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'76 XS500 wicked engine vibrations


bwellervt
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Hey All,

My first post here. Thanks for having me. My bike is a '76 XS500C.

BACKGROUND:

So, down to business: I recently had a break down on the way up a twisty mountain road here in VT when I shifted into a false neutral between 3rd and 4th during a pass. Over-revved the motor, and blew out 3 of the 8 valve tappet adjustment screws on the rocker arms.

Limped her up and down the mountain, got closer to town, tire locked up as expected. Grabbed clutch and coasted her into a country store to call a buddy.

FIRST STEP:

This was all two weeks ago. Since then I pulled the valvecover... Found 2 of the 3 adjustment screws, and 3 lock nuts... That's right, 1 screw is MIA in my mill.

Fished and fished, drained the oil, flushed and flushed again. No screw has surfaced. I found a slightly longer adjustment screw from a more modern Yammy to fit. Slapped it all together. Set clearances x8 valves. Locknuts tightened down good. (Can't fit my torque wrench in the tight space, though!) Valvecover back on, new oil filter, new oil.

PROBLEM:

Started up the XS after putting her back together. Sounds great. Some valve noise, but sounds like everything is working as it should. The bike starts easily and quickly now.

After warming the bike up, I took her for a rip down the street... At 5500 - 6500rpm, the motor produces severe vibration. Enough to numb the hands almost instantly.

SUSPECTED CAUSES:

- bad job setting valve clearances (doesn't seem too likely to me)

- bent valve(s)

- balancer issue (Is that part of the balancer I see through the oil fill? If so she ain't moving with the motor!)

- loose motor mount

Any input will be greatly appreciated! The warm-weather riding season is short in Vermont, and I want to get out and ride. Thanks a lot.

--Ben

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Hey All,

My first post here. Thanks for having me. My bike is a '76 XS500C.

BACKGROUND:

So, down to business: I recently had a break down on the way up a twisty mountain road here in VT when I shifted into a false neutral between 3rd and 4th during a pass. Over-revved the motor, and blew out 3 of the 8 valve tappet adjustment screws on the rocker arms.

Limped her up and down the mountain, got closer to town, tire locked up as expected. Grabbed clutch and coasted her into a country store to call a buddy.

FIRST STEP:

This was all two weeks ago. Since then I pulled the valvecover... Found 2 of the 3 adjustment screws, and 3 lock nuts... That's right, 1 screw is MIA in my mill.

Fished and fished, drained the oil, flushed and flushed again. No screw has surfaced. I found a slightly longer adjustment screw from a more modern Yammy to fit. Slapped it all together. Set clearances x8 valves. Locknuts tightened down good. (Can't fit my torque wrench in the tight space, though!) Valvecover back on, new oil filter, new oil.

PROBLEM:

Started up the XS after putting her back together. Sounds great. Some valve noise, but sounds like everything is working as it should. The bike starts easily and quickly now.

After warming the bike up, I took her for a rip down the street... At 5500 - 6500rpm, the motor produces severe vibration. Enough to numb the hands almost instantly.

SUSPECTED CAUSES:

- bad job setting valve clearances (doesn't seem too likely to me)

- bent valve(s)

- balancer issue (Is that part of the balancer I see through the oil fill? If so she ain't moving with the motor!)

- loose motor mount

Any input will be greatly appreciated! The warm-weather riding season is short in Vermont, and I want to get out and ride. Thanks a lot.

--Ben

Hi Ben

:welcome: to the forum.

First off you need to find that screw that's missing in action ..... it could do untold damage to the engine do you think the screw could have droped down the cam chain tunnel ? If so its going to be lay in the bottom of the engine waiting for its chance to ruin the engine.

Bad valve adjustment will just give you mechanical noise or poor performance (loose or tight)

Bent valves will give the bike mechanical noise and no performance/ bad starting

The balancer chain is visible throgh the plug on the l/h crankcase cover but adjustment takes a while as you have to strip the cover off (drain oil etc) to get at the adjusters .... back in the 70's it was a normal mod for the blanancer chain & shaft to be removed as you didn't get too much more vibe but did gain a pony or two.

