A few questions about fuel delivery in an oldYZ490

[zoommx]

I bought a '82 yz490 as a winter project for the upcoming spring post-vintage ultima 500 class. Yes, even after reading a few items about their various shortcomings. Well, I just got to the top end, and it is only at +.5mm over right now (much to my surprise and happiness); I'll have to mic later to see if I need to bore it. Now for the questions:

1. the jetting confuses me. I just want to verify that it was completely wrong before I go further. The Yamaha motors online parts lists these std.: 50 pilot, 6F16 needle, 440 main. What I found in carb.: 40 pilot, 6F16, clip in top position, 200 :scream: MAIN!!! The only test ride I did was get it running and shift through the gears to make sure tranny was o.k. It WAS hard to start. I am leaning to someone not really knowing how to jet and dropped a 200 in, not that the Yamaha site is wrong or that the main could be that far off.

2. It has steel reeds, Who votes that I replace them with fiber?...i.e. Boyseen.

3. It has a boost bottle. do they really DO anything or should I toss it and plug the hole?

Thanks,
Roger

 

[nitropimp]

I had the same issue with the main on my 92' WR500. My manual and tech info at the dealers indicate it should have a 440. The number on the jet could not be read, so I went to a shop to get some extra jets and compared the one I had to a 440, not even close. I'm not familiar with how the number coresponds with the actual hole size, but the mechanic came up front and told me it measured out to be a #260 Mikuni main.
I'm in the Phoenix, AZ area and I run a 270 main, 6F16 Jet needle, Q-8 Needle jet, 3.5 cutaway, 35 pilot. I have only changed the piston 1 time and was able to replace with stock 87MM.

I run the Boyesen 2 stage reeds.

All I know about the Y.E.I.S. is that if it's removed, the performance will be effected, read that in the manual. I have heard that you can tune by adjusting the length/size of the hose. I have not modified mine in any way.

Maybe someone can give us some headsup.

 

[Rcannon]

I had a 82 YZ 490. Running at 4500 ft elevation I was running a 380 main. I believe the parts guy also gave me a "Q4" needle jet.

I spent entirely too much time trying to make th ebike run clean. If I would jet it correctly, the bike would sieze. I truly hated the bike.

 

[marcusgunby]

The YZ490 needed head mods to run clean-it kept a few tuners in business in them days.

 

[zoommx]

Originally posted by marcusgunby
The YZ490 needed head mods to run clean-it kept a few tuners in business in them days.

Thats funny :) Any idea what they did besides, I presume, mill it some?

Sounds like I need to start with another main anyway, and I might as well get another needle jet also! Thanks guys.
Roger

 

[marcusgunby]

It was to do with squish shape , not a compression issue, eric would know.

 

[Lorin]

I had Noleen do the head mod on my 92 wr 500 for $80. Consisted of a 2 angle squish (13 & 35), with 20 thousands off of the head. They said it added a total of 2.4 ccs to the head volume. It did help with the pinging on the motor. As to jetting, mine ended up with a 430 main jet, 45 pilot, and the 6f16 needle in the 2nd clip. I later put a 39.5 PWK keihin on the bike. I'm suprised that you can run a 35 pilot jet on that bike. I thought mine was jetted pretty sharp with a 42 (summer-time).