the bike, it magically appears in the tank again. Look up wet sumping.

Dave
69S

Hi ghostbuster,

really, I would just clean both plugs good and put them back in and check them again after say 100 miles

if that same plug is again quite a bit darker than the other one than ok maybe you got a problem somewhere

and even if it is, so what?

you can elect to just ride and enjoy your bike, I would

the solution could be as simple as leaning out the mixture/idle screw setting on that carb, maybe too rich now

or maybe you solve the problem by dropping the needle one notch to lean out the midrange on that carb

if these are tried and the darkness persists, so what if it starts and runs ok!

and yes maybe you have a worn valve guide on that side letting in some oil mist to darken the plug,
and then it become a lot of work to pull the head and have a good mechanic see what he can do

Thanks for all the responses, this Norton Commando is by far the most charming bike I've ever owned, but there is a lot of technical stuff to learn in order to be able to tune it yourself