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Old 10-04-2013, 12:12 PM   #31
JR Greenhorn
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I'm partial to Kehin PWK's. Mikuni VM's are the default choice on anything prior to the late-'80s. They're easy to tune and run pretty well. My cousin's '91 CR125 has a Kehin PJ on it. I haven't liked that carb. There's nothing really wrong with it, it just doesn't seem very precise. Among the guys I ride with most often, we've got 3 PWK's. They're not much harder to tune than a VM (I've even dialed in needle tapers and diameters on my 440), but they meter so much more precisely. I'd love to try the Air Striker version. Seems ideal for the low-throttle modulating you do on a big bore in the woods and on the trails.

Bottom line, if I were in your shoes I would stick with a VM, but if swapping carbs, I'd only go with a PWK. If the bike already has the stock VM, might as well run it for a while and see what you think. I don't think the PJ is an upgrade.



The 490 should be an absolute tractor in the woods. (I wish I could say I've ridden a 465 for comparison!)

My buddy usually runs his quite rich, and doesn't mess with jetting much. I'd be chasing jetting all the time if I owned his bike, but I like crisp jetting. I've bugged him for years about trying the head mod everybody raves about, he hasn't given in yet. After rebuilding the lower end and getting his bike dialed in (it was a mess when he bought it), he got eight solid years of use out of it without ever going into the jug. He ran it until the piston slap got really bad, and it turned out he had wore through the first overbore, and right up to the second.


I once looped my buddy's 490 on a hill climb. It was all sand and I picked a bad line and didn't carry enough momentum up the hill. The 490 just tractored along at fairly low RPM in whatever gear I was in, all the way to the top. What I didn't know is that I was carrying the front wheel in the air the whole way up. When I reached the top, the front wheel skipped off the sod cornice at the top of the hill, and the bike just rotated around the rear wheel. At least it was sand and the hill was steep enough to make it easy to get out of the bike's way!



How about some pictures?



Tight woods, new trail; had to wag the bars around these little trees to get through.




Sandy hill climb.




Out on the ice with 1200 gold ice screws.




Crossing a bog after days of heavy rains.




Starting a run through more mud.




My own 440 in the mud.
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Old 10-05-2013, 07:30 AM   #32
mudmantra OP
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Cool Pictures. Yes. I'm going to try jetting the VM in a little better. It's good enough to ride and starts good. I've found so far, that if I get the low to midrange perfect the top end suffers, and if I get the mid range to top end perfect, the lower throttle settings suffer. Right now I've got it to where it just runs good, but not perfect everywhere. Yes it would be interesting to try a PWK carburetor. I couldn't find any PWK jetting info for a YZ490 though. Maybe one jetted for a CR500, would be close enough to start.
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Old 10-05-2013, 09:59 PM   #33
Rot Box
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I know it's apples vs oranges but my 84 CR500 (air cooled) suffered from pretty much the same motor issues that plague the 490's. Between the impossible jetting (can you say ping king?!) and light switch power band it was very hard to work with on the trail.

After the PWK carb and Eric Gorr head/port work it is a whole different bike. Crisp, smooth and predictable power from bottom to top. More-usable-power overall and no more pinging I honestly cannot recommend those two mods enough.

As for jetting the pilot, main and fuel screw are pretty easy to dial in. The needle on the other hand is going to require some homework. For me it was lots of trial and error and staring at needle charts and my notes but I finally nailed it. Sudco's needle chart helps a lot I would try to get one with a taper(s) close to your stock needle and go from there. Hope this helps,

Andrew
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Old 10-06-2013, 02:48 AM   #34
Twin-shocker
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I wonder what exactly are the mods to head and porting, which transform an open class MX bike into a usable woods machine?
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Old 10-06-2013, 07:19 AM   #35
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Originally Posted by Rot Box View Post
I know it's apples vs oranges but my 84 CR500 (air cooled) suffered from pretty much the same motor issues that plague the 490's. Between the impossible jetting (can you say ping king?!) and light switch power band it was very hard to work with on the trail.

After the PWK carb and Eric Gorr head/port work it is a whole different bike. Crisp, smooth and predictable power from bottom to top. More-usable-power overall and no more pinging I honestly cannot recommend those two mods enough.

As for jetting the pilot, main and fuel screw are pretty easy to dial in. The needle on the other hand is going to require some homework. For me it was lots of trial and error and staring at needle charts and my notes but I finally nailed it. Sudco's needle chart helps a lot I would try to get one with a taper(s) close to your stock needle and go from there. Hope this helps,

Andrew
Thanks. It helps a lot. The 490 is still running, so I need the extra motivation to pull it apart and fix it right. What size PWK did you use? Did you get a new one, or a used one from another machine?
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Old 10-06-2013, 07:37 AM   #36
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Originally Posted by Twin-shocker View Post
I wonder what exactly are the mods to head and porting, which transform an open class MX bike into a usable woods machine?