Now the engine sieze has some bearing on your problem perhaps the MIA screw has already damaged the motor in some way ... it may be worth dropping the oil out again removing both left & right crankcase covers and the sump where by you may find the screw & the engine damage.

One last question after the bike has got warm does the oil light flash on & off (when the engine is running) occasionally ?

Hope this helps

Regards Jim

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One last question after the bike has got warm does the oil light flash on & off (when the engine is running) occasionally ?

Hope this helps

Regards Jim

Hi Jim,

Thanks for the reply. I think I'd have to agree... I'll be draining the oil and pulling both covers and the sump.

When the bike is warm the oil light remains off (it is working!), no flashing. Current symptoms are the vibration that I mentioned, and poor performance (like one intake valve isn't functioning) after she gets to full temperature. Still puzzled, but getting the engine apart might reveal the solution. That screw can't be doing anything good in the motor. Thanks again.

--Ben

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Hi Jim,

Thanks for the reply. I think I'd have to agree... I'll be draining the oil and pulling both covers and the sump.

When the bike is warm the oil light remains off (it is working!), no flashing. Current symptoms are the vibration that I mentioned, and poor performance (like one intake valve isn't functioning) after she gets to full temperature. Still puzzled, but getting the engine apart might reveal the solution. That screw can't be doing anything good in the motor. Thanks again.

--Ben

Hi Ben,

now you have said that I guess the bike is only running on one cylinder (most of the time) ... The compression is perhaps a bit low on the one cylinder. It would be worth doing Compression tests (wet & dry) on both cylinders. If you find this to be low on one cylinder you will have to get the head off which also means the motor has to come out of the frame :o, to inspect/repair the valve train, but on the plus side with the motor out of the frame you do stand a bigger chance of finding that screw ! A word of Warning tho, stripping an XS500 motor is not for the feint harted as it was a bit over complex ....

It's good the oil light does not flash when the engine is running and hot as a flashing light does indicate problems with the big ends (shells, rods etc). So perhaps the engine strip you may need to do will be confined to the top end of the engine ... so you only end up with a couple of hundred bits on the bench rather than what appears to 1000's !

Regards Jim

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Whoo Doggy!

So, I jumped into a tear down of the left side crankcase cover... Certainly found a problem there.

Opened the cover, and found two chains in there. The frontmost chain was intact. Behind this chain was a second chain. This chain is completely destroyed, and the respective sprocket on the crankshaft is done. Still searching for the screw that must have caused this. I've got some pictures to upload after I scrub my hands.

The plate behind the rotor is cracked. Anyone know what this is called? Also, anyone have the specs for the thread that is just inside the rotor? I'd like to bodge together a puller and get the old rotor out of the way. Looks like I've got a hell of a project on my hands. Any and all help is appreciated. I'll be posting updates as I get along here.

Thanks.

--Ben

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Pictures as promised... No good happening here.

crankcasecarnage1.jpg

crankcasecarnage2.jpg

Hi Ben,

I guess the screw has knacked the balance chain but how it got there would be a mystery !!! the vibe problem will be down to the balance shaft just 'swinging in the wind' this would also cover the seize up you had (when the chain became derailed) best option is to find that screw !!!

Regards Jim

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Hi Ben,

I guess the screw has knacked the balance chain but how it got there would be a mystery !!! the vibe problem will be down to the balance shaft just 'swinging in the wind' this would also cover the seize up you had (when the chain became derailed) best option is to find that screw !!!

Regards Jim

Thanks for your advice Jim! I'll be finding that screw. When I do I'll epoxy it to my keychain as a reminder of the work I've caused myself!

--Ben

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Thanks for your advice Jim! I'll be finding that screw. When I do I'll epoxy it to my keychain as a reminder of the work I've caused myself!

--Ben

Hi Ben,

Happy hunting !!! but tbh I would guess the whole motor will need to come apart which leads you on to obtaining the gaskets and other parts before the strip is done.

Regards Jim

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