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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.

2 things, does it have a metal supressor cap on end of coil ht lead. They were common for breaking down electrically.
Are you using the same one when replacing coils?
Also your bio shows its a snapper. Now certain models at around 2007 08 had wrong opc cables fitted which did not allow the freewheel brake to release far enough and so the coil earth switch was slightly still connected to ground and cause rough running and bad starting.
To make lever go further tie a knot in cable to shorten inner.

I would remove the kill wire from the coil and see if it makes a difference. Pugs suggestion about a possible short in the kill wire, at the parking brake, or along the wire is a definite possibility. If there is no change, I would rule out an electrical problem, because of what you have told us. The next place I would look is either a sticking intake valve or valve clearance too tight, allow slight blowback when the engine get warm.

Hi pugaltitude, The coil lead has a rubber cap at the plug end not a metal one. Also the new coils each had their own plug leads that came with them. Your comment about the brake release/coil shorting switch is interesting because this mower did have that problem several years ago. I corrected it by grinding some metal off of the shorting contact to provide more clearance. What's interesting is that the way the motor behaved with that shorting switch just making light contact or maybe intermittent contact is very much like it is behaving now with a resistor plug and after warming up for several minutes. More recently I removed the shorting wire from the coil, at the coil, and this problem still occurred.

Hi Rivets, Please see my post today 1/11@3:26PM. I did remove the kill wire from the coil and it had no effect on this problem. I set the intake valve at 0.006" according to the operators manual spec of .005"-.007". What have you seen that would cause an input valve to stick? Also, I wouldn't think the input valve would heat up much in only 2 minutes of running. ( The motor typically starts easily and runs well for about 2 minutes before exhibiting sputtering and 200-500 RPM reduced speed erratic running).

You say this is an L head engine. How did you set the intake valve clearance?

You would be very surprise at how fast an engine can heat up. My thoughts were a carbon buildup on the valve stem, reducing the clearance. This could cause the valve to not fully close when it gets hot. Not very common, but I have seen it. The question of what procedure you used to adjust the valve clearance is one I to would like to know. May give us an idea of your problem. Did you clean the valve faces and seats before you set clearance? Did you lap the valves in? You must remember that the members who are trying to help you are not there with you to see what and how you did things. We may ask the same questions more than once, because we are trying to picture what is going on and don’t remember your answers to other reply’s.

Hi cpurvis, I removed the head and valves and cleaned all carbon. I then lapped the valves, cleaned everything well, and re-assembled the valves. Using a feeler ga. I set the intake to 0.006" and the exhaust to 0.008".

Hi Rivets, Please see my reply to cpurvis today 1/11 @6:44 right after your post @5:05

Patr said:

Hi cpurvis, I removed the head and valves and cleaned all carbon. I then lapped the valves, cleaned everything well, and re-assembled the valves. Using a feeler ga. I set the intake to 0.006" and the exhaust to 0.008".

Click to expand...

yes, but what process did you use to "set" the valves?

Hi cpurvis. To set the valve clearance I had the piston essentially at top dead center. I then moved it slightly to find where there was maximum clearance between the end of the valve and the cam follower. This is the position where I set the clearance to be 0.006" on the intake valve. On the exhaust I was careful to avoid the effect of the compression release. As far as the actual grinding of the valve stem, I held it perpendicular to the side of my grinding wheel and rotated it while grinding. I would take off a little at a time, put it back in the motor to recheck the clearance and repeat until I obtained 0.006"

Patr said:

Hi cpurvis. To set the valve clearance I had the piston essentially at top dead center. I then moved it slightly to find where there was maximum clearance between the end of the valve and the cam follower. This is the position where I set the clearance to be 0.006" on the intake valve. On the exhaust I was careful to avoid the effect of the compression release. As far as the actual grinding of the valve stem, I held it perpendicular to the side of my grinding wheel and rotated it while grinding. I would take off a little at a time, put it back in the motor to recheck the clearance and repeat until I obtained 0.006"

Click to expand...

That's the part I was interested in. Setting clearance by grinding valve stems is usually beyond the ability, or willingness, of most people. You're the exception!

Do a compression test after the two minute run, because I would also be looking a valve problem.

Hi EngineMan. That's a good suggestion. I'm located in IN and it's cold here so the mower is put up until Spring. I am looking for ideas in case this problem exhibits itself again at the start of the mowing season. If so, I will check the compression when it starts to exhibit the problem.

Thanks to everyone who have responded to my problem with helpful suggestions. I especially want to thank Boudreaux who was first to respond and kind enough to speak with me by phone. I hadn't intended to work on this mower this Winter but was looking for ideas in case the problem returned in the Spring or got worse during the mowing season. Because of this forum and related suggestions my curiosity caused me to retrieve the mower and investigate further. I attempted to measure compression, even though the motor has a compression release on the exhaust valve, by driving the flywheel nut with a drill and socket. It indicated the compression was very low. This caused me to decide between repairing the motor or replacing the short block. I purchased a short block because one was available from Small Engine Warehouse for only $75. After incorporating the new short block, I tore down the old block to determine the cause of its problems. Before I get to that, I had indicated earlier that the flywheel key was OK. Before I removed the flywheel I looked at the alignment of the flywheel to the crank using a jewelers loop and found they were misaligned by .015". The key had been slightly compromised. Comparing this misalignment to the circumference of the shaft shows the timing was advanced by 2.8 deg. Back to the old block, the rings were worn significantly. Compression ring end gap measured .060" to .062" along the bore. This compares to a replacement spec of .035" let alone what a new ring dimension would be. In addition, there was significant scoring of the bore in a 1" wide section ranging from 1/4" down to 3/4" down from the top of the cylinder. This area was located roughly between the 2 valves. Some of the areas were deep enough to see light between the ring and cylinder wall by back lighting from inside the block and looking with the jewelers loop. There was significant carbon buildup on the cylinder head and top of the piston. The exhaust valve also had quite a lot of carbon and some of it was on the face of the valve and the face of the seat. This occurred even though I had removed the carbon a year ago and replaced the pitted exhaust valve and lapped it. I don't have a way to measure valve guide clearance but I thought the valve was a little loose in its guide. The exhaust valve plastic cam lobe was 1/16" wide at the top and pitted compared with the intake valve lobe which was smooth and rounded on top. The lift of both lobes appeared to be OK. All other internal parts were in good shape. It turns out that spinning the old motor at 425 RPM wasn't fast enough to eliminate the effect of the compression release because the new motor indicated low compression also. So, it appears I replaced the short block based upon an incorrect measurement but it got me to do what was needed and at an attractive price. I want to add that most of these posts, and my experience also, don't deal with motors that are worn out. Most are carburetor problems probably followed by ignition problems. Worn out motors are harder to deal with because compression is difficult to check with compression release involved and while carbon can be removed it returns fairly soon with worn rings or valve guides.