Standard spark plug on my 95 bike is a 5. But looks like a       previous owner has put a 6 plug in it. Any  obvious  reasons for this, or would it just be a mistake.

If it was changed deliberately, it could be for several reasons.   Maybe he ran with more oil in the fuel, and needed a hotter plug to keep it clean.  It could be his choice of Winter plug vs Summer plug, especially if he only ran the engine for three or four minutes at a time in the sections, rather than flat out for hours on end.

I wouldn't think a change of just one grade would make too much difference, are the colours on the electrodes looking Ok..?

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If it's an NGK plug, a 6 will run colder  than a 5 so will tend to foul sooner in  typical trials going.

Edited March 1, 20215 yr by b40rt

Thanks for your input lads. I changed it to the 5 plug today. Haven't ran it yet on that one, but the 6 I took out looked a bit dark. It seems not be running hot enough for the 6. I'll keep it in the toolbox, and check after a bit of running to see if the 5 is getting too pale. But I would agree that there couldn't be that much difference between them. And everything else looks fairly standard on it, now that I've cleaned all the crap off the bike.

Update on the spark plug issue. Ran the bike round the yard yesterday evening for a while.     No high revs really. Checked the plug (5) this evening when cold. Looks like its burning a lot cleaner than the 6 that I took out. Started easier too. Didn't really need to use the choke at all, and it was cold. Hope this info is of some use to someone else.

 

3 hours ago, AlanC said:

. Didn't really need to use the choke at all, and it was cold. Hope this info is of some use to someone else.

 

Good that your getting there. You should always have to use the choke from cold. If it will start without it you have a carb problem, float level or jetting, leaking needle valve. Always switch petrol off when storing or transporting, and completely drain carb and tank for longer periods.

Thanks b40rt, all info appreciated. I did use the choke to start it, but didn't need it on for as long. And I always turn the petrol off when its parked. Bearings, bushings and brakes done, so time to get the cobwebs off me now!