Motorcycle Repair: 1975 xr75, air screw, dirt bikes

Question
QUESTION: gday mate. my 1975 xr75 started back firing and splattering, when i took out the spark plug it was covered in soot and crap. i replaced the plug and it ran good for a short cruise then started playing up again. i took out the plug and it was covered in soot again. any ideas would be appreciated, cheers mate

ANSWER: I'm not very familiar with dirt bikes, especially the older ones, but I would check to see if the carb is properly tuned and adjusted. It sounds like it is running rich. You could try draining the float bowl, as there might be contaminated fuel in it. Also, check that the choke isn't stuck in the "on" position and that the air filter is clean.

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QUESTION: cheers mate. so if its running rich would that be caused by the air screw on the carb and if so do i screw it inwards or outwards :)

ANSWER: Generally speaking, yes, however it could also be incorrect jetting or a stuck float valve or another ailment that is causing more fuel than needed to be sent into the cylinder.

Try slowly turning the screw gently clockwise until it just bottoms out, then back it off 1.5 full turns. This is only a starting point, you may need to further adjust it in or out until the bike idles properly.

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QUESTION: mmmm... ive cleaned out the carb, float bowl, filter, exhaust and ive adjusted the screw in and out in various positions with little success.it seems to be sparking fine. the spark plug seems to be getting covered in soot almost straight away tho. any other ideas? if it helps it sat in the shed for ages then i rode it hard for two weeks with no problems, it sat for a while longer and now im at this point

Answer
The soot on the spark plug, is it oily? If so, you are looking at a valve job or piston rings. If it is a dry, powdery soot then it is an excess fuel problem.

Beyond that, I can't think of anything else beyond what you've already tried to cure the problem. Check the float bowl pivot and needle to make sure it isn't sticking open and causing too much fuel to get into the cylinder.