<<<What exaclty are valve seals? I thought the valve closed metal to metal against the port, by replacing valve seals do you mean have a machine shop grind them?>>>

The good metal to metal fit between valves and seats is commonly referred to as valve seal.  Valve seals are an entirely different thing, they are rubber seals on the valve stems that keep the pockets of oil on top of the head around the valves from leaking down between the valves and guides into the combustion chamber causing higher oil consumption and smoking from the exhaust.  That little "s" makes for a big confusion if your not yet familiar with engines.     Bargain older bikes with very low mileage can have dried out seals from just sitting too long.  Doubt you need valve seals if they really only have 8k miles use on them.  Hard to determine on non running engine.

<<<Unfortunatley I don't fully understand how the valves got bent (Blueknyt is convinced i turned the engine backwards and caused valve to hit piston, but i don't think i did that). And due to my cleaning efforts i caused 2 huge explosions in the engine which somehow resulted in bent intake valves and 0 compression.>>>

We might never know, I find it highly unlikely the explosions bent the intake valves and a small amount of turning the engine backwards is unlikely to bend valves unless the cams and crank got out of time somehow.  A loose cam chain could cause it if you removed the chain tensioner and then later turned the engine backwards.

<<<So i'm not sure what needs to be replaced. Exhaust valves are kind of expensive so will try to measure them for straightness and try to reuse. Do valve buckets go bad? was the damage visible in your case?>>>

Lap the valves and find out, it's best way to check for bent valves.  If you work a valve back and forth and around and around with lapping compound on seat and valve face you will get a continuous gray circle the width of the seat on the face of the valve if it's straight showing it's touching all around  Lapping will also improve the seal.

My valve bucket didn't go bad it was just too big for the hole and thus too tight in the head. Misfit at the factory, there have been quite a few of them reported by members here.

Quote from: musicmarley on July 30, 2013, 07:01:46 PM
Scary how thin those valves are.  How often should the shims be checked?

Thx,
Sean

Manual calls for every 3-4k miles I think.  I checked twice a year on my 97 GS for about 80k miles, that was about every 8k miles or so.  On the 02 my annual mileage was less and I've done it once a year every 10k miles or so.  Half way thru the 02 I started setting the exhaust valve clearance wider to .003-.005" and they went another 40k miles without a shim change.  Last valve check at 86k miles, now at 99k miles, and doubt I'll ever check them again.  Exhaust valves now holding like intakes which have never required a shim change on my GSs.

http://www.gs500.net/gallery/data/500/GSvalvelogs.jpg

the cam chain tensioner takes up slack on one side of the engine.  its setup so that minimal effort is needed to keep that slack taken up. when you turn the engine in the opposite direction you are pulling that slack out and putting it in the other side.  it doesnt take much slack when it comes to cam timing and piston interference especialy when spun in a direction it was not designed to operate. most of all, ONLY the intake valves were bent..

Folks, im not saying this to accuse someone of screwing up, nor to sound as a mechanical guru.  I did this same thing on a bench when i had to fix the counter balance shaft bearings in one of my GS engines, turning the crank one handed with a wrench,it was not hard to do.  turning backwards, the intake valve can not get out of the way of the piston fast enough before each piston is TDC and shifting the slack worsens this issue.  the smaller faced exhaust valve doesn't open as far and is setup to be open at nearly TDC for the exhaust stroke to finish.  the intake is not meant to open until after the piston starts its travel down and is clear within a few degrees of crank travel. 

it sounds plausible to my thinking also Blue ... not a mechanical guru either .. but a little chain slack can make a big difference when tolerances are close! .. and valves/cylinders are usually awfully close! .. 

Is it possible that an uncontrolled backfire explosion in both cylinders reversed spin direction to cause the bent intakes?

.. plausible? i think?... how likely .. dunno ..

again ... to my 'bush mechanic/hillbilly fix it' thinking ...you had a butt load of carb stuff in there... sustained firing/flame/bang 'might' result in enough force to 'stop' and reverse cylinder direction of motion/rotation ?! ... and if as Blue says the intakes can clash with the pistons in reverse because of chain slack then it could be the thing that did it.. ?

.. you watch top fuel dragsters when they go bang ... flames out of blower hats,  holes and flames out of engine blocks and heads , head separated from block ... seized motors and or bent heads/crushed valves/broken rods/cracked pistons ... ..... OK thats huge power and a massive fuel load ... but its in a big engine too!! ...

we have a little engine ....

regardless of size an explosion is by its nature 'uncontrolled energy release' .. sure they have 'controlled explosions' but thats kinda the amount or direction of force .. an unplanned explosion goes everywhere! ..

anyway .. probably just talking out my butt again.... it just makes sense to my 'order of things' in my head. 

  I dunno and don't need to know, if the valves got bent they need to be replaced.  Only one word I'd change in my post above,  change we might never know to we will never know.     Your comments on valve timing are interesting but I just don't see that much difference between the intake and exhaust timing relative to tdc.  The exhaust is normally closing as the piston goes over the top and is closed at 37* past tdc.  Run it in reverse and the intake is closing as the piston goes over the top and is closed at 34* past tdc.



I agree that if the chain tensioner collapses or is removed and the engine is then turned backwards the valve damage could occur as the slack changed side.  But the tensioner clicks out notch by notch as the chain wears and even at top rpm the centrifugal force of the chain doesn't over ride the tensioner. 

jack, when the engine is spun opposite direction, the cam timeing is changed. its not the same on paper as the came chain tension does not remain a constant in the reverse direction.        something looks funny with your diagram,   intakes begin to open almost imdiately after exhaust close. the exhaust valve has the clearence to be open at TDC, the exhaust closes and the piston is just completeing its travel up. as the piston starts down intake starts to open.  the cam chain tensioner on the GS is a sleved screw system with a spring, its not a rachet jaw system and the chain guides take up and redirect most the centrifical forces  hence their length all the way down to crank level. the tensioner is prone to being pushed back in, it doesnt take much movement.

Well I don't believe that the sashkar induced explosion directly bent the 2 intake valves, it took metal to metal contact to do that, and I don't think it's likely to have been caused by slowly turning the engine backwards; sashkar believes he only turned it forward by hand.

From first post: "I thought it was cleaning itself out so I continued cranking. Then a massive backfire explosion happened in the intake manifolds and smoke went thru the carbs and came out from below the seat."  The bent intake valves were evident after the explosion.

My theory, and it's only speculation, is that the engine was in a position with a piston a little before tdc where the big explosion could have driven the piston downward  spinning the engine backwards at least 180*, probably more, to bend both intakes, collapsing the tensioner at the same time, and all evidenced by the smoke coming out from under the seat.

We'll never really know for sure but it's broken now and needs fixed. 

Thanks For the explanation gsJack, That's what I suspect happened as well. Now why is it so hard to get parts quickly? Looks like no one stocks them on the east coast so it takes over a week to deliver...

Quote from: sashkar2000 on July 22, 2013, 04:13:15 PM
hmm, would a massive backfire possibly screw up timing?

If there is something stuck to the intake valves, what are some options to get it off? I tried  flushing it out by spraying seafoam and running with sparkplugs disconnected, and then with spark plugs removed so the liquid was coming out. It was bringing up some particules with it. maybe i didn't run that long enough.

If you have filled the cylinder with liquid then turned the engine over you have hydraulically  locked the engine hence the bent valves.

I ended up opening up the engine and replacing the 2 bent valves. It was quite an adventure doing this for the 1st time and overall this has been a success story and i'm grateful for the guidance i got on this site. thank you guys!