Carbs off by dinner

Same evening. The dual carburetor assembly came off the bike and onto the bench, and then it came apart.

I told myself I’d just look. Four hours after the bike rolled in, both carburetors were on the bench in pieces.

The XL350R runs two carburetors on one cylinder: a cable-operated primary and a vacuum-operated secondary that opens as airflow picks up. They come off the bike as one assembly, joined by a linkage and a fuel crossover tube, and the first job on the bench is separating them. Take photos of the linkage from every angle before you touch it. I did, and I still needed them.

What I found

Varnish in both bowls, as expected. Drain screws seized. The slide in the secondary moved freely and the bore was clean, but the diaphragms (there are several on this carb setup) are all hard and cracking at the edges. Those are the parts that actually stop these bikes from running right, and they’re the ones worth spending money on.

Lesson

The photos in this entry are the ones I wish I’d found online before I started. Every one is tagged to the carburetor & intake region of the atlas.

Photos

The complete dual carb assembly, engine side. Primary carb on the left, secondary on the right, joined by the linkage and the fuel crossover.
Carburetor & intakeBefore

Airbox side. The two bores where the airbox boots clamp on. Four decades of grime on everything.
Carburetor & intakeBefore

Three-quarter view showing the throttle linkage between the two carbs and the cable bracket.
Carburetor & intakeBefore

Looking down the secondary bore. Butterfly closed. Note the brass fittings on the side of the body.
Carburetor & intakeBefore

Float bowls, from below. Both drain screws present, both stuck.
Carburetor & intakeBefore

Top side. The two float bowl vent caps and the linkage spring. Photograph this angle before you take the linkage apart, you will want it later.
Carburetor & intakeBefore

Straight down the bores from the airbox side. The secondary (right) uses a vacuum slide, the primary (left) is cable-operated.
Carburetor & intakeBefore

Diaphragm on the side of the carb body, exposed. The rubber is hard and cracking at the edge. Expect to replace these.
Carburetor & intakeDuring

Primary carb separated from the assembly. Throttle linkage arm still attached.
Carburetor & intakeDuring

Secondary carb on its own. The long arm on the right is the linkage that the primary drives.
Carburetor & intakeDuring

Secondary carb from the engine side with the throttle return spring visible on the right.
Carburetor & intakeDuring

Float bowl off. Float, float valve and main jet visible. Varnish, but not the worst I've seen.
Carburetor & intakeDuring

Primary carb body, bowl removed, showing the jets and the accelerator pump passage.
Carburetor & intakeDuring

Throttle shaft and butterfly, secondary carb. The shaft turns freely, which was a relief.
Carburetor & intakeDuring

Same, other side. The small screw is the throttle stop.
Carburetor & intakeDuring

Linkage bracket and the cable stop, separated from the body.
Carburetor & intakeDuring

Keihin casting, primary body, with the linkage pivot. Made in Japan, as it says.
Carburetor & intakeDuring

Secondary carb with the bowl off, held up to the light. Float pivot pin and float valve seat visible.
Carburetor & intakeDuring

Looking into the secondary float bowl cavity. Brass float still attached, pivot pin in place.
Carburetor & intakeDuring

Vacuum slide and spring out of the secondary. The diaphragm is the part to inspect carefully here.
Carburetor & intakeDuring

Slide, spring and the body it came from. The slide bore was clean, the diaphragm was not.
Carburetor & intakeDuring