Recently I decided to wash a few years worth of leaking oil from my 170 cowling and fuselage belly. I of course could use a sprayer with some kind of degreaser and a garden hose. No this job was to big. I needed a POWER washer. AR AR AR AR .
Well needless to say and as expected the ignition got a little wet. I didn't even try to run the engine the first day. On the second day I fired her up and sure enough one mag was fine. I'll say that one more time. ONE MAG WAS FINE. The other mag had a large drop and it seemed to be loosing one to maybe two cylinders. No problem it's still wet, I wait another day.
Next day arrives and I do another check. Great the one mag is still OK and the other is as well but I still detect an occasional dropping of one cylinder. No problem I'll just accelerated this drying process by pulling both mag harness from the mags and hosing down the wire ends and the mag caps with WD-40. After all that is what WD-40 was made for, not cleaning the engine compartment or the belly though I've been told by someone it works for that as well.
Yes I must admit that while at the auto parts store reviewing what degreasers were available there was this BIG gallon can of WD-40 staring me right in the face. And every time I gazed at that can a familiar voice with a Texas draw would say "buy the WD-40. It's the best stuff to wash your airplane with". I ended up buying the Super Purple Degreaser the Cajun voice was telling me to get. But I digress...
Surely on day four, specially after the use of the magic WD-40, the run up and mag check would be fine. That could be farther from the truth. The engine started but it was shaking to beat the band. And it didn't matter what mag it was running on it ran like crap. Holly cow what did I do. Could I have put the mag harnesses back on wrong. How could this water that was almost gone the day before be back worse that it was. Dam water. I'm through with that power washer. The light gets brighter as all I can think about is I had to pressure wash the dam engine. The little Texan voice says "should have used WD-40", the cajun voice says" Don't worry the convention is a year away. Plenty of time to fix this. Where is the St Pauli Girl".
Well there is nothing more to do but remove both mags for disassembly and cleaning, never mind they have only 100 hours on them since new. We'll get all that water out and while we're at it we'll change plugs just for good measure. Darn it I don't care what Frank says this engine is just going to leak oil if it ever runs again.
Well the mags came off pretty quick and disassembly of the Slicks is pretty easy and straight forward. I had every thing I'd need to swab out that puddle of water in each but there was no water to be found. I wiped it all out anyway in case there was some of that invisible water in there and confident I'd solved the problem I reassembled the mags and reinstalled and timed them. Man this thing was going to run sweet. Nope. It ran like crap and it didn't matter which mag. The light was so bright I wonder how I ever made it to a friends house where we decided I must have contaminated the gas with water. That's all it could be.
Back to the plane and drain the carb bowl. No water. Check the finger filter. Clean as a whistle. Start it up and it runs like crap. Feeling the heat from the cylinders one was colder than the rest. I must have had two bad plugs in the batch I swapped in and some how I put them in the same cylinder. I'll need to go borrow the torque wrench as I'd left mine at home and swap them out and it will be fine.
As I was borrowing the torque wrench from friend Buck and telling him I washed my engine and now it runs like crap he blurts out in an uneducated guess "think you have a stuck valve".
Just like that Buck turned out that bright light I'd been staring at.Well remember way back when I said one mag was fine. Then both mags were pretty good but I'd make them just a bit better. Then the next day the engine ran like crap. What do you think happened? That's right a valve stuck wide open and our engines, no matter what condition the mags are in, run like crap on 5 cylinders. I was so blinded by the light that said to me the problem must be the water and pressure I'd used.
Now I'm going to have to listen to that little Texan voice saying "use WD-40 to wash your airplane and you shouldn't need to use MMO to keep the valves from sticking".
Ole Murphy must have been hiding just waiting for you to do something totally unrelated to the engine (wash the plane) and jump in and screw with the valves and just to add insult to injury he messed with your mind too. A lot of time when we have a problem we get so focused on it that we can't see that forest for all the trees 
)
May there always be and Angel flying with you.