® Reasons your compression test reads low


In my article, "Where did the compression go?", I addressed the nonsense of the sohc4 forum calling Honda's published figures being unrealistic on the basis of the tool they used. While it's true that the Kowa Seiki compression tester shown in some factory sohc four manuals and sold to both Honda and Kawasaki dealers is long discontinued, it nonsensical to think that A) only with that exact tool can reliable measurements be obtained, and that B) mechanics haven't been getting correct readings using a myriad of other quality compression testers for several generations.

I explained in that article that extreme valve recession is virtually always the cause of low cylinder compression in Honda's single-cam fours, and that there is even a Honda service bulletin on the subject, Service Letter #84. That the forum "experts" are ignorant of valve recession being the sohc four bike's biggest problem; that this almost defining feature of Honda's 1970s engines has somehow escaped their notice, is an astonishing and rather humorous mystery. All vintage Hondas have this problem.

This then is cause number 1, because all sohc fours suffer from it. But there are also other issues that can contribute to low cylinder compression. Here is a short list in no particular order.

  • The compression tester
    Now it may seem like I am validating sohc4 in this. I am not. There are good quality compression testers and bad quality compression testers, another thing the forum fails to address. Two things stand out in telling the difference. One, if the tester does not include separate, dedicated hoses for each size spark plug hole, you got a bad'n. Testers with screw-on threaded adapters are the bottom of the barrel, the bane of the industry. It warrants another article to cover why, but at the very least be reconciled to the fact that more often than not they will read low. The same with those tapered rubber tip jobs. Pedestrian, both. Two, another giveaway of a bad conpression tester is if it has a cheap-looking pressure gauge, especially if it's plastic. Professionals aren't drawn toward toy tools such as these, so this analysis is recent. But expect to spend at least $150 on a compression tester, not $49. Note that I have said nothing about the tool's hose diameter or length. That subject is steeped in myth and lore. It's a non-issue, ridiculous.

  • Altitude
    Honda's compression numbers are relative to sea level. There are charts all over the Internet that will help you determine corrected compression for when your test is conducted at altitudes offering less oxygen. Where I live in Prescott, elevation 5300 feet, the correction factor (i.e. the expected loss) is 15 percent. You simply must take this into consideration when testing cylinder compression. And this is not an Internet fallacy. It is real and something I have confirmed by testing the same engine at different altitudes.

  • Throttle and battery
    Most folks know you must hold the throttle wide open while testing cylinder compression. If the carburetors have been removed, that is the same thing. But the battery must have a good charge also. I have found that I have had occasionally to recharge the battery at least once in the middle of testing compression on all cylinders. Of course, we're assuming electric starting. Something else: Forget warming up the engine first. Although promoted by Honda in many of their manuals, it is neither necessary nor beneficial. Every Honda engine I have ever tested, and that has to run into the thousands, has delivered accurate numbers stone cold.

  • Attitude
    Above I said, "altitude". Now I'll add, "attitude". What I mean is the attitude forums advocate of always blaming the other party. "My compression is low, Honda's numbers must be wrong!". It doesn't take very much time spent on forums before you will be affected by this irresponsible ethic. It is shameful and idiotic. It's also patently bad mechanical practice. "Bike's not running right, better rejet the carburetors. Charging system seems weak, gotta modify it. Headlight could be brighter, time to alter the wiring with a relay. Starting and storage are problematic, must be the ethanol". No one thinks any more. The problem with society isn't parenting or politics or pedagogy. It's that we have been trained to no longer think.

Recommended reading:
Building and using the cylinder leakdown tester
Where did the compression go?
Compression
The truth about cylinder compression


Last updated November 2022
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