May 19, 2001
My friend just bought a very low hour, unraced 00' 400 SX. He rides tight woods, and found the bike overheats and the over flow burps onto the exhaust when he rides slow through the tecnical stuff. . Kinda messy. He has installed DEVOL rad guards, and is about to get some REDLINE coolant wetter, which will hopefully help with the over heating. He wants to run the bike without the T-Stat but I told him not to, you will get hot spots in the engine and possibly damage it. Are there any other suggestions he could possibly do? Like removing the rad guards, as they restrict the air flow thru the rads?

Also he is finding the bike does not have smooth clutch engagement, When he releases the clutch the bike sorta wants to lurch ahead. Maybe the 4 speed tranny i guess. The bike has a 14T front sprocket and a 52T rear. It is geared down but not enough. Would it be safe to run a 12T on the front or is this sprocket to small for the power of the bike.

And another question! HE was also wondering about using Stainless steel oil filters because the previous owner told him to change the oil and the oil filters approx every 4-6 hours of riding time. THAT can get expensive! are there any draw backs using the stainless steel oil filters?

Any input would be very appreciated. I just dont want him to be dissapointed with his first KTM and it will not work for him for the type of riding he does.
Thanks again


Jul 20, 2001
There is a KTM electric fan kit I put on my 520 EXC, seems to do the job. Thermostatic switch screws into bottom of left radiator, fan mounts on right radiator. Might take some adaptation to make work on non-electric start model though.


Jul 21, 1999
The 00's had some overheating problems but mostly when stopped. The common wisdom was to shut them down when ideling. Easy on the MXC and EXC as the start was a button away but harder to want to do on the SX, of course. The '01's have one column wider radiators and do not seem to suffer the same problems although it's still best to shut them off when ideling. Water wetter and fresh antifreeze might help some. Mix a little anitfreeze in as you feel safe with.

I run a 13t but I don't know anyone running a 12. My bike eats up CS sprockets pretty fast. That's the drawback to going with a smaller one. He may have to bleed the clutch line if the engagement is not smooth.

SS filters is an ongoing debate. The official word out of KTM, the last I heard, at least, is that they will void your warranty. That's good enough for me so I use paper. He can get pairs of filters from Cycle Zone KTM for under $10 per pair. Toll free 866-843-7433. I change the filters every 3rd oil change.

Smokin Joe

Sponsoring Member
Apr 21, 2000
Thoughts on over heating and c/s sprocket.


Although I don't have an RFS, I do have a '97 250 EXC that is prone to over heating. I installed Devol guards which just made it worse. After posting here and on Holeshot's forum I made the following changes:

1. Switched to Water Wetter at 4 capfuls per quart of Distilled Water. This helped but did not completely eliminate the problem. I also spaced out the rad shrouds as the '97's did not have a wide front opening.

2. I used a cut off disc in my Dremel and cut out the front grid of the Devol guards. I then shortened the height of the stock louvres to fit in the opening. The other thing I did was to drill 1/2" holes in the back of my front fender to allow more air to get through. It did not overheat after these last mods.

The fan is a good idea also and it was something I was going to try next if I still have problems. I think KTM makes an overflow bottle that fits inside of the rad shroud but I am not sure what models it fits however.

As far as countershaft sprockets go I seem to be one of the few people that regularly run a 12 toother. I definitely think it is the killer way to go for tight woods. It gives you a granny low that a spode like me fully appreciates. Plus it tightens up the ratios such that being in the absolute correct gear is not as critical, that really helps out in situations when it gets real gnarly and you'd rather not shift or can't. You don't need to rely on the clutch quite as much either. The downside is reduced top end speed. Not a big issue with me as I ride trails where I rarely get above 3rd gear and can still get up to 50 mph or so (fast enough for me where I ride). Also as mentioned it wears out chains and sprockets faster but as far as I'm concerned that's a normal wear item anyway and they still last a good while as long as you buy top shelf items. Economy chains and sprockets could be another matter though.

Good Luck!



Jan 5, 2001
You have stated that he has a 400SX. The SX models did not come with a thermostat, so removing it would be a difficult task. The rad guards are reducing the amount of air flow thru the rads and making his problem worse. I would like to see what kind of jetting he is running. Most times an overheating RFS can be helped with a little jetting work. I do not know if the rad fan will fit the 2000 models, I believe that the kit was meant for the 01's with the bigger rad. It will be difficult to keep his bike form overheating in a tight woods environment.


May 20, 2001
Originally posted by kdxdude
Also he is finding the bike does not have smooth clutch engagement, When he releases the clutch the bike sorta wants to lurch ahead.

I find the hydraulic clutch, while having a lighter pull, also has less effective (between fully open and fully close) travel. On my dad's 380 sx's the clutch is completely consistent, but harder to fan than my regular cable clutch on the YZ250. However, the more you keep the revs up, the better the clutch gets. At least the 380 seems to work the better the harder and faster you ride it. It's not at it's best at slow speeds and non-aggressive riding.


Apprentice Goon
Mar 16, 2001
Charlestown, IN
Have you (err...or your friend) considered heat wrapping the pipes? I have it on opinion from a very reputable 4-stroke man, that all 4 strokes will benefit from wrapping.
I'm not an authority, but something about the effect of gasses actually being able to return to combustion due to the effects of the wrap at high rpm's, and a dramatic decrease in temp at low rpm's.
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