Problem with carbon build up on spark plug

Bike in question 79 XR 500

So my plug turns black quickly with carbon. That means it's running rich doesn't it?

Could an incorrect float level be the problem or would it be in the screw settings? The plug looks black and dry when they stop working after only maybe 1 or 2 hrs not sure on the time but they don't last long. Other than that the bike runs great and has lots of power. Oh black exhaust to when revved.

Can someone point me in the right direction to resolve the problem?


quick fix,try going to a hotter plug. works for me . down 1 # if you use NGK.

Is your air filter clean?

Is your compression good?

Is your carb clean?

If all the above are Ok, I would lean it out on the needle or main jet depending on what kind of riding you do?

Good luck!

The air filter is clean.

The compression is good I think. I have not tested it but feels good when it comes up on the fireing stroke.

No I have not cleaned the carb. Yes that had occured to me but I didn't want to do that enless it needed it. maybe it does but it runs so good I figured it didn't need cleaning.

So the float level wont effect this? Also the valves are set.

Well, I'll tell you a story.

I rebuilt my XL500 about a year ago. I cold seized it, and there after it ate oil. Come to find out, I was missing an oil oring that you put on before you install the head. I may have been burning oil all along, but I'm not sure.

Fast forward to now, my plug came out superblack and sooty, also a lot of carbon on my piston and valves.

The black plug could be from super rich, or could be from burning a bit of oil.

Black plug is rich or oil.Oil will usually have buildup also.

YES, the float level is very important on these bikes it will make it run rich if adjusted high. If the motor is old or has a lot of miles, do yourself a favor and check the compression with a gauge to know where you start. Cleaning the carb is easy and you only need the bowl gasquet to start with. (if your careful, not even that). Clean to see if it makes a difference, then buy a kit if you need it. Be sure to check the needle seat as it can make the bike rich as well. I always hook a clean rubber hose up to the fuel inlet push up gently on the floats and make sure I can't blow air through the needle seat.

Ok, thanks tons guys:applause: I'll work on all these things and post back after I make some headway.

I had no idea that I had to have my bike licenced to ride the logging roads up here in WA.

Ran into the National Forest enforcment duds up their today and they let me slide but said to get the bike licenced. My riding partner wasn't so lucky. He didn't have a valid drivers licence and was fined 275.00 3rd deg no licence + his bike can never be on the logging roads because it's not a duel sport like mine. What a pain in the azz!

:ride::D:thumbsup: :thumbsup: :ride::confused:

May I suggest: when cleaning carb also use a thin piece of wire like guitar high E string and clean out the tiny air bleed holes in carb throat. You don't want to ream them out so run the end of the wire across some emery paper to remove any burs before using. Wire and some carb spray cleaner helps the carb's air/fuel balance throughout rpm range. Also helps to cut the tip on carb cleaner tube at a 45 degree angle so you can hold it tight against carb throat over the air bleeds when you clean them out.

I well do that and it just so happens that Fenderstratman has acess to guitar wire. :thumbsup:

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