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'88 yz490


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Okay, so I finally have almost gotton my bike perfect! When i got it, almost all the gaskets were deteariated. It ran, only by pull starting. But obviously, with all the air getting in between the cases and the cylinder, it was running SUPER lean. So i replaced those, cylinder and piston looked.. perfect, to me anyways. Put it back together and actually was able to start it without pulling it! ­čśĆ

Still running pretty lean, I put new oil (75w gear oil) is that okay?? The case say's 750cc's and i thought that it was ml's so i had put 750ml's in it.. and it was leaking oil out of the case breather hose. I was stumped for like.. two days. Couldn't figure out where it was coming from! The I seen it. So, first question. How many mililiters do i put in it?

The bike, as i mintioned was still running a little lean. I put a new throttle cable on it, and moved the clip down one while i had it apart. It did backfire horribly, but since i moved the clip, none. I still have to use eather to start it the first time, but after it's warm. No problem. ANYWAYS, back to my second question. The scilencer is completely hollowed out. Could this cause lack of back pressure, and cause it to over rev some? I'm not sure if it's just over reving a little, or if its still running lean. Either way, it is running great compared to three weeks ago.

All help is much appriciated!!

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I had a 88 also, the only way i could get it running right was with modifing the carb slide. Many post on this at yahoo yz 490 group.

if your pumping out oil from the crank breather with the right amount of oil in it, then you may have a bad crank seal which can pressurize the tranny.

Who ever designed that oil fill hole was a prick!

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  • 2 weeks later...
Okay, so I finally have almost gotton my bike perfect! When i got it, almost all the gaskets were deteariated. It ran, only by pull starting. But obviously, with all the air getting in between the cases and the cylinder, it was running SUPER lean. So i replaced those, cylinder and piston looked.. perfect, to me anyways. Put it back together and actually was able to start it without pulling it! :banana:

Still running pretty lean, I put new oil (75w gear oil) is that okay?? The case say's 750cc's and i thought that it was ml's so i had put 750ml's in it.. and it was leaking oil out of the case breather hose. I was stumped for like.. two days. Couldn't figure out where it was coming from! The I seen it. So, first question. How many mililiters do i put in it?

The bike, as i mintioned was still running a little lean. I put a new throttle cable on it, and moved the clip down one while i had it apart. It did backfire horribly, but since i moved the clip, none. I still have to use eather to start it the first time, but after it's warm. No problem. ANYWAYS, back to my second question. The scilencer is completely hollowed out. Could this cause lack of back pressure, and cause it to over rev some? I'm not sure if it's just over reving a little, or if its still running lean. Either way, it is running great compared to three weeks ago.

All help is much appriciated!!

I purchased an 85 yz490 in october and went thru alot of stuff you are talking about. I got it fixed and won the FLVMX B class on her, so i will give you the advise I got and tell you what I did to fix the problems,,,here goes.

First the oil: I run 10w40 in the bottom end. Remove the bolt on the right side, of the case, in front of the foot peg It is about 2 inches up from the bottom of the case. Start putting oil in it, till oil starts to drip from the hole you removed the bolt from. Put the bolt in and start her up. let her warm a few, then cool of for about an our. Then remove the bolt again..if it drips, you are good, if not, add more.

Second: the revving: obviously the gasket issue not help. Once you have all the gaskets replaced, make sure your head bolts are torqued properly. I had same problem. Torqued head and problem went away. I suggest a new head gasket.

third; the every loving carb!!: I eventually ended up with the stock carb, with the stock slide, 430 main jet and??? pilot(stock) I set the clip on the needle one ring from the top of the needle. Air screw ended up at 1 1/4 turn out. This was the best setting I found, for 85-95 degrees, little elevation, in Florida. Now,,,I have heard a flat slide carb is supposed to do wonders,,,I just recently purchased one and I am going to attempt to get it set up on the new motor I put in, which is a 1989.

When checking how lean it is(Main jet) run it wide open for a few seconds, then pull in clutch and use kill switch to stop it. Check your plug then! As far as the air screw,,,have some one help keep it running, turn the idle all the way out. Rev it up and listen to it come back off rev. It should be smooth. Once you achieve that, set your idle. If you have to back air screw past 3 turns out, you need different pilot jet, but you should be able to get it in the ball park, with the stock one.

Fourth: Exhaust: i was told the back pressure issue does not affect 2 strokes. I lost silencer in first turn of a race, and it did not bother the performance at all,,,just the noise sucked.

This should get you started. If you need further help, feel free to email me at bill_petitt2000@yahoo.com.

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