Roadside Mechanics series #1 engine compresion test

So I finally broke down and bought an engine compression test kit by Actron, that includes the right size 12mm spark plug adapter for my engine.

Unfortunately, the compression testers you can borrow from Autozone, or O'Reilly auto parts, through their loan-a-tool programs, do not have an adapter for the 12mm spark plugs that our motorcycle uses. The loan-a-tool program has been great for me in many other situations, and this tool is still valuable for most people because they do work for standard size plugs on cars and trucks, and some motorcycles. Beware though, if you do borrow from them, make sure the gauge works, and the rubber gaskets are in good shape.

With my fancy new tool I checked the compression and found out it was low. Real low. I pulled the heads to adjust the valves, a common source of pressure leakage, and this is when I found out why we died on the road.

In order to adjust the intake valves and the exhaust valves on the engine it is necessary to rotate the engine until you have one cylinder at top dead center (TDC) on the timing plate. This position is the beginning of the firing and exhaust sequence, when all valves are closed so the spark plug can ignite the fuel/air mixture. For my motor the sequence begins on the left cylinder side.

When I rotated the engine to TDC for the left side, my valves were engaged! Right away I knew something was amiss. I stepped back from the whole thing and took a second to think about why my exhaust valves were open when my timing plate said my engine should be at TDC. After thinking a moment, I figured the valves were probably where they were supposed to be and the timing plate was probably wrong. Bingo! I looked at the timing plate and the nut that holds the whole thing together was stripped and about to fall off. This my friends was the cause of our breakdown outside of Mission, Texas.

Well, I figured we still have hard starts, and that happened before I ever touched my timing, so I will continue on my line of work checking the compression. Plus, I had the valves open, and I have been wanting to adjust them, and now things were lining up right.

My first check of the compression, before adjusting the valves, in both cylinders, was 60 pounds per square inch (PSI). The importance of having good compression cannot be overstated. This is the third element that is imperative to have in a running combustion engine. The first, is fuel, and that includes air and lots of it. Next is spark. Getting a good reliable spark has been my goal with replacing almost all the parts in our ignition system. I also figured we should do it now while we can still get the parts for a good price. Lastly, and just as important is good compression. Compression in the cylinder is what creates a good seal in the cylinder to draw the air/fuel mixture in, and then compress it, so it can be easily and readily ignited by the spark. That sets into motion the crank, to turn the gears, and get the wheels going. When the piston reaches the bottom of this stroke it comes back up completing its forth stroke, and forces the exhaust gasses out of the chamber, ready for the intake, and TDC. That is the principle behind a four stroke engine. Most cars and trucks run a four stroke engine. Some motorcycles, the really really loud ones, can be two stroke engines.

After adjusting the valves I started to get 90 PSI consistently from each cylinder. This is better than before, but our 1976 Yamaha xs500 likes to have between 128 PSI and 146 PSI on each cylinder, without too much difference between them. So I know that my cylinders are acting the same, and are both producing low compression.

I have some more testing to do tomorrow, so we will leave off here.

I am starting this as the first of a series in Roadside Mechanics. I will go back and change some posts to reflect this, so they will be beta posts of the series. Some will be long, some will be short, but I will try my best to include all of them, and with good pictures. There will be internal linking to cross reference to ideas and things that I have already discussed so look for the links to past entries.

Benjamin