Carbs or Coils...or something else???


'78 XS400-2E
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Janesville, WI
Hello everyone, haven't been here in a while because I haven't really putzed around with my XS400 for a few years.

I've got a '78 XS400-2E that I had running exquisitely back in 2016 until I started having electrical issues - basically the original mechanical voltage regulator was fighting me (with a full-size battery I was fine but I tried rigging up a small 2.something Ah battery and the bike died on me after about 2mi once so I had to return to a bulky battery - obviously no charging at low RPM), and due to work and other life excuses, I hung up on the project for a while...until earlier this summer. Also worth noting is that a few months before hanging it up back in 2016, I adjusted the valves and installed the electronic ignition - man did that make a difference!

Over the last couple months I completely redid all the lighting - except headlight - new turn signals, tail/brake etc, new mini speedo and tach from Dime City - everything is now wired up beautifully and I installed a modern solid state voltage regulator from a thread on this site (massive help - I've got great idle voltage now and I've ridden around my neighborhood almost 10mi over the last week with no charging issues); at this point, there's zero electrical issues to speak far as I know.

However, bike performance is a different story. I had to disassemble and clean the carbs because I didn't properly store it back in 2016ish so I took care of that, screwed around with the floats, again, based on threads I found here. At this point, the bike starts from stone cold with no more than two kicks and idles like a clock - it's beautiful...until I ride it.

My issues arise above a certain throttle position. At low end - she takes off great! However, with more throttle, I'm getting sputtering at higher RPM, (seemingly from the right cylinder), and at a point, it seems like the right cyl just goes cold above a certain load/throttle position. My neighborhood (Janesville, WI) has a very steep hill - tried going up that in 3rd gear with a running start....hell no...downshifted to 2nd...hell no, almost died, 1st gear, I was able to make it up with no shortage of sputtering - again, throttle position seemed to be the issue.

I figured, maybe with all the bending of the floats I jacked them up so I rigged up the clear tube method of getting floats set correctly - did that last night, - yeah, the floats were definitely off even though the standing float height was 26mm from the gasket surface for all four - (important lesson there!). Now both float bowls fill up to midway up the "rim" of the bowl which, according to other posts on this site is correct (can someone confirm?).

Figured at this point, the bike will now be golden...nope...exact same issue persists. So I tried a vacuum gauge style manometer to check idle vac on both cylinders to balance them. This is where things get weird...the vacuum pulled...was basically zero...the needles just fluctuated around -2psi of vac to 0psi (atmosphere) it supposed to be -18psi of vac like a car? When I revved up the motor, RPM remained around zero and once I released the throttle, the vac would drop down quite a ways very briefly but as the RPM returned to idle, the vac returned to basically nil. I cannot tell you what the vacuum looked like when the bike was running back in 2016 so that doesn't help. With respect to balance, the carbs were slightly off but not drastic - got them balanced, test rode it, problem was identical - sputtering above a certain throttle position, cannot go 40mph. Brought it back to the garage and adjusted the idle air mixture screws to about 2turns out...test rode - unsurprisingly, performance is still the same. I'm running Uni air filters, they're freshly cleaned and oiled as of a few days ago.

Please help!

Thank you,

- James