I was recently picked up a Craftsman DYT 4000 model #917.274641 with a B&S 18.5hp model #31P777-0348-E1. when I got it, I replaced the battery. It fired right up but was running rough and seemed to lack power at full throttle. After about 5 minutes of running, as I was mowing on an incline the engine backfired and shut off. I tried to restart it but no luck. I pushed it over to level ground and tried cranking it over for a couple of minutes. It would turn over fine but would backfire with every unsuccessful attempt. I have done the following troubleshooting with no luck. Replaced the plug and put on a brand new coil. Coil is gapped at .011 off the flywheel. Removed the flywheel and replaced the flywheel key. The old one was fine. Removed the head and checked for blown head gasket. Gasket was fine. Adjusted the valves to .005. Removed and cleaned the carb twice. I did cut the titter off on the anti backfire solenoid to eliminate that as a cause. The bowl fills with gas as it should. Adjusted the choke cable to ensure that was closing all the way. Engine cranks over nice and strong but doesn't seem to want to catch. After cranking it over for a awhile the plug seems to be wet. I do have spark but it's yellowish white. What am I missing?
First check oil level, adjust level or change oil & filter with OEM B&S filter, fuel & air filters, fresh gas, suggestion: in the future don't cut anything from engine like solenoid.
I have not done a compression test yet. When I turned the engine by hand, compression was never a concern. I did reuse the head gasket. I realize you should always replace your gaskets, but it was in perfect condition. Checked the oil and dropped the level a bit, to the top of the full line. Air filter is new and the fuel filter is flowing. I'm in the process of draining the gas and will report back. Thank you for the responses so far.
#5
grumpyunk
If you have fuel, air and compression, a spark will set things in motion. I would remove the spark plug and connect the coil wire, laying the plug on a conductor. Crank the engine and observe the spark produced. You should get a healthy spark. The coil you replace might not work properly, or your ignition wire to the switch has compromised insulation, and is grounding the magneto. The wire grounds the magneto to kill the spark, to in turn stop the engine. If the wire is grounded, the engine won't start. In addition, the ignition needs some good rpm to produce a spark. If the rpm is too low, you won't get a good spark. You can leave the ignition wire disconnected from the key switch and try to start the engine to test that the switch is not grounding your spark. A wet plug indicates presence of fuel, but no ignition. You have verified spark timing with the flywheel key, so you can also verify that it is not a fuel problem by spraying carb cleaner into the intake, and cranking. If the engine sputters and tries to run, you have a fuel/carb problem.
tom
I'll get my compression tester back and check. What should I be expecting for a reading? I have checked for spark and I am getting a yellowish one. It appears to weaken a bit the long I crank it over. Would this indicate that the magneto is not grounded? You may have to explain disconnecting the ignition wire test, not sure I understand.
If the coil was overheated it can put out spark at cold, but perhaps not enough to start. If bad it will not put out consistently at hot. These coils apparently cannot be tested by owners, and although a few have indicated the dealer may have such testers , testers are expensive, require maintenance and really go against the sale of replacements. Unless you have a procedure calling for it (please share) ohm readings would only be useful for the plug wiring and contacts.
When buying a used mower, never asume anything is right till you have confirmed it.
In your case I would now be checking the valve lift and the valve timing after double checking all else.
Squirt a little starter fluid down the plug hole and see if the engine fires.
Next try some down the carb throat.
If it fires with the fluid down the plug hole but not with the fluid down the carb then this suggests a valve problem.
you can use the teeth on the flywheel as a defacto timing disc, just divide 360 by the number of teeth then each tooth will represent that number of degrees.
It is a bit rough, but good enough to pick a cam that is 1 tooth out
