1. Your carb tuning is probably way off, which makes a huge difference in power.
It probably should be spinning tires in all gears on loose gravel.

2. If your plug keeps fouling out, your carb is definitely not tuned correctly.
You probably don't get into the throttle too much when trail riding, so I'd bet your needle position isn't correct.
Move the needle down into the carb one step at a time (that means put the clip one notch higher on the needle) and see if that fixes your fouling issues.

3. When you're tuning a carb, you want to start off with the lowest circuit and work your way up to the main jet, and you also start off with a clean plug.
Start the bike up and let it idle for a while, occasionally blip the throttle, but try to keep it at idle speed.
Shut it down and check the plug...if it's black you need to tune the idle (pilot) circuit.
Turn the pilot air screw all the way in until it stops, then back it out 2.5 turns.
Clean your plug (use a sandblaster if you don't have a plug cleaner) and re-install.
Start it up and see how it runs, check the plug.
If it's still black and fouled up, you need to go to a smaller pilot jet and try again.
There's a lot more to it than that, but it'll get you started.
Next, you'll want to test out the needle jet.
Use the same procedures as before, except don't run it past 3/4 throttle (if you can find a way to limit the throttle to that point, even better).
Check your plug visually and see if there's any soot or fouling.
If there is, you're rich on the needle.
Adjust the needle until the soot goes away and you're left with a brown color.
Now you're ready to test the main jet, and the procedure is a bit different.
Find a spot where you can run full throttle for an extended period of time (at least 10 seconds, but more is good too).
You'll need a new plug for this part (2 plugs at least).
Start the quad, get it warmed up by driving it around, then shut it down and put in the new plug.
Keep your old plug on hand.
Start it up and immediately get on the throttle and do your run.
Using the kill switch, shut it down without letting it idle, clutch it and make the engine stop right then and there.
Remove plug, replace with old one, drive back home.
Cut the threads off the plug all the way down to the base and look at the insulator (this is called a plug chop).
There should be a small ring of discoloration just above the base about a mm or two thick.
That's where the main jet leaves an indication of how the engine is running at full throttle.
Light to no color or white - LEAN (go up at least 2 sizes for piston safety)
Brown/chocolate color - PERFECT
Black/sooty - RICH (drop a size on the main and try again)

Now, if you have good eyes you can eliminate the physical plug chop by looking all the way down at the base of the insulator, but it's not recommended until you get some experience doing it.
You'll be buying lots of spark plugs if your main jet is off...so get the cheap ones (BR8ES, unless you have a resistor spark plug cap, then get a B8ES).
Later on, when you get the hang of reading plugs without cutting threads off, you can take a plug reading, clean it on a plug cleaner/sandblaster, and re-use it for tuning.

There's a lot more to tuning than that, but it's the basics.
Somewhere on the forum there's a big writeup on jetting, read up.
This also only takes into account that the rest of the engine is in good shape and it's only the carb giving you problems.
You could have all sorts of issues causing a loss of power...
-compression
-reeds
-air filter
-ignition/timing problems
-detonation/fuel quality
-intermittent fuel flow issues

Some are identifiable by how they act, some just affect the entire operation of the engine, some are hard to troubleshoot.
Start with the carb tuning and work your way from there.