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How hot should these old motors run?

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I have a 79' YZ400 and of course it's air cooled. Now that it's getting warmer out the motors running hotter as well. Easily burning my hand at the touch. Being this a motor somewhat rare I'm paranoid about seizing it up! How hot is too hot??

Thanks...PB

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Thats a good question id like to know myself ,But engines reach certain temps and should stop .This is if the engine is tuned right if it is lean of course its goin to run hot ,if its rich its goin to be gutless and colder.... So if your carb is tuned, the bike runs ok and there is oil in it i don't think it will seas up . Also i dont think i can touch the head on my xt 200 after ive been ripping it realy hard

I know that i can touch the head on my friends kx 250 and its just like a coffee cup because it is air cooled

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I'd say don't worry. Air-cooling is perfectly effective. The switch to watercooled engines had very little to do with actual cooling and a LOT to do with tolerances inside the engine. A LC motor can be built with tighter tolerances, for more power.

These old motors may appear to run hot, but as long as you're not getting seizures, pinging under acceleration, you'll be fine. Make sure the jetting is on target and you should be good.

I run a 490 Maico which of course is air-cooled and never had a hint of a problem. Your YZ was built to be air-cooled. It's not some kind of design flaw.

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I'd say don't worry. Air-cooling is perfectly effective. The switch to watercooled engines had very little to do with actual cooling and a LOT to do with tolerances inside the engine. A LC motor can be built with tighter tolerances, for more power.

Uh, no. The reason for the tighter tolerances is because the liquid cooling keeps the temps down to a more limited range. The air cooled motors got a lot hotter and the loose tolerances were so the metal could expand and not seize the piston.

I would agree with the rest though. I wouldn't worry about it too much. They operating conditions that get you into trouble are slow speeds and lots of throttle. When there is no air flow there is not much cooling. Going slow in the sand or a technical trail on a hot day are the kind of things that cause overheating. It is harder to tell when an air cooled bike is getting too hot. It's easy on a liquid cooled bike since they boil over to inform you of the overheating.

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Thanks alot for all the info guys. I'm thinking of getting one of those temp guns. Anybody know off hand the max temp this motor should see? I would like to add that this is mounted in a Blaster chassis and might not be getting all the air that it did in the dirtbike frame. Thanks again...PB

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Uh, no. The reason for the tighter tolerances is because the liquid cooling keeps the temps down to a more limited range. The air cooled motors got a lot hotter and the loose tolerances were so the metal could expand and not seize the piston.

I would agree with the rest though. I wouldn't worry about it too much. They operating conditions that get you into trouble are slow speeds and lots of throttle. When there is no air flow there is not much cooling. Going slow in the sand or a technical trail on a hot day are the kind of things that cause overheating. It is harder to tell when an air cooled bike is getting too hot. It's easy on a liquid cooled bike since they boil over to inform you of the overheating.

I think we're saying the same thing different ways about the tolerance aspect. When it's LC, the tolerances can be tighter.

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Thought this was a bike forum. Anyhow, if you're so concerned about overheating, why did you install an air-cooled motor in the first place? I know there were air-cooled ATV's out there back when, so why would this setup be any different?

When you build a frankenstein creation like this, it's hard to figure out what the results will be. The actual answer to your question is to drive it until you have a problem.

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Thought this was a bike forum. Anyhow, if you're so concerned about overheating, why did you install an air-cooled motor in the first place? I know there were air-cooled ATV's out there back when, so why would this setup be any different?

When you build a frankenstein creation like this, it's hard to figure out what the results will be. The actual answer to your question is to drive it until you have a problem.

I'm sorry did I offend you in some way? This is a 79' Dirt bike motor so yes this is the correct forum. If I drive it until there is a problem and that problem is a seizure then I'm SOL being this is a rare one year motor that's hard and expencive to get parts for. That's poor advice.

The reason I put this motor in really is because I got a good deal on it and because it make very respectable power even for today's standards. Anybody know the right temp these motors should run at???

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I'm sorry did I offend you in some way? This is a 79' Dirt bike motor so yes this is the correct forum. If I drive it until there is a problem and that problem is a seizure then I'm SOL being this is a rare one year motor that's hard and expencive to get parts for. That's poor advice.

The reason I put this motor in really is because I got a good deal on it and because it make very respectable power even for today's standards. Anybody know the right temp these motors should run at???

The problem is that it is difficult to tell. The fins are deep and the inside part will be hotter than the outside. Even with a temp gun it may be hard to get a consistent reading. The best reading may be the bottom of the piston. It is kind of hard to access though. If it gets black stuff bruned to the bottom then it's running too hot.

The most important thing is to be sure that the mixture is not too lean. I'd shoot for a little on the rich side to be safe.

It is an air cooled motor so don't expect to be able to touch it. Normal will burn you. My XR600 may be a four stroke, but it is also air cooled. I'm sure that I have had the cylinder head temp as high as 350F maybe hotter. I base the measurement on the fact that on frying pan water starts dancing on the surface right at about 350. I have done the drop a little water on my cylinder head test and I have seen the water dance. Since the fins are so much larger on a two stroke, if the outside ever gets to 350, that would be far too hot. The inside may get that hot though.

Wiseco still makes pistons for the YZ400 motor so don't be too worried about getting one.

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cleonard, thank you for the info and explaining things. I'll try to jet a little rich. I do feel better about the temps now.

Wiseco makes YZ400 pistons for every year but mine.

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No, no offense. Just surprise that you'd take a vintage bike motor and put it in an ATV.

But again, you've created a one-off monster and there's no way to answer some of your questions short of actually using it until something breaks. Knowing how hot it should be on a bike where it's out in the breeze, compared to tucked away inside an ATV chassis isn't easy or maybe even possible to answer short of actually trying it.

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Follow up info. I've never had this motor open since I got it so just thought I'd check a few things. I have to fix a small hole in the case from a broken chain anyways. Given it's age and still on the standard bore this motor is a tank! But... There is distinct burns under the piston that was mentioned earlier. I'd say it's running lean.

IM000636.jpg

IM000640.jpg

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If it is specs right... i would not worry. Use a good oil...

In high school i was riding a moped.... and stop and go traffic did not kill the engine in 5 years of very harsh driving conditions.....

cheers

Michel

83 XR350

80 DR400

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