Back in September I thought that things were in pretty good shape with the mechanical systems of the 1981 Suzuki GS650L and I was enjoying cruising around the local area. This turned out to be short-lived.
I was rounding a corner when a car pulled out of a driveway in front of me and I needed to quickly come to a stop. When I attempted to resume my travel, the engine was near stalling, I had no power at all to get going, and it just didn’t sound good. I put on my emergency flashers and slowly made my way along the shoulder. Luckily I was only a couple of miles from home when the problem occurred and was able to limp the bike back to the shed for investigation.
My most recent work completed was within the electrical system. The regulator/rectifier was upgraded and I did the coil relay modification to improve the voltage drop at the ignition coils, so I suspected an electrical problem, but I still had strong spark on all four cylinders.
I then checked the compression. Cylinders 1, 2, and 4 were good at 175 psi, but cylinder 3 was extremely low at 105 psi. Early on, before the engine was even in a running condition, I checked compression on the cold engine and this cylinder was at 165 psi. 142 psi is the low service limit for compression on this engine. Adding some oil into the cylinder did not improve compression, so it is starting to look like a valve issue.
I pulled off the valve cover to verify my valve clearances. Everything was good again except for the intake valve on cylinder No. 3. The clearance was more than double the spec now, 0.24 mm clearance. The valve shim bucket still moves freely, not stuck, so something is preventing the intake valve from closing. Is it bent?
I bought a cheap WiFi endoscope camera that would fit down the spark plug hole to see if I could tell if there was any valve or piston damage before taking off the head. There is no going back once you take the head off.
It connects via WiFi to your smart phone rather than buying a fully dedicated unit. Not pictured are thread on accessories for a magnet, hook, and right angle viewing mirror. It also has adjustable LED lighting. The picture is OK and it works fine in the cylinder looking straight down, but I had no success with the right angle mirror to see the valves clearly. Couldn’t focus that close to the lens.
Here is a shot of the No. 3 piston. Looks fine…
As I am cranking the engine over by hand to check the valve clearances, I noticed that there were locations that were harder to rotate through than they should be since there were no spark plugs installed and therefore no compression to get past. So I started to look at where the cams were in relation to the hard spots.
If everything is timed properly you should have the following alignment of the crankshaft and camshafts:
On the exhaust cam, the arrow adjacent to the 1 should parallel to the surface of the head gasket (removed).
Then the 2 on the exhaust cam and the 3 on the intake cam should be pointing vertically with 20 pins between the two timing marks.
Looks good so far. Now the crankshaft should be at Top Dead Center (TDC) which is found by aligning the T on the crankshaft in the viewing window under the electronic ignition signal generator and should look like this:
In my case, when the cams are positioned correctly, the engine is not at TDC. The cams are actually ahead of the crankshaft and the cams look like this when the crankshaft is at TDC.
A significant rotation of the cams which probably bent at least the No. 3 intake valve enough to prevent it from closing fully. Most likely the cam chain tensioner is not functioning properly and the emergency stop caused some lash in the chain, the timing skipped, and valves were bent.
No matter the cause, I am now back to square one and more. The carbs and exhaust will need to be removed and the top end of the engine torn down and rebuilt. Then reinstall everything, check valve clearances with the new valves, re-sync the carbs… Should be fun!