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Do what tstidham suggested first, but what oil are you using - is it rated for use for a motorcycle with a wet clutch? If the oil is not JASO MA or MA-2 rated, it may cause the clutch to slip.

Right on! +1 on this. Prep has to be extremely thorough for the sealer to last and work. I believe too many think sealers are the magic bullet to take care of rust scale when the truth is you have spend effort to get that crap out of the tank and down to bare metal before the sealer even gets...

Yup. They sell only OEM Yamaha parts - same as the dealers. I have used them all of the time after price shopping and never had a bad experience.

Welcome. You might get Yamaha cables to work from a different style of bike. I originally had buckhorn bars on my '81 XS400H Special II, but decided to go with lower bars that would be found on a standard. Luckily the '78 XS400-2E and '79 XS400-2F have the same wheels/engine config as well as...

You know, I forgot to mention one thing. People have had some luck opening up the TCI, cleaning them and repairing the solder joints that may have failed because of vibration. There are some threads on here as I recall and also lots of other things to read on the general internet from other...

Good move. The only way I know of to test what is up at this point is to swap in a known good TCI. If you know of other DOHC XS400 owners near you, now is the time to show up with a big smile, a case of good beer to leave and a small request from them. Otherwise, eBay is going to be your best...

They tend to idle higher when warmed up, but it could be the pilot jet is bigger than it needs to be. A couple of questions to narrow it down: - do you know what pilot jet is in the carbs? - how many turns out are your pilot mixture screws?

Welcome. Either the TCI is not connected properly or not working or the pickup coil is not sending signal to the TCI. I am assuming you have a good battery on the bike?

Welcome. While I don't have the DOHC bike, I have had my SOHC bike starter apart and can offer some opinions of how you might proceed. Mitsuba made a lot of models of starters for all of the Japanese makers. Some are four brush models (like the SOHC XS400s) and there are also two brush models...

Build a hidden fender hump into the seat pan to allow for the rear wheel to have full travel. It will also allow you to get the look you want for the seat without compromising the ride geometry. So you have to sacrifice a bit of seat foam in the very back - who is really going to sit back there...

So, here is the link I was thinking about: https://www.xs400.com/threads/81-back-running-now-what.20446/

I second using the on-line catalogs. They also list what other bikes the part is used in which helps to broaden your search when trying to find used bits.

There has been a recent discussion of this very thing within the last six months or so. I recall @JPaganel responding to it. Let me see if I can locate it, if I don't get beaten to it.

It is worth getting the question out there, but those two links are probably the best info you are going to get. Yamaha did "parts bin" a lot of stuff, using engineering, dimensions and parts over and over, so I wouldn't be surprised that the swap works.

¼" clear vinyl tubing is the best tool I have in my tool box. When I saw the factory manual used this method, I was all over it!

What RD front brake plate part number do you have? When I check it, the XS brake plates cross over to RD250 plates as matching parts.

Ohhh, Avon front tire.... You XS400 owners in Philly know how to roll! You have to stop rubbing this stuff in my face. On the '80 and later SOHC bikes, Yamaha specified setting the float height using a piece of tubing from the drain port and measuring the level down from the bowl/body seam: I...

Sounds like a plan. On the prior cleaning, if there is even a speck of grit in the tank or the petcock, it will find its way into your freshly cleaned carbs and absolutely wreck your day. Running 40+ year old tanks makes the chance for this even greater. See how it goes with the spark end of...

Welcome! First, check for solid spark on the right side. You might want to put in a new set of plugs if the ones you are running are old. Second, you should clean the carbs, the right one especially. There is probably some clogging in the pilot and needle jets. Make sure you blow through all of...

Welcome! I think you have a pretty good starting point, and I love the color. There are some oddities (like the seat), but stuff like that is sort of expected. Fortunately, parts availability is pretty good for these bikes, both new and used, so whatever you want to do is possible. But, that...