Briggs L head motor starts well but sputters after warming up

I have a Briggs motor 126T02-0675-B3 that starts easily and runs well for several minutes but begins to run unevenly, spits droplets of gas out of the carb, and may die after warming up. This appears to be an electrical problem because the only clue I have found is that this happens consistently with the specified resistor plug installed but it runs noticeably better with the equivalent non-resistor plug. I have a fair amount of experience with small engines and have addressed such things as carburetor, valve condition and clearance, ignition coil, kill switch and wire, high voltage lead to plug, cleanliness of gas tank, venting of the gas tank cap and condition of the flywheel key. I have also tried to evaluate the strength of the magnets in the flywheel and they appear to be OK. I am apparently missing something relatively obscure that I haven't encountered before. I would appreciate any suggestions of anything else I should check.

Sounds like you got all the bases covered.. It's still in the carb, welch plugs need to come out and a good soaking or a ultrasonic cleaner....
First try the intake tube and see if the 2 screws might be loose..........

Boudreaux thanks for responding. This motor has a very simple carb with no idle circuit, only normal operating speed. I am quite confident of my carb cleaning but regardless I replaced the carb with on new one. It wasn't because of potential cleaning issues but rather because the carb casting was wallowed out at the top of the throttle shaft allowing the shaft to move around and also to leak air thru the enlarged opening. Neither the cleaning nor carb replacement had any effect on the issue of running rough after several minutes of operation. Also I have ensured that the air tube screws are tight. I keep focusing on the fact that the motor runs noticeably better with a non-resistor plug J19LM compared with its resistor equivalent RJ19LM even though the owners manual calls for the latter. Another thing I am curious about is that even with a resistor plug the motor starts and runs perfectly for 2 minutes. The rough running and spitting droplets of gas out of the carb begins after that. I wouldn't think many components would get that hot in only 2 minutes. The problem continues after that but doesn't get worse. I want to point out also that this is not a problem that appears to have happened abruptly. It became a problem in the Fall of 2015. The problem continued in the Spring of 2016 and throughout the year. It continued to be a problem in the Spring of 2017. Various things were addressed over this period of time but it was early June of 2017 that I realized that I had changed the spark plug, as I do every Fall, but that I hadn't tried a second plug to make sure it wasn't a defective new plug. This mower belongs to a friend of the family and when I indicated I was going to try a second plug she mentioned she had an old plug. It dated from some years before I have been tending to her mower at the end of each season. It was in far from new condition but what was significant was that it was a J19LM not a RJ19LM and that the motor ran significantly better with this old non-resistor plug. After that I put in a new J19LM and the motor worked OK for the remainder of the 2017 mowing season. I might have let it go at that except for 2 reasons. The first is that I will always be curious about why the resistor plug that is called for isn't useable with this motor. The second reason is that this Fall, after adding stabilizer to the gas and letting the motor warm up prior to changing the oil, I witnessed the motor sputtering again after warming up for several minutes. Perhaps I added a little more stabilizer than I should have since it's hard to see how much gas is in this tank. I didn't pursue it any further because my focus was on winterizing the mower and changing the front drive bearings in the adjusters, the drive gears and the front wheels, which took quite a bit of time. With this post I'm looking for additional ideas in case I need them in the Spring.

Get another coil from a older mower that is a 6 HP also....... Any engine in the range will work. Try that and see......

Was it a Briggs carb you put on ?? I have had mowers come in running rough and there were chinese carbs on there ~!~!

From your description I would look at electrical problem, like coil starting to breakdown as it gets warm. Not a complete open, just a higher resistance as it warms up. This could be why the nonresistor plug does help. Could be as simple as a bad or loose ground on the coil. A nonresistor plug does not need as high of a voltage to work properly. If you have a neon spark tester, you will probably see a difference in spark color between the two plugs. There were actually two coils listed for this engine at on time, depending on the date code. I believe that the part number for the one you need is 590454, but have your supplier double check with all your numbers before ordering a new one. At most places these electrical parts are nonreturnable. If they order the wrong one you have recourse to return it, because it will be their fault.

Boudreaux thanks for continuing to offer suggestions. The carb replacement is an actual Briggs part. I have been so convinced that this problem is an ignition one that I have actually replaced the coil on 2 different occasions months apart. These were 2 different new coils. Each time I put the original coil back on and returned the new one because there was absolutely no difference with either of the new coils compared with the original. I don't have access to any old coils. One of the interesting things about this problem is that it is usually easy to determine if a motor problem is related to fuel or spark. After as many hours as I have spent on this problem I'm still not positive which is at fault, but I keep leaning toward ignition because of the definite improvement with the non-resistor vs resistor plug. I have checked or replaced every part in this simple ignition system except for the flywheel. What has kept me from replacing it is the approx $75 cost. What I have done is attempt to evaluate the strength of its' magnetism by loosely dangling a medium sized screwdriver by its handle with its blade approx. 3/4" away from the magnet and verifying it will attract the blade and hold the screwdriver vertically by its blade. I also borrowed an inline spark indicator from a neighbor. It produces an orange colored light when a spark is produced. The result was inconclusive because it pulsed regularly when the engine was started and running normally but when the problem began and the motor would operate unevenly and a few hundred to 500 RPM less than normal, the indicator continued to flash and at 2500 to 3000 RPM it was impossible to tell if the motor was operating unevenly and causing the indicator to be a little less steady or if the indicator was showing that the spark was slightly weak or occasionally missing and that was what was causing the motor to run unevenly. It would be easy to tell the difference at an idle speed but not at 3000 RPM.

If you want to pay the shipping costs I will give you a flywheel..... You got me curious about that engine...... If it was in front of me I may be able to cure it.....

I can't tell the year of manufacture from that number, so here comes a stupid question: By chance, is this engine old enough to have points and condenser?

Hi cpurvis, The motor is a 2008 model. It doesn't have points and condenser.

Hi Rivets, Please see my reply at 8:20 right after yours. I have changed the coil twice during the course of troubleshooting this problem and each time I put the original back on because the new coil had no effect on preventing the rough running/reduced and varying RPM after warming up for approximately 2 minutes. The coil P/N's were 590454.