Will not start.On occasion it will but never stays running.replaced primer ball and spark plugs.Since it does fire occasionaly can I eliminate not getting spark?

Sounds like the carb(s) need a qualified professional cleaning and setting...The bulb should stay tight..
.. the lower cylinder was not = 1. Firing.. or 2. Not getting fuel..
.. I would replace both spark plugs,,,( NGK BR8HS Spark Plug ).. after getting the carbs professionally cleaned and set..

It may not have a primer ball. If it does, it would be attached to the carberator. If it starts, but won't stay running, that could mean your carberator needs to be cleaned/serviced.

If it has not been run for a long time I would remove the float bowl on the carburetor and clean the jet.

I would take it to your Stihl dealer. They have a quick carb check than can diagnose it very quickly at (usually) a very small expense.

First things to try is -
Clean the air filter..
Replace the spark plug..

check your gas and oil mix remove the plug take some carb cleaner spray in plug hole pull start replace plug pull start if it fire you mix is wrong.....Good Luck

My Kracher 2500 pressure washer has no pressure unless I remove the nozzle. If, I remove the nozzle and pump the trigger several times and quickly replace the nozzle it works fine until I release the trigger. Karcher (online) suggested I remove and clean the unloader which I did. It still doesn't work correctly. What another solution?

Remove the spark plug and squirt a little bit of the fuel from the gas tank into the spark plug hole, replace the spark plug and crank it over several times.

If it runs, or tries to run... that tells you that your engine, ignition system and gasoline are good (or atleast good enough to get it started)

Next check your fuel line and the primer ball. Make sure that when you pump the primer ball that you can see it pumping fuel through the fuel lines in ONE direction only. (It's kind of hard to see, you might have to start the test with empty fuel lines).

If that fails, you need a carb kit because that's where the check valves are. (unless your primer ball is cracked, broken, or clogged)

Next check the metering diafram and needle in the carb. If the diafram is hard (not easily flexible) or if the needle valve is stuck shut the engine will not be able to get the fuel that it needs to run.

If the Needle valve is stuck, just clean it and it'll most likely be fine.

If the diaphram is hardened, you need a carb kit.

If all the above is ok and it still doesn't work... check the gasket at the base of the carb to make sure that the posative pressure pulses from the crankcase are able to make it through the little holes into the carb (that's how it pumps fuel into the carb).

That's pretty much all the easy stuff. Keep in mind that a new carb only costs about $28 at most places.

my heater will not stay running