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socalduner

XR600 crankcase vent

8 posts in this topic

I was turning off the choke on my 600 and felt alot off air coming from the crankcase vent tube. I was thinking that it must have a compression leak to blow that hard. It seems to run fine. Any ideas on what to check? Thanks :thumbsup:

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We'll maybe hear from others but that particular vent tube, I think, is an active one. Mine also pumps air. It's the reason the tube is there. The crankcase has lots going on that will change air pressures in the case. Those pressures need to be relieved.

Unless some else suggests something is abnormal I won't worry about it.

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That's normal. When it starts blowing oil out the tube all over the place, then it's time for some rings. I just did the rings on my xr600 because of oil out the vent tube, and afterwards it still blows in and out quite a bit. When the piston goes up it sucks in 600cc's of air and that goes out again when the piston goes down. Any ring blow by just adds to that 600cc's blowing out. At some point so much air is going out that it carries some oil along for the ride.

So, no oil, no problem...

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Yup all singles do this. The multi cylinder motors have the other pistons

going up while others are heading back down, so they cancel each other

out pretty much. But like cleonard says, it's sucking and blowing all the time.

It'll blow more than it sucks, so never block or restrict the airflow. Doing

this will cause the pressure to blow out someplace else..... where it's not supposed

to. :thumbsup:

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I think I'm going to need to rebuild the top end and replace the rings on my XR600 soon. I did a long desert ride last weekend and it used a bunch of oil. My riding buddies said they could smell oil and sometimes see puffs of it when I would let off the gas. And I can see see oil on the frame below that crankcase vent tube. So did you do the work on your XR or did you have someone do it?

That's normal. When it starts blowing oil out the tube all over the place, then it's time for some rings. I just did the rings on my xr600 because of oil out the vent tube, and afterwards it still blows in and out quite a bit. When the piston goes up it sucks in 600cc's of air and that goes out again when the piston goes down. Any ring blow by just adds to that 600cc's blowing out. At some point so much air is going out that it carries some oil along for the ride.

So, no oil, no problem...

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I did it myself, starting in late January. My bike is a 93 and (I think) it has about 15000 miles on it. Look for some of the threads I started asking questions. It's not too hard, but it wasn't the first time I've been in to a XR or XL engine either.

I had head issues a couple of years ago, and got a valve job and new timing chain and tensioner. The head usually goes before the rings wear out. First I took the head to the local Honda shop and they didn't fix it right. Six months later I took it off again and sent it to XR's only. They fixed it right, but it took three weeks to get it back. Two new valves and two new seats. It worked much better after that. Then six more months and it was making noise. It was the cam chain slapping around. A new cam chain and tensioner and it was much quieter.

Last fall it started blowing oil out the breather tube. Time for new rings. Much to my amazement, the piston measured new. The bore was OK as well. They measured almost like new. The rings were toast. The test is ring end gap. Take the top ring off and put it in the bore. Use the piston to get in square in the bore. Measure the gap between the ends. The max spec is .020 I think. I didn't even measure mine. It must have been .150 or .200! I honed the bore rather heavily to clean it up, and was ready to put it back together.

However, there were a few other problems. When the cylinder is off you can see the transmission gears. I grabbed a flashlight, and :thumbsup: 2sd and 3rd gears had heavily pitted faces. Bought 4 gears ($250!) and split the cases to put them in. I was kind of planning on a new piston, but the cash went to gears instead.

The other issue was the choke plate on the carb. It's a known problem with XR600s. It beaks off and gets sucked thru the engine. Sometimes it passes through with no problems, and sometimes it wrecks the motor. The plate is pot metal and has a little flapper. My flapper was hanging by a thread. The pot metal plate had a big crack going thru it. I just pulled the choke out completely. Makes cold starts harder, but I have been able to start it in the morning when it was 35F. If it doesn't want to start, I tip it over so it floods. Then I can start it.

The performance improvement was a lot more than I was expecting. As the engine wears, you lose power bit by bit. You don't really notice it. If I had known, I would have done it earlier.

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Thanks for the info. My XR has approx 12,500 miles on it. So I guess it is about time for some motor work.

I replaced the stock choke plate with one of those billit choke plates about 6 years ago when somebody first told about what could happen when the stocker wears out. :thumbsup:

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