QuestionHi Ben.
I am having problems getting my motor running and when I do it misses and backfires.Here is what has been done to it since last fall.I had standard rings put in and also had the ignition changed from points to electronic ignition.I had problems getting it to run right properly from the beginning of this work done.Last week I had the intake valve replaced .Now I have 85 # compression.I cannot get fuel up into the engine,even by choking it .It will backfire and sputter.Even when I pull out the spark plug and pour gas into the cylinder it might start and if it doesn't I can pull out the plug and it will be dry and caromed up.It seems as though the timing it way out as when the arrows on the mag and flywheel match up the piston is not at top dead center.Top dead center mark would be over to the right.If I hold down the governor lever control to stop it from fluctuating when it is trying to start it will run but if it does run and I stop the engine and come back a half hour later I cannot get it running.Is there a way to set the timing?Is the compression to low and thus causing it not to draw fuel up?
I would appreciate ALL the help I can get.
Thanks Larry
AnswerLarry
There can be numerous reasons for the engine to quit running when it gets up to operating temperature.. expansion of metal parts where bolts/screws aren't tight enough, breakdown of the condenser for the ignition points (if equipped), or a possibility of a crack somewhere. An overall inspection of the tightness of fasteners would be a good place to start.
If the miss is a miss, the problem probably has to do with ignition or the fuel mixture. Check the tightness of the carburetor mounting bolts and where the intake manifold/air-fuel mix tube fastens to the engine block.
If instead, the miss is popping through the exhaust, this might be caused by a bad exhaust valve or valve seat.
If there is puffing or popping near the engine head, the head bolts might not be tight.
If the engine spins over too freely when it's hot.. indicating much lower compression, check the torque of the head bolts.. if they were loose, the engine should now restart.
If it won't restart until it's cooled down, the exhaust valve (and/or valve seat) may be the problem. If you can remove and disassemble the engine yourself, this isn't expensive to have corrected. If not, the shop labor for tear-down and reassembly will be fairly costly. A common reason for exhaust valve failure is that grass clippings become embedded in the cooling fins around the exhaust port area.
If the engine is just failing to fire the spark plug when it's hot, there's a good chance that it's time to change the (points and) condenser. This can usually be confirmed by quickly removing the spark plug after it quits running, and grounding the sparkplug base securely to the engine (away from fuel vapors).. then spin the engine to see if there is a hot blue spark with an audible snap, snap noise
To set the timing and The advantage of these systems is that there is no maintenance as there are no points to wear out or gum up. Timing is fixed by the relationship of the flywheel magnet and sensor. There are also no adjustments except to set the magneto coil pole piece to flywheel air gap.
Replacement modules are available for many small engines to take the place of their original point type ignition system. These can be designed in several different ways but all must emulate the behavior of the point type ignition replacing just the magneto coil/pole piece assembly and without requiring alterations to the flywheel.
One design uses the flywheel magnet passing the coil pole pieces to induce a moderate voltage - a few hundred volts - and store this in a capacitor. A sensor coil then triggers a solid state switch which dumps the charge in this capacitor into a high voltage transformer (ignition coil) to produce the spark.
Another possible approach is to provide a coil in which current builds up until the moment when the points would have opened and a transistor switch to interrupt the current flow at that instant. So, perhaps something like a switchmode power supply or TV horizontal output transistor with a current coil like the ordinary magneto coil and a network to control base drive to the transistor so it turns on and off at the proper times. Some designs may use a separate sense coil to control this more precisely.
Compression testing
Special compression gauges are available at auto parts stores or small engine parts suppliers. These will catch and hold the highest pressure reached so you don't need to be in two places at once.
Note that this procedure may always yield a very low reading if there is a compression release mechanism on your engine - which is very likely. In this case, the crankshaft must be spun in the opposite from normal direction by the flywheel (counterclockwise when viewed from the flywheel end, with the starter removed).
If the compression gauge has a screw thread, install it in the spark plug hole so it snug - it doesn't need to be really tight. If it just has a rubber boot, have a buddy hold it in place in the spark plug hole as you perform the following tests.
Pull the starter cord several times or use the electric starter in the normal manner.
New Tecumseh engines should have a compression reading of at least 80 psi. If your reading is at least 60 psi (or the specification found in your engine manual), then compression is acceptable.
If it is less than expected, squirt a small amount of engine oil in through the spark plug hole so it coats all around the edge of the piston and cylinder.
Repeat the compression test.
If the pressure reading is now acceptable, then the rings need replacement or the piston/cylinder are excessively worn. If there is little change, there is a valve problem.
If the measurement is very low under both conditions, there may be a blown head gasket or damaged (punctured) cylinder or head. (Or your engine has a compression release mechanism which is reducing the reading - see the note above).
For Briggs & Stratton engines, the manufacturer simply recommends spinning the crankshaft by the flywheel in the opposite from the normal direction with the normal spark plug installed. A sharp rebound (as it compresses the trapped air since the compression release mechanism is not active in reverse) indicates good compression. Little or no rebound indicates low compression and need for service.
