John Deere Z225 cranking but won’t start

[IsaacPhillips]

I’ve been working on getting a Z225 back up and running. It’s had 2 engine replacements (it’s about 20 years old), the last one being 8 or 9 years ago. It’s currently cranking, but won’t catch. If I spray some starter fluid in the air intake, it’ll ignite a few times, but then continues to just crank. So far, I’ve replaced the starter, starter solenoid, carburetor, fuel filter, and fuel pump. I’ve confirmed the fuel is flowing through the pump and getting to the carburetor, checked the magneto gap, run a spark plug tester and confirmed there’s a good, constant spark. Any thoughts on what could be going on or what else I should check?

 

[Shane¹]

Did you check valve clearance? And possibly make sure exhaust isn't plugged?

 

[GMB74]

I agree with Shane, check those items first.
Also be aware these single-cylinder Briggs engine blow head gaskets when they get old or have been run hot. The gasket is not well supported between the cylinder and valve pushrod area. Do a leak down test and see if air is going into the crankcase.
Replacing the head gasket is not a big job and you are off and running again.

 

[IsaacPhillips]

Did you check valve clearance? And possibly make sure exhaust isn't plugged?

I have not checked valve clearance. I'm still kind of new to this, what tools do I need to do that? The exhaust is not plugged, as I can feel air flow when I spray starter fluid in the air intake.

 

[IsaacPhillips]

I agree with Shane, check those items first.
Also be aware these single-cylinder Briggs engine blow head gaskets when they get old or have been run hot. The gasket is not well supported between the cylinder and valve pushrod area. Do a leak down test and see if air is going into the crankcase.
Replacing the head gasket is not a big job and you are off and running again.

Pardon my ignorance, but what is a leak down test?

 

[hacskaroly]

Pardon my ignorance, but what is a leak down test?

Doing vacuum and pressure testing to make sure the engine is holding pressure and vacuum where it should. For example on a chain saw, if you pressurize the cylinder and get air bubbles from one of the oil gaskets then you know you have an air leak and that spot needs further inspection and/or parts repair. You can do pressure and vacuum testing on the carb to ensure no air leaks, the needle seats like it should.

An air leak can mean that you are not getting the compression you need to get the fuel to start. Its possible you have enough compression to light the starting fluid, but not enough to light the gas. You can get a compression tester to see what your cylinder compression is at.

If you can inspect the valves, make sure a push rod has not slipped out of place, if that happens the engine will not start, I had that happen on a generator before, it was an easy fix to get it back into place (unless it is bent).

 

[GMB74]

A compression test on a small 4-cycle engine will give a misleading low reading due to the automatic compression relief.

A leakdown test is performed by bringing the engine's piston to TDC on the compression stroke and locking the crankshaft to prevent it from turning. The cylinder is then pressurized by using a fitting that screws into the sparkplug port. The other end is connected to an air compressor.
Pressurize the cylinder slowly and see if air escapes somewhere.
Out the carburetor? The intake valve is not seating.
Out the muffler? The exhaust valve is burned or not seating.
Into the crankcase? Broken piston/piston rings/blown head gasket.
Externally? Blown head gasket or damaged cylinder or head.
These tools can be fabricated for a cheap and easy test, or you can buy a kit that will give you a percentage reading of the leakage. This can tell you how badly worn an engine is.
Here is a cheap but useful set:

https://www.harborfreight.com/cylinder-leak-down-tester-64918.html

 

[hacskaroly]

A leakdown test is performed by bringing the engine's piston to TDC on the compression stroke...

This is great information, thanks!! I have a tiller that I will need to do a leakdown test on.