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620 over heats

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My 96 620 RXC overheats easily. Things I have done.

Chemical clean cooling system.

Running the waterless coolant from zip-ty racing.

Running the oil additive (supposed to run cooler)

Added Radiator fans

Running fuel mixture rich as indicated on plug (off road mod and pumper to quick silver carb up jetted next bigger needle in carb)

Changed radiator cap

Changed thermostat

Checked no kinks in hoses,

radiators look good, no banged up fins free air flow

FMF mega max pipe with 13 diskks in it.

Currently have water pump apart and it looks good, no rubbing and good tolerances (but I broke a fin pulling the impeller and will replace it (ordering it tomorrow).

I am so perplexed. Ive had this bike since 97 (bought new) it has been over heating since around 2000 anytime I get below about 10 MPH for over 5 minutes it starts puking coolant and eventually the high temp light comes on.

Hi temp has caused me to have to replace two stators (internal grounds)

Any ideas what to do next? Does anyone know of an aftermarket waterpump manufacturer (higher volume)?


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I have had similar experience with my 1996 620RXC

I replaced the crap QuikSilver carburetor with a Dellorto. I use Evans NPG (same as ZIP-TY) to stop boil off. I think the jetting is good.

My major recommendation is to back off on the ignition advance. I found that the stock timing marks resulted in a bike that was difficult to start, stalled easily and was prone to overheating.

I can now use the bike on easier trails but I still stay out of the slow, tight woods to avoid overheating. I have considered adding a radiator fan with a view to running tight trails. Do you think that the fan will have a major effect on temps? You stated “fans”…are you running more than one? Did you use the KTM fan kit 59035041044? (see H&H page: http://www.ktminfo.com/ktm.asp?p=9&s=1&h=2&o=3&t=16&pg=4 )

Were any mods required to fit that fan kit to the RXC?

My opinion is that KTM (at least at that time) was willing to let the buyer complete development of a very flawed bike; it is the only bike I have ever regretted buying. It has kept me from buying another KTM.

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pulled the impeller from the water pump and replaced it. Talked with a KTM guy here, he said run the RPM higher.

Put a thermometer on the side of the primary radiator, let it idle till it got to 205 F. Raised the throttle up to about 1/3 (probably 2000 or so rpm) temp shot down to 180 in about 30 seconds.

Seems the water pump is inefficient at low rpm, he told me 1800 is minimum with low air flow.

I put 6 hi volume CPU cooling fans (3 each radiator) have been running them for about 3 years, does seem to help a low speed. Manufactured the mount myself, fits between the radiator and the plastic inlet fins.

Rode the bike for about 2 hours sunday. low speed but high RPM, no overheating, temp was about 80 where I was riding.

This may be the fix.

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