Bubbling spark plug

Mar 16, 2016
Basic 2 stroke engine kit. There is bubbling liquid around where the spark plug meets the engine. It seems like oil or gas is leaking out from around the plug or maybe has collected there? Should i just tighten it down more or is it a sign of something going wrong? It's been sitting for awhile but it started right up.


Staff member
Sep 15, 2008
Littleton, Colorado
Unfortunately there are some engine kit instructions floating around out there that tell you to "finger tighten the spark plug". That is the dumbest thing I've ever seen. Plugs need to be tight and checked after a couple of heat/cool cycles. Use a torque wrench if you have one. 70 to 80 inch pounds will suffice. If you don't have a torque wrench then an old rule of thumb is 'finger tighten then use a socket and give the plug another 1/4 turn. That should stop leaks around the plug.

Also I've seen damaged threads in cylinder heads or rough places where the plug is supposed to seal against the head. I've also seen cracked cylinder heads with the crack running through the threads where the plug goes. Check these things but most likely your plug just wasn't tight enough.

Let us know what you find.



Active Member
Mar 1, 2008
Aztlán, Arizona
I've had a few where the machining was so bad no matter how tight you tightened the spark plug you could never get a flush mount with the plug and head.

Been a while but I believe I used a combination of a file and dremel to finally solve the problem. Never got them perfect but good enough to get them to stop leaking.


Well-Known Member
Sep 30, 2012
I believe that 'finger tighten' part is just for covering the hole while working on it so nothing falls inside, but it can easily be removed to turn motor.

came out funny in engrish - as shown at 'engrish.com'


Well-Known Member
Jul 13, 2010
I've with an old plug that I removed and installed a few too many times had it leak even though torqued down. The metal gasket on some plugs can be removed and if you have a gasket from another less compressed metal gasket it can be swapped in. For the cost of a spark plug I guess you may not bother this way if you have a new one and this turns out to be the reason and can tell it is so.

More recently no mater how you try with some plugs to remove the metal gasket it cannot un-thread sort of. I suppose Dremel will work by cutting a broken plug to save a metal gasket, but how to install?

Only place I hear of spark plug gaskets has to do with some fanatics about spark plug gasket shims to exactly set the depth it reaches in the cylinder.

What I noticed was not bubbling, but I have a four stroke engine, it was a carbon deposit near one side of the spark plug. After cleaning the spot and newer plug with better metal gasket it never showed carbon around the outside of the cylinder head near the spark plug again.