1985 YZ490 jetting

[firesubie]

Hello all,
New to this forum, but I'm glad I found it. I have found a wealth of knowledge here.

I have a 1985 YZ490. Got it in a trade for putting a new crank in a TTR125. I had not owned a two-stroke for many years and decided I would restore this one for trail-riding. After reading many peoples opinion about this bike, I was wondering if that was a good idea. Anyway, I am going through the bike and checking it out. Seems to be in decent shape. Many shade-tree repairs. Rides good and has a ton of power. Low end is terrible. Blubbers and smokes. I took the carb off and cleaned it. Current jetting is CC-0 needle jet, 60 pilot, 460 main, 7f7 needle on 4th position from the top, air mixture screw 1 turn out, 3.0 slide that a previous owner filed down to about 4.2 mm. This seems like a very rich jetting with a very lean slide. I'm using pump gas with yamalube mix at 32:1. It has already had the head mod with a DG pipe and FMF silencer. I plan on replacing the slide with a 3.0 or 3.5 depending on what you guys say.

I have found several jetting recommendations on this site and others, but they are for the 38mm carb. This bike has a 40mm and I haven't seen jetting recommendations for it. I tried to join the yz490 group on yahoo, but no word yet after a week. I know there are several yz490 riders on this forum and I am hoping to get some help on this. I may put a 38mm carb on it to give it a little more low end if I can't get the jetting right on the 40mm. I also thought about putting an IT transmission in it for the low 1st. What do you guys think? Thanks

[firesubie]

I found a jetting recommendation in an old thread for the 40mm carb. I put it in and the bike runs great. I still need to adjust the float height because it floods like crazy and fuel runs out of the overflows. And I need a new throttle cable before I get killed on this thing. It's sticking and scaring me to death. Perfect bike for a fat guy like me.

[merlyn1965]

The throttle sticking on a 490 isn't an option at all. I sure hope that your kill switch is working fine and within easy reach while hanging on to the handlebar.

[kwmotocrosspunk]

i would look up Faded on this site, its been a while since i been on here but i seem to remember he is the man when it comes to jetting...and everything else

first of all though i would go back to a stock slide, try to make it as stock as possible to help eliminate variables, as for the gas ive always had the best luck at 40:1

and for more torque in first throw on a smaller drive sprocket, just 1 tooth will make quite a difference, especially from stock

[firesubie]

I used a jetting posted by yz490freak with just a little modification. This is for a 40mm carb which I believe is stock from 84-90, but I could be wrong about the year range. The jetting I used is 3.0 slide, 470 main, AA-5 needle jet, 7DH5 needle in 2nd slot from the bottom, 45 pilot, and air screw 1.5 turns out. It revs clean and pulls hard all the way up (at least as far up as I had room to run) but it kind of misses at about 1/4 throttle if I hold it steady. I've been on 4-strokes atv's for so long, I may just be forgetting how these 2-strokes run. I have a vm38 mikuni that I'm going to stick in there and see if it runs a little better. I also have vm36, but I think that would choke it down too much.

By the way, what is the stock gearing for the yz490? The rear on mine is a 48-tooth but I really haven't looked at the front to see what it is. It kinda looks like a 14 or 15 tooth. What gearing do most people use for general trail riding in the mountains?

[kwmotocrosspunk]

if it is missing around 1/4 throttle you are going to want to look at the pilot circuit, the slide like you mentioned and the needle, you could try going to a bigger pilot or move you needle clip to the bottom

after you get your carb issues squared away and you still want more low end, just drop one tooth in the front

[bill petitt]

Originally Posted by firesubie

I used a jetting posted by yz490freak with just a little modification. This is for a 40mm carb which I believe is stock from 84-90, but I could be wrong about the year range. The jetting I used is 3.0 slide, 470 main, AA-5 needle jet, 7DH5 needle in 2nd slot from the bottom, 45 pilot, and air screw 1.5 turns out. It revs clean and pulls hard all the way up (at least as far up as I had room to run) but it kind of misses at about 1/4 throttle if I hold it steady. I've been on 4-strokes atv's for so long, I may just be forgetting how these 2-strokes run. I have a vm38 mikuni that I'm going to stick in there and see if it runs a little better. I also have vm36, but I think that would choke it down too much.

By the way, what is the stock gearing for the yz490? The rear on mine is a 48-tooth but I really haven't looked at the front to see what it is. It kinda looks like a 14 or 15 tooth. What gearing do most people use for general trail riding in the mountains?

Where did you find their jets and the slide?? I can only find up to a 460 main, also.

[firesubie]

I got the jets from Niche Cycle on the web and the slide from some parts I had. Really good service and fast shipping. Give them a call if you don't see what you're after, they may just not have it listed.

I'm still having issues though, so this jetting is not spot on. I tried a 55 pilot today and it still misses/surges at steady throttle. I've got the air screw 1 turn out. I think I'll try a 60 pilot and drop the needle to the bottom. I forgot to try it with the choke on to see if it gets better, which would mean its still lean. I'll try that tomorrow. The first gear on this thing is high and it makes it hard for me to climb technical hills. I need to slow this dude down. I think maybe a 12 or 13 tooth front sprocket may be the ticket. I did get the throttle cable fixed, it was just binding from a hard bend.

[bill petitt]

Thanks alot!
I have heard so many different set ups for this carb,,,geez,,,getting a headache. I am in Florida so sea level may be easier jetting. I got the bike from a guy in Tennessee and he had a 428 main, with a 45 pilot. I only started it up and rode it in 2nd gear briefly, but it was bogging. It is completely stripped down right now, but when it goes back together I wanted to be somewhere in the ball park with the jetting. I appreciate the info. Keep me posted and Ill let you know what goes on with mine.

[firesubie]

Well, the money pit strikes again! I thought I would change the oil and noticed the plug had been "customized" via vice-grips. So I broke out the trusty vice-grips and proceeded to turn. About 1 turn later the plug broke off and left about half the threads up in the case. I'm hoping an ez-out will bring out the remainder, but I'm not going to be able to work on the jetting until I get a new plug.

I did play with it a little before the incident and I'm not sure which way to go. I checked for air leaks and didn't find any. It's not too bad until it gets warmed up and then it misses badly at steady throttle. Under a load it pulls hard, but surges as the rpm's decrease, especially right at the bottom of the rpm range. I tried to run it with the choke on, but it has no power and bogs, but it doesn't surge as bad. I lowered the needle to 2nd clip position from the top, but it didn't change. I hope somebody on this forum can give me some direction on this jetting thing. I would like to try my 38mm carb but I need an angled cable adjuster to try it out. The cable adjuster for the 40mm is different. Looking at the specs for the wr500, Yamaha chose to go back to the vm38 for some reason. Maybe, they found it works better with this engine.[COLOR="Red"]

[bill petitt]

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This may help!!

[firesubie]

Well, I got the oil plug problem taken care of without too much strain. I still have not been able to get the jetting correct. I have looked for the stock setting to get a baseline, but apparently they are not to be found. Oem parts diagrams show a needle jet for a vm38 which will not work in a vm40. If anybody has the baseline jetting or which jetting works for them with the vm40, I sure would appreciate it if it could be posted.
But right now I can't do too much experimenting due to the fact my kickstarter nut fell off and now my kickstarter is in the weeds somewhere along a half-mile section of road. This is the second time this happened, but I found the kickstarter in the yard the first time. It's getting expensive to drive 20 miles to the dealer for a new nut. I guess if I ever find my kickstarter, I'm going to locktite that nut on. If anyone has a spare kickstarter for sale, please contact me.