Lunchbox/Needle Shimming Question

[travisceral]

2004 GS500F

This past weekend I attempted a lunchbox conversion with stock exhaust. I have the jets (20/65/142.5) and thought I had the correct washers (I picked up #4s when I really needed #3s).

Therefore, I couldn't shim the needles. I still went 3 turns out on the mixture screws. When I ran the bike, it sounded amazing for the first 30 seconds. Then it revved extremely high at ~6k RPM. I tried adjusting the throttle idle (bike was already warm from riding), but then it just kept dying even with choke. I would start it back up, it would turn over and sound rough for a second, then die again and again.

Would this be a symptom of not shimming the needles? If not, what else should I check?

Can anyone also explain what shimming the needles does? I am both fascinated and frustrated by carburetors.

I reinstalled the stock jets and airbox and it runs as the factory intended.

[The Buddha]

20 is fine with airbox and exhaust, 65 is too much, you could use 132.5 mids, however sounded like you had the cable or something binding for it to rev to 6k.
Cool.
Buddha.

[travisceral]

I was getting my rejet specs from the wiki and another thread:

http://wiki.gstwins.com/index.php?n=Upgrades.Rejetting

http://gstwins.com/gsboard/index.php/topic,59796.msg691135.html#msg691135

I'm going to try this again now that I've picked up M3 washers that should fit.

If anyone has any further advice, I definitely welcome it!

[mr72]

OK first of all, if you reinstalled the airbox and stock jets then... problem solved, no?

I mean, there are people who have made the lunchbox work ok but is it really worth the hassle and reduction in filter performance?

Anyway, the reason your original washers didn't work is because they were probably M4 and not #4. #4 are 1/8" and M3 are close to 1/8".

But what shimming the needles does is basically enrichen the mixture throughout the so-called "mid-range". Part throttle basically, rpm really doesn't matter. The reason you may have to do that if you make an intake change like this is due to reduced vacuum so the slide doesn't come up as much at part throttle. Putting the stock airbox back on restored the vacuum conditions that the engineers designed in so the carbs work correctly.

[travisceral]

Yes, it's back to working order, but I do like the way the bike sounds with the lunchbox and the extra room without the stock airbox. I would like to reattempt with the M3 washers. I've taken the carbs apart so many times at this point I can disassemble/reassemble like clockwork lol. I replaced the carburetor JIS screws with M4 and M5 socket head screws, so that also helps.

If it doesn't work, I'll stick with the stock airbox and jets. Maybe shim the needle with a couple of washers based on some feedback from other riders.

[mr72]

I get that you like the sound and I can appreciate that, but bear in mind that there is no performance gain to be had from the K&N, and in fact it might be worse because the airbox improves consistency of airflow and it is what the carbs were designed to work with. And the filtering capability is far worse than the stock paper filter, so you will be accelerating wear by quite a bit. So there are some rather significant downsides aside from just frustration in basic tuning.