In the Shop with Ron

T o P - E nd H EA LT H

This month in the shop, we will be going through the steps necessary to check the top-end health of a typical internal combustion engine. If you notice a loss of power or maybe the motor isn’t running quite as well as it did, it might be time to check to see how healthy your top end is—or isn’t. The two main tests I use to determine top-end health are a compression test and a cylinder leak-down test. To test compression, we will need a compression tester, which will tell us how much compression each cylinder is producing.

In order for the engine to run properly, the pistons and rings must compress the fuel and air mixture so it will fire and must be able to transfer the pressure from combustion into power. To test compression, first we choose the correct flex-hose adapter for the cylinder we’re checking and screw it in to the sparkplug hole on the cylinder being tested (with spark plug removed). Next, we can connect the compression gauge to the flex hose on the cylinder being tested.

Now we can turn the motor over several times with the starter while watching the needle on the compression gauge stop turning the motor over when the needle on the compression gauge stops climbing. Record your reading and repeat test on remaining cylinders, recording the readings for each cylinder. Each manufacturer has their own recommendations for minimum compression requirement and also for maximum acceptable difference in compression between the cylinders. As a general rule, if there is a 10% or greater difference in compression between cylinders, we need to do additional checking to see why. One motorcycle manufacturer recommends acceptable compression between 90 psi and 120 psi per cylinder, but both cylinders should be with in approximately 10% of each other. To explain, if one cylinder has 110 psi of compression and another has 80 psi of compression, that is a difference of roughly 15%, which is not acceptable. To test to see where/why we are losing compression, we will use a cylinder leak-down tester.

The leak-down tester has two gauges on it: one monitors the amount of air being introduced into the cylinder (the one on the left) and the other monitors the amount of pressure being held by the cylinder being tested (the one on the right). If there is a difference greater than 12% between the two gauges, there is a problem. To determine the cause, I like to have the engine properly prepped for testing by removing the air cleaner and having the crankcase drained and drain plug removed. Now, while pumping air into the cylinder, listen for air escaping through the exhaust or intake also from the crankcase. Air escaping from the intake means the intake valve is not seating properly or burned, air escaping from exhaust means the exhaust valve is not seating properly or burned; air escaping from crankcase means it is getting by the piston rings. If you’re testing a liquid-cooled engine, check the radiator for bubbles. If there are bubbles forming in the coolant, it usually means a head gasket has failed. There are several other factors that could cause a cylinder to fail a leak test; by determining where the air is going points us in the right direction to determine the cause. If your motor seems a bit more tired than usual, it may be time to give it a bit of a physical to accurately determine its overall top end health.

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by Ron Johnson

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