When you go to the doctor they will typically do some baseline tests on you like taking your pulse and blood pressure. The results of these tests and comparing the results to previous results can give the doctor some indication of current health and can help with diagnosing problems. If long term test results are available they can be reviewed for indications of future problems.
The same method can be used to determine your Big Red mechanical health. Taking the time to do some a baseline mechanical health check (compression test) can really help with troubleshooting current problems and if done at regular intervals can give you an indication of possible future issues. I see so many questions here and on other sites that involve a Big Red not running correctly and many basic checks have not been done. It can be difficult to diagnose a mechanical issues over a video or through text but results of some baseline tests can help eliminate some possibilities and point you in a direction towards a solution.
Before performing the compression some things should be checked first. Make sure the air cleaner is clean and flow to the carburetor is unobstructed. Now on to checking the valve clearance. First I remove the plug on the left side if the engine to see the flywheel and markings to find top dead center (TDC). Then I remove the spark plug and the caps on the cylinder head cover. I then rotate the motor to TDC on the compression stroke. How do you know if it is the compression stoke? Either put your finger over the spark plug hole and rotate the motor (your finger will feel outward pressure on the compression stroke) or watch the intake valve when it opens you know you will see the compression stroke next. Once at TDC on the compression stroke use a feeler gauge to check the intake and exhaust clearance between the rocker and the stem of the valve. The spec is .002" clearance (cold) for both the intake and exhaust valve for the 1982 1983 200E and 1984 200ES and .003" clearance (cold) for both the intake and exhaust valve for the 1985 1986 and 1987 250ES. The clearance can be adjusted using the threaded adjuster and locknut.
Checking Valve Clearance picture above
Next I will start and warm up the Big Red to normal operating temperature. I then install the compression tester in the spark plug hole, make sure the choke is fully open, fully open the throttle, and crank the engine for a few seconds until the compression tester shows it's highest reading. The factory compression spec is 159 PSI +/- 14 PSI (145 - 173 PSI) for the 1982 1983 200E and 1984 200ES and 170 - 185 PSI for the 1985 1986 and 1987 250ES.
I typically use 120 PSI as a low compression limit for the 200 and 250 cc Big Red engines. Yes the engine can run with lower compression but it may not run well or have good power output. Low compression can cause low power output, misfiring, and hard starting.
Per the factory service manual low compression can be caused by:
Improper valve adjustment
cylinder head gasket leaking
Worn piston ring or cylinder
Per the factory service manual high compression can be caused by:
Carbon deposits in the combustion chamber or on piston crown
I have disassembled many 200 and 250 Big Red engines and I am surprised how many of them are still the original bore and have not had a top end rebuild. These are very reliable engines and can run even if they have not been rebuilt in some cases 40 years. I mention this because especially if you are a first time Big Red owner you may not know how a fresh engine should perform. You may be wasting a lot of time trying to tune and get more performance out of a worn engine. In my experience it's much more difficult to get a worn engine to run well consistent and compared to a fresh engine. I know it's obvious but I mention this because I have answered questions from Big Red owners who advise there Big Red isn't running/starting well and they automatically want to take the carburetor apart or they have taken the carburetor apart and advise no change.
Trending your compression test results.
I record the compression test results of almost all of my engines typically on an annual basis. I will put the results in the service manual so I have an indicator of the engine health and if I see a decline I can hopefully catch a problem before it happens or schedule a rebuild.
Compression testers are available on line and at many auto parts stores. Some auto part stores will rent/loan them also.
If you have low compression a leakdown test can help indicate the potential problem. I am not going to go into the details of a leakdown test in this article but HERE is a link for more information. The leak down test is a good tool because it can quickly identify the cause or causes of low compression and will get you to focus in those areas during engine teardown and inspection. If you don't have a service manual for your Big Red they are available on line for free. HERE is a link to the manuals. The manual gives more information on the valve clearance checks and the compression test.
HERE is a link for more information on checking valve clearance (250ES)
HERE is a link for more information on checking valve clearance (200E 200ES)