Cylinder compression basics
One of the many things I hear from customers is, "Well, sure, my compression is low, but at least they're close to the same in all the cylinders." As if that makes up for it. It doesn't. Despite forum drivel to the contrary. While it's true that being close to the same is beneficial, it is not so important that it will ever trump what the actual number is. Let me say it again. I will trade compression evenness across the board any day for a nice high average number. No contest. What the average compression is, is many times more important, and this is ever more critical as the age of many vintage Hondas are now in the antique realm and are exhibiting lost compression. Cylinder compression is the foundation of engine performance. I ask my customers to take compression readings before they send me their carburetors. I get a surprising amount of pushback on this. Remedial education is plainly needed.

Cylinder compression's importance
Cylinder compression is so basic a lot of us overlook it. Here are a few wrong ways of looking at this important test. One, it isn't a test relegated to when your bike has a lot of miles on it. I hear this all the time. "No, I haven't checked compression. My bike has only 12,000 miles on it." Wrong. It's important at any time, regardless of mileage. Rather than thinking of a compression test as a yardstick of old age, instead consider it a routine checkup able to catch any number of possible issues before they take you by surprise. The most important of which is valve wear. So many vintage Honda owners seem unaware their valves need adjustment nearly ten times as frequently as in modern bikes. Two, forget all that Internet nonsense about warming up the engine first. I know, some (though not all) Honda manuals advocate it, but no professional mechanic has ever done this. The results on a cold engine exactly match those in the manual. Third, forget the teaspoon of oil. That's not really part of a compression test. It isn't even a good test, certainly not as good as the leakdown test that replaced it in the 1940s. Fourth, best practice is always to adjust the valve clearances before performing a compression test. Put them a little on the loose side. And use a good quality tester. Stay away from those rubber tipped jobs and testers having screw-on adapters. In fact, if you haven't paid at least $200 for it, your compression tester is likely junk.

A note on valve recession
1970s Hondas have serious issues with valve recession. The very first determination in any vintage Honda revival effort then is to square away cylinder compression. If lower than 150 psi, loosen your valve clearances 0.002" to get some of it back, and in the meantime perform cylinder leakdown tests to pinpoint and confirm the source of the loss, and to plan future repairs.

Compression and carburetion
Cylinder compression directly affects carburetor mixture. Higher compression makes carburetion richer, lower compression makes it leanner. It's an efficiency thing--combustion efficiency. Probably 90 percent of the modifications folks make to their carburetors are attempts to unknowingly compensate for lower than normal cylinder compression. This includes carburetor jetting modifications.

Compression and leakdown tests as a pair
One of the first things career mechanics learn is how to use both a compression test and a leakdown test in engine troubleshooting, that is, use them together. People who are not vocational users of these two tools may not understand this, and might think one is simply a substitute for the other. But this is not so. The best use is to compare one against the other. Kind of like how exhaust gas analysis works, wherein two gases compared reveal a third picture not possible with either alone. Here are some examples of how that works with compression and leakdown tests.

Let's say the vintage Honda engine is running poorly. A compression test indicates good compression, well over 150 psi and nearly the factory's 170. But, the subsequent leakdown test is not so promising, showing a 20-30 percent loss. Wait. How can this be? How can the engine have good compression but at the same time high (bad) leakdown? Easy. Receded valves. Even badly grooved valve faces manage to seal adequately and average some sealing when repeatedly banged closed during a compression test. However, the completely static nature of a leakdown test will expose the bad valves. Engine experts are well aware of this.

Okay. Now another example from the opposite viewpoint. How about low (good) leakdown combined with low (bad) compression? Huh? Is this even possible? Sure is. But what does it mean? This is the classic indicator of retarded valve timing, typically from worn cams, or mistimed cam chains or belts, which results in low cylinder compression, despite the good valve and cylinder seal. (And incidentally, in the case of cam chains, often caused by the use of K&N air filters.)

So you see, the two tests--compression and leakdown--work best not individually but in tandem and that is the best way to use them to get a complete diagnostic picture.

Compression Chronicles part 2