dilema,help please

ok this is the situation

i curently have a 98 yz400.

ive spent a lot of time fiddling with this bike.

and i can honestly say it suits me pretty perfectly now.

i ride enduro and hare and hound type events.

however it over heats and boils over.

so i tried a higher pressure radiator cap, this stopped the boil over but put extra pressure on the rest of the cooling system.

ive blown the O'ring at the front of the cylinder 4 times now, and get water dripping from it.

ive gone from a 45 pilot jet to a 48,engine runs crisp, without being lean.

ive also tried watter wetter.

the impellor works fine.

so what next,, i know 426 and WR radiators are one core bigger which im sure would help.

but i just cant find any.

There is a guy in the UK who will make me a set of bigger radiators ,which will cure my problem,,, but the cost is £400


I dont really want to spend this on a 4 year old bike, if i can help it.

What are my options?

Is it posible to fit a x400 oil cooler

or could a small extra radiator be fitted behind the the front number plate?

so in effect id be running 3 radiators?

or should i just give in buy a KTM520 and start all over again?

any help would be kindly received,,,thanks

are cr500 radiators bigger,,

cud they be made to fit?


You really need to post some more details on the bike before you jump to conclusions.

I'm afraid that the direction that you have and are taking with a solution is not in-line with the culprit of the problem.

In other words, your bike should function without these problems, as is, from the factory.

This could be an issue of fuel, timing, or you may have a blown head gasket...for starters.

Have you done anything to verify that the water that is "boiling over" is really at a temperature in which boil over should occur?

Have you examined the plug? What is its condition?

What other mods have you done to the bike?

Have you taken temperature readings in various engine and radiator locations to narrow down hot spots?

Let's start here, but till then, save your money.


dave its always had a problem with over heating , as i believe many 98 yzf have when used for trail riding.

relevent mods to the bike.

WB E-series silencer with 12 discs on non tapered high boy header pipe.

Higher presure radiator cap

WR over flow bottle

Fuel squirt dropped from 3 seconds to <0.5seconds

pilot jet from 45 to 48

running mainly at sea level.

type of riding varies from gunning it across the top of a welsh mountain. to bog hoping at 5 mph

yesterday i rode for 10 hours in wales

top speed was 38 mph average speed was 13mph

the trouble is the radiators on the 98 yzf is 1 core narrower the the WR's and 426

So id say the reason the bike over heatsis, im riding this yzf like it is a WR,, not just blatting round an MX track.

on top of that the radiators look like they are off a 4 year old bike. fins flattened in a few places etc.

the header pipe exhaust clearance is at most 1/4 inch.

i know deep down the solution is a new bike, but to honest ive not yet ridden a bike that i feel is worth the extra cash.

Ive budgeted for a new bike next year but,ill probably still keep this bike

also 426's and crf450, just dont seem as fast as my 400

[ August 12, 2002: Message edited by: Mark_UK ]

Add a lighting coil and a small fan from a road bike. :)

This was happening to me yesterday on some slow, steep, rocky trails. All I needed to do was get some air flowing back over the radiator. I could feel the heat from the exhaust and knew that this was going to happen soon. Sure enough...it did. I got off the mountain and rode around on the desert floor and all was fine. I just kept the bike doing what it was originally built for. Going fast!!


Before we get into this too far, I'll assume that you have checked the little things like fuel screw, the air filter, completely cleaned all of the passages for the air to flow through the radiators, and that your exhaust system is free and clear including a quick look up into the head.

These little things can make a big difference, so go at them with as much attention as everything else.

If all is well, then proceed with the following.

1. Choose and mark about 5 to 10 areas on the cooling system that you can monitor. Places like bottom and top of left and right radiator, cylinder left, right and front, head left right and front, and various hoses leading to and from the engine. Note each area on a piece of paper by number or by a small drawing of the bike. Your manual may have a good cooling route diagram to copy from.

2. With bike fully cooled, say from overnight, put it on a stand and start the bike up, getting it to a slow idle as soon as possible.

3. Then either by feel or by infrared, measure the temperature of each area as the bike goes from cold, to warm to hot. Do a round of tests every 30 seconds or so and note on the paper according. Continue to do this until boil over occurs.

4. Examine the notes. You should find that the cylinder should have been hot to the touch first, followed by the head. During this, the radiators should have stayed cooled. Then, after 3 to 5 minutes, the radiators should have taken the first flow from the engine and dramatically climbed in temperature. Both left and right should have increased together, top and bottom. If this is to not the case, you may have a flow problem.

5. If you don't have a flow problem, let the bike cool and drain all the cooling fluid from the bike. Examine the fluid very closely for any signs of oil or contaminants. Small signs of oil would be worst-case scenario. Rust and particles are second case. If you find oil, you have other problems and can stop here. If you have a lot of other small particles or rust, you’ll want to flush the system until it’s perfectly clean.

6. If you don't find oil or excess particles, refill the bike with nothing but distilled water. You can add some water wetter if you wish.

7. Adjust your fuel/air screw 2 to 3 turns out.

8. Start the bike up as before, with the radiator cap removed so that you can clearly see the water flow over the top of the radiator core. Watch the water very closely (with safety glasses on) for any small bubbles. Rev the bike occasionally as well. If you have any small bubbles you have a leak in the system. The other option would be to pressurize the cylinder, engine hot, with about 150lbs psi at BDC.

9. If all of the above checks out well, you may want to verify the ignition system. Check readings on the TPS, CDI and coil, (when hot) and verify the timing mark on the magneto to TDC. These are not likely culprits, but still worth checking. Again this area is unlikely, but should not be over-looked.

10. Beyond this, your options are to richen each of the four circuits within the carb. Start with idle screw and pilot jet, then main and needle. Of course, make sure the air filter is clean when you do this. Small changes can make a big difference.

11. Then if all of this fails, I would begin to look at other options, the first being a vented front number plate. I run one of these and it makes a huge difference. Pro-Circuit has them, or you can make em as well.

12. From there you could upgrade to larger radiators and/or a fan. An oil cooler would be useless and not recommended.

Obviously, the process is to verify all other possibilities before you re-engineer the bike. I have had plenty of bikes that ran hot due to other problems I resolved with a simple turn of the screwdriver, or an engine rebuild. Do this wisely or you may waste a lot of time and money. Besides, you’ll want to be able to sell a bike that is fully functional as well.

Best of luck and let us know how things turn out.


thanks for the advice dave ill work through all those things this week and let you know how i get on

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