Top end rebuild = Running HOT

I did a basic top end rebuild on my 2008 kx450f, cleaned the carburetor, and installed an hour meter last week.

The bike runs, sounds and idles perfectly, but the head pipe began to glow during my first few startups. I was not alarmed because I know that the break-in requires a few heat cycles.

I now have about an hour on the motor, but the coolant still boils over after about 5 minutes of mild riding. I recleaned the carburetor and realized that the plunger for the hot start was stuck (now fixed). I also made sure that the airscrew was set to the stock setting- 2 turns out. The plug indicated a lean mixture before, but I have not checked it since my recent 1/2 hour ride.

What else could this be??? Or is this normal??? :thumbsup:

Not normal to boil over as long as your keeping a pace with your "mild riding" Mine and most others pipes will glow while idling for any period. That being said, a lean mixture under load is the worst and most dangerous to your engine. It causes lots of extra heat and can bring on detonation and melt parts if severe. Add fuel if in doubt.

You say "mild riding" and "it runs fine".

Is the carb jetted right so the bike wants to rip your arms out when wide open in 2nd gear? Or is it running lean, thus hesitant or bogging on throttle opening, and/or taking ages to rev out when you hold 1/2 to it wide open?

If not lean, and you have air flowing through the radiators, and quality fuel, then it's NOT normal. Perhaps your coolant is not pumping, or your radiator cap is broken.

I have no twisted past half-throttle yet, but the throttle response feels the same as before the rebuild. The carb settings are the same, but I guess I could richen the airscrew by a half-turn or so.

I will keep a close eye on the overflow. The timing appears to be precise, but would 1 or 2 degrees cause this to happen? Since it sounds and runs so smoothly, this probably isn't the case.

The bike runs, sounds and idles perfectly, but the head pipe began to glow during my first few startups. I was not alarmed because I know that the break-in requires a few heat cycles.

Nothing to do with break in - 4 stroke pipes glow when left idling for even a short time - very noticable under low light conditions.

You need to know (or remember) these bikes were not meant to travel slowly at low rpm's.

When you idle them for extended periods that is bad, they will over heat. Same goes for moving slowly at low rpm's. The cooling system was designed to be efficient at or near wo throttle and significant air moving through the radiators.

With that being said you can get creative and check operating temp. in a number of ways. (heat strips, heat gun, or temp. sensor) With what you have described so far all sounds normal but there may be a few things that need checked. First verify the temp. then check the water pump and make sure it is rotating with the crank shaft. If it was too lean to be causing this you would notice it in performance.

Thanks guys. I will be sure that the plug looks healthy first. If not, I will make small adjustments to the carb and ride it slightly more aggressively.

Just for the record... what is approx. normal engine operating temp? I have a heat gun and temp strips on my head and radiator, and would like to know whats operating temp and too hot?

An engine is an engine, is an engine. I was searching for normal operating temp. numbers but lost track. I'm sure anybody with a FI bike that has the factory tool to remap could respond with what is normal for our bike but I would treat it just like most any race engine. 200 degrees fahrenheit to 220 would be acceptable. Anything above 240 is cause to worry. The best place to measure would be on out let of radiator.

Plug looks perfect. I guess I will go out and ride it a little more aggressively to see how it reacts

Rode it today at a little higher RPMs and a little cooler weather. Runs Great! Must have been building up too much heat and not circulating the coolant quick enough.

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