The dual overhead camshafts (DOHC) spin in relation to the crankshaft via the camshaft chain. The lobes on the two shafts push open the valves, held closed by springs, at the correct time to allow the intake and exhaust gases to enter and exit the cylinders. The clearance between the camshaft lobes and the valve shims must be within a certain specification.
If the clearance is out of specification, the valve wont be open for the proper length of time to allow the gases to enter or exit the cylinder, thus causing a major decrease in performance. This could burn the valves, wear the valves, and be noisy. Ill-adjusted valves can cause a whole skew of issues (idle problems, running rich/lean, low compression, et cetera), so it's important to do first when diagnosing a seemingly unrelated problem (as well as for regular maintenance, of course).
The clearances are adjusted by using different sized shims, which sit in a bucket located between the valve stem and the camshaft lobe. The shims range in thickness from 2.15 mm to 3.10 mm and have a diameter of 29.5 mm (not 29.0 mm, which is common with other manufacturers).
The clearance specification on my model bike is 0.03 mm to 0.08 mm. It's the same for most Suzuki engines of the same time period, but double-check your owners manual. Going a bit looser, say 0.09 mm, wont hurt, but I wouldn't go looser than that. The valve clearances tend to get tighter (smaller) with time.
The engine must be dead cold to have an accurate reading of the valve clearances. Dead cold meaning 12 hours since the engine was last running.
A quick overview of the actual process: Remove the valve cover, measure the valves, replace valves if necessary, replace the gasket, reassemble.
Note, there are a lot of pictures loaded on this page. I apologize to those with a slow internet connection!
Take the seat and gas tank off. You will have to remove or move the horn out of the way, remove the breather hose, and disconnect the tachometer cable. Next, loosen the 16 bolts in the cylinder cover. The breather cover (4 bolts) needs to be removed for my model, but it might not be required for other bikes. The bolts are all 10 mm hex. I would suggest always using the proper sized socket as opposed to a wrench.
Remove the breather cover if necessary to fit the valve cover out. Normally, there would be a gasket and some metal mesh pieces here; the picture below was taken after I removed and cleaned everything, as well as replaced the bolts.
Remove the cylinder cover. Again, there would normally be a gasket here. Do not pry off the cover with a screwdriver or other tool, as this will score the surface and potentially cause an oil leak
In the image below, see the valve cover before I cleaned it. Notice the metal mesh. There are four pieces: two in the top of the breather cover (not seen here) and two down in the valve cover (shown here). These separate out any oil mist and let it drip back down into the sump.
In my case, the gasket stuck quite well to the cover. I had to remove it using a razor blade. It took a hour or so to get all the gasket material removed, but it is essential to remove all material from both mating surfaces. That includes the breather cover gasket too! Be careful not to score the mating surfaces, as this will potentially cause an oil leak
Note the location and numbering of the cylinders. Sitting on the bike, cylinder #1 is on your left and #4 is on your right.
Remove the ignition cover. We need to access the nut in order to rotate the crankshaft manually. You may want to replace the screws and gasket on the ignition cover if they look worn.
Use a 19mm wrench on the outer bolt to rotate the crankshaft and camshafts as needed below. Only rotate in a clockwise direction. You will have to loosen (almost entirely) or remove your spark plugs to avoid pushing against the compression of the engine. If you remove the plugs, I'd recommend stuffing some shop towels in the holes to prevent anything from falling in.
Rotate the crankshaft clockwise until the #1 exhaust cam lobe is horizontal and pointing forward. In this position, you can measure the exhaust clearances for the #1 and #2 cylinders.
Using a feeler gauge set, determine the clearance between the cam lobe and the shim of the #1 exhaust. In the picture below, 0.07 mm fits, but 0.08 mm does not. Thus, the clearance is 0.07 mm, which is within specification.
Next, without moving the crankshaft, measure the #2 exhaust. My clearance is a bit smaller here, 0.05 mm, but still within specification.
To measure the intake clearances, rotate the crankshaft clockwise until the #1 intake cam lobe is vertical and pointing upward. In this position, you can measure the intake clearances for the #1 and #2 cylinders.
Without moving the crankshaft, measure the intake clearances for cylinder #1 and #2. Below, you can see that my intake on cylinder #1 has a clearance between 0.10 mm and 0.15 mm (my feeler gauges skip from 0.10 mm to 0.15 mm). I'll have to change this shim.
To measure the exhaust clearances for cylinders #3 and #4, rotate the crankshaft clockwise until the #4 exhaust lobe is horizontal and pointing forward.
