CheckValves

Valve clearances should be checked every 4000 miles (6000 km). This thread from GStwin.com explains valve clearances and gives some details on why it is important to check valve clearances: http://gstwins.com/gsboard/index.php?topic=38827.msg436160#msg436160

The best resource for this procedure is Kerry's Valve Adjustment Video. He walks you through step by step how to check and adjust your valves. (very long, but simpler technique. Does not remove carbs, or side cover)

Also, see a nice picture write up on GSTWIN.com link (removes carbs)

Remember that you need a metric feeler gauge, preferably with the thinnest blade of 0.03mm. You can check this eBay listing. I got one for less than $11 USD shipped. I know of two German brands for metric feeler gauges - Elora and Horex.

You'd better check the actual shim dimensions (thickness) with a Digital Vernier caliper or a micrometer. You can get one for under $20 shipped from eBay

The Suzuki shim tool can be obtained the same way for around $18+S&H. Take a look. While rotating the tool to compress the spring, push the handle down and slightly towards the valve, that way it makes a better contact with the bucket and helps avoid slipping. If the tool keeps slipping off, try using a short screwdriver or similar tool on the other side of the shim bucket to keep the spring compressed.

If you need shims try this online place, Suzuki shims are $4.95 a piece, got them delivered in two days to Mass.

Shim Size Calculator

This calculator (MS Excel) can be used to determine which shims need to be replaced. Input the shim size and clearance for each valve. The spreadsheet determines which valve clearances are out of specification, and which replacement shims are required to correct the clearances (Suzuki part numbers are also provided).

GS500 Valve Clearance Calculator

Also, keep in mind that you can always loosen the camshaft to get more clearance if you can't get enough with the Suzuki tool.