QuestionHi Howard we have a 79 mg engine cranks have spark but wont start.Changed plugs cap but still wont start.need help.
AnswerHi Carl,
All piston engines only require three items to run with conditions on each of the three. The three are, Compression, Fire and Fuel.
The conditions are,
Compression -- Enough compression, 125 to 180 PSI on a compression test.
Fire --- A good spark at about the correct time, A thick blue spark at the timing set down by the manufacturer.
Fuel --- A combustible fuel available to the combustion chamber, atomized by carburetors or an injection system and delivered to the combustion chambers in close to the correct fuel/air ratio.
First you must confirm that the engine itself is ok by running a compression test on all four cylinders. (a compression gauge is not expensive) The engine must turn over at least 4 or 5 revolutions and the throttle must be held open.
Spark can be roughly tested by removing a spark plug and reconnect the plug wire to it and lay the plug on a metal part of the engine and spin the starter and watch the plug. You should see a thick (thick as a pensile lead) and blue in color (not is bright sun light).
If it is orange or yellow and thin as a hair, it is not good enough. To do a rough test to see if the timing is so far off as to make an engine not run do this test, With the plug attached to it's wire and laying on a metal part of the engine, put your thumb over the plug hole and have someone spin the starter. When the piston comes up on the compression stroke it will push your thumb off of the plug hole with a "Pop" sound. The spark plug makes a "Click" sound. When the timing is close enough for the engine to start, it will seem as though the "Pop" sound has caused the "Click" of the spark. If it seems as though it is "Pop - Click" or "Click - Pop" then the timing is probably far enough off for the engine not to start. This preliminary quick test is sometimes used by mechanics to tell what is needed to be done.
To set the timing exactly, you either need a timing light or a static test light.
To do a preliminary quick test of fuel, most mechanics will just spray starter fluid into the intake while someone spins the starter to see if it will start. WD-40 will work in place of starter fluid. If it starts on the spray then they know fuel is the problem and thus know where to look for the problem.
There is no magic screw to turn to fix an engine that does not run, it requires testing to get down to a failed item.
Howard
