250-500cc Wr360 boiling and surging

On my last ride it started to surge a little when i let of the throttle and otherwise ran fine then i noticed it boiling so i let it cool a bit then rode faster but easy on the gas and it cooled down and shortly after headed home and it started boiling again and surging worse.. coolant only lost about 100ml from overflow im guessing, has no leaks or damaged radiators (at least nothing worse than itd always had) so im wondering what i should start testing first?... i was thinking of starting with coolant change qnd radiator flush but id hate to put coolant in and have to drain it if theres another issue all along

Air leak, head orings, dodgy impeller/blockage

How do i test for air leaks, and do i have to pull apart water pump to check that ? Also if head o rings was dodgy wouldnt it be burning coolant?

Radiator cap? My 250 was boiling when going slow. The rubber in the cap was drying out. no seal

Spray aero start around intake/cylinder/carb when running see if it revs up. Just take rad cap off & make sure coolant pumping. If orings gone then yeah pushes coolant out overflow, mighten be enough coolant to cool/cavitation/gas build up in cooling system. Just throwing em out there

Yah CCP i thought the same and im going to replace it though it boiled on way home going at speeds under shady tree areas just after i had cooled it from going easy for a while at speeds lol

I can't find any obvious leaks with spray and the pump is working and there is coolant there thats only about 10 houts old and if the head orings was gone would it not push it out the overflow all the time not just time to time.. for example it would not have cooled and stopped inbetween being hot first time anf the second would it?

Must have an air pocket?
If you have a slight tear on the o-ring, sometimes it rears it's ugly head when the motor is stressed rather than idling.

so far I cannot find the fault I took it out last weekend after having fixed it I thought (and had new coolant in), ran fine for a hour or so generaly though a couple times it got fairly hot but not boiling or overflowing (as far as I could tell when looking at the overflow pipe a lot) and on the way home it started overflowing out whilst revving the bike a fair bit to try hop a log but couldn't get traction (stupid me forgot to check if it was boiling though It didn't feel as hot as when it had boiled previously), anyhow ive now pulled the head it does not appear to be warped in any way and the o rings looked brand new and can not find a single nick or crack or anything in them (there less than 30 hours old anyhow) and the water pump appears to be recirculating as far as I can tell via a good rev with cap off (possibly not properly under load I cannot tell)... only thing I can think of is radiator cap it self could be the fault as its running a 1.1 cap and it did leak tiny drops out overflow till I put cap in and out couple times and the old bad conditioned rubber piece seated a bit better then it was totally fine as far as i could tell at least until it fully overflowed though I wasn't riding it as I normaly would either I wasn't hitting power band real hard and long or as often for a bit to make sure it was safe... so besides air pocket or maybe a worn out water pump I'm thinking all that's left is the cap, so my question is should I buy one from a dealer or some shop locally or are the ebay ones alright (specifically one with the temp gauge built in for 40 dollars?) also if that would be alright to run what is a normal kind of temp when fully running the bike the way it should be ran?

Edit: also if it was an air pocket how do you bleed that out, theres no bleed screw like on some bikes and I figured that the cap is highest point in cooling system that you just open that after its been running or whilst its running maybe?

How caked in crud are your rads? Give then a jet wash and you may have a nice bike again.
But yeah replace rad cap as they're cheap.
Also 100ml of coolant is a but load think my 360 only holds 800ml .

yeah my radiators are clean I normaly wash my bike well every ride, and I'm going to replace cap without question I just don't know if they ebay ones with gauges built in are shit or would last a while, and yeah myn holds around the same id say and I reakon I may have lost between 50-100ml if I had to guess... I'm unsure but am I correct in thinking the rating on caps are pressures, so a 1.1 would release before a 1.8 (modern) one would? only thing with that if its the case is the bikes prev never had a problem lol

The Chinese caps look good, and are right.
Have been using one for a year now no issues.
Well if rads are clean head orings are mint and no warpage then it's got to be your mix of anti freeze or rad cap .
Not sure how you check impeller is working but an air pocket is highly unlikely.

basically as far as I know you take radiator cap off and if you see it swirling and come out of the return pipe at top its recirculating and working.. just unsure how its working under load as cant check it whilst moving, and yeah I thought a air pocket though I'm unsure how to get air out without a bleed screw and also its not first lot of coolant ive put through it since prob started so I'm not sure that it would have a pocket each and every time

Lean the bike side to side and kick it over to get any air pocket out.

my 98 RM 250 was doing the same thing you are talking about. I pulled the carb off and all kinds of crap in the bowl.milky looking oil seperated from the fuel, drained all fuel cleaned carb mixed up fresh fuel and oil and also drained all coolant and replaced with engine ice coolant. cured the surging and boiling over. definitely worth a check .

ranhcer1 that's a awesome tip thanks, and wrx ive cleaned the carb multiple times and it hasn't changed the surging at all and I'm unsure on the boiling as ive only had 2 shorts rides since and it didn't seem to boil but got hot (unsure if its to hot though as ive never really checked heat before it boiled that one time), and ahh I fixed the surging for now using air screw and idle screws its not 100% but its almost back to normal.. id say I need to change a pilot jet size from what ive been reading though I just don't know why as the weather and such hasn't drastically changed or anything lol

I think you are on the right track by checking the pilot jet. It might be starting to get plugged and cause your surging. Good luck and hope you get it sorted!

I cleaned the jets well as they was blocked when I had the boiling issue to start with, since then it still surged a bit though I'm thinking might be just enough of a change in the climate to make an issue with it so will trry changing it at some point, trying to fix the over heating issue first so bike is still ridable... at end of the year I'm buying a brand new bike so its really only gotta last the year and possibly a spare bike after that if I don't sell it