Cylinder compression is marvelous! It’s an efficiency thing; squeeze the combustible harder and it will more completely convert the fuel’s thermal energy into pressure--it'll push back harder. Compression also determines how your engine uses fuel; a mechanically sound engine is less wasteful and thus carburets better. In fact, vintage Honda performance shortcomings that virtually everyone blames on carburetion can in many cases be traced to low compression. Unfortunately, compression is a problem on 40-50-year-old Hondas. That's the soft valve thing we spoke of earlier.

Every vintage Honda owner needs a compression tester. And try to spend more than $49 on one. Compression testers having screw-on adapters often indicate low quality. No professional uses that kind of tester, not because of the stupid debate raging on forums about the volume of the adapter or the hose, but because the adapters are glitchy sources of leaking and durability issues, and they most often exemplify very cheaply made compression tester kits. If not American-made and costing at least $200, your compression tester is likely junk. This is not hyperbole, it is experience.

If you’re above sea level you will need to correct your readings for altitude; at 5,000 feet you’ve already lost 15 percent of the available oxygen and this gets worse as you go up, naturally. 1970s four-stroke Hondas were mostly manufactured with 165-170 psi cylinder pressure. Some say the book is wrong out of frustration with their results and an unawareness of the soft valve issue. But these numbers are correct and career Honda mechanics have confirmed them through multiple generations of engine testing and rebuilding. Naturally, fifty years later we can’t expect non-rebuilt engines to yield as-manufactured compression. Therefore consider the decision point for a 50-year old engine that had 165-170 psi new to be 150 psi today. If you have that, you are in good shape. If less, don’t spend money on carbs or ignition or whatever. Fix the engine first. On a related note, it is a fallacy to believe that a freshly-rebuilt engine can have lower than expected cylinder compression because it, “has to be broken in”. Proper machining and factory parts result in exactly the Honda spec and the most compression the engine will ever have by the end of the first test ride if not sooner.

A test related to compression is cylinder leakdown. A leakdown test is easy. The cylinder being leak-tested is put at top dead center on the compression stroke (TDCC) and air pumped in from an air compressor. Before attaching the tool I usually watch the intake valve open and close so I know that the next thing in rotating the crankshaft is this important place. Twin-cylinder engines can sometimes lack the internal friction needed to make the piston stay at top dead center—you may need to hold the crankshaft with a wrench.

Just as with the compression test, do not put oil in the cylinder, and do not pre-warm the engine. Normal in-service leakdown readings are 10-15%, with the small bore, multicylinder vintage Hondas centering on the 10%. A recently rebuilt old Honda engine may be as low as 5%. Readings below that are suspect and point toward a faulty tool. Readings below 5% are in the realm of racing engines especially modified. Either the tester is not reliable or the tool’s input air pressure is too high. 80 to 100 psi is the normal leakdown tester input pressure, while some air compressors are capable of 160 psi. Results just slightly above 10% indicate some wear but nothing terrible. But beyond 15% indicates a level of wear that demands rebuilding sooner or later. While the test is underway I like to roll up a piece of paper and use it to listen to the air escaping from various places. Hissing from the carburetor of course points to intake valves and at the exhaust to exhaust valves. Air sounds at the oil dipstick hole or under the valve cover point to rings and cylinder wear.

Some people view a leakdown test as just another form of compression test, albeit somewhat more sophisticated. That's wrong. A compression test can sometimes be less informative, but it is far from dispensable. The correct diagnostic approach is to use the two together—first the compression test, then the leakdown test. Beware of retarded valve timing, typically from worn cams (common on vintage Hondas whose cams can wear half a millimeter in just 6,000 miles) or worn or mistimed cam chains or belts, all of which result in low cylinder compression despite the good valve and cylinder seal.

Here's a really cool leakdown tester trick. Lower the input pressure until it is possible to rotate the crankshaft without the air pressure taking over and moving the piston. This Smokey Yunick (Google him) technique is used to find cylinder flaws and was how I found a damaged cylinder many years ago in a ZX11. A compression test overlooked the wear (returned a good reading) because cylinder compression is built up in the upper one-third of the cylinder and the trench in this cylinder was below that, and incidentally causing massive oil consumption. Another advanced technique is to, while the leakdown test is underway, carefully rap on the valve tappet and watch the leakdown tester’s gauge. I do this whenever seeing a high reading. If a high leakdown reading decreases and stays decreased, you know that it was mostly due to carbon buildup and not bad valves.

Compression Chronicles part 6