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04 YZ250 Running Hot

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Greetings everyone,

A friend of mine started up his YZ250 the other day after it had been sitting for a few months, and the header got so hot once it warmed up that it melted a hole in the hose coming off the water pump. The bike ran perfect last time he used it, so we are at a loss trying to find the problem. I suspect a vacuum leak but he can't find anything obvious, and the bike runs so hot he can't even run it for more than a few minutes before it smokes the hose (on his 3rd hose now).

I tried using the search function but didn't have any luck. Is there some shaft seal or other rubber part on these bikes that can crack from age and cause a lean condition? Any help would be appreciated - thanks.

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Pilot jets clog when not cleaned before long periods of storing. Try replacing the pilot.

If the pilot jet is plugged, it would be very hard to start and probably wouldn't idle at all.

The header glowing red is not an indication of lean jetting. Even when they are jetted perfectly they will glow red if you have the bike running motionless.

If the hose melted, it was probably because something was bent (radiator, header, etc.) allowing the hose to get too close to the header.

Edited by 2grimjim

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If the pilot jet is plugged, it would be very hard to start and probably wouldn't idle at all.

The header glowing red is not an indication of lean jetting. Even when they are jetted perfectly they will glow red if you have the bike running motionless.

If the hose melted, it was probably because something was bent (radiator, header, etc.) allowing the hose to get too close to the radiator.

spot on

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Thanks guys. I'll tell him to take a closer look at the geometry - that should be fairly obvious if that's the problem.

Although I am still not convinced he doesn't have a vacuum leak somewhere. Reason I say that my WR450 had a similar problem with running hot once and I pulled my hair out trying to find the cause. The bike would boil coolant and was hard to start. Problem turned out to be the choke knob - it had gotten slightly bent in a low speed tip over, and that was enough to let air in around the little shaft seal. You almost wouldn't believe me if I showed you the bent knob, but when I replaced it the bike ran normal again. These types of problems can be very subtle, so I thought I'd post here to see if the 250s were prone to any weirdness.

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