Carb and a half

The carb on the Yamaha XT600 is unlike any I've ever worked on. It's almost a double carb — but not really.

Almost a double carb

There's only one cable and one pilot circuit, but there are two mains and two slides. One slide is the more typical kind, operated by the cable. The other is vacuum operated.

That second half of the carb is like a boost when the RPMs get high. It acts like a double main.

A vacuum-operated slide isn't connected to the throttle. It lifts on the engine's own suction, so it only opens once the engine is pulling hard enough to need the extra air and fuel.

Simple carb, tricky jetting

The carb itself is simple enough, like most carbs, but the extra jetting makes getting it right a challenge.

I also had a very difficult time sourcing the jets. There were a few options, but I could never find the stock sizes.

Jetted for Colorado

The previous owner had the bike jetted for Colorado. I live at a much lower elevation, so my fuel mixture was too lean — the bike was bogging at higher RPMs.

Air is thinner up high, so a bike jetted for the mountains gets less fuel. Bring it down to thicker air and the same jets don't keep up — the mixture goes lean.

Jetted for Colorado, ridden much lower down — the mixture was too lean.

I put in larger jets all around, trying to match the stock jetting sizes. There was a very noticeable improvement in performance, and after some messing with the fuel screw, I got it running mostly perfect.

Not perfect yet

It still isn't perfect, and I'm not sure if it's the jetting or the screw.

After I posted the video of me working on the carb, someone chimed in with the jetting they used — and it's not what I have currently. I think I'm going to try what they said they used. These kinds of commenters usually really know what they're talking about, and they're often a better resource than the manual.

These kinds of commenters are often a better resource than the manual.