What do you think of this bike for sale?

[Trfsrfr]

Dude after you crack this egg open the only valve you will ever want to see is a power valve.

I just got a very small taste of that, and can see where you're coming from. But I am unfamiliar with this powervalve thing, although I have heard of it.
What is it?
And is it exclusive to Yamaha, or do all 2-strokes have one?
This engine definitely new to me, and has some extra doo-dads that I am not used to seeing.

 

[SRAD97750]

Many two strokes have an exhaust valve which blocks some portion of gas flow through the exhaust port.
It ultimately changes port timing/duration so the engine can make power at more than just a narrow RPM range.
Typically at lower RPM, the valve makes the exhaust port shorter/smaller. As engine RPM increases, and exhaust duration should increase, a speed governor (typically spring governed) opens the valve.

Exhaust valves come in many shapes and sizes and I am just amazed at the ingenuity that goes into the hundreds of exhaust valve designs. Some are like saloon doors, some are additional ports, some are like a garage door, and I've seen rotating pillars with variable cutouts on each side. Amazing stuff.

The exhaust valve system typically requires a good cleaning on every inspection because it operates directly in the exhaust gases which are rich in unburnt oils. Usually you set them from the factory specs and never think about them again, other than routine cleaning.
-BIG DAN

 

[Trfsrfr]

Whoah. That is a shit-ton of information.
Thanks Dan.
Is this exhaust "valve" a physical object? Something that I can view on a fiche somewhere?
It helps me tons when I need to understand something if I have a visual.
I looked at the Motosport OEM site but didn't find it, but maybe wasn't looking in the right spot.
I thought 2-strokes we're supposed to be less complicated?

I took it for a ride tonight that verified that I didn't make a bad purchase, but I expect that eventually a top-end repair will be necessary, and I would like to learn-up as much as possible before that happens.
Any and all info, links, or other help is always much appreciated.

I foresee a schooling in my near future...

 

[2Wheels]

No worries man you got a couple YZ experts in house.
And yeah they are less complicated but none the less brilliant.

 

[SRAD97750]

I thought 2-strokes we're supposed to be less complicated?

Once you see it, it's very easy to understand. Mechanically, there is nothing complicated about it. The actual theory behind 2T engines is VERY complicated, though.
I happen to be starting a complete top and bottom end on a 98 CR125 and the cylinder is on the kitchen table.
NOTE: YOUR EXHAUST VALVE IS VERY DIFFERENT FROM THE HONDA.
Here you can see them in the closed position, just kinda hanging in the way of the exhaust port, making it shorter. These slide outboard into slots exposing the rest of the exhaust port.

Closed


Open


Here is the linkage on the RH side, linked to the LH side so they work in unison.


There is a rod that comes out of the clutch cover from the governor, which is driven from the crank. The rod moves either up or down, commanding the valves at the proper RPM.

-BIG DAN

 

[Trfsrfr]

Okay, I sorta get it now. I'll have to physically get my hands on one for it to sink in completely, but it makes sense. It also connects a couple dots that tie-in to the exhaust system as well. The word/phrase "back-pressure" comes to mind. Brilliant and ingenious indeed.

I'll clean the bike up today and take some pics. It leaks fuel from the petcock and the carb right now so I'm gonna tackle those things first then re-assess. Hopefully if all goes well I'll be able to take her out into the desert later and run it through the gears.
My buddy "borrowed" me his ratty KX85 for my son to transition with and it needs carb work too. So I'm gonna grab a bowl of cereal then head out to the shop. I'll be back later.
Thanks again y'all.

 

[2Wheels]

I just took it for a spin around the block...holy christ this thing is fast.
I hit the powerband and holy-frigging wow.

welcome to the dark side!

 

[Trfsrfr]

 

[PALMER84ONE]

Welcome to the YZ family. You'll be in good hands from all the knowledge from Dan. I spend 90% of my time on TT because it's a bigger base of people on the YZ forum. AND I CAN POST PHOTOS from my phone without resizing.

Really, if you get in a jamb and need imediate help, PM sent.

 

[Trfsrfr]

Thanks y'all.
I am quite excited to get this thing running like it should and expand my skills, and am looking forward to the learning experience.

 

[Trfsrfr]

Okay, here is what is next;
A CR125. This bike is guaranteed mine, from a friend of mine. It could be my son Zach's bike when he is ready.
The only thing keeping it from being in my shop is an agreement on price. I'm told it is a 98.
I'm also told it runs, but haven't seen it in person. What do you guys think?

 

[Trfsrfr]

Welcome to the YZ family. You'll be in good hands from all the knowledge from Dan. I spend 90% of my time on TT because it's a bigger base of people on the YZ forum. AND I CAN POST PHOTOS from my phone without resizing.

Really, if you get in a jamb and need imediate help, PM sent.

Thanks Gary, I am sure I will be in touch soon enough.

 

[2Wheels]

'98 cr250 is a rough one my friend. If it were me I'd pass. Great motor, terrible everything else. Like a rocket powered sawhorse.

 

[2Wheels]

I always seem to get along better with the steel frame bikes though. Even my newer crf and YZF frames feel rigid and unforgiving.

 

[Trfsrfr]

'98 cr250 is a rough one my friend. If it were me I'd pass. Great motor, terrible everything else. Like a rocket powered sawhorse.

Sorry, my bad, it's a 125. I'll edit that post.
Plus it's for my kid, who's 11 and still learning. But thanks for that!