Reworking the valves...

[Revvy Kevvy]

On my XS400 that has not been ridden for 26 years I am slowly working towards it actually runninng.

Initially it would not start because the Mikoni carburettas were all gummed up and every adjustment was wrong, I bought a kit and put it in and then had a long time of getting them back to working again including working out the float heights (23mm) but I am confident the carburettas are working.

The first time I started it, it was "backfiring" out the carbretta on the left side, according to google AI it was the timing. I had very weak spark and I could not set the timing as the points were worn out so I bought a new HV ignition transformer and new points and I have set all those up with good timing on LF/RF and points gap (0.35mm) and dwell (40 deg) all that seems ok now.

But I am still getting the backfiring problem through the carburetta and if I hold my hand in front of the inlet and give it the bejesus (rev it) I can see vapour pouring out. Putting my hand in front will result in a wet hand of petrol, only on the left side. The right side is fine.

I have come to the conclusion the valve on this side is not sealing properly (or worse I have a timing issue) so I will open up the cylinder and work on this.

Those that know me will know I am way up north almost in the artic circle and getting parts up here is hell. Delivery is very hit and miss and because I travel for work sometimes I miss a delivery and after a week its sent back to the sender...they will not deliver to homes its held at the local supermarket its very strict. Collect in two weeks or it goes back.

I have a business trip to Poland comming up and this is a good opportunity to buy and carry home the bits I need.. Has anybody opened up their heads, maybe to the cylinder and I guess worked on the valve and maybe honed the cylinder since it will be open anyway. What parts should I buy (and tools) remember I get only one shot at this for the summer and soon winter will be here. I had hoped to have my bike at least running by the end of this summer and that's only a few weeks away. 24 hour sunlight has dropped to darkness by ten PM now and soon it will be like the movie "pitch black" with 24 hour darkness but unlike him I also get snow and minus 20 degrees farenheit!!!

I think I need to buy:
A gasket kit for a XS400 SOHC engine.
A valve stem seal??
A valve spring removal tool
A lapping tool
Some valve grinding paste.
Some of that blue stuff that indicates you have lapped the valve properly. (forgot its name)

I guess for the cylinder walls I will need a hone?
I seem to recall honing is done dry.
Can I use the same rings and oil ring or should I change them too.
If I change them I will need a tool to get the pistons back in but I think last time I did this (back in 1986) I used a jubalee clip and grease.

Anybody who did this job what did you have on hand to do it??

I have a pretty good workshop up here that if you are rolling past on your way to view the Aurora or looking for Polar bears and need help you are welcome to use but my biggest issue is getting parts as getting stuff deliverred is really not easy and there is no automotive or motorcycle store anywhere near by.

Pictures below was me testing my Mikonis that the float height was correct and as far as I can tell it is, the fuel should go up to the height of the bowel +/-3mm and this method of using a glass jar was a good one. AI said that the reasons for having fuel comming out was over filling of the float bowel...but mine is being blown out and that indicates a valve not seating properly.

 

[Revvy Kevvy]

AI opinion...
Yes, valves on a Yamaha XS400 engine can get jammed or stuck if the bike sits for a long time. Old engine oil turns into a sticky varnish or hard gum. This sticky layer catches the valve stems inside their guides and stops them from moving. [1, 2, 3]

Why Valves Get Stuck

• • Old oil dries out and turns sticky.
  • Moisture gets inside the engine and causes rust on the metal parts.
  • A stuck valve stops the spring from pushing it closed. [1]

How to Check for Stuck Valves

• • Remove the valve cover on top of the engine.
  • Turn the engine over slowly by hand.
  • Watch the rocker arms and valve springs.
  • See if every valve moves up and down smoothly. [1]

Addressing Stuck Valves
Resolving stuck valves is a delicate process that requires precision to avoid permanent engine damage. General safety and maintenance standards include: [1]

• • Lubrication: Applying specialized penetrating lubricants to the affected areas may help break down carbon deposits or varnish over time.
  • Safety Precautions: Avoid using the starter motor or forcing the engine to rotate if resistance is felt, as this can lead to bent valves or damaged pistons. [1, 2]
  • Professional Assessment: Because the XS400 is an overhead cam engine, internal repairs often require specific torque settings and specialized tools. It is highly recommended to consult a professional mechanic or refer to the official factory service

 

[CaptChrome]

What parts should I buy (and tools)

I think your list of parts and tools is a pretty good one. But here is some additions and a comment.

The comment is that the "blue stuff" is called Prussian Blue or sometimes mechanics bluing or just bluing. While is is nice to use, I don't think you should bother getting it because after lapping in the valves with the lapping compound, you will be able to see a line on both the valve and the valve seat in the head where they are making contact. Just make sure that they are continuous around the valve and seat and that the width of the thin line is the same width all around.

