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I pushed the oil temperature of my air cooled Honda XR250R upwards of 350 degrees during 2 hour hare scramble races (it the sole reason I started using synthetic oil) and if the oil was 350, the actual engine temperature was somewhat higher. My liquid cooled Honda CRF250X never sees oil temperatures above 250 degrees, but the engine temperature is never above about 230 - the cooling system on the CRF does not have a fan, it simply relies on sufficient forward movement to provide air flow through the radiators. I have heard the coolant boil on rare occasion, but the engine has never been overheated and there is a fine line between boiling coolant and overheating.
 

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Wow 350* oil temp that's pretty hot. Was that an early XR250R with a wet sump oil system or a later dry sump model with the oil tank in the frame? Even the crf seems pretty hot.

The unicam CRF probably isn't helped by the fact that the engine is wet sump and only holds about .5 quarts.

It's been my experience that fast dirt bikes are for the most part undercooled, or barely adequate to keep up, they HATE to sit idling once hot. Gotta keep moving, because, as you said most don't have a radiator fan. Some do, but they are still a tiny fan. (I actually though that CRF "X" bikes had a fan?) I know some of the XC bikes have a fan and coolant recovery tank and are harder to overheat. 2 stroke in general seem less prone to overheat than the comparable 4 strokes but still will. My CR was pretty bad for example.

I've owned a CR250R and a YZ450F and they were both pretty easy to overheat to the point that coolant would start coming out the overflow hose. Most notably while not moving, on really tight 1st gear trails or in the sand with a paddle tire. If not in sand and you kept a good pace it wasn't an issue. The CR did boil if pushed in the sand, never had the YZ450F boil.

On the other hand both the YFZ450 ATVs that I have are sand monsters, and never overheat. They both have cam mod, zip tie mod, oil mod, FCI intake, carb jetting NVCQ needles and exhaust. They are very close to their dirt bike brother for power. The ATV engine is extremely similar to the dirt bike. Most parts even interchange. But the ATVs have twice the oil tank capacity, a bigger radiator, coolant recovery tank and a fan that is considerably bigger than the trail bikes have.

I've seen too many unicam CRFs eat valves and seats to want to own one, how has your been? The Yamama valvetrain has really impressed me, they rarely need shim changes and if not abused usually last way longer than the recommended service intervals. The 5 valve Yamahas anyways. I don't have any 1st hand experience with the newer 4 valve engines.
 

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I had a 79 XR250 when I got back into racing in the late 80's - it was a wet sump and I never checked the oil temperature, but I never had a problem with it either. I just ran (at the time it was one of the best) Castrol GTX 20w50 in it. Then in Dec of 97 I got the 97 XR250 which was dry sump oil in frame. I broke it in on Honda GN4 10w40 as recommended and switched to Castrol 20w50 (I don't remember now, but I believe Castrol dropped the GTX brand and introduced Actevo). I was not having any problems as it is my habit to change the oil after every race, but one day I saw a dip stick thermometer advertised for the XR and I bought one. It was then that I found out how hot the oil was getting.

It was a revelation to me and it explained why I had to replace the steering head bearings with less than a year of use on them. The frame was hot enough all the factory grease had melted out of the bearings and they rusted. I installed new bearings packed with High Temp Waterproof grease and between that time and 2004 when I got the CRF, I never had another bearing issue, but I was still keeping my eye on the oil temp. Mineral oil starts to break down around 300* F. I figured as long as I was changing the oil after every race it would not be a problem, but it was stuck in the back of my mind. One day I was talking to oil guru's at an industry trade show and they were extolling the advantage of synthetic oil. I got into a conversation with one who explained and demonstrated what happens to oil when heated over 300*. I came away with a better understanding, a confirmation that my oil change routine would prevent the problems associated with overheated oil and a sponsorship for Silkolene Synthetic oil. I started running Silkolene 15w50 oil and while the oil temp did not change dramatically, I was comfortable knowing that the synthetic oil did not start to break down until heated above 350*. I still changed the oil after every race.

Now when I got the CRF, I broke it in on the GN4 by trail riding for a couple months and keeping track of the mileage (the 250X came with an odometer). When I got about 250 miles on it trail riding, I switched the oil over to Silkolene and went racing with it. It holds 750cc oil in the wet sump engine and about 750cc of two stroke transmission oil in the transmission. I immediately found out it consumed about 200cc of oil in a 2 hour race. I knew the engine consumed oil as it was a warning in the owners manual (it's a cost of racing), but it consumed about twice as much synthetic as the mineral oil it was broken in on.

I found out it did not need synthetic as the liquid cooled engine never got over about 230* coolant temperature. I used a laser thermometer to check the oil temp and it was approx. the same as the coolant temp. The X offered a fan option, but I decided I did not want the extra weight and the bike never ran slow enough to need a fan. The X has a coolant recovery tank so any boil over would be captured and returned to the cooling system upon cool down. The only time I heard the coolant start to sizzle was once on a photo shoot, once while riding with a group of youths and once while waiting fro a traffic jam to clear an obstruction on a hare scramble course.

I never had a problem with a valve seat and the only owners I know of who did liked to let the engine idle while they peed or stopped for a 'short' break. The intake valves are Titanium and when the special surface coating is worn out it's bare Titanium banging on the steel seat and they wear out very quickly. If the valves are replaced routinely during normal maintenance, you will never have a problem with them. I did get tired of replacing them each year and my racing was not serious anymore, so the last time I rebuilt the top end, I installed stainless steel valves. That was about 3 years ago.

