Step by step: From general maintenance to complete restorations, we share tips and tricks for working on classic bikes.
Suzuki’s 1977-1983 2-valve 4-cylinder GS series engine was one of the best. Whether in 550cc, 650cc, 750cc, 850cc or 1,000cc guise, these air-cooled twin-cam fours were praised when new, and over the years they’ve proven themselves to be remarkably robust, able to withstand gross negligence and abuse. That’s good news, because it means that with proper care, you can expect a GS to run, well, almost forever.
An often neglected maintenance item on these engines is valve adjustment, although experience shows that gently ridden machines tend to hold to spec for tens of thousands of miles. Our subject 1977 Suzuki GS750 is a perfect example of this. An original condition survivor, it shows a low 12,500 miles on the odometer and looks like a bike that was only ridden when it was nice outside, and put in the garage when it wasn’t. Checking the valves confirmed this: All the valves were within specification and no adjustment was needed.
The valve clearance (or lash) on the 2-valve GS750 is very tight; 0.03mm-0.08mm, or 0.001in-0.003in. Loose is better than tight, and in our case the valves were all on the high end at 0.0025in-0.003in. The valves are shim over bucket, with adjustment shims carried in a recess in the actuating bucket. Removing shims requires the correct valve shim tool, readily available from Motion Pro for $16.50. Shims are available from various sources individually or in kits, with individual shims costing $12-$15 and kits around $75-$100. Local shops typically stock the shims, and once you know what you need you can usually purchase them from the local Suzuki dealer or independent shop.
You’ll need a good set of feeler gauges, making sure the set includes the thin sizes needed for the Suzuki. Besides that, all you’ll need are standard hand tools. The actual checking doesn’t take too long, but be prepared to either let the bike sit as you locate the necessary shims — or do the job twice if shims aren’t available locally and you need to keep your bike on the road while waiting for them. As always, have a good manual at hand to confirm proper specifications.
1. The first step is to remove the gas tank. There are two lines to the petcock, a fuel line and a vacuum line that opens the petcock when the engine is started. They’re different sizes and only go on one way. Remove and plug the lines. Remove the single bolt at the rear of the gas tank.
2. Remove the gas tank, then remove the horn from its mount. Remove all four spark plugs. This makes it much easier to rotate the engine as you index each camshaft lobe as you won’t be working against compression.
3. Next, remove the four camshaft cover end caps. Use the proper JIS Phillips head driver. One end cap screw on our GS was damaged to the point we had to drill it out, using successively larger bits until the head of the screw broke off. From there it was easy to remove the remaining screw shaft.
4. Working in a cross pattern, remove the valve cover retaining bolts and the valve cover breather cover; the valve cover won’t clear the frame for removal with the breather cover in place.
5. Remove the right side ignition cover. Using a 19mm wrench, rotate the engine clockwise until a camshaft lobe is straight up. Here it’s the #4 exhaust.
6. Check the lash, then rotate the engine as necessary to get each camshaft lobe in the position shown, checking each one. As it turned out, our engine needed no adjustment, each valve coming in at 0.0025in-0.003in clearance.
7. If necessary, adjustment is done by changing the adjusting shim. To do this, insert the shim tool as shown, hooking it under the camshaft and positioned so its raised center ridge contacts the edge of the valve actuating bucket. Rotate the tool down and toward you and depress the bucket.
8. With the bucket depressed, remove the shim using a magnet or tweezers. Any oil under the shim will tend to hold it in place. If necessary, use a dentist’s pick to lift it from its seat. It will then pull free.
9. The 2-valve GS750 engine uses 27.5mm-diameter shims. The shims come in 0.05mm increments from 2.15mm-3.10mm. Choose the proper shim by subtracting the readings and shimming up or down as necessary. If the valve is loose, you’ll need a thicker shim. If tight, you’ll need a thinner shim.
10. Once the valve lash is correct, clean the valve cover gasket surfaces. Install the half moon end plugs, two on each side. If the new valve cover gasket has strengthening ribs as shown, cut them off before installing the gasket and valve cover. Install the gasket and plugs dry.
11. Install the valve cover followed by the valve cover breather cover. The valve cover bolts on our engine were mildly corroded. We wire brushed them clean, then coated the threads with engine oil before installing them.
12. Tighten the cover bolts evenly, working in a cross pattern. Install the spark plugs, side covers and ignition cover. Install the horn. Install the gas tank and reconnect the fuel and vacuum lines. MC