1998 yamaha big bear black spark plug

[sandy]

What is wrong when the spark plug turns black?

[Mech]

It's probably because the mixture is too rich..  How's the quad running ?

It could also be oil burning..  Does it smoke or use oil ?

[sandy]

No the oil is clear and and very little smoke only when starts it first.  Runs good for about an hour or so then idles bad, hard to keep going.

[Mech]

Well that smoke when it starts could be the cause. Oil, even clean oil, burns black.

But, since it runs bad after an hour, it probably means you need to adjust the idle mixture. The idle mixture might need adjusting because there are other problems in the carby, which may need attention, but you should try adjusting it first to hopefully cure the black sparkplug, and also to check that the idle does adjust as it should. If the idle doesn't adjust properly it's a sign of carby problems.

When you adjust the idle mixture, you should do it at as lower idle speed as possible, then adjust the speed and mixture in turns getting the idle speed as low as possible, with the mixture set half way between the point where it starts to slow because it's getting too rich, and the point where it starts to slow because it's getting too lean.. The thing to take note of, which indicates other work being needed, is how far you turn the idle mixture screw before the speed starts to drop. If the carby is all good the idle mixture screw should not need turning more than about a half a turn between too rich and too lean.

I'd adjust the idle, and then see whether the spark plug starts to clean itself after a few hours use. .

Edited November 21, 2022 by Mech

[Gwbarm]

The smoke at start up, especially if its been sitting for a while, is oil leaking past the valve guide seals and getting into combustion chamber, very common on older engines. If your black plug is shiny its oil burning, if it flat black its too rich. Strange that it runs and idles fine for an hr, is seems like something is heating up. When it cools will it idle again. You might check your choke cable to make sure its not sticking and is doing what its supposed to do. Mech gave the correct procedure for adjusting carb I would start there . The only thing I might add is change the plug, iv had plugs not fire as well when they get hot, drove me nuts trying to find that.

[Mech]

Yup, good point Gw.. the shinny or matt black is an indicator of whether it's oil or fuel burning..

 

 The spark plug, yeah I thought of suggesting changing it but they hadn't mentioned misfiring.. but a different heat range would perhaps keep the plug cleaner.. You could check what heat range the plug is meant to be Sandy..  It will be in the manual or a bike shop will tell you.. or anyone that sells spark plugs really..  And, I'd suggest you buy an NGK plug.. they seem to last longer than any other make in japanese bikes

It would probably be a good idea to check the aircleaner isn't dirty, or over oiled, and it might pay to check the valve clearances too. If they are too tight they might be preventing a valve from fully closing once it gets really hot.