If you were as astute as you think you are you'd know that answer.
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Old 10-06-2013, 09:22 AM   #37
Rot Box
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What size PWK did you use? Did you get a new one, or a used one from another machine?
My PWK came from a late model KX500 which I scored locally for $25. They're all over eBay for fairly cheap as well. I thought well the KX was a 500 so the jetting should be close so I'll start there----wrong---not even in the ball park Again I would use a Mikuni chart and compare the needle you're running now to a Keihin chart and see if you can find one that is close or the same as what you have and go from there. Because it's uncharted water and every carb delivers fuel differently there's a good chance it won't be exactly what you need but I think it should get you a nice starting point. Order a couple clips as well they're so easy to loose when you're pulling the needle out multiple times.

Anyway the later KX500's used a 39mm PWK. Many bikes used or still use 38-39mm PWK's and PWK air striker carbs. Look for the "D" shaped slide. A lot of them had electronically actuated 'power jets' if I'm not mistaken you can just leave those unplugged. You can't beat JD if you want a brand new one. http://www.jdjetting.com/xcart/produ...age=1&featured

Here's a nice schematic showing the dimensions of the PWK. http://www.sudco.com/Diagrams323728/exppwk.gif
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Rot Box screwed with this post 10-06-2013 at 09:24 AM Reason: clarity
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Old 10-07-2013, 01:40 AM   #38
Twin-shocker
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If you were as astute as you think you are you'd know that answer.

Not got a clue.......................but would have thought anyone who has had the mods carried out would know exactly whats been done?
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Old 10-07-2013, 07:42 AM   #39
mudmantra OP
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Not got a clue.......................but would have thought anyone who has had the mods carried out would know exactly whats been done?
I haven't had anything done to the YZ490 yet, But I did a lot of checking into it. Even Yamaha put out a wrench report to correct the cylinder head and porting on the 1982 YZ490. I'm not an expert, But from what I understand cylinder head shape needs to be correct to keep the piston from over heating. Once that's done, you can get the jetting dialed in. I have an old spare YZ490 cylinder. The port casting is so rough and irregular, that I think any kind of clean up, would be a worthwhile improvement. The YZ490 already has a ton of low end torque. So I'm real interested in seeing how the porting works out.
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Old 10-07-2013, 08:27 AM   #40
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Not got a clue.......................but would have thought anyone who has had the mods carried out would know exactly whats been done?
Here's the easy way.

Machine the cylinder base -.75MM. Widen exhaust (do not raise)port keeping in mind the intended use. Open/widen/lower the boost port W/O changing height. Clean the transfers up and correct cylinder entrance angles if needed. Open intake to facilitate flow because that won't change timing due to the reed controlling it. Gore makes a Boyesen Port in his 465 cylinder but I don't see the gain being worth the effort. But he has been doing this longer than I.

Correct the head for compression,MSV and clearance again keeping in mind usage. Keep the stock carb 'cause a 38MM is small for a 465/490 and suits low-mid range power. If you want to make the most of the new port timing a couple pipe mods and tuning ignition timing would finish the job.

All rather elementary if you know anything about smokers. Perhaps turning off the computer,purchasing a motorcycle and riding it would lead you down the path of modifying for performance instead of just buying new. Then again maybe not. Too many good motorcycles rot behind barns and garages because of that attitude.

What say you now?
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Old 10-07-2013, 09:12 AM   #41
Twin-shocker
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I guess those changes would make some difference, but I get the feeling it would be difficult/impossible to make an open class MX bike into something that wouldnt be easily shut down by any competent rider on a 200, when it comes to riding in the woods.
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Old 10-07-2013, 09:54 AM   #42
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Your guesses and feelings mean little when compared to my and others real world experience. You assume that others posses your meager riding skills. Not so. So in the end it would behoove you to...................

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Old 10-07-2013, 11:55 AM   #43
Rot Box
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Dear Twin Shocker,

We understand you don't believe an open class bike will make a competitive trail bike. Many feel the same way and there's nothing wrong with thinking that. We heard everything you had to say in the 'What's a better sand bike CR500 or KX500' thread. Everyone tried their best to explain the appeal of an open class bike to you and explain what these modifications do to improve the open class bike off road. As a result that thread completely shit the bed for several pages. I think I speak for a lot of people when I say we would hate to see this thread go the same direction. Love,

Rot Box
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Old 10-07-2013, 10:28 PM   #44
dynodave
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Originally Posted by Twin-shocker View Post
I guess those changes would make some difference, but I get the feeling it would be difficult/impossible to make an open class MX bike into something that wouldnt be easily shut down by any competent rider on a 200, when it comes to riding in the woods.
Twin-shocker...read this:

http://georgiaoffroad.com/forums/vie...0%27s+are+best
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Old 10-07-2013, 11:07 PM   #45
The Flyin Hawaiian
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If you have good clutch and throttle control you will be fine in the woods on a open class 2t mx bike. Its something that Ive noticed that very few people have though. I used to race a 84 IT490 and a 88 KX500 in enduros in my late teens and never had any issues. I currently race a 04 KX250/500 hybrid and love it!
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