Carb issues, XS400 Maxim

[pulmenti]

Hey guys, hate to start another carb thread but I couldn’t find what I needed and there’s a lot of dead links out there. My ‘82 Maxim is running on brake cleaner but won’t start on gas. One time, seemingly randomly, it fired up first crank and idled for about 2 minutes but hasn’t since. Just ordered a carb rebuild kit from mikesxs, unsure what to do, since I already rebuilt these… I matched everything to the manual available on this website, but did notice that all the main jets that came with the kit are different sizes, and remember reading something about differently jetted carbs on DOHCs. What size jets should I be running, and should I be running different sizes left to right? Thanks!!

 

[JPaganel]

There is a difference in left and right main jets. Pilot and air jets should be the same.

The stock jetting for the 1982 Yamaha XS400J should be this:

Jet, Pilot 42.5 (4G0-14142-42-A0)
Jet, Main Left #125 (620-14231-25-A0)
Jet, Main Right #115 (620-14231-23-A0)
Jet, Pilot Air #170 (3G2-14231-34-00)

 

[pulmenti]

That’s really interesting, definitely explains why my cylinders are running different temps and i’m getting iffy throttle. You know why it’s done this way? Is this common with many models? Also - where’s the info coming from? I’ve yet to find a good manual online.

 

[gpounce32768]

My '82 Seca is supposed to have different main jets but when opened them up they were identical. I drilled out the left one and it made no discernable difference. What did help was balancing the carbs, getting the AF screws set reasonably and cleaning out the enrichener fuel passages.

The way I heard it was the left cylinder tends to get air more easily via the airbox, so has a larger jet to match. On a little 2 cylinder engine like the 400's I'm a bit skeptical of the actual importance though. But assymetric jets do happen elsewhere- my old '99 R6 had the inner two carb jets different than the outer, apparently to help smooth the power curve or maybe compensate for temperature differences related to the inner pair vs the outer pair. OTOH my old Bandit was also 4 cylinders but jets were the same on all 4 carbs.

 

[JPaganel]

That’s really interesting, definitely explains why my cylinders are running different temps and i’m getting iffy throttle. You know why it’s done this way? Is this common with many models?

Airflow is different somehow.

I had this on a transverse four, inner two cylinders were different from outer.

Also - where’s the info coming from? I’ve yet to find a good manual online.

Parts fiche. That's what factory specifies for them.