Outdoor Power Equipment and Small Engines - Briggs 20hp Running bad & Dying

"I have John Deer L120A lawn mower with a B.S. 20 hp Intek Twin V OHV. I was running it and noticed a slight miss. As things progressed it become worse and worse to the point it would not run. Once cooled down a tad it will fire back up but run very rough and steadily decrees rpm to the point it will die again. I removed both coils (I was corrected later by being informed they where in fact magnetos) I cleaned and reset the gap using a business card. This did not correct my problem. In fact I had difficulty get it to even start. I found that if I remover the plug wire from the plug on the left side it will fire up with a miss, where it would not with both plugged in. If I unplug the right wire from plug it will not fire up at all. I do show spark on both plug wires.

Also want to add my first thought was fuel. I did check to see if the engine was in fact getting fuel. As far as I can tell it is. When I had the cover off and got it to barley run I did see a fuel fog in the carburetor.

Thanks for any help..."

Try taking your kill wires off the ignition coils (the things you reset the gap on). See if that makes any difference. That will remove all the safety switches from the system, as well as your ignition switch. Keep in mind you'll have to choke the engine to kill it.

The mag gap wouldn't have made any difference in what you have there.

If taking off the kill wires doesn't help, check the compression = checks valves/cam/pistons/cylinder, but it really sounds like an ignition problem.

I would suspect the coil on the left is not working properly or is weak, causing the plug not to fire under compression. I would replace the coil on the left side, and just as a matter of normal maintenance, set the valve clearances on the valves on both cylinders.

Best of Luck...

I am about to go out and do a compression test and try to kill wire test. I was searching for the gap clearance for the valves and cant seem to find it. Anyone happen to know what clearance it is?

By the way. Thanks for the helpful tips guys. I am an automotive technician by trade and feel lost when it comes to small engines...lol

.004" - .006" on both intake and exhaust.

removing the kill wire is not going to make a difference on your engine as both ignition modules are wired together. If there was a problem in the safety interlock on your rider, it would cause both modules to fail together and not separately.

Although I agree about the safety system causing a problem on both coils rather than one, I would remove the kill wires anyway and do the test because there is a diode in the kill wire which, if bad, can cause one coil to bleed down through the other and cause strange ignition problems similar to what you've described.

Well I guess it would not hurt to test with the kill wires unplugged, I only dismissed this as a possible cause, as diodes generally are good or bad and are not likely to get hot and quit, then start working again when cool. But it costs nothing to try, and theres always a possibility.

Also when you reinstalled the modules, make sure both are installed with the correct side facing out. It's possible to install magnetron modules upside down, and if so they will not fire. They are usually marked as to which side faces out or the cylinder.