OzVMX Forum
Marque Remarks => Yamaha => Topic started by: subarumy98 on February 28, 2009, 07:27:56 pm
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HI All
I am lookig to help a buddy of mine here with his YZ490.
Apparently there was a Dirt Bike article with Broc Glover in the day that described how to sort out this beast.
Seems the article covered head, porting, ignition and jetting changes to smooth out the beast.
All I have found so far, is a diagram on a head mod where the edge of the squish band is radiused to eliminate pinging.
I have not been able to find the actual article itself.
Anyone able to help or point me in the right direction ?
many thanks
Trevor
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A radius at the squish curtain will do very little for pinging.First is reduce the compression to 10:1, next is squish clearence to 1mm. Harder is to rid it of the 'top end' porting but it can be done. There are some additional ports you can put in to improve the abyslmal auxilary transfers. Fatten up the jetting and retard the ignition. Then it will go marginaly better than a 250.Best advice is to find a 465 head, cylinder and piston
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Cool - thanks for the quick reply.
I am mailing Rick Siemen Super Hunky as well - let's see if he can trawl up anything too.
I will repost here on findings.
T
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If his past record on this model is anything to go by, Rick will recommend it as a boat anchor
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as i remember the hot setup back in the day was to sell it and buy a CR500 or KX500
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There are some additional ports you can put in to improve the abyslmal auxilary transfers. Fatten up the jetting and retard the ignition. Then it will go marginaly better than a 250.Best advice is to find a 465 head, cylinder and piston
yaeh I wouldn't get too excited about the auxiliary transfers, conversion to 465 is much better solution.
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what year is it first ? the head mod for the j /k is machining a blend from the squish to the combustion chamber , 8mm wide , i,ve never done it on any of mine ,besides they only ping when u back the throttle off ,done in a lathe using a mandril that fits the plug hole , use the highest octane u can get ,98 is good 33 to one oil tts is good ,for j or k ,ign timing is std at 2mm btdc, i use #50 pilot jet , (richer than std#45) std nozzle and needle q8 , 6f16 clip in #3 , get a new set if it,s got any wear on it , std main is 440 , but with the leaner oil mix u might have to go smaller , boisen reeds work well but roll them over regular and/or replace them to keep the lip down on the block , fiber reeds will probably knock the main down a size or two , dg pipe or similar is a must reeds and pipe and any porting may affect jetting , the 490k i bought off 090 had a main 5 sizes down from std ,it,s no good doing anything to a worn engine , check cylinder for clearance and blow by scorching ,check big end and mains , it,ll ping less if it,s worn , gearing , start from std 15/50 for a std bike , 15/49 for an open track , 15/48 , for a fast bike and fast track , drop the front end as far as u can for a tight course , adjustment to the twin link shoes is vital for them to work , use the sintered shoes with groves , use the linkage between the brake cams to adjust the slop out of the system , idea is to get both shoes expanding together , with new shoes and the brake plate in your hand adjust the linkage so the cam on the second shoe contacts it at the same time as the first shoe , give it a run then check wear pattern and tweak if required ,unless u r riding full on mx soften up the forks , if it,s a j i,m not sure how to get the rear suspension to behave , just make sure it,s all in good nick , or pissit all off and fit a 465 swing arm on it and soften the shock , if it,s a k their good in the rear , if it,s an L -lucky u the 490,s aren,t the best in the class but are easy to maintain and get parts for ,
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i forgot , u gotta have 490,s so,s those honda boys can skite about how fast their bikes go !!
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I think this anti-yz490 thing is a bit of an urban myth (as any Montesa rider can attest to) created by Mr Sieman and ran with by other journo's over the years. I can only go by a mates bike that was used in the 80's and ridden by a pretty tidy rider. He can't understand all the hype about how bad they were as his ran flawlessly and was reliable as a rock. Same goes for all the 'pinging' that is talked about.
He's building one up as we speak for Viper and is more than comfortable with his choice.
Also, as pointed out, parts are easy to get and nicely priced.....
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I have a 1984 Motocross action here that has a five way shootout that states that the YZ490L was a winner hands down ???, I was always of the opinion the L model was a great bike...
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fella,s i couldn,t agree more , anyone who says the 465 is a better bike is just getting on the bag the 490 band wagon , i believe the 465 was better against it,s opposition , where as the 490 wasn,t as comp. against it,s rival,s esp the j , i have a very good 465 , and believe me i,d rather ride one of my 490,s , and if we are talking pre 85 , yes the 84 L is a little bit better ,
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That's a load of crap that you need to put a 465 top end on a 490 to get it working. The 490K and 490L can be made to run sweet with a little machining of the combustion chamber and correct jetting. You can be sure the A graders of the day ran 490 top ends and they didn't detonate or sieze in a 20min moto.
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bang on jim , in fact the opposite switch is far more interesting ! with the 5 speed box
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I have that article your chasing, let me know how to get it to you.
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If you can't hear them pinging you must not know what to listen for they won't ping on the stand but as soon as it under load.........
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lozza ,what model yz490 do u ride / own ?
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Never owned one,I just work on the engines, what I do own though is a very good ear.
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Then you should've heard that coming... :)
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i have heard both sides of the story and was wondering how many have just taken what super hunky has said as gospel, but i have also heard that even the pros could never really get the 490's running right for the Finke desert race no matter what they did.
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I had 490 S and then more horrendously the WR500, WR500 had the head and barrel machined by former top 5 Mr MXer. So he knew a thing about 490s. The WR500 got a lot of sand time like at Finke, it pinged enough to make you shift up to put it out of its misery.
But then again I am not saying that someone with more talent for jetting and more patience than me cannot find the perfect set up. What the WR500 had going for it was the WR250s suspension.
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well my patience has paid of for my 490j it was pinging big time but now it doesn,t this what i done timing set to standed new needle, needle jet, new main jet, pilot jet, needle and seat and mixture screw theres no point trying to tune a rebuilt motor with a carbie that has 26year old internals in it, i tryed it doesn,t work for around 200 bucks you will have a near new carb. the carbie was set to standed specs found at http://www.stadiumyamaha.com the only change i made was a 460 main form a 440, needle is on the thrd grove i use bp primium fuel motul 800 at 40;1 and pro stength octane boster 60ml to ten litres of fuel. this motor has no head mods it has dual stage bosien reeds,dg exshust i live at sea level the bike is runing great nice at crisp down low can,t keep that front wheel on the ground yz490s are a great bike like any old bike you just need spend the time to dial them in, same said with the rear end on this model i put a whitepower spring on the shock along with all new swingarm bushes and got the shock serviced and revalved and it handles good there is no need watsoever to go backwards in putting a 465rearend on or a 465 topend,moding 490 heads or running standed reeds i think thats bs :o
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jason how do u get it to run a 460 main ? do u have a very small n/jet it ? also what gearing are using ?
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STEVE, IM RUNING 14/48 GEARING AND A 6F16 NEEDLE, Q2/247 SERIES NEEDLE JET. :o
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q2 that is small , std 465 , and with the low gearing seems to work ok , i,ve got the j running at the other end of the spectrum , 40 pilot r0 440 , high gearing