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Knowing the compression of a cylinder is one of the most important things.
Poor compression will reduce the hp output, top speed, acceleration, fuel
economy and make the carb mixture harder to
adjust.
There are a few ways to check compression in an engine.
- Use a gauge that seals by being shoved into the spark plug hole.
Very poor and not worth doing.
- A screw in connector on a short piece of hose. Then plug a gauge
into the hose. This is far better and probably the most common way.
- A "leak down" test is the best way. It is the standard of the aircraft industry. This
puts pressure into the spark plug hole and one can measure how much leaks out.
One may also listen for where it is leaking and learn if it is rings or valves
and which valve.
The first thing I always do is to slowly kick it through and "feel" the
compression. With some experience, one can learn to guess the compression quite accurately. At the minimum you should be able to feel if they are
equal and if one is lower, which one it is.
I am most familiar with the second method and will describe it. The engine
must be warmed up to get an accurate result. When removing a spark plug always make sure that there is no
dirt or crud at the base of the plug. Loose dirt may cause real trouble.
Many mechanics use a blast of air to clean out the area. That just isn't
always good enough. The air blast may not break all of the crud loose.
I prefer to break the spark plug loose a 1/2 turn and then blast it with air.
I like to remove both plugs on a BMW twin. Screw in the hose on the
cylinder to be tested. Give it a few slow kicks to feel the cylinder when
it is on compression and the one that has none. Lay the gauge head where
you can observe it easily. Open the throttle completely. That allows
maximum air to get into the cylinder for compression. Failure to have it
open will give false results. All human errors in measurement will give a
low reading. I have seen engines torn down just because someone failed to
correctly measure the compression.
Bring the kick start lever up to compression and lift the lever up to the
top. Now give it one healthy kick and observe the number. Is the
needle moving down quickly or staying right there? That number should be
about 1/2 of the total that you reach. Now give it the second kick and
note the number. A third, forth, fifth and sixth kick. If you
arrived at a total of 125 psi. note that too. Here is how it should
have looked.
- 60 psi
- 90 psi
- 105 psi
- 113 psi
- 118 psi
- 120 psi
The first kick produced about 1/2 of what the total turned out to be.
Each successive kick produced almost 1/2 of that number. The first was 60,
so add in 30 more and get 90, now add in 15 more and get to 105, but you get the
idea.
Below is an example of a poor cylinder.
- 25-35 psi
- 45-55 psi
- 65-75 psi
- 85-95 psi
- 100-110 psi
- 120-130 psi
This time each kick produced about the same increase. The first
produced about 30 and each one after that did almost as good. Towards the
end it would diminish a bit and only add maybe 10-15. You would have the
feeling that it should get kicked many more times to finally "get there."
The kicks should be made in quick succession. The amount of "leakage"
between each kick can mean a leaky hose or valve. That is what is so nice
about the leak down test and why it is the standard in piston aircraft.
How much for each type engine in healthy condition?
R50-R50/2 120-140 psi
R60-R60/2 125-145 psi
R69S 150-185 psi
Compression tests on the /5 and later
The /5 and later usually have an electric starter. This does not mean
that because it is easy to crank over, you should just crank until it reaches
maximum. We still restricted the test to 6 compression strokes and
carefully watched the gauge as it "stepped up" on each stroke.
The CV carbs must be removed a bit to get air in easily. The older
slide type carbs on the R50/5 and R60/6/6 and /7 should just have the throttle
opened up fully during the testing.
Tuning evidence
As the compression gets lower and lower, the tuning changes in the engine.
An experienced tuner will "feel" this as the carb adjustments are made. It
just won't respond to the idle air adjustments properly. The sound out of
the exhaust just isn't crisp either.
R51/3, R67, R67/2, R67/3, R50 and R60 tuning
When the compression gets down to around 100 - 105 psi, the carbs won't respond
well to adjustments.
The bike will still start easily. The top speed will be reduced by 10-15 mph,
but how many "open it up" to find out? At 75-80 psi, the bike will still
start, but not pull out without some warm up time. It may take a lot of
"tickling" to get enough gas into it to keep running. It will be really
sluggish at medium highway speeds. The carb tuning will be rather flat too.
At 50 psi and lower it will be really hard to start and barely keep up with
traffic on the highway. The gas mileage will suffer badly too. On
the R50 it is not so accurate as it is hard to get much resistance from a good
engine. This
description applies to the singles too. They are not "sport" tuned.
R68, R69, R69S and R50S tuning
The R69 might be a few psi lower, but is nearly as high as the other sport
models. These models are easy to test by foot. With my weight of
175, as the kick starter goes through the compression stroke, my whole weight
will be on the lever. My other foot will be off of the ground for a
second. It will slowly sink through the compression stroke. That is good compression. The R68 has a different
transmission and feels easier to kick. By going through compression
several times, one can easily feel which cylinder is low and about how much it
is. After you have a lot of experience with feeling the resistance, this
"kick method" will allow one to get within 10 psi of what the gauge will show.
At around 125-130 psi it starts to be a bit harder to adjust the idle air,
but will still start easily. At 90-100 psi it will start, but not so
easily and take some warm up time. The carbs just won't respond well at
all. At 75-80 psi, the bike will still go well on the highway, but top
speed is down at least 25 mph. But the R69S is a 110 mph bike, so the
limit of 80 mph isn't something that many owners test.
General info
An engine with a sport cam may not show these results. The valve
overlap can affect the measurement at cranking rpm. It may give an
artificially low reading.
At cranking, or kicking, speeds the valves are operating under ideal
conditions. They operate so slowly that they can fully seat, while at road
rpm they may wobble and chatter in the loose valve guides.
When we had a head off of the bike, we would put some solvent into the intake
port and exhaust port. If the liquid leaked through into the combustion
chamber just sitting, think about how it may leak in a dynamic test.
I saw a BMW engine with very low compression of 25-30 psi in each side still
go down the freeway at almost 60 mph, but that was about all it would produce and it
had to warm up to even get there. That particular motorcycle lived on top
of a hill and the owner would use the entire hill to "bump start" it. It
got so bad that even after a couple of blocks of that type of running it wasn't
running well at the bottom, so he brought it in so I could "tune it up" and fix
everything. The valves were completely burned up and solvent ran through
them instantly.
A carb that has a partially blocked idle gas jet would produce a "flat spot"
on an otherwise good engine. On one with lower compression, it is harder
to "feel" the flat spot. The entire performance is reduced and the whole
response is flat anyway.
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