Fast dirt bikes are not supposed to sit idling or go slow - they are meant for going fast - if I want to trail ride, I have a trail bike - I don't race a trail bike. On the other hand, Trials Bikes are meant for going slow and idling, but a trials bike can go fast. Just have to choose a machine designed for the intended use. I don't ride a dirt bike on the street either.
 

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FPD218, thanks for the msds, I'm not a chemist but that looks like standard ethelyne glycol, correct?

Latebird, thanks for sharing your experience. I'm shocked that that 97 XR got so hot. I was under the impression that the dry sump arrangement helped to control the temperature better, but apparently not. I use synthetic for the cold start flow and longer drain intervals on my cars, but that highlights another reason to use synthetic in small engines. Currently I just use mineral oil in my ATVs and change it very often. but am thinking about switching to synthetic. I use rotella by the way..


About valves, your experience matches mine. I haven't owned a unicam CRF, my brother does and he loves it, but he is always in that engine (sometimes because it needs it sometimes because he's after more power) most my experience comes from working in an independent powersports shop from 07-13. We had alot of unicams through that needed valves a few that ruined the seats. If the head was good, then stainless valves seemed to give the best longevity. Granted alot of those bikes were absolutely thrashed. I can't think of one Yamaha that wore the valves out (maybe I'm just forgetting because its convenient lol) the 5 valve yamaha heads are all coated Ti valves as well intake and exhast. they still fail of course but were catastrophic failures related to people thinking that the rev limiter should be tested frequently, and not changing pistons.

I agree, you have to choose the right bike for the intended purpose. But I have a problem where I want the best of both worlds, I want the power, light weight and suspension of a motocrosser, but a wide ratio gearbox, decent charging output, and a radiator fan big enough to keep up.. I'm currently without a dirt bike, as ATVs fit my requirements better ( where I currently live there aren't many single track trails) I'm currently weighing my options but most bikes that fit my requirements don't fit my budget for a dirt bike, suprise suprise lol. I like the BRC 500 the best right now, the new yz450fx, wr450f, and ktm 300 xc-w are on my list of contenders. I also detest the tire maintenance of dirt bikes and like riding in sand and snow, so I'm very interested in the big wheel conversions and snow bike kits. It will probably be a while before I get another 2 wheeler though. I'm enjoying switching between the yfz450s and the big bore 4x4 ATVs.
 

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Mendomotox - the temperature of the oil is what made me change me to synthetic oil.

If you had been in the shop in 01-02, the YZ250F underwent a recall for intake valve breakage - the smaller intake valves in the 5 valve Yamaha do not wear as quickly as the intake valves of 4 valve Honda due to lighter springs, less mass and cooler running temp.

I will gladly trade the tire maint. of a dirt bike for the decrease in maint of a quad. It cost me about $10 at the car wash to wash off my CRF, but it cost over $20 to wash the quad and then it is still to dirty to work on. All we have to ride in central IL is either flat farm land or tight timbered ground. I hate riding a road through the woods, so I prefer two wheels and making my own trail. I really love riding where no one has ridden before, so I'm well known for cutting new trail.
 

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Latebird, yes that seems like the best explanation on the 5 valve longevity, lighter weight, requiring less spring to control the valve and I would add, less required valve lift to achieve the same flow/power.

I still think the ATVs are easier to maintain I just use a hose and brush to wash at home. Seems like the vast majority of maintenance I do is fluid changes and tire maintenance the sportsman xp in particular is very well laid out for fluid changes and I just prefer ATV tires over motorcycle tire changes. But I guess it's all subjective :) ride what like.

I do miss the freedom of a dirtbike, there is no faster way to cover ground in the hills that on a dirtbike. "Hills" real mountain with tight trails, are about 3hrs in any direction for me though (south east washington state) it's all wide open deserts, sand around here. There are still some places I'd prefer a dirtbike but it's not often enough for me to buy another one at this point. One day :)
 

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I use Prestone in everything, ATVs snomobiles, motocross bikes, sportbikes, domestic/japanese/german cars, diesel trucks, etc. I've never seen or heard of any failures or accelerated wear due to using "cheap" or "regular" coolant. In fact the "best coolant" you can buy is actually just plain, clean distilled water as it has the best thermal properties. Although that's not advisable for extended periods of time as you do want the corrosion resistance, freeze protection, higher boiling point and lubricity provided by the ethylene glycol.

So in conclusion I run a 50/50 mix prestone and distilled water and have never had an issue. I think any brand coolant will work but I like the color of prestone and its widely available.

It is advisable to make sure that whatever coolant is in the system is compatible with what you add to it as some formulations can react badly together causing gelling or possibly other side effects. It would be my advise to drain the old coolant and re fill with standard coolant if it is an odd color. It's probably due for a change anyway.

Checking for leaks is good advice. At the water pump weap hole, radiator hoses, radiator, thermostat etc.
Thanks to you both. I know absolutely nothing about atv’s and just bought my first. So this was extremely educational. It may have seemed like a debate. But I felt like I was learning from two people who know a lot. I’m officially educated and can’t wait till I’m in a position to have this conversation with someone. Thanks
 
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