I don't think your carb is %26quot;fine%26quot;, it sounds to me like someone has been messing around with the jetting.
I would go ahead and pull the carb from the manifold, and open the top of the carb up. Take a look at the needle jet. See where the %26quot;C%26quot; clip position is. This determines the height for the needle jet. I would put that clip about two to three notches from the bottom. If I recall, that might be about half way up, or thereabouts. That should get you somewhere near the stock setting for your carb.
Next, pull the float bowl. See if the float moves freely. Get a manual and find out the clearance of how far that float is supposed to travel. I don't think it should be messed up, but its something that you should check when you are putting your carb back together. That float should move freely, and it should be in the right clearances.
Re-assemble the carb. Make sure that all the jets are clean, re-installed, and good to go.
Kick your bike over and see how it does on idle. Set the idle screw and make sure your motor is revving up to factory specs. Once that bike is running good at idle, then it should be good to go throughout the entire powerband.
Check your fuel mixture. Make sure you're not running too much pre-mix in your gas. Too much oil will definitely get your plugs fouled out. Make sure your plug gaps are set to factory specs.
You really need to find another air filter. You can't run the bike without it. Neither can you safely run it with the old one. In an emergency, you might be able to soak the filter in some solvent ( air filter foam cleaner ) and try to get it cleared out a bit. And then put a small dose of filter oil on it to trap any dirt. But in reality, you need to find another filter and install it. Just leaving the airbox empty is going to affect the air/fuel mixture in the carb, and that will affect the way the bike runs. Right now, with an empty airbox, it's pulling in more air than it is really supposed to.
Try that, and see if this works for you. I don't see any problems as long as you are not having any leaks, or electrical problems. I think this problem is strictly carburetor, in my humble opinion.How do I get my Honda CR250 to run good?check the connector between the engine and the carb for an air leak
it could also be the crank seals
clean the low speed jet and its passagewaysHow do I get my Honda CR250 to run good?you can check with e-bay.How do I get my Honda CR250 to run good?with a bike this old you will most likely have leaking crank shaft seals that will cause excess oil from the gearbox to foul the plug.check the compression as well,and don't run it without an air cleaner the engine will die very quickly.take the old air cleaner to a bike parts store and see if you can find something that you can make fit. good luck
