Bent intake valve #4 WHY ideas !

[vedgar]

Head is removed and new valve ordered . Put on thinking cap. Bought this pristine 80 SG with 2200 miles from the original owner. No keys and a lost title. Just for background info, he remembers last registering it in early 1982. After ordering new keys and running a compression check I found all cylinders ok but #4 at 0 psi. Crosshatch pattern is abundantly visible on the cylinders so the mileage rings true. What would be some probable reasons for a bent intake valve with such low mileage ? The exhaust Valve on #4 is perfect, spinning in a chuck. Hmmmmm. Just thought I would ask before re assembly .(No nicks or marks on the piston)
Thanks,

Vance

[TopCatGr58]

Even though these valves are strong enough to withstand the heat and pressures of the combustion chamber, their shafts are surprisingly weak!

With the engine sitting for years/decades, it can develop RUST around the shaft of a valve that is pried/wedged open from a cam lobe when in the stopped position. Then when the engine is finally turned over, that valve will NOT retract back into the head, held in place by the rust/corrosion around the shaft between the valve head and valve guide! And since the engine is turning over relatively slowly, the piston can still come up and bump the valve bending it without necessarily doing any damage to the piston.
OR....these heads are interference designed, so that both valves can not be open and occupy the same space at the same time, and with the valve wedged open, the opposing valve then opens and hits the already open valve bending it. So...even though you say the opposing valve isn't bent....you'll want to be sure to perform a leak test once you replace the bent valve, along with doing a lapping of the expected good valve again to ensure that it's sealling properly!

T.C.

[vedgar]

thanks Topcat

I was making the assumption that the original owner may have caused the damage somehow at the 2200 mileage mark. The bike has been in storage since 1982. I believe that the bent valve is what took it down many years ago. Treading carefully , I will be sure to assemble and leakdown test. My working career was as a Jaguar technician , so I am familiar with what bad stuff happens in an interference engine. Fixed many ham handed repairs from owners who turned cams while installing head.

[ESJr1642]

Clean!

Very clean from what I can see, would love to see more pics!

Nice find!

[TopCatGr58]

Hey Vance,

Glad to see that you're a knowledgeable mech. At 2200 miles, the engine and cam chain didn't have enough wear to stretch much, and aside from setting the cam chain tensioner incorrectly which could have allowed the chain to skip a tooth or such, but then it would have gotten all of the cylinders out of time, and bent more than just 1. Another scenario could be that the PO mishifted and hit an rpm oops above redline and floated a valve and got it bent?

Don't know if you knew to check the cam timing dots prior to disassembly just to see if it was still in time?

With this level of rebuild I hope you've seen the notes about the old cam chain tensioner and it's faulty design? We suggest either getting and installing a newer ACCT (automatic cam chain tensioner), or perform the mod to the OEM unit to make it a more stable/strong failsafe manual adjusting/setting design!

Yes, very nice clean engine, but what would we expect from only 2200 miles!

T.C.

[vedgar]

Will post more pics later. Brain foggy by 11 pm

Cam timing and crank timing correct when disassembled. Compression 135 all around with carbs installed and throttle open. Cylinder #4 at 0. I guess I never took into account the PO could have been trying to revive the bike and a valve stuck ! He probably just got tired of trying to start it and gave up. He had new fuel lines on the bike but that was about all. I first found this bike three years ago and finally got the PO to give it up. He just wanted assurance the bike would not be parted out and he could test ride when finished. I am giving it a good home with six of its' stablemates. Back to the valve, I am just trying to wrap my head around why the PO never rode this bike after 1982 or so and some theory of how the valve got whacked. Topcat, I think you got it right with the oops throttle. The owner would have been single and about 26. Testosterone fueled rampage of speed. Ha Ha.
Vance

[vedgar]

In POs' garage (notice original tires) Mag Mopus

[Schming]

Wow !!!
Sweet !!!

Nice Score !!!

I think that may be the cleanest, low milage, original XS11 on this site.

[trbig]

Just a suggestion while you've got it down that far... Swap the washer on 2nd gear so you'll never have to worry about that again.

[vedgar]

Tank mess

I posted a want for a stock tank as the inside was unsuccesfully coated. If that is the worst of it I can Hollah for a Dollah !
Vance

[vedgar]

No Speedp swaps on this one

[skids]

On assembly, you might want to use a cam shim that provides extra gap so that the valve does't open very far. Maybe the shim gap from the factory was too narrow, eventually causing piston/valve contact. Then after you are done reinstalling the chain and sprocket, replace the shim for the correct gap. Maybe this is a bit on the cautious side, but that is what I would do.

[vedgar]

good idea !

The 80 and 81 call for a revised gap , closer than previous models. I think I will go with the 79 gap. 78-79 is .16 to .20 mm. 80 up is .11 to .15 mm intake only.

[TopCatGr58]

Hey Vance,

Part of the reason for the closer gaps is because Yamaha changed the design of the heads, and put slightly larger/wider headed valves in the later bikes, along with reducing the valve lift, all for trying to meet the EPA requirements and still provide a decent amount of power, so they set the clearances a bit closer to help unsure enough valve opening lift action especially with the lower cam lift profile. The closer specs also provide a quieter experience. You may know this but may not, the clearances on these engines actually get CLOSER with wear instead of wider, due to the valve hammering the seat into the head. Some folks will set the gap/shim to the widest spec to allow a longer interval before they may have to reshim, others set the gap as narrow as possible for the best performance and noise level.....YMMV.

T.C.

[vedgar]

Thanks again T.C.

Gonna look for mid spec, shim if it is on the tight side. I dont have extra shims to shufffle around. Have a 79 SF with 26k on the clock and have not touched it (adjusted) in 2500 miles. Still sounds quiet.
Vance
P.S. Have a couple airhead BMWs', have to check them every oil change. Simple as pie to do them, not like the cover on my XSsssss' . I love all my 2 wheeled children.