Desperate after Cafe racer build ( difficult problem )

[Zero]

Hi Guys,

Not really desperate.... . But...I rebuild a GS500 to a caferacer. I have changed the Exhaust to an open and have open airfilters. It wasn.t running good, I didn,t get it further then around 5000  RPM. I have changed the nozzles from 110 up till 150. No change. Sometimes it seems that it will go further in RPM, but only sometimes. I filled up the open filters a bit and tried all sizes of nozzles.  Last thing I could think about where the valves, so i brought them in specs. Still no result.
Any tips ? Nozzles I used where 110, 120, 135, 140, 147.5 and 150. I think 150 was a bit overdone.

[herennow]

what year carbs?

Which jet did you change, you sure you chaned the right one? (not the choke jet)

[mr72]

You have set up a classic example of the impossible-to-tune GS500.

If you want to get it running, put the airbox and stock exhaust back, then you can make it run correctly. If I am interpreting your mods correctly, they are well beyond what can be adjusted with just jetting because you've made major changes to air velocity. There is no way to know what could be wrong because the intake and exhaust are obviously very wrong.

[Zero]

I am sure i changed the right jets... It is not the first bike i have build.
And about the changes i have made. It is just a matter of finding the right mixture between air and gasoline, so I dont believe it can not be adjust.
I was just hoping someone had the same problem...

[mr72]

Except it's not just "the right mixture between air and gasoline" when you are trying to use constant velocity carburetors. They rely critically on air velocity to work correctly. Once you change things like the air filters, air box, and exhaust, that dramatically changes air velocity and those carbs have other features besides jets which will require changes to accommodate the change in air velocity.

The flexibility of the diaphragms in the carb, which also has to do with damping; the spring rate of the slide return spring; the size of internal orifices in the carbs where air flows (like the pilot air orifice), the size of the main venturi itself, needle profile, emulsion tube hole size and number, etc., all of this is tuned to match an expected/known air velocity. Jetting can help with very small adjustments like for altitude or a different muffler but big changes like airbox-delete are far easier to solve with different carbs altogether. It's not a tuning problem, it's a design problem.

Put a lawn mower carb with one circuit and a simple throttle plate as the only air valve and you can tune it with just jets. But with a CV carb, there's a lot more to tune than just the jets when you make very big changes like this. Go ahead, read the archives, there are plenty of stories of folks trying to do this with basically zero success. Mine included.

[Bluesmudge]

Are you running pod filters? The GS500 carbs don't like pods.
Some luck with a "lunchbox" style K&N filter.

[mr72]

Yeah even the K&N lunchbox is notoriously difficult to get it to idle but can be made to run on main jet with a big enough jet. pods, not likely. add to that open exhaust and all bets are off.

I mean, you kind of have to decide whether you want it to look a certain way or to run correctly.

[qcbaker]

I mean, you kind of have to decide whether you want it to look a certain way or to run correctly.

Ahh, yes. The cafe racer paradox.

[crackin]

I 'm calling bullshit, put in a dyno jet kit and it will go like shaZam! out of a shang eye.
It wont run properly with the stock needle with the mods you've done. The mid range is all wrong and the dyno jet needle fixes that.
  regards johno