Advice Required Regarding Carbon Deposits:

[dotti]

The brake cable broke on my bike last night, so while I got that repaired, I also got the sparkplug replaced, as there were signs that it needed replacing (misfiring, backfiring, getting harder to start etc).

If you have a look at https://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html , you will find that my sparkplug is a carbon copy (no pun intended) of sparkplug number 3.

Now first of all, the sparkplug I removed could have been in the bike for years and years. It is near impossible to find a tool to remove it. Even the bike shop struggled to find one to get the plug out. some of the other things that were lacking when I bought the bike signify that it was not well looked after in the past. I am not going to jump to any conclusions unless the new plug undergoes the same fate.

The website above suggests that there are four main causes for such a problem:

1. A plug with too high a heat rating (it is running the standard, suggested plug. This was confirmed today).

2. Use of a too rich air, fuel mixture.

3. Trouble in the ignition system

4. Improperly functioning cooling system (it's air cooled. How can it be too effective?)

The question I have is what steps could I undertake to establish which of these four points is most likely to be the cause?

Other factors:

1. The carb had a blocked jet which was recently fixed. However, surely this would make the bike run lean, rather than run rich (or is my understanding of rich/lean incorrect?)

2. Until very recently (which is why I changed the plug) the bike always started first time.

3. The bike burns about 500mls of oil per 1000 miles, but according to the guy I spoke to at the bike shop today this is normal for GN250's.

Off topic, but on the bright side, the guys at the bike shop were very impressed with the condition of the bike. They said it is in "awesome" condition considering it is 23 years old. Incidentally only 2 years younger than me
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[mchaggis]

Mucky air filter? That'll make it go rich. I'm not sure if a dodgy HT lead will do much for it either. An unblocked jet would make it run richer, but if it's what it should be anyway, I shouldn't have thought it'd make much difference. If anything, unblocking jets would make it run more smoothly I should think.

No blue smoke from the exhaust I presume? Leaky valve seals, rings that kind of thing perhaps if so.
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[dotti]

I should have mentioned the new air filter 1200 miles ago. Incidentally, I already checked that and it is still in perfect condition.