Newbie, Found this mystery little piece in the engine . . . (THE PLOT THICKENS!)

[SloBoi]

Hey everyone! I'm a newbie, both to this forum and to working on engines. And to riding lol. I'd really appreciate any input on this.

I made a mistake - I was taking apart the engine on my 1978 XS400 and as I was splitting the crank case, I found this little piece had fallen onto my workbench! I definitely didn't intentionally unscrew this from wherever it came from, but somehow it came out and I have no idea what it is or where it goes. I almost wonder if it was just rolling around loose inside the engine, but I can't be sure. Can anybody help me out with what this is or where it's supposed to go?

Thanks in advance!

 

[CaptChrome]

To me it looks like the clutch play adjusting screw (#23 in the exploded parts diagram below.) It should be under the output shaft gear cover on the left side and not in the crankcase.

 

[SloBoi]

To me it looks like the clutch play adjusting screw (#23 in the exploded parts diagram below.) It should be under the output shaft gear cover on the left side and not in the crankcase.
View attachment 38045

Awesome, I bet that's it! I'll check and see if that fits when I get home. Thanks a lot!

 

[SloBoi]

To me it looks like the clutch play adjusting screw (#23 in the exploded parts diagram below.) It should be under the output shaft gear cover on the left side and not in the crankcase.
View attachment 38045

Okay so unfortunately that wasn't it. I found that part in my bike and they didn't match. The one that was already installed matches what Partzilla has for that part as well.
https://www.partzilla.com/product/yamaha/90113-08021-00?ref=c88448deea51cbbea3c0cc30c2f3cee8521df936
Thanks though! That's one possibility eliminated still.

I think I'm going to just start scanning through all these exploded parts diagrams and see if I can find something that looks like it.

In the meantime, any other guesses are still appreciated! I'll post here if I find what it was.

 

[tstidham]

Looks like a valve lash adjuster screw. If you have all four on your rockers, someone in the past likely dropped one and it fell down to the sump.

 

[SloBoi]

Looks like a valve lash adjuster screw. If you have all four on your rockers, someone in the past likely dropped one and it fell down to the sump.

Bingo! It was a valve lash adjuster. However! The plot thickens!

I checked my valve lashes and there was, sure enough, one missing. But I didn't manually remove it - it had somehow actually loosed itself out and then fallen out completely while running. I think. And I also noticed that this one is slightly different than the others installed, and NONE of them match the OEM part exactly. Somebody in the past replaced all these with non-OEM parts and one came out.

Quick back story: The reason I'm taking a apart and rebuilding the engine in the first place is because after I first bought the bike it was doing this weird thing at idle where it would suddenly rev up and out of control. It would increase to high rpms, sometimes up and down, and sometimes just staying stuck at redline until I used the clutch to apply some load, which would bring it back down. Then one day as I was riding along, the engine suddenly completely seized up on me. I couldn't make it turn forward with the kickstarter, and even taking a wrench to the bolt connected to the crank shaft couldn't make it budge. It literally went from running relatively fine, and then when I went to take off from a stop sign - clunk, death.

So what I think happened is, the revving issue was caused by the loose valve lash not actually pushing down the valve properly. And then the loose screw eventually fell down and got caught in something, which is what caused the engine to suddenly seize up while running.

There is visible damage both on the inside of the cylinder head where the screw unscrewed itself while running (pictured), and on the screw itself (pictured with damaged screw on right, screw I just removed on left). I found the nut that goes with the damaged one too.

With the OEM parts, there should be a nut on both sides of the lash rocker which would help prevent this from happening (8, 10, and 11 in the attached parts diagram screenshot).

Thoughts? Do you folks think I'm on to something?

 

[SloBoi]

Upon closer inspection, it seems line that #8 which I thought was a second nut is actually a "cotter valve."
https://www.partzilla.com/product/y...?ref=8f0d1667515b62489cfbd971b3c69160fdefe43c

 

[tstidham]

There is only one nut for the lash adjuster, someone didn't tighten it enough to lock the adjuster bolt. Number 8, as you found, are just the valve keepers to attach the valve stem to the valve spring cup.

 

[SloBoi]

There is only one nut for the lash adjuster, someone didn't tighten it enough to lock the adjuster bolt. Number 8, as you found, are just the valve keepers to attach the valve stem to the valve spring cup.

So I'm going to replace the adjuster screw since the one I have is damaged, maybe just replace all of them since they look different than the OEM parts.
And I'll make sure nothing else is rolling around in there.

Any reason I need to replace the cylinder head which got damaged on the inside where the screw/nut was hitting it? Or doesn't really matter?

 

[tstidham]

It's just the access cap for lash adjustment. If it didn't wear a hole through it, it'll be fine.

 

[CaptChrome]

If my eyes are right, the bastard adjuster is a larger diameter than the stock one, so that means whoever did this must have drilled out and tapped the rocker arm. You might have to chase the threads and reuse the screw or get a new rocker arm (and adjuster) for that cylinder. The stuff you find inside the engines on old machines! Good sleuthing on this, just be on the look out for other "oddities" as you proceed.

Also, the adjuster port cap that has the impact wear is still available from Yamaha for not too much money should you need to replace it.

 

[SloBoi]

Awesome, thank you both! And good catch, I'll take a closer look and maybe order a new rocker arm / lash too.

Really appreciate the help