BK: Help!! radiator boiling over on starting line

9 replies to this topic
  • blakemx124

Posted February 22, 2001 - 12:39 AM


Brian, we race MX and alot of the tracks in our area have concrete starting pads. While sitting on the line, our 250F radiator starts blowing water out the overflow, getting the concrete wet & screwing up our starts. What do you recommend? Overflow bottle?higher pressure radiator cap? Boyesen impeller?What do you guys do with Ferry's 426? Anyone else out there with some input? We are using "Water Wetter" presently to help but still have the problem.

  • Boit

Posted February 22, 2001 - 01:14 AM


If you are filling up the radiator to the neck when cold, you aren't leaving room for thermal expansion. What you are experiencing may NOT be abnormal. I thought my 426 was running hot by boiling over a little when on the starting line until I quit filling the radiator to the top of the neck. If you leave about a half inch or so below the inside of the neck flange(when cold), the boil-over should stop. You must leave a little room for thermal expansion.

  • dirtdad

Posted February 22, 2001 - 05:42 AM


blake, you could also try a 65%(water) to 35%(coolant) mix. Water actually cools better than the coolant, but this still leaves enough coolant to lube the internals of the cooling system. I've also read alot of favorable comments on Water Wetter though I have not tried it yet! Hope this helps.

  • forloop

Posted February 22, 2001 - 06:33 AM


Try some waterwetter.

01 YZ426F #85 Vet C

  • fastkevin

Posted February 22, 2001 - 06:36 AM


I know somebody that switched to a higher pressure rad. cap. Said it worked for him. I think Fontana Radiator ( they advertise in Cycle News) is where he got it.

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  • JBM

Posted February 22, 2001 - 07:07 AM


I read somewhere that some of the factory guys use a small overflow bottle to catch the coolant. They are trying to avoid the exact problem you are having, spilling over on the concrete starting pads and losing traction.

  • daveyg

Posted February 22, 2001 - 07:30 AM



Whenever you mix coolant with water, make sure your using Distilled water. It's free from any impurities, minerals, etc and won't corrode your system. Also, using a larger water to coolant ratio will allow the bike to run cooler. I've heard of many racers using 80% distilled to 20% coolant. Just my two sense....Maybe BK can clear up some of the myths....


  • brooks

Posted February 22, 2001 - 03:41 PM


You didnt state how long your sitting on the starting line with the motor running. I dont think the four stroke was made to be revved like a two stroke while waiting for the gate to drop. The bike warms up quick while sitting still. Try to remember this and the above good ideas. good luck

  • BK

Posted March 27, 2001 - 07:09 PM


First, Four strokes don't need to be warmed up as long as two strokes, and they hold their heat longer. The overflow is normal and will seem excessive when you first top off the radiator. There needs to be room for expansion. The coolant will find it's happy place after a few rides. You might want to try a stronger cap though. We run a cap that is rated at 1.8 kpa that is converted roughly to 22 psi.
let me know how it works out.
Yamaha Factory Race Team

  • Dr_Cox

Posted March 28, 2001 - 05:44 AM


Originally posted by JBM:
I read somewhere that some of the factory guys use a small overflow bottle to catch the coolant. They are trying to avoid the exact problem you are having, spilling over on the concrete starting pads and losing traction.

Ideal thing to do is purchase the expansion bottle and hoses for the WR400 (or WR426). One of the hole fixings is already on the bike, (one which secures the mudguard) and the other mounting point you have to make from a piece of metal (Aluminium or steel), in the shape of a 'T', then fis this to the frame with a couple of jubliee clips. Drill a hole int he 'T' and you have your second mounting point. one hose then replaces the overflow radiator hose and is routed to the bottom of the expansion and the other hose from the top of the expansion bottle to the same sort of place where the carb venting/overflowing hoses are located.

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