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I have a riding mower with a Briggs and Stratton 18HP OHV motor. model 31M777 - 0234e1. I know for sure that the carb needs a rebuild, so I've already ordered a new carb for it.

While I'm waiting on that I pulled the plug and checked for spark to make sure electronics are working and i get spark just fine.

I then did a compression test and I get ZERO compression, nothing at all on the gauge. Then I noticed that I'm getting all the compression coming through the intake pipe right past the carb. I held my hand over the carb inlet while cranking and it seems like ALL of the compression is coming right through the intake. I'm not very familiar with these motors so I figured some folks on here may be able to help point me in the right direction before I start tearing things apart. Is there some sort of compression release device on these that may be holding the intake valve open? Or is it usually a valve problem?

I haven't pulled the valve cover yet, but will do that probably tomorrow.

Thanks in advance for any help!!

Remove the valve cover and watch the valves move as you rotate the engine by hand, also check to see that they both travel the same length and return to the same height, might be the intake valve stuck open.
It could also be that you're feeling the Compression release, release compression back into the intake, it makes for easier starting, but even with the Compression release you should still get a compression number on the gauge.

Two usual causes are a stuck valve or a shifted valve seat.
For the price of a head gasket I just pull the head off & check them both.
You can stake the seat back in position most times
You can stake the valve guide back in position most times
However they are a symptom, no the problem.
The problem is excessive heat
So take the blower housing off and clean the cooling fins as well.

Good advice guys. Ya, there is no compression at all. I tried holding my hand as tight as i could over the intake and my gauge read around 20psi. So something with the intake valve is probably not right. Going to pull it apart hopefully tomorrow. Where's a good source for headgaskets and parts for these motors? I ordered the replacement carb from amazon.

parts Tree, Pro Parts direct, Jack's small engines are the main ones I use.
Parts Tree is located in Austin Texas,Pro Parts is in New Hampshire, and Jack's is in Maryland.

ProParts Direct, Jacks, Parts tree, Ereplacement parts.
Just about anywhere other than Amazon or Ebay unless you are the sort of person who is always winning raffels.

Just be aware if the OP needs a new head (796026) they are currently on back order from Briggs. And with Briggs in Chapter 11 it might take a while to get a new head.

One question though. When you intake was at the side of the engine's head or the where the air filter attaches?

Scrubcadet10 said:

parts Tree, Pro Parts direct, Jack's small engines are the main ones I use.
Parts Tree is located in Austin Texas,Pro Parts is in New Hampshire, and Jack's is in Maryland.

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Thanks! I'm in Round Rock (suburb of Austin). I'll check out parts tree first.

StarTech said:

One question though. When you intake was at the side of the engine's head or the where the air filter attaches?

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No air filter on, carb still attached though.

A good place to get a good used head is your local small engine shop. They'll probably sell you a good one for $20 to $50.

baktasht said:

Thanks! I'm in Round Rock (suburb of Austin). I'll check out parts tree first.


No air filter on, carb still attached though.

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The reason I ask is because it can be a severely blown head gasket which these engines are known for.

StarTech said:

The reason I ask is because it can be a severely blown head gasket which these engines are known for.

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even with a blown headgasket shouldnt I still get some compression reading? What are typical symptoms of a severely blown headgasket?

It is possible to have zero compression with a blown head gasket if blown severely enough. I know for sure as I just repaired one a few weeks ago that was that bad. It had nearly an inch of the gasket missing where the metal core melted away. The head also melted away the two plastic push rods guides. With only a half of oil left in the engine I was thinking more along the lines of a broken rod but it was the head gasket instead.

But normally it is the heavy oil assumption that kills the engine where the head gasket blows between the cylinder and push rod galley; unless, caught before severe damage occurs.

From my experience.
Popping out the intake means the exhaust side isn't opening. (Compression has to go somewhere).
Just my 2 cents on this.

Got the valve cover off and it looks like the Intake valve is moving a very small amount and the exhuast valve not at all. Pulled the cylinder head and it looks like the previous owner tried a DIY repair and made a few mistakes:
* He put the intake pushrod on exhaust and the exhaust pushrod on the intake
* Bend the aluminum pushrod which was placed incorrectly on the exhaust side
* He lost one of valve caps. only the intake had a cap

Both valve seats look okay. TONS of carbon build up everywhere which I will clean up. The valves both seem to move freely. There was also a broken piece of the plastic pushrod guide in there which I took out. Was also missing the intake tube o-ring. Was also missing one valve cover bolt and a few other bolts on shrouds. So I don't know what on earth happened to this thing before I got it. So many things put back together incorrectly.