The item that is missing is some sealant for the rocker cover to the head. There is no gasket for this, so you will need to carefully put sealant on one side before you join them up. The recommended sealant is Yamabond, but you will be able to use Threebond, Hondabond, Permatex Motoseal or similar. This is the same stuff used to seal crank case halves on air cooled VWs, aircraft engines, small gas engines and motorcycles. So, finding something should not be too hard.

You will definitely need a cylinder base gasket, the cylinder base o-rings, the o-rings for the oil passages up to the head and a head gasket. I do think you should replace the valve stem oil seals while in there. All of this stuff will be in one of the inexpensive gasket kits you will find from Taiwan on-line. The only other thing that caused me an issue was the seal at the points drive end of the camshaft. Mine leaked after putting the engine together. It is not very hard to replace after the engine is back together, so perhaps you go with what you have, but I would replace it and the tach drive seal while in there as they may have hardened up after so many years and they will not be in the gasket kit from Taiwan.

 

[CaptChrome]

I guess for the cylinder walls I will need a hone?
I seem to recall honing is done dry.
Can I use the same rings and oil ring or should I change them too.
If I change them I will need a tool to get the pistons back in but I think last time I did this (back in 1986) I used a jubalee clip and grease.

Yes you will need a hone. either a ball hone or a flat stone hone will do the job, just make sure the size of the cylinder bore is within the range that the hone is made for.
Hone with oil. I used a lightweight oil like mineral oil. But, spay oil, baby oil (mineral oil that has a nice smell), or motor oil with work.
You can reuse the rings if they look okay. I re-used mine after a thorough cleaning and careful inspection.
You will not need a ring compression tool to put the pistons back into the cylinders as the bottom of the cylinder bore is beveled and the rings will self-compress as the cylinder slides over the top of the piston.

Also I forgot this in the post above - If you do take the pistons off of the conn rods, you will need new wrist pin circlips (per the workshop manual.) The copper sealing washers used in reassembling the head and rocker arm cover can be re-annealed with a gas torch and then reused.

 

[Revvy Kevvy]

Thankyou so much for the list Captain Chrome, I really do need it as I cannot just pop down the local automotive shop where I am. I have quite a good garage up this way but not everything.

Like honeing tool. Oil I am good for looks like (baby oil from when I had a girlfriend, before motorbike) (I have a honing tool from honing the brake master cylinder but its way too small for a cylinder)

Sealant, I will absolutely grab some of this, hopefully these brands exist in Warsaw Poland

Looking up the circlips idea now, and the kit I can get sent to Poland to the hotel.

Thank god for business trips, I have not made one to USA for a very long time so I have to learn how to find stuff around and about cause getting it sent here is tricky and I have lot too much stuff on return.

Thanks ever for your advice, I hope one day to either buy you a beer as you cross the 66 lattitude heading north on your XS400 or if I am ever in southern New Jersey. I used to work on the cyclotron in John Hopkins in Baltimore and I know you love cycloltrons. The only place I lost my car to a hijacking and got a gun pointed at me was leaving John Hopkins late one night....happy days. Never stop at stop lights around John Hopkins is my advice...and the advice of the cop who responded to my 911 call too )

 

[CaptChrome]

Glad to help. Next time I plan an XS400 trip from Stockholm to Nordkapp I will look you up! Sorry to hear about having a gun pointed at you here is the US; what are the odds? I have also had a gun pointed at me while traveling, both times in Asia though. At least those incidents make for some colorful stories now.

Perhaps the Polish cyclotron mechanics have the sealant you need?

 

[Revvy Kevvy]

You will need a beer if you are comming from Stockholm, I am quite a few more hours north from there.

 

[boldor750]

Hey, just read this thread. I had a similar situation with an xs 400 2a2. It was in a barn and wasn't moved for over 10 years . The forums and especially a Canadian guy on YouTube,the mighty don alan, rebuilt his xs 400 and recorded his work motivated me to tear down the motor.Anyway, i removed the cylinders and head, was mostly cleaning. I did the valves like you plan on doing. Like Cpt. Says I didn't use any blueing , didn't hone the cylinders, they looked fine. I did get a complete gasket set from Louis though. The valve stem gasgets were included in the set. Louis is a large motorcycle parts shop, maybe they have them in Poland. Anyway, i got lucky and things worked out for me. I still ride this bike now and then and always enjoy it. I really hope that you are also successful too. Just imagine some guy being chased on a xs 400 by a polar bear.... Jesus.

 

[Revvy Kevvy]

We have brown bears here in the north artic but not polar bears. The closest would be Svalbard for these, I occasionally encounter Brown bears but they ignore me and I am happy about this. I hit a moose once but never a bear. We have wolverenies, lynx, and a lot of mosquitos. In the winter the sun is down all the time but we have a nice aurora from time to time.

I did the valve work as described and everything is runnig quite well. I have a bit of tappet knock but I will fix this after the tecnicial inspection in September...

It will be cold and dark soon after so I hope to get at least a little bit of ride time in.. soon is 24 hour darkness....like in the movie pitch black except its also bloody cold.