Hi,

Im documenting all my work in the below topic so please read for more details. http://gstwins.com/gsboard/index.php?topic=72904.0

Bought a gs500 with 30psi and 100psi compression...

Checked the valve clearances. Both intake and the right exhaust were reading somewhere between .1 and .14 so Im hoping a new set of shims will bring the right cylinders compression back into spec (was reading 100psi before). The left exhaust on the other hand would not allow me to get a .04 feeler inside.

Any ideas? Possibly valve springs?

Just for reference, this cylinder was reading 30psi and had a slight oil leak from the rocker cover bolt above the exhaust valve. There's was also some scoring on the exhaust cam lobe... Doesn't look like a big problem but may have been the result of one.

The site wont let me upload pics as they are too big. I will include a link to a google doc where I will upload pics.

https://docs.google.com/document/d/1rVSgGW9B3AsRqJMbZ1u_7-uqaGRz1oogRYiR2cUbjYA/edit?usp=sharing

If you are exceptionally lucky then valve shims will restore compression. Both cylinders are way under spec. Should be more like 140-160 psi. I think the chances are much higher that you have a bent valve and likely need rings plus a valve job on both sides. If it were me, I'd be planning a top end rebuild and stop wasting time and money on band aid fixes.

Thanks allot for your reply!

I get the feeling I might get away with a shim job on the right hand side but I'm sure there's another issue on the left given that I couldnt get a .04 feeler in there...

At this point the engine will need dropping and the head will come off so while im there ill do rings and gaskets... I will get a professional round to double check the bearings on the piston rod. Anything else need doing?

And you said valve job... do you mean lapping? Sorry, Im new to this engine overhaul business

Thanks again mate

Top end rebuild typically includes rings, honing the bores, grinding/lapping valves, new valve seats and all seals and gaskets north of the block. That's just off the top of my head. Probably worthwhile to consider new intake boots and surely do intake boot o rings. You can do this job with the block still mounted in the bike. I hired mine done for just under $1k but should have done it myself for more like $200 or less in parts and machine shop fees. I get the impression that top end rebuild jobs are somewhat common on air cooled motorcycles of a certain age.

Perfect, thanks again for all that. I have bought a complete engine gasket and seals set so should be all good there...

Would you recommend I replace all valves or just the potentially bent one?

Tight valves will lower compression, loose valves will not.
And - you cant get a .04 feeler gauge in a valve means - precisely nothing.
Not a bent valve - why would a tight valve be bent if at TDC. It guarantees, its not bent. A bent valve will be hanging open - making for a large clearance, not a small one.

Burnt valve ? Maybe, valves go to 0 and then negative clearance and then burn ... but on a 2 valve 2 cyl bike - the bike runs like crap for a long time after startup even before the clearance is actually negative. You get near 0 and warm up's are terrible. You do get negative and the deeper your negative number is, the worse it will at all operating temps of the engine, as long as the revs are low, and when revs come up, it wont run much better, you can ride it in low gear with high revs though and burn your valves more.

Cool.
Buddha.

Hi Buddha,

What do you think then? or is there no way of telling without taking the head off?

To check valves i lined up the RT marker, made sure the notches on the cams were facing inwards, measures both intakes and the right exhaust. then rotated 360 so the notches were facing away from each other then measured the final left exhaust valve.

Simplest way to look for a bent valve - Take off carb, exhaust and sparkplug.

Its best if its dark. Night time in a garage works well, or just night. And make sure you have atleast the spec valve clearance - not 0 or negative.

Shine a flash light in the sparkplug hole - look for light in the exhaust, intake - NO light - Good.
Repeat with flashlight in intake looking in spark plug hole and exhaust - NO light = Good.
Repeat with flashlight in exhaust - NO light = Good.

Repeat with other cyl - NO light = reassemble and run the bike.

Cool.
Buddha.

Quote from: 09dudharb on April 10, 2019, 07:17:17 AM
Hi Buddha,

What do you think then? or is there no way of telling without taking the head off?

To check valves i lined up the RT marker, made sure the notches on the cams were facing inwards, measures both intakes and the right exhaust. then rotated 360 so the notches were facing away from each other then measured the final left exhaust valve.

I have no idea what these marks are. I never take the right cover off (or atleast when I have to look at valves)

To put chamber in TDC at the top of compression stroke - Put it in 6th gear, pull the valve cover and spark plug.
Turn the rear wheel till the cyl you want has the intake valve opening - you're now going down the intake stroke, then intake closes and turn the wheel till intake and exhaust are both closed - you're on the compression stroke - TDC is a bit irrelevant. Here you should have both valves shut tight.