I straightened the aluminum pushrod with a metal working hammer and put things back together, seems everything moves correctly now. At least visually.

I have the following on order. Let me know if you see anything else I'm missing:
* head gasket
* valve cover gasket
* exhaust and intake gaskets
* Intake oring
* Intake pushrod
* Exhaust valve cap
* Carburetor, gaskets, and fuel shut off valve

I bought this thinking it was just a carb replacement it needed. But if I can get it running after all this clean-up and some new parts hopefully it will run well for a while.

Thanks again for all the help so far!!

Richie F said:

From my experience.
Popping out the intake means the exhaust side isn't opening. (Compression has to go somewhere).
Just my 2 cents on this.

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You were right, the exhaust valve was not opening at all. See my post right above this one.

Measure the height of the valves from the floor of the head. It should be identical
Pushrods bend because the valve can not move
Only 3 reasons for this
Valve is hitting the piston
valve is stuck
valve guide has moved & the valve spring is coil bound.

bertsmobile1 said:

Measure the height of the valves from the floor of the head. It should be identical
Pushrods bend because the valve can not move
Only 3 reasons for this
Valve is hitting the piston
valve is stuck
valve guide has moved & the valve spring is coil bound.

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Good points. Where should I be measuring from and to? Are you revering to the valve stem end of things? Or the valve face?

Valve stem
When you do it regularly you can see it without measuring
The valves are the same length and so are the valve guides
Both are set to the same depth in the head
so the stem ends should sit even when the rockers are removed.

bertsmobile1 said:

Valve stem
When you do it regularly you can see it without measuring
The valves are the same length and so are the valve guides
Both are set to the same depth in the head
so the stem ends should sit even when the rockers are removed.

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Gotcha, makes sense. Will check that tomorrow to confirm how they line up.

Okay, got things cleaned up a little bit. TONS of dirt and old oil/exhaust residue clumped up everywhere, so I would guess it probably did overheat at some point.

Got most of the carbon buildup on surfaces that I could reach cleaned up. Thoughts? Anything obvious I'm missing before I start re-assembly this weekend?

All the parts arrived. Will be working on it this evening.

Good compression on cold motor:

Replaced the fuel line, cutoff valve, and fuel filter. And pre/post air filters too. It started right up without even a hiccup. I was actually surprised how quick it starts up. Let it warm up and adjusted the idle screw and fuel screws and it is running perfectly.

NOW THE NEXT PROBLEM.... haha... Drive belt is on, but I get zero motion when i put it in any forward or reverse gear. Not sure if it's even the right belt with all the mess the previous owner left on this thing. Going to order a new drive and deck belt for this weekend pickup to see if that fixes it. Fingers crossed!!!

All fixed up. Needed an idler pulley replaced, one of the "lips" had been chewed off. Also had a couple of support arms missing from the deck, got new ones ordered and put on. Ordered up a two mulching gator blades, too. And also a mulching "cap" thing that blocks the outlet of the deck.

This thing will now start right up at the blip of the key, even without choke engaged. It runs at slow medium and high throttle speeds perfectly, and the deck is operating smooth as butter.

Thanks all for the help and suggestions!!

sounds like it's running great!!! Good job.

Glad to hear you are up & running
Gators do not work with a mulch cover,
They are designed to cut & recut the clippings but not to drop them strait down into the spot they were just cut from.
They throw the fine clippings which will vanish into the grass in a day or two.

When used with a mulch plate you end up with blobs of grass shake pulp all over the lawn.

bertsmobile1 said:

Glad to hear you are up & running
GAtors do not work with a mulch cover,
They are designed to cut & recut the clippings but not to drop them strait down into the spot they were just cut from.
They throw the fire clippings which will vanish into the grass in a day or two.

When used with a mulch plate you end up with blobs of grass shake pulp all over the lawn.

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You're right! I noticed that there were lots of clumps. Any recommendations on blades to use to get the best mulch functionality with the mulch cover?

Try it without the cover and the chute raised about 30 deg up

bertsmobile1 said:

Try it without the cover and the chute raised about 30 deg up

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Tried it without the mulch cover and it cut great. No more clumping. I just have to be careful to cut more often I think. Because if I wait too long the clippings will be too much I think.