Compression through carb and intake

[baktasht]

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!!

 

[Scrubcadet10]

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.

 

[bertsmobile1]

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.

 

[baktasht]

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.

 

[Scrubcadet10]

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.

 

[bertsmobile1]

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.

 

[StarTech]

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?

 

[baktasht]

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.

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

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

No air filter on, carb still attached though.

 

[PTmowerMech]

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.

 

[StarTech]

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


No air filter on, carb still attached though.

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