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Clubroom => Tech Talk => Topic started by: 1983_Enduro_YAMI on April 22, 2017, 06:45:17 pm

Title: 1984 YZ490 40mm carb jetting??
Post by: 1983_Enduro_YAMI on April 22, 2017, 06:45:17 pm

G'day all,

I'm a bit unsure about jetting the 1984 YZ490....because the stock jetting on these 84 models are not working at all!!! Anyone would have any jetting suggestion for it??

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: 80-85 husky on April 22, 2017, 07:15:32 pm

apparently, extending the pilot jet into the fuel bowl with some hose helps alleviate the pinging issues which are supposedly caused by the vibration frothing the fuel and leaning the slow circuit off. with the husky 400, there are no such issues and it runs like a clock.. ;D ;D sorry, just taking the piss a bit... ::)

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: 80-85 husky on April 22, 2017, 07:16:38 pm

http://forum.ozvmx.com/index.php?topic=36868.msg357563#msg357563

info here  may be of help

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: John Orchard on April 22, 2017, 10:02:22 pm

Quote from: 80-85 husky on April 22, 2017, 07:15:32 pm

apparently, extending the pilot jet into the fuel bowl with some hose helps alleviate the pinging issues which are supposedly caused by the vibration frothing the fuel and leaning the slow circuit off. with the husky 400, there are no such issues and it runs like a clock.. ;D ;D sorry, just taking the piss a bit... ::)

Maybe if it was the full 500 mans bike things would be different ;-)

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: Momus on April 24, 2017, 11:21:15 am

Experts opine that there are fundamental engine porting and head/compression issues that may make simple jetting tuning not simple.

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: 80-85 husky on April 24, 2017, 11:26:01 am

never seemed to worry gally ;D

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: evo550 on April 24, 2017, 06:50:37 pm

I have a MXA article somewhere that goes into detail on what Broc Glovers mechanic did to his 490 race bike in '84 to get it to run right. Amongst other things like head work, he would send Glover out and get the motor to ping and ask Glover to remember his throttle position when it started to rattle, bring the bike back in. Pull the carb off the boots and get Glover to turn the throttle to the position he remembered. Mark the needle where it is coming out the venturi, pull the needle out and file it down with some emery paper at that point..send him out again and repeat.....

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: 80-85 husky on April 24, 2017, 07:06:13 pm

custom jetting eh ;) nothing like Yamaha to be helping...I wonder what they called that needle....the "wtfhydtt"

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: Momus on April 24, 2017, 08:41:16 pm

Quote from: 80-85 husky on April 24, 2017, 07:06:13 pm

custom jetting eh ;) nothing like Yamaha to be helping...I wonder what they called that needle....the "wtfhydtt"

probably not that simple. Over scavenging, exhaust gas dilution and heating in the transfers, non homogenous mixture at certain rpm etc.

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: John Orchard on April 24, 2017, 08:45:30 pm

Filing-down my CR500 needles allowed me to determine the correct needle to order from Keihin USA. Biggest problem with my stock Keihin is unavailability of needle jets.

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: Ekka on April 25, 2017, 11:02:24 am

Wasn't there a acticle in vmx Mag on Glover's bike as well

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: Slakewell on April 25, 2017, 04:47:56 pm

Have you tried BP 100 fuel?
For me I would buy a new 38mm and start from there. If the 40mm is old and worn out you wont jet it regardless.

Title: Re: 1984 YZ490 40mm carb jetting??
Post by: g465b on April 25, 2017, 05:53:42 pm

 hi ,   Motocross action , Dirt bike and the lesser known Motocross ,  magazines from 1984 all recommended the following for the YZ490L
       stock is   440 main    needle Q8     slide 2.5    pilot 50    air screw  1 3/4 turns out
  recommended     440 main      needle Q4 on 3rd clip    slide  3.0    pilot   35     air  2 1/4 turns out , if it pings go richer on needle pos.    g.