'01 YZ250 Cylinder Suction Pipe?

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#1
We have an '01 YZ250 that was recently acquired and am curious about this suction pipe/Vent hose thing mounted on the left side of the cylinder. We're waiting for the service manual to come in, so besides that, we're relying on parts diagrams and common sense. Does anyone know the actual purpose of this?, appears to me to be factored in with the power valve system, but just not sure yet?

The problem with this is that when you start up the bike and then after warm up and you get on it, the connector pops out and leaks spooge. Lots of exhaust comes out this port and hose. Should that be the case? Since the part is called a suction pipe, I'd think its a vent to suck air in, not push out.

We're just not sure yet how to troubleshoot this. Other than that, the bike appears to run just fine - its used, so its hard to know what's been done previously (or not done for that matter)

Thanks for any help you can lend.
 

RM_guy

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#2
It's a breather hose for the power valve chamber. It will blow smoke and spooge if the jetting is off or the chamber is full of spooge. If you just got the bike I'd remove the cylinder and clean the valves out.
 
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#3
hey rm guy have you had issues with your 250 seizing because the intake port is to narrow. the piston is scored from the crown all the way down the skirt. what do ya think? :rotfl:
 
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#4
Finally received the repair manual and will be tearing down the cylinder shortly, followed by a fresh top.

I'm just curious, would you know what drives these valves?, is it merely the air/compresssion/exhaust force that works them? or is something mechanical tied in?
 

RM_guy

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#5
It is driven by a mechanical linkage that moves based on engine RPM.
 
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#6
Your power valves are actuated by the vertical movement of the actuator rod. That rod is pulled up and down from the rotational motion of the crank in the bottom end. The rod pulls on an arm tied to a rod in the cylinder causing that rod to rotate. This rotation in turn pulls the power valves in and out using an elaborate linkage system.

You have three power valves - a main valve that opens first and then two side valves that begin opening when the main is opening fully. The side valves look like plugs or barrels and the main looks like a wide slide.

The power valves actually change the exhaust area volume and the exhaust gas speed. In order for this system to work properly, there must be normal atmospheric pressure on the back side of the valve mechanism. That is what the rubber hose is for. It allows the valves to push back and not be restricted by a sealed air chamber.

Some exhaust gases will pulse through this hose under normal operation. If your tube blows off, then there is either too much pressure getting past the valves or the tube is clogged. In either case, I would service the valves through disassembly, cleaning and replacing any worn parts. You should be aware that the main valve on your year 250 is prone to fracture over time.

Good luck.
 
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#7
Thank you for the overview, very helpful!! I'm becoming familiar with this mechanism since I've just torn it down and will begin dismantling the powervalves for cleaning tonight.

A couple remaining questions:

Is it safe to say that with the engine on the bench, and when manually cranking the kick starter and watching the valves that you will not see the mechanism move, since RM_guy mentions that the movement is based on engine RPM?

With the linkage separated from the power valve shaft, I can turn it and see the mechanism rotate fine - but very,very dirty!

Also, when you mention "prone to fracture", where should I look, will I see a crack or something noticeable on the main valve?

Thanks Again!
 

RM_guy

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#8
SteveTroi said:
...Is it safe to say that with the engine on the bench, and when manually cranking the kick starter and watching the valves that you will not see the mechanism move, since RM_guy mentions that the movement is based on engine RPM?...
You won't be able to turn it over fast enough by kicking it.
 
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#9
SteveTroi said:
Also, when you mention "prone to fracture", where should I look, will I see a crack or something noticeable on the main valve?
That's a good question. As I recall, the fracture occurs at the back part of the valve where the contact is made by the mechanism that moves the valve. Do a search on DRN and you should see a picture of a broken valve and some descriptions of the failure. I seem to recall that some people attribute the failure due to the valve shutting quickly and causing splitting/fracture of the stop mechanism built into the back part of the main valve. I also seem to recall that Yamaha had produced a stronger valve in subsequent years. I do not know if the newer valve can be retrofit or if someone supplies a stronger one. Perhaps some other member can chime in on the replacement.

It would seem prudent to go ahead and replace it when you clean things up as fracture of the back part of the valve causes the valve to move fully forward and into the space normally occupied by the piston! I also seem to remember that the valve is over $100.

Good luck.
 
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#10
01 YZ250 Power Valve Re-Assembly

Solid State said:
Do a search on DRN and you should see a picture of a broken valve and some descriptions of the failure.
I consulted the DRN link above and it led to some very interesting reading, but now I'm a little more nervous.

I dismantled and cleaned up my power valve assembly real nice and plan to reassemble with all the textbook lubes in place. Good news is that I did not find any breakage, especially no ugly broken main valve. Based on the above thread, I have a couple questions:

1. How was bclapham able to see that his valve was not moving in proper calibration with the the exhaust ports and moving into the flow? Is this done only by twitching shaft back and forth by hand? I just want to confirm once again that there is no real way to see this mechanism move under crank control?

2. As for re-assembly, I guess it's just bolt back together as the shop manual instructs and go? There's no calibration required, correct?

I'm just afraid I'll create some problem that did not exist previously! This all started just as a project to de-gunk the mechanism and I don't want to find myself into the chronic PV problems others have had!

Please tell me its gonna be OK !! :cool:
 
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#11
SteveTroi said:
1. How was bclapham able to see that his valve was not moving in proper calibration with the the exhaust ports and moving into the flow? Is this done only by twitching shaft back and forth by hand? I just want to confirm once again that there is no real way to see this mechanism move under crank control?

Rotaing the power valve shaft by hand will actuate the valves without the cyclinder installed. You will be able to view the operation and timing of the side valves and main valve this way.

2. As for re-assembly, I guess it's just bolt back together as the shop manual instructs and go? There's no calibration required, correct?

I found that the stock calibration was lacking as the side valves did not open simultaneously with each other as I assume the designer intended. I was able to 'time' the valves by tightening the set screws for the linkage at slightly different locations on the shaft. When I was done, both side valves openened simultaneously and at the oppropriate position in relation to the main valve. Can't really explain in any additional detail. You'll have to play with it during reassembly to see what I mean. Maybe by then you'll just be satisfied that it's all back together and not care too much about fine calibration.

Good luck.