Hello,

I have a 4 year old, 20hp B&S V-twin engine mounted on a craftsman lawn tractor with a few problems. First let me put up all the info on it though.

Model: 407777
Type: 0174 E1

Alright. So after i have been mowing the lawn with the thing for about 20 or 30 minutes (engine is good and hot) the engines power will pretty much drop to the floor. The RPM's go way down, it starts shooting out black smoke (not continuously but a lot of puffs), and it starts misfiring, this all happens while i have the blades engaged. If i turn the blades off and let the engine stay at full throttle it stops running as bad, however it is still kind of rough. If i leave the blades off and just try to throttle up from idle to top speed quickly it exhibits the same qualities as it does when the blades are on.

If i turn the engine off and let it sit for about 10 minutes it will start back up fine and run perfectly for another 15 minutes or so. Then it gets hot and starts having the same problems.

Another possible related symptom is what appears to be air bubbles in my oil (when i pull the dipstick out it usually has a bunch of bubbles on it), again this is only after it has really heated up (i think, need to double check).

Another possibly related symptom. The spark plugs have about 4 or 5 hours of use and there is already a very large amount of carbon build up on them

*Note: In all cases the engine never has any problem starting

Here is what i have done/ checked in an attempt to fix this.
-Replaced air cleaner
-Replaced plugs
-Replaced fuel filter
-Replaced fuel line (some of it was crack, so i only replaced the short section between the fuel pump and the carb)
-Checked compression, 145psi (i wasnt able to check the other cylinder because my battery died)

Thats all i can think of that would be relevant, hope i got everything you need.

Sounds to me that when your engine gets hot, you may be loosing spark from one of the CDI modules effectively dropping one of your cylinders. The black smoke comes from the unburned fuel from the cylinder lacking spark and entering your muffler. Turning off the blades reduces the load on the engine so it seems to run better.

I dont think that is the case. I have checked the spark on both plugs when it does this and both plugs have a strong and clean spark. I will however double check that as soon as i get a chance.

Well it's certainly possible that there is some other problem, this just seems to me the most likely.

It could be a valve problem or an air leak possibly.

Check the engine for running when this happens with one and then the other spark plug disconnected and see if you notice any difference with the way the engine runs. It is possible to still have spark when a CDI fails but rather the spark gets advanced or retarded to a point that the engine will not run properly or the intensity drops to a point that the plug will not fire under compression. This condition is rare usually you just loose the spark, but I have had this happen before.

My 6.5 hp B&S ohv self propelled mower is doing exactly the same and it's a one cyl. Can someone please help?