GS450 Valve adjustment question....

[450twin]

For lack of a better word, I am "Restoring" a 1980 GS450s, with help from a similar vintage GS450s parts bike. I have spark, starter doesn't work, so I checked Compression and got readings of 90, 78, 80, 70, from the left Cylinder. Compression decreased with oil(Go figure), so I decided to check the valves instead of looking at the rings as the culprit. Long story short, I can't get a accurate measurement. I watched a bit of the GS500 valve adjustment video, but I still cannot get a measurement between cam and shim/bucket. .0015 won't even fit. I turned the engine by hand(wheel) in sixth till the lobe was facing away from the bucket, but I still couldn't slide a measurement shim between them(cam/bucket.) Is there something I'm missing, or has anyone had this issue before?
Any help greatly appreciated.

[Steve]

Can you still spin the bucket with the cam lobe pointing away from the valve?

If you can, you might be able to drop just one shim size, but you will still have to remove the shim to see what size it is.
Put that shim back in, record it, move to the next valve.

One way to handle a situation like this is to get a shim that is smaller than all of the ones you have, and use it for a "checking" shim.
Let's say, for example, that your shims are all 2.65 or 2.70. You don't really know if a 2.60 will work, it may need a 2.55, so order a 2.45 or 2.50 and try that. Yes, you will have extra clearance, but that is EASY to measure, then figure out how much too big it is and order the proper shim.

May I also suggest that you read my sig for what I (and 529 others) have found to be a reasonably nice tool?
My spreadsheet will help you with all the math and will also help you keep track of what shims are in your engine, along with other maintenance items.

.

[450twin]

Thanks! I'll try to see if I can spin the bucket, and I have another head full of shims I could scavenge, so I'll see what I can do tomorrow morning. If I can't spin the bucket with the cam lobe facing away am I going to have to mess with the tensioner and take off the cams?

[pete]

Welcome to the forums!

I had to do exactly this myself a little while back, and I had exactly the issue Steve was talking about where I couldn't rotate the buckets on a couple of the valves.

I ended up going down 2 or 3 sizes on those ones.

If you can't rotate the bucket, it just means going down one size won't be enough and you will need to do 2 or more.

The other thing is that it may mean that valve never closes properly, and if it doesn't close properly then that could potentially be the cause of your compression loss...

[2BRacing]

No, DON'T mess with tensioner or remove the cams! If you can't move the bucket with your finger when the lobe points up (put your finger in the notch of the bucket and try to move the bucket left or right), it means there is no clearance at all. So instead of going down only one shim size (0.05) to obtain clearance within spec, you will have to go down at least two sizes (0.10) or maybe more to get clearance within spec.

NB: NEVER turn the engine when a shim has been removed or if a shim is not properly seated in the bucket.

[2BRacing]

I have spark, starter doesn't work, so I checked Compression and got readings of 90, 78, 80, 70, from the left Cylinder. Compression decreased with oil(Go figure), so I decided to check the valves instead of looking at the rings as the culprit.

You are correct in suspecting valves instead of rings when compression decreases after adding a little oil into the cylinder. However, compression readings are only meaningful when the correct procedure has been followed: i.e. warm engine; throttle held wide open; and engine spinning fast enough to build compression over a few cycles.

If your starter doesn't work, how did you turn the engine over consistently enough to obtain repeatable compression readings? Maybe I would not read too much into those readings at this point in time. First get all the valves set to the correct clearance, and then follow the correct compression test procedure to obtain meaningful, repeatable results.

Those readings will then help any further diagnosis, if required.

[450twin]

Thanks for all the replies! I can spin the buckets, at least on the parts bike head. Since both bikes are in the same boat and this is a case of "My first Motorcycle" I do my wrenching on the parts bike first, so I don't make a mistake on the one I can get running.

You are correct in suspecting valves instead of rings when compression decreases after adding a little oil into the cylinder.

If your starter doesn't work, how did you turn the engine over consistently enough to obtain repeatable compression readings? Maybe I would not read too much into those readings at this point in time. First get all the valves set to the correct clearance, and then follow the correct compression test procedure to obtain meaningful, repeatable results.

Those readings will then help any further diagnosis, if required.

Well.... I turn the engine over with the bike in sixth by hand when it's on the center stand. Grab the back wheel and spin. I do the same to turn the cams, and check my spark. I've been pushing it to try to get it started, I decided to not go that route when checking my compression.

[pete]

Push starting if it hasn't been run for a long time will be interesting as there will be no fuel in the carb float bowls, so even if it's sparking nicely I suspect it will take quite a few pushes before it sucks enough fuel in there to be able to start successfully...