Compression: Before and After Valve Adjustment

[DaveP]

Curiosity question concerning the relationship of the valve clearance adjustment to compression. This is on the GS650 engine.

Purchase a bike that had not been running for a few years. One of the first things that I did was to check the compression to make sure there were no major engine issues before proceeding. At that time, with a cold engine the readings were good for a warm engine, 150-180 psi. I thought that was great.

Since then I performed the valve clearance adjustment. Some were very tight and now everything is at the higher end of the clearance range. 0.08 to 0.10 mm clearance.

After the carburetor cleaning I was having trouble the first time trying to start the bike, so decided to check the compression again and was surprised to get much lower numbers. Now 120 psi on the four cylinders while still cold.

Does it make sense that the tighter valve clearances had higher compression values?

As I write this, I realized that the first test had no carburetors installed and on the second the carbs were installed, but I didn't open the throttle. Would that lower the compression reading?

[wymple]

Of course it matters. You can't compress air that isn't there. Open the throttle wide for a test.

[DaveP]

Thanks, makes sense. At least 120 is perfectly fine for starting compression.

[Griffin]

People get really hung up on compression numbers from time to time. Compression is a tool to be used when trying to diagnose a specific problem, but if one's bike is well tuned, it really doesn't matter on these bikes if readings are in the 160-170psi range, or the 120-130 range.

I don't get too bent about compression numbers unless they are below 110psi. Even that low, the bike will still run ok, but could be a little hard to start and will probably be noticeably down on power compared to an identical model engine with 150psi readings.

Compression numbers will not change after performing a valve clearance adjustment, unless the clearances were zero and some valves were not fully seating. Proper clearance adjustment would then result in higher compression readings, not lower.

I'd inspect the carb and airbox installation to make sure everything is properly seated and sealed (you do have the airbox on and a proper filter installed, right?). If it all looks good and you're still having issues, pull the carbs and reclean them in carb dip, making sure to thoroughly clean out all the passages with small wire. Also check each spark plug to make sure you have good spark.

With the electronic ignition (no points), it usually either works fine or not at all. If cleaning your carbs again and sealing all leaks doesn't help and you have good spark on all four plugs, I'd probably pull the valve cover off and double check timing just to make sure it's correct. If the timing is off by just one tooth on one of the cams, it could make a BIG difference in starting and running ability.