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Yz490/dt

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Posted by: Circleburner

I know there are a few YZ490 guys on here and I could really use your help. I have two problems, first, trying to get this motor to work on a dirt track. Regardless of gearing, it will bog coming off the corner and then spin the rear wheel all the way to the next corner. While impressive, not very productive. How do I get some bottom end?
Second problem, does anybody have a base line jetting for a flatslide Keihin (38 PWM carb) I can't seem to get the low speed lean enough.
This is in a Bultaco Astro frame and the handling is sweet, If I could just get this motor to be predictable I would have a real weapon. Almost forgot, the motor is an 83 YZ490....Thanks


Posted by: dar373

it pipe reeds and jetting?????????????????????


Posted by: Circleburner

dar373,
The pipe is a Jemco(down pipe) as for reeds,I have Boysen. The jetting w/38 mikuni is stock: mj 440, sj 50,slide 3.0 Forgot the needle,but it is what the stock needle is.
However I'm trying to convert to a flatslide Keihin. Timming is at 1.85mm
Don't know what the spec's are for the It pipe so that I could compare to my Jemco.


Posted by: dar373

i only now in stock form the it had heaps down low and nothing up top..the opposite for yz...my 465 pulls like a tractor off the bottom but shes not peaky at all.rips your arms out of there sockets in all gears.lol.lol good luck with your project sounds like it will be an animal once sorted...cheers dean


Posted by: XRpredator

I hope you can find someone to help you with your 490, but I've never ever heard of one that ran well.


Posted by: Enduro_Nut

Quote:
Originally Posted by XRpredator
I hope you can find someone to help you with your 490, but I've never ever heard of one that ran well.


I agree, the bike was never that good compared with the 465.


Posted by: Circleburner

Come on guys, the motor can't be that bad. Maybe just misunderstood. There has to be somebody on here that has the key to this motor. There's too much potential there and I'm not going to give up on it at this stage. I just didn't want to trash a bunch of parts trying to find a good combination.
Wasn't the 490 just a punched out 465,I haven't done any comparisons on spec's.


Posted by: TRexRacing

The key to getting a 490 to work is the head.It must be modified by someone that knows 490s.Here ya go:see post #9


Posted by: Circleburner

Now that's the kind of stuff that gets me pumped..........Thanks a bunch TRex


Posted by: TRexRacing

Sure 'nuff.


Posted by: Circleburner

Well the "head mod." is complete,even tapped it for a compression release. Will be on the track tomorrow nite. Also added an inch to the head pipe,hope I find some lowend grunt to get this gal off the corner.Plenty of power every where else.


Posted by: TRexRacing

Great.keep us posted on how it worked.


Posted by: Circleburner

Well it's 3 am and I'm back from the track and the motor ran sweet. Between the head mod.(dropping some compression) and detuning the pipe(adding an inch)to the head pipe.The power was very predictable and usable. Still plenty of wheel spin on the straights,but controlable. Aced two classes by a straightaway.
Thanks again TRex


Posted by: Tom68

Haven't played with one but when I had an IT250 back in '83 my mate had a IT490 and it accelerated clean and strong in any gear so they may be the specs you need.Smaller diameter on the fat section of your pipe will reduce total horsepower and maybe the hit, Trials 2 strokes don't have a fat section.Advancing the timing is meant to boost the bottom and hurt the top, lowering the barrel gives more bottom end, raising compression is supposed to give more bottom less top and programable ignition can move things around but I'm betting the bad name the yz 490 has comes from transfer port issues or possibly the head as stated earlier. When you get it right it 'll be a weapon.


Posted by: Circleburner

Tom, I hear what your saying, but on short track I can't afford to give up my power. On a hard pack slick track it's all about a smooth transition of power, that's why it's hard to beat the new 4 strokes.


Posted by: Tom68

Quote:
Originally Posted by Circleburner
Tom, I hear what your saying, but on short track I can't afford to give up my power. On a hard pack slick track it's all about a smooth transition of power, that's why it's hard to beat the new 4 strokes.

I hadn't read page two sounds like you're doing just fine, any tips for my XR500R I just picked up?


Posted by: zoommx

Hey circle burner,
Did you do any of the carb mods? For good carb specs, check out the yz490 forum at yahoo.com. There's some good information there. Good luck, as if you need it, sounds like you got that running real good already.


Posted by: Circleburner

Zoommx,
I wrote down some of the carb. info for the mikuni,may try it later. Right now I'm using a Keihin (PWM 38) when I get it the best I can, I'll switch to the mic. and see which works best for me.
As for the YZ490 got balz on Yahoo, I tryed to get in but it says I'm still pending.Not sure what the problem is? Was looking forward to more good info on the 490. The more I learn, the more I realize how little I know about this motor. It's been fun so far.


Posted by: Circleburner

Tom,
It depends on what kind of riding or racing your planning on doing. A good friend of mine has been racing XL600 motors until we ran out of cases. We are on our first XR600 now. It was running strong last nite till the chain sheared 4 teeth off the counter shaft sprocket and pitched the chain.


Posted by: Tom68

Circleburner
Back on trailbikes after a 24 year break, would like to set up XR500 for short dirt track, tight turns. Need to be able to slide easy don't need speed yet just want to see if I can do it any suspension tips would be appreciated, Thanks.


Posted by: Circleburner

Tom,
Don't know much about the stock frame, anything is doable I guess. Depends on the class you want to be in. Is this a local short track or are you talking ARHMA or simular org.? You don't want to mod. yourself out of the class your interested in.
Once you get that resolved, find "Flattrack for Dummies", not trying to affend you, it has some really good set up info: wheelbase,frontend rake,etc. or you could find a used speciality frame: My friend likes Knight Frames, I prefer Champion, there are many others: C&J, trackmaster, Star Frames and many more. If you can find a good deal on a rolling chassis and leave yours alone(for trails and such) Then all you have to do is get the motor fitted for the frame. Which is easier than all the changes you would need for your stock frame(and you still have a heavy frame) Hope this hasn't scared you off.


Posted by: Tom68

Thanks, no offence taken, you could scare me off financially but the works not a problem I got out of bikes to build Speedway cars and Nascars (Australia), know I build Drag cars and competition 4WD's. Classes I don't know about yet but I'll just get running with what I've got to try it out, Thanks again.


Posted by: Ken King

We used to run two head gaskets on the YZ490's - worked pretty well and made it easier to start too. Otherwise, in stock form those motors really liked high octane fuel.


Posted by: Circleburner

Ken,
Went with two base gaskets and the head mod, too much chance of an air leak with two head gaskets. I use 116 race gas and a remote starter, so that's not a problem. Just had to de-tune the motor to make it work on the short track, way too much wheel spin with stock compression.


Posted by: Circleburner

2 strokes,
Do you know the weight diff. between the IT and YZ Flywheel ? I've never seen an IT flywheel on Ebay, may have to start looking a little closer.


Posted by: Circleburner

2 Stroke,
Thanks for the offer, but .............I'll play it safe and keep what I got. I'm still going to try and find one though. It could be the right ticket for my application.


Posted by: TRexRacing

You need a YZ flywheel 2 Stroke?I may have a good one out there somewhere.


Posted by: Circleburner

TRex,
Do you know what the difference is between the two: weight,size, use the same stator,etc.?


Posted by: TRexRacing

No I looked around and found no info on the IT.I do know lighting coils are available for the YZ from Electrex.As are ignition coils.No stator though.


Posted by: Circleburner

TRex,
Thanks for checking, I'm just interested in the flywheel for the added weight. Wasn't sure if the "electrics" in the YZ were compatable with the IT flywheel.



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