Repeat the process to determine the clearances for the exhaust valves on cylinders #3 and #4.
To measure the intake clearances for cylinders #3 and #4, rotate the crankshaft clockwise until the #4 intake lobe is vertical and pointing upward.
Here's the sheet I recorded all my valve clearances and the size of the shims currently in my engine so you can see how I organized it. Be careful not to confuse the cylinder numbers and the intakes with the exhausts!
There's a little bit of math involved here. As I mentioned before, the clearance specification is 0.03 mm to 0.08 mm.
As an example, if your clearance is measured at 0.02 mm and the current shim is 2.70 mm, you want to replace the shim with a 2.65 mm to add 0.05 mm to the clearance, thus bringing it to a final clearance of 0.07 mm (within the specification).
Old Shim - New Shim + Current Clearance = New Clearance
My example: 2.70 - 2.65 + 0.02 = 0.07
You have the values for Old Shim and Current Clearance, and an acceptable range for New Clearance, so just find a shim size for New Shim that makes it within the clearance specification.
I hope that's not over-complicating it. Basically: If your clearance is too small, you want to use a thinner shim to open up the clearance. The amount it will open up by will be the difference between the old shim and the new shim.
NEVER rotate the engine without a shim in every bucket!
To remove a valve shim, you will need a tool to compress the valve spring, a tiny screwdriver, and small pliers or tweezers. The valve shim tool is used to compress the shim bucket to allow enough room to remove the shim.
Rotate the shim bucket until the notch is visible, then use the valve shim tool. It can be tricky to use, don't worry if it snaps off the shim bucket suddenly. Go slow and apply some horizontal pressure to keep it on.
It's also a good idea to take an inventory of the shim sizes currently in your engine, even if all the clearances are within specification.
Notice the notch in the shim bucket below.
Use the tiny screwdriver to break the surface tension of the oil on the shim through the notch, then remove it with pliers or tweezers. Do not use a magnet to remove the shim. The shim is easily magnetized. You don't want metal shards from the oil sticking to the shim.
This is a 2.60 mm thick shim in good shape. When putting the shim back in, always coat them with a small amount of engine oil and face the numbers down, away from the cam lobe, to prevent it from wearing off.
A new 2.60 mm shim below. This is an aftermarket replacement shim (K&L, if I recall correctly), not Suzuki OEM.
The shim below is marked 2.55x. The x means it is a bit bigger than 2.55 mm. A general rule of thumb is to add 0.02 mm for x sizes, so this shim should be around 2.57 mm thick. The x sizes can be used to fine-tune your adjustments, but I have yet to see one for sale (either OEM or aftermarket).
Don't mind the blue coloring, that's just how the shims look sometimes (especially after getting hammered on by a cam lobe under high temperature for 35 years).
Some additional notes on shims: Although they are specially hardened metal, they do wear over time. Your 2.60 shim may not measure 2.60 mm exactly. The shim may have even been ground down at some point. You can get a caliper/micrometer to measure to be sure. I took this into consideration when determining the sizes of new shims to purchase. The math of calculating the new shim size might not be exact due to wear of the old shim. For example, if you replace an old 2.60 mm shim with a new 2.60 mm shim, the clearance is likely to change. You can use this to your advantage for fine adjustments.
After removing shims, make sure you rotate the crankshaft a few times to ensure the shim is seated correctly in the bucket before re-checking the clearance. Again, NEVER rotate the engine without a shim in every bucket!
When the valve clearances are all within specification, it's time for reassembly. I replaced the rubber half-moon seals and the gasket. I use and strongly recommend OEM gaskets. The half-moon seals I used were aftermarket, and they work fine. To ensure a good seal on the "D" groove in the cylinder head, I used a tiny amount of Permatex Gasket Sealent & Dressing.
Only put the gasket sealant in the "D" groove to seal the half-moon. Make sure you clean up any "spillage" of the gasket sealant (as you can see on my intake cam in the second image below). It can harden up and become loose in your engine.
Put on the new gasket. There is not a top or bottom, but there is a front and back. Just make sure the holes align.
Put the valve cover back on. Do not over-tighten the bolts! They only need to be a bit more than hand-tight. They strip very easily! The gaskets are design to swell upon contact with oil, so don't worry about making it too tight!
Put the two metal mesh pieces back into the breather. This mesh separates any oil mist and allows it to drip back down into the sump.
Two more smaller mesh pieces go into the breather cover.
Reconnect your horn, spark plug wires, breather hose, and tachometer. Finally, ride away!
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