Motorcycle Compression Testing – How to Avoid False Results

Compression testing is a basic measurement of the mechanical health of your engine. Errors and faults usually are caused by incorrect tools or faulty procedures. In almost all cases incorrect readings will result in lower than actual numbers. Do not rely on compression test results by themselves. Before doing mechanical work do a thorough investigation and isolate the problems first.

Correct Tools for Motorcycle Compression Testing

Of course, you will need a compression tester. Make sure it is decent quality, name brands are safer bets. Do not buy the cheapest tester possible, go for mid to higher-level price range and quality. For the average person, you will not need a professional unit, relative pricing will be your best guide. This type of equipment will last decades if properly cared for. I recommend tester sets that come in a case and have multiple adapters and extensions. The case is a bonus, keeping everything organized and protected.

When buying a tester kit, or when testing the motorcycle, make sure the spark plug adapter has the correct thread length, one that matches the length of your spark plugs. If the thread is too short it will create more volume to be compressed and low readings. If the adaptor is too long it could decrease the effective volume and worse may hit the piston and cause damage.

Avoid compression testers that do not thread in, and use only a rubber cone for sealing between the plug hole and the tester. The only case for these is you have no other choice and do not have a proper adaptor for the engine being tested. Other than this, there is no reason to use this type of design. Rubber cones style is also very hard to use single-handed and get accurate readings.

When using extensions (rubber or metal tube), make sure there is a functioning check valve in place. If you use an extension without a check valve the volume in the cylinder is increased (inside the tube is added) and will give lower than actual results.

Make sure the check (Schrader) valve is seated and not loose. The Schrader valve is the same one found in car tires, you can use the tool that is built into some valve caps to check tightness and seating. If the valve is defective it will not hold the gauge’s last reading. To inspect the valve do a compression test, then put some soapy water or spit on the value – look for bubbles forming (just like a tire valve test for the same issue).

Check all unions for missing or damaged O-rings. This can allow air to escape during testing, resulting in lower than actual readings. Missing O-rings may be harder to spot, typically every shoulder in the connection will have an O-ring in place.

You may have an older, beat-up maybe even a hand-me-down tester. This is a delicate and precise tool, if the site glass is cracked or missing, or has Teflon thread sealing tape on it, then it is probably time to replace it.

Correct Procedure of Motorcycle Compression Testing

The basics of a proper compression test require a full and unrestricted flow of air to the cylinder, and enough compression strokes to build to maximum pressure. Test results will fall into the acceptable psi/kPa range, and each cylinder should be within a 10% range of the others. For exact values check your service manual, there are no standards that apply to all makes and models.

For safety, it is recommended that the ignition system be disabled (fuse removed or disconnect coil, as per your manual). For fuel-injected motorcycles, the EFI fuse is removed in most cases. If carbureted just turn off the fuel supply. Again, check your service manual. Also, make sure the bike is neutral. Remove all spark plugs.

To run a test, hold the throttle fully open, do not engage the choke. Turn over the engine at least 4 or 5 full cycles. You will need a fully charged battery capable of easily turning over the engine. The compression readings should jump up in steps, and finally reach a maximum level. Write down the results, release the gauge pressure and repeat. Test each cylinder at least twice. If you get inconsistent readings on the same cylinder to check your gage, adapters, and procedures first.

To confirm operating compression, you need to test a warm engine (tip, loosen the spark plugs while cold, it will make removal easier). If the bike won’t start you will be testing a cold motor, in this case, you are looking for very low or relative compression between cylinders. Some builders do a cold test to ensure all is well mechanically before the first start. Also, for a motorcycle that has been in long-term storage, or is a “barn find” a preemptive cold compression test is the safe way to go.

For “well-loved” equipment you may feel or hear blowby at the spark plugs. Hopefully, this is caused by a worn-out compression test adaptor. If the cylinder head plug threads are worn that badly (or cross-threaded) this will have to be fixed (Heli coil repair kits, a new cylinder head is not required). If the adaptor is worn you can try using Teflon thread tape to temporarily make a good seal.

Do not add oil of any type before doing the first set of readings. Adding a squirt of oil will temporarily increase sealing around the rings. If you are trying to start a non-running machine use any kind of “Quick Start” spray very sparingly. Too much will remove oil from the cylinder walls and can lead to rings scraping and gouging. However, if the machine has been idle for a very long time, some oil is a good precaution, but re-test after letting it sit for a few hours.

If you need to use an extension on one cylinder only, then use it on all cylinder tests. This will make the testing equipment the same for all and remove it as a potential for misleading results.

Do not cycle your starter motor longer than 30 secs without rest – it could burn it out or cause heat damage.

Common Reasons for High & Low Results in Motorcycle Compression Testing

  • Bikes that have been in storage and not ridden or turned over can suffer from sticking rings and valves, resulting in low readings. This can often be corrected by running the engine, allowing the rings, valves to free up.
  • Dirty air cleaners can cause enough airflow restriction to lower readings
  • Recent mechanical work such as new pistons, rings, valve work, can result in low readings. This is because the new parts need to be broken in. Typically, break-in takes at least 300 miles.
  • Rebuilds with incorrect gaskets (too thick, or doubling up), will lower compression readings.
  • Blown gasket(s) will allow compressed air to escape and provide low readings.
  • Incorrect valve adjustment can leave the valves open far too long and cause low readings.
  • Sticking or burnt valves will have low readings. Adding oil will not boost readings if this is the condition.
  • Worn piston rings, low readings until oil is added – this will boost psi readings temporarily.
  • If there is excessive carbon build inside the cylinder you will get high readings. The carbon has taken up space and effectively raises the compression ratio.
  • The tester may be sticking, or malfunctioning – it will read low. Check it against another tester, or blow compressed air into it, see it reaches expected pressure AND holds it.

Proper Diagnosis

You must have the correct values for your machine, consult a service manual. There is a relatively wide range of acceptable readings, across makes and models, don’t rely on unqualified information. Having said that, in general, 100 psi is a baseline number, below your engine is “getting old”. Another general rule is that proper compression is 15 to 20 times the bike’s stated compression ratio.

If the numbers are low overall, the next step is to identify possible mechanical problems – most likely with rings or poor valve condition. If a small amount of motor oil temporarily boosts readings rings are a prime suspect.

If there is a significant difference (10%) between cylinders this points to a damaged gasket, that is affecting only one cylinder. Note that one low reading could be caused by worn rings or valves for that one cylinder also. As before, adding oil may improve readings if rings are the cause otherwise, readings will not change significantly.

If you are not able to find and correct the problems without disassembly the next step is a leak down test. This is where the cylinder is filled with compressed air, simulating compression. You then can use various techniques to identify if, and where the air is escaping…leading to what needs fixing.

Summing It Up

Compression testing is a first diagnostic step only, do not jump to conclusions after only doing this test. Consider all other factors, such as how the bike was performing before the test, if the motorcycle is running fine don’t go looking for problems. Learn to “read” spark plugs, and add to the diagnostic procedures. If the bike has been idle for 2 weeks or more and is running poorly try a fresh tank of gas, run it in and retest.

If you find problems, double-check all readings, test equipment, and procedures. Review all service items that may be related (air filter, valve adjustments). If no improvements are seen, move on to a leak-down test to isolate this problem.

Since doing a compression test is a fairly infrequent activity, make sure you are following the correct procedures each time. Always refresh yourself by reading over the instructions in your manual first. Working with factory specifications is also key.


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