1980 XS400 Special, jet issues.


XS400 New Member
Reaction score
East Tennessee
I recently purchased this bike from a fellow that couldn't get it running after parking it for the winter. It ended up being a combination of problems, the electrical had been hacked up, the ignition switch was sticky and then after I had it running I couldn't lean out the carb. That ended up being because somebody had installed an ebay carb rebuild kit that was poorly fitted, fuel was making it through the main jet at idle making it run even with the pilot screws all the way in!

Anyway, long story short, the previous owner had saved the original carb parts, so I cleaned them up and just put them back into the carb. I bench synced the carb, and then built myself a manometer and synced it again with that; now it runs pretty good, except I sometimes have a little bit of a hanging idle after revving. And I have lubed all the cables. Also it runs best with the mixture screw at 4 turns out or better, with popping and throttle hanging worse below that.

The Old main jet was marked 135, the old pilot jet was marked 42.5. The main needle is a single position needle. The float level is good, and she has tons of power and response when riding.

Do I need to change the pilot jet? I sprayed around the carb and the boots and all, no idle change no leaks, factory original filters.

Thanks in advance for your help.

Popping and hanging revs after throttle roll off are both signs of a lean idle condition. The usual culprits are leaking carb holders (intake boots), leaks from the vacuum nipple caps/hose/petcock or leaking throttle shaft seals. If you didn't replace the throttle shaft seals, my money is on them as the source of the leak. Changing the jetting will not solve your issue.
I didn't replace the throttle shaft seals, I started to attempt to remove the butterfly but I was worried about stripping out the screws that hold it. I guess I'll look at getting a seal kit for the throttle shafts.