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gas coming out of lawn mower muffler and no compression? Answered

Briggs and Stratton 8 hp lawnmower engine. Engine has no compression and when running the electric starter gas comes out of the muffler.


best bet would be to use a leakdown tester. This pressurizes the combustion chamber with the piston at top dead center of the compression stroke through the sparkplug hole and then gives you a % of leakage reading. you then have to identify if it's going past the intake valve, exhaust valve or past the rings. do it by listening at the muffler, air intake (with the airfilter removed) or at where the dip stick screws into the engine block. This will help pinpoint the problem. But if you don't have the correct diagnostic tools, It seems like everyone here already has the basics ideas and procedures covered. I don't believe that the oil on the rings will be a problem, as long as the oil is at the correct level, engines are fairly good at keeping the oil where it should be, even if an engine does get flooded out. Not to mention that the engine most likely has a decompression system for starting that doesn't disengage until 500 + RPM is achieved. But that's just from my own personal experience and opinion

I took one apart fer that same problem to find that th' exaust vlv seat had worked out of th' block and has hanging th' vlv wide open , tapped it back down and staked it in (deformed th' edge of th' hole around its perimeter to lock th' ring in place) and ran it fer two more years !

The description is of a valve problem, most likely the exhaust valve (see Re-design's answer). It doesn't sound good, but all may not be lost. *Note: these instructions are for the "flathead / side pocket valve" engines. OHV engines should be troubleshot as per the engine manual.

Step 1.) Confirm it is just the exhaust valve by taking the air cleaner off of the carburetor and crank the engine. If gas shoots out there, it may be the cam. If not, go to step 2.

Step 2.) Remove the head and inspect the valves. Hand crank the engine and see if they move. If one valve doesn't move, it could be: burned, bent, the valve spring broke, or the valve spring retainer clip broke or came off.

Step 3.) Open the engine block. You shouldn't need to remove the flywheel as most blocks come apart on the opposite side of the flywheel. Inspect the cam, valve springs, retainers and the part of the valve stem you can see. If everything is intact, use a screwdriver to try to move the offending valve (at the cam face). If the valve doesn't move freely, you must remove the retainer and free the valve. Clean the valve stem and guide as thoroughly as possible and put the valve back in with a couple of drops of oil. If the valve moves easily, reinstall the spring and retainer and verify movement. If all works fine, reassemble the engine and you should be good to go.

IF you find broken parts in the crankcase (other than the spring retainer), you must investigate further to ascertain the cause and whether it is justifiable to repair the engine.


.  I'd go with Re-design's flooded-the-engine-and-washed-the-oil-off-the-cylinder-walls theory first. It could be a mechanical failure, but I'd guess loss of oil on the rings.
.  Drain the gas tank (and carb bowl if possible).
.  Remove the spark plug and blow compressed air into the hole to get rid of excess gas. Watch you eyes and don't breathe the vapors.
.  Leave spark plug out and crank until there is no more gas coming out the spark plug hole. You'll be blowing gas vapor into the air, so be careful.
.  Squirt a little bit of oil into the cylinder through the spark plug hole and crank a few times to coat the rings and cylinder wall.
.  Inspect spark plug for fouling. Clean/replace, if needed, and re-install. .  Fill with gas and see if it will start.

Broken valve spring.

That's two problems.  Unless your spark plug is missing.

Raw gas coming out the muffler means you are not igniting that gasoline.  Check for spark.  It might just be flooded.

But the no compression is a different story.  If compression is totally gone then you have a stuck valve, badly burnt valve, broken cam shaft gear or something that is not allowing the valve to seal properly.

If it's just low compression then the raw gas has wash all the oil off the rings and they are so warn that they are not sealing.