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Overheating after rebuild

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I just had to rebuild the top end on my bike Because the primary nut backed off and my valves were wrecked. Since it was apart any way I ported and polished the head and put a higher compression piston in (original 12.2-1 and new is 13.5-1). When I put it all back together and started the break in I followed all the instructions for refilling the fluids but the bike seems to be overheating if left to idle for 20 min or so. I rode the bike for a half hour or so and it seemed to be fine but once I stopped and left it to idle it overheated again. Is this normal with the new piston? If so is there something i should do to make it run cooler like a lower temp plug? Or is there something more going on? any feedback would be great!

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It could be lean? It could be the piston cyclinder clearance is tight? Or it could be nature of the beast, I know a couple of folks with high compression DR-Z motors setups and neither will sit and idle for an extended time without overheating.

Edited by Windblown

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Lol. Thanx for the help guys. There's no fan on the drz "e" model but I did unscrew the bleeder plug on the left rad when I filled it up. I was trying to be a little bit nice to it by letting it idle for a bit after the rebuild but I guess I might as well just start riding it again and see what happens :bonk:. The guy who had it before me bent the right rad a little bit so I'll probably just replace that and buy a temp Guage and just keep an eye on it. Thanx again!!

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A new motor should not be left to idle. Start it, ride it, break it in. Idle not good.

OK you are beyond that now. What model is it? Does it have a fan? Does it run hot when riding it? You are running higher compression, I would expect a little more heat.

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It's a klx400r which is the same as a drz400e. There is no fan or thermostat. There's no temp Guage on it so the only way to tell if it is overheating as far as I know to watch for it to start puking coolant out of the overflow reservoir with is tricky to do while riding but I did take it out for about a half hour and it was fine. I think it's probably just running hotter due to the new rings and higher compression but I wanted to make sure. I'm going to get a temp Guage on it before I take it out for too long so I can keep an eye on it but I'm wondering if a cooler plug might be a good idea too?

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Colder plug affects only the temp of the plug electrode. It has no affect on the temperature that the motor runs at. Google spark plugs for a better explanation.

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You should check out the motorman's break in secrets power news magazine. Idling is not part of the procedure. You want to warm the engine up to operating temp and then give it a couple of full throttle blasts, 1st 2sd 3rd, I would stay out of the higher gears for now, do this a couple of times then let the engine cool for a bit then repeat. You want to be at operating temp but not hot. You should probably do this several times. The combustion pressure in the cylinder at full throttle goes behind the rings and pushes them out against the cylinder wall breaking them in. This should be done with in the first few miles of the rebuild. If you wait too long the cylinder will glaze with burnt oil and become very slick preventing a good break in.

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1st thing that comes to my mind is air pockets in the rads..

i've never messed with the bleeder screws. i just lean the bike over to the left side to get the left rad full.

after letting the bike cool down, lay it over to the left side, stand it back up & check coolant level in rad.. if it's full, this is not the problem. if you need to add coolant, it may be the problem.

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I agree that this is not how I break in a new piston / cylinder - but if you let any bike w/o a fan sit and idle for 20 minutes - it is going to overheat. Nothing abnormal here. :bonk:

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You should check out the motorman's break in secrets power news magazine. Idling is not part of the procedure. You want to warm the engine up to operating temp and then give it a couple of full throttle blasts, 1st 2sd 3rd, I would stay out of the higher gears for now, do this a couple of times then let the engine cool for a bit then repeat. You want to be at operating temp but not hot. You should probably do this several times. The combustion pressure in the cylinder at full throttle goes behind the rings and pushes them out against the cylinder wall breaking them in. This should be done with in the first few miles of the rebuild. If you wait too long the cylinder will glaze with burnt oil and become very slick preventing a good break in.

carbide glazes ?

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Alright. I layed it over a few times when I filled it up so it must be fine. I'll just start riding it how it says to in that article and switch my oil and hopefully I didn't ruin it already. Lol it's suposed to be +3 here tomorrow so ill be able to get out again. I'll let you guys know if I blow it up or not !!!!!

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carbide glazes ?

It's not really carbide, it's something they call Suzuki Composite Electrochemical Materials (SCEM). More like a hard chrome. If you look at a used cylinder they have a brown tint to them. Thats the glazing.

I don't think there's anything wrong with your KLX. You have a higher compression piston in it and that makes more heat. The 400s have always overheated if left idling for too long. Recheck you coolant level and go have fun.

Edited by james509

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Rode the bike for a couple hours today and no issues. Runs great and is way more powerful with the new piston and head work. Thanx for the info guys!!!

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