Engine time

Hi,

I purchased a '99 YZ 400 that is in excellent shape (engine runs strong and body has not been abused). The gentleman I bought it from said he estimated it had 100 hours or less on it. My question, I am reading about complete rebuilds, replacement of valves and cams after 50 hours or less on these bikes. Is this if you are racing or really pushing the bike to its max? I do recreational riding and perhaps an enduro or HS once in a while. If I do the routine maintenance what sort of life should I expect from this bike? While I have no smoke or starting problems, am I wise to replace the piston, rings and knuckle now so I have a benchmark hours wise? Or just have fun and ride it until I notice start or smoke problems. I don't want to have it grenade and cost me a small fortune.

This question is actually a good one for any second hand bike purchase, as I also bought a KTM 300 EXC that I have no historic records on.

Your input is greatly appreciated.

I am in the same predicament as well. Mine has about 3 times that many hours, and I dont do any sort of racing. I do get a hint of blue smoke if I blip the throttle on startup. After 30 seconds or so it will not do it.

I dont really wanna spend money because it seems to be running good, but the way I see it $200 or so is alot cheaper than having the thing gernade on you when you're in the middle of nowhere.

I am in the same predicament as well. Mine has about 3 times that many hours, and I dont do any sort of racing. I do get a hint of blue smoke if I blip the throttle on startup. After 30 seconds or so it will not do it.

I dont really wanna spend money because it seems to be running good, but the way I see it $200 or so is alot cheaper than having the thing gernade on you when you're in the middle of nowhere.

A puff of smoke on startup or until engine is warm is a sign of leaky valve guides, just so you know. But anyway, I would tear it down with that many hours and do at least a piston and timing chain.

( As for the guy with the 400, I would tear it down and just do a camchain, piston, and rings. cheep security for a good bike, that will last a long time.)

A puff of smoke on startup or until engine is warm is a sign of leaky valve guides, just so you know. But anyway, I would tear it down with that many hours and do at least a piston and timing chain.

Since it dosent smoke unless I blip the throttle, I'm assuming its the rings, and not the valve guide seals, right? This would be because the rings are still cool, and have not yet expanded, allowing a little oil to get past. Also, if it was the valve guide seals, wouldnt it smoke on deceleration too?

I guess I'll see when I tear her apart.

... if it was the valve guide seals, wouldnt it smoke on deceleration too?
Not normally, no. If they're really bad, they can cause a puff as you get back on the gas after decel.

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