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| At a basic level, troubleshooting a 4 stroke twin cylinder engine
is not much different than it's single cylinder counterpart. Procedures and considerations in regards to testing
the various systems (ignition, fuel,compression, etc.) is pretty much the same and most often accomplished in the
same manner for both types of engines. The primary difference of course is that you normally have to repeat each
test for each individual cylinder. There are however, some other considerations involved with a twin cylinder that
we will try to cover in this section. |
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| Ignition System: |
On twin cylinder engines (depending on model and manufacturer) you
may have either one ignition coil or two individual coils. In the case where there is only one ignition coil with
two spark plug wires, you might be inclined to think that having good spark for one cylinder would automatically
mean that spark at the other cylinder will be good also. Not
necessarily true! While the coil itself may be common
to both cylinders, the spark plug wires, terminal ends and spark plugs are not, so those components must still
be tested separately.
The flip side to this scenario is that if you have an engine with two separate coils, one producing spark and the
other not, you cannot assume that one is "bad" because you still have to take into consideration the
common wiring between the two. One side may be grounding out through the kill wire or spark plug wire, where the
other is not. This type of situation can become even more complicated when the engine design has diode(s) wired
into the harness to control the direction of travel of electrical current.
The bottom line is that you have to determine what is common between the two cylinders, test both sides individually
and factor in everything that can have an adverse affect on either or both. Having access to a repair manual may
be mandatory in order to resolve some of these types of problems. |
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| Fuel System: |
The most common type of fuel system configuration on twin cylinder
engines makes use of one single carb to feed both cylinders. In this scenario, most of the fuel system components
that can cause problems will be common to both cylinders and affect both cylinders fairly equally.
The usual exception in this case is most often caused by an intake leak from one end of the manifold and not the
other, which can often be determined by one spark plug becoming severely fuel fouled (loose black soot) while the
other spark plug is burning lean or dry.
In general, intake leaks can be caused by loose mounting bolts, damaged gaskets or cracks and/or defects in the
casting of the manifold. An easy method of checking for and pin-pointing leaks is with the engine running at idle,
spray an aerosol cleaner or lubricant along the entire length of the manifold and mating surfaces. You will almost
always be able to determine a significant change in engine speed any place where a leak is present. |
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| Compression: |
| Compression on twin cylinder engines is definitely a much easier
situation. Generally, a problem with compression on one cylinder has little or nothing to do with the other cylinder.
However, I'd like to emphasize that it's difficult to get an accurate compression reading and in some cases specifications
are not even published by the manufacturer. The major factors that may influence the reading are: |
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| Compression release. |
| Engines with a compression release system for easier starting (usually
6HP or greater) will generally show a much lower reading while turning the engine over at cranking speeds. |
| Cranking speed and duration. |
| Can the engine be turned over fast enough or long enough to obtain
a correct reading? |
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| Bottom line: A compression check will give some indication of the "health"
of the engine, but a "leak down" test (which is beyond the scope of this guide) is a much more accurate
method of determining the cause for compression related problems and/or output potential. |
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| Comparing Cylinder
Output: |
One very important aspect of twin cylinder performance is in having
a fairly equal power output from both cylinders. One weak (or completely dead) cylinder can be a major factor in
a "low power" situation and it's sometimes difficult to distinguish by regular troubleshooting methods.
For example, a twin cylinder with one weak cylinder will often have no problems starting or running and may pass
testing procedures for ignition spark and fuel related problems. It may even pass a compression test due to the
above mentioned accuracy situation.
However, there are some fairly easy tests that you can do that will clearly indicate a weak or dead cylinder (regardless
of the cause), which I'll describe below: |
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| Checking for a dead cylinder: |
Tools required - Test light (continuity tester).
This test is done by grounding each spark plug wire separately, which effectively "kills" the cylinder
that is being tested. With the test light attached to a good ground and the engine running slightly above normal
idle, touch the tip of the test light to one of the spark plug terminals. The normal result of this test will be a noticeable
decrease in engine speed since the engine will at that point be running only on one cylinder. Should the engine die,
that is an indication that the other cylinder is dead or not working sufficiently to allow the engine to run.
If you get a normal result, repeat the test on the other cylinder and note the result. Should both cylinders pass
this test, you can move on to the comparison test below: |
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| Testing for a weak cylinder: |
Tools required - Test light and tachometer.
If the engine passed the test above, we know that both cylinders are capable of keeping the engine running so our
focus now is in measuring the overall effectiveness of both cylinders. In this test the procedures are exactly
the same with the exception that this time, using the tachometer, you will measure and record the drop off in engine
speed (RPM) for each cylinder, then compare the two readings. |
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**Remember that the cylinder being grounded is actually a test for
the opposite cylinder.
As shown in the examples above, as long as the differences between the two cylinders are reasonably close (10-15%),
there should probably be no great reason for concern unless both are very low. Any percentage much greater would
indicate a weak cylinder, with more testing needed to determine the cause. |
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