Running hot
 

My bike steams some coolant after longer climbs. I'd say that I'm at mid throttle during most of the climb. I'm moving at a decent speed so there's air moving through the (clean) radiators. It's fine when back on level terrain.
After a normal 30 mile ride, with some hills, the coolant level is an inch or two below the fins.
My 1st thought was that the cap (1.5) may have gone bad. I switched to the stock (1.2) and it was no worse and no better.
I looked at the (LTR) impeller and it looks fine and turns when I kick the starter.
My next thought was that I was lean (2nd clip slot, 178 main). My buddies said my exhaust is smokey and think I may be rich. My needle seat could be wearing/worn, too ?
I should have taken a plug reading after the hill but I figured it'd be better to just ride easy and keep some air moving over the radiators and coolant flowing.
It's an '03 EC300 with a fresh top end. The issue existed before the new top end. Oil has looked good at changes.

You could be rich off the bottom and lean in the middle, and the bike will smoke as it clears out. Going through this now on my brother's Husky 250, rejetting a Keihin carb conversion and RB head mod for the cooler weather. The bike exhibited lean symptoms climbing in the midrange but still smoked as the spooge left in the pipe burned off.

Maybe try a NEW 1.4 - 1.5 bar cap too?

The GasGas 2 stroke should not overheat with the stock rad cap if it is 1) jetted correctly, 2) you have enough airflow and 3) the cooling system is in good working order. My riding buddy, son and I run our GasGas 300's at high elevation (up to 6000ft), warm temps (80F), climbing long hills (miles of climbing w/tight switchbacks in 1st and 2nd gear). I often give my 6 year old son a ride. Our combined riding weight is about 370 lb. The only times I had overheating were a) when the waterpump seal failed and the coolant went into the case and b) when I had radiator guards. Otherwise, it does not overheat. I have a temperature tab on the outside of the cylinder near the head. After a long hard climb on a warm day with my son along the temps are around 200 deg F.

I would check the plug right after you get to the top of the hill to get an indication of how lean you are during the overheating condition.

Some radiator guards significantly restrict airflow. If you have radiator guards, I suggest you remove the guards and put the stock louvers back on and see if there is a difference on the overheating.

I think you might be lean in the middle as well. Just try the #3 position and see what happens.

What needle are you runnig, what altitude, temp, Pilot jet, and so on. Thsi will help in finding the culperate.

I'll try the 3rd clip spot. That was my plan but the smoking pipe had me 2nd guessing myself.
I'll move the clip and check the plug after a climb. Thanks.

Are you using pump fuel with ethanol?

My bike was doing the same thing on E-10. I ended up going to a 180 main jet.

Lucky for me, a place up the street sells ethanol-free gas.
I buy all my gas there.

Yes, especially now that its cooler. The Husky 250 pulled a 185 main clean with pump gas and my 250 with a 182 is right on the edge in cool weather.

I think all pump gas in the Northeast is E-10 now, correct?

I thought it was gubment mandate but maybe there's a grace period ?
The station has a hand-written sign on the pumps "Our gas has no ethanol."

My bike started running hot long before the E-10 signs went up at my local gas station. Later I found out local stations were already selling E-10 and shortly after that the signs went up.

When I went out to western NY for an enduro I bought gas for my pickup and didn't see any E-10 signs. I get a lot better mileage with non E-10 fuel.