Butterfly seals

[LeBouc]

Hi all. I am still having problems with the engine not being able to reach its redline, whatever gear I am running. First, I think that any air leak that would affect the fuel mixture must be after the needle jet because, otherwise, it would not change the quantity of air being drafted by the piston downtroke through the venturi of the carb, just the quality. I found that the boot that joins the carb to the cylinder was not sealing properly so I went with silicone joint maker on both side. The tube protuding from the right side boot could slide in so I glued it with enough glue to completely seal that as well. Those 2 corrective actions cleared the problem with the idle and the rpm not going back down when the throttle was closed. However, even after I had uncrewed the pilot jet 3 turns, the engine was backfiring while still in gear and coming to a stop. The engine was not able to rev past 6000 rpm and I had to close the throttle a little bit to reach that point. So I went on the highway and brought the bike to that point in 6th gear; the engine wasclearly sputtering; to verify my theory (lean mixture), I pulled the choke and, immediatly, the engine ran fine and would even accelerate a little bit more. All those informations led me to think that the mixture is lean; the spark plug was also showing a light tan. I had in hand a couple of 150 main jets so I went with them to verify if it would clear the problem. The engine is now pulling to 7000 rpm but I don't believe the real answer is going even bigger on the main jet. The only seals that I have not changed on the engine side of the carbs are the butterfly shaft ones. On the diagram, they don't show those parts but I read that the part number is 256-14997. My question is: how many of them do I need?

 

[CaptChrome]

On the diagram, they don't show those parts but I read that the part number is 256-14997. My question is: how many of them do I need?

The part numbers that will work are 256-14997-00-00 or 2A2-14997-00-00 (one might be cheaper than the other from Yamaha even though they are the same). There are two in each carb, so you will need four seals total.

When removing the screws on the butterfly it is very important that you use a JIS screwdriver for the job (pretty sure it will need a #1). The screws are staked into the shaft and if you don't use a screwdriver with the very best engagement on the head of the screws, you will risk ruining the slots on the heads and will have to drill out the screws (a nightmare).

 

[LeBouc]

The part numbers that will work are 256-14997-00-00 or 2A2-14997-00-00 (one might be cheaper than the other from Yamaha even though they are the same). There are two in each carb, so you will need four seals total.

When removing the screws on the butterfly it is very important that you use a JIS screwdriver for the job (pretty sure it will need a #1). The screws are staked into the shaft and if you don't use a screwdriver with the very best engagement on the head of the screws, you will risk ruining the slots on the heads and will have to drill out the screws (a nightmare).

Thanks for the info. I have contacted Yamaha Canada and they sent me the catalog of parts for the XS400 1981 in PDF. I could surely download it if it could be useful for others. What's funny, though, is that they show every single parts for the entire motorcycle except those related to the butterfly mechanism! I also contacted a Yamaha dealer to see how I could get the parts because nobody seems to have it in Canada; if I am lucky, they will receive some tomorrow (tuesday). Today I rode the bike again on the highway and with the higher temperature (10 degrees more), I could only get 6000 rpm showing that I am really low on the fuel ratio. But the bike pulls really great until almost 6000 where it seems to run out of breath! I can even run it in 6th at 50kph (30mph) at around 2200 and it will accept to pull up to its limit! I love that!

 

[Revvy Kevvy]

Any chance of putting up that catalogue of parts XS400 from 1981, could be useful

 

[LeBouc]

Any chance of putting up that catalogue of parts XS400 from 1981, could be useful

If you go into the Tech section, you will see

Maintenance and General Troubleshooting​

XS400 Manuals

Go in there, scroll down to the sixth post.
Have a good day!

 

[JPaganel]

The part numbers that will work are 256-14997-00-00 or 2A2-14997-00-00 (one might be cheaper than the other from Yamaha even though they are the same).

In the Yamaha part numbering scheme the first three digits are the code for model of the bike they are first listed for and the second set of digits is the actual part. The last two sections are revisions/updates.

they show every single parts for the entire motorcycle except those related to the butterfly mechanism

They stopped listing those parts separately right around 1977 or so. Which is when XS400 came to be.

They decided shaft seals aren't replaceable, which is why the screws that hold the butterflies in are peened on the end.

Note that unless you want to strip them, you really want the correct JIS screwdriver and not a US-spec Philips.

 

[tstidham]

I'd recommend using a Dremel with a small grinding stone on the thread ends. Even with a good JIS screw driver, I had one screw shear off when trying to remove. Luckily I was able to grab the thread ends and remove it out the other way. Patience and a steady hand and they'll be out.

 

[LeBouc]

In the Yamaha part numbering scheme the first three digits are the code for model of the bike they are first listed for and the second set of digits is the actual part. The last two sections are revisions/updates.


They stopped listing those parts separately right around 1977 or so. Which is when XS400 came to be.

They decided shaft seals aren't replaceable, which is why the screws that hold the butterflies in are peened on the end.

Note that unless you want to strip them, you really want the correct JIS screwdriver and not a US-spec Philips.

Somewhere here, I found the part number for the seal and called a Yamaha dealer. It was backorder but they were supposed to get some a couple of days later. I should be able to verify that next week. Thanks for the advice for the JIS screwdriver. I'll make sure that this is what I use!