Cool.
Buddha.

Quote from: 09dudharb on April 10, 2019, 07:17:17 AM
Hi Buddha,

What do you think then? or is there no way of telling without taking the head off?

To check valves i lined up the RT marker, made sure the notches on the cams were facing inwards, measures both intakes and the right exhaust. then rotated 360 so the notches were facing away from each other then measured the final left exhaust valve.

Thats the correct way to do it. Sounds like you have one of the shop manuals, thats good.

I agree with The Buddha that not fitting a .04 shim means nothing. Could be .03 or .02 which would be fine. Buy the shims you need to find out for sure.
Many on this forum who don't ride their bikes hard set the exhaust valves on the looser side of spec to slow down the interval between shim changes (.08 -.13). Keep the intake within the Suzuki spec (.03 -.08 if I remember correctly).

Do The Buddha's described flashlight check to see if its a bend valve.
If it is bent, you can swap a low milage head onto your bike for around the same cost as doing a top end rebuild yourself, and a lot less hassle. Thats the nice thing about the GS500, you can just throw an entire "new" low milage engine in it for $500. Most GS500s get swapped around between beginners until one of them crashes it bad enough to end up at a bike breaker who sells the 12,000 mile engine for $500 on eBay.

Excellent points bluesmudge.

Only thing I'd add is on an engine with this mileage and any risk of bad rings (which in this case I'd say the evidence is there), if you have the head off you might as well pull the jug and hone it and fit new rings. And if you are putting on a used head, it's easy to freshen the valves along the way. In other words, this is mostly labor and not parts. Removing and reinstalling the head is like 70% of the labor of a full top end rebuild.

BTW with the head off you can check valves for sealing the way my mechanic did, which is remove the cams and then pour liquid like gasoline into the intake and exhaust ports directly and see whether it leaks out of the closed valve. In my case both exhaust valves were bent and it flowed like a river when doing this test. If you pull your head and it passes this test then your compression problem is either rings or maybe a bad head gasket.

Also fuel in the oil can be an indicator of bad rings on an engine with extremely poor compression.

And don't try to measure valve clearance after swapping a shim.
If you swap a shim - the shim will be floating on oil - some people say turning it over a few times cleared it and made it seat, others (me) have felt it needs to be run a few sec atleast ... no idea what the truth is. May even depend on the motor and type of oil - cos I used thick as molasses 60 wt in the bike I did these experiments on.

TBH, I'd check the clearances, set them correctly and check compression again, and most importantly, ride it. That would tell you if it would even need a rebuild or you can simply keep an eye on its oil level and simply ride it.

Cool.
Buddha.

This is great, thanks all!

Yes, got the haynes manual

I believe the shims in there were 2.2's... So would I want a 2.25 to bring they back into spec? Making the x3 valves between .05 and .09? And would I leave that dodgy exhaust valve as it may be still in spec (e.g. .03)?

Then do the bent valve check that buddah detailed...

Then turn the engine over a fair few times to get rid of trapped oil then measure compression and see what happens?

Sorry for all the noobie questions lol

The aim of the build really is more of a repair than replace game. Trying to learn a bit more.

You likely didn't have 2.2 shims in all the valves. But you need to do that math per shim.
However a loose valve is fine for the bent valve check - don't worry about that before you get that proved out as - not the problem.

Also - if you did your math (simple addition and subtraction) right, there is no need to turn the motor over etc. The shims will get the oil out by the time you get it started. That shim float isn't a problem where the shim "floats away" The shim just doesn't get you an accurate measurement.

I think the repair part is correct, you don't need to rebuild it. I'm sure you don't have the expertise to do so, else you wont be asking these questions. So don't take on a job that requires 10X your current level.

Cool.
Buddha.

Hi all,

My friend crashed his GS the other day so I was considering it as a donor bike... £100

Forks are bent, 50k miles and pulse generator was damaged along with the right crank case cover.

If I take the top end off the donor and stick it on my bike that will be relatively simple right? Off, on then reset timing?

At 50k miles do you think the compression will be in spec? He a friend and said it was running okay.

Was thinking of swapping out the top end then I can take my top end to the shop and get the valves lapped, barrel sorted and stick on new rings so I can stick it on in a years time or so.

Thoughts?

Much of the engine work is easier with the engine out of the frame. In some cases the head wont even come off with the engine in the frame.
The engine also goes in easier if the alternator, rotor and starter clutch is out, though I have stuffed it in even fully assembled.
I'd suggest you get the engine out of his, fully do everything it needs and put that in your frame instead of doing a head swap.

BTW have you found out what's wrong with yours ?

Cool.
Srinath.