FC420V Scag Mower Engine Low Compression No Start

[wverd]

Hello everyone, first post here.

I have a scag 48” walk behind mower with a Kawasaki FC420V engine which does not currently run.

It would not start with starting fluid sprayed in the carb or carb cleaner sprayed in the spark plug hole, so it didn’t look like a fuel delivery issue.

I checked spark with an in line tester, and it sparked in the tester. The plug was dirty and oily, but seems like it should probably still work. I’ve seen worse from running engines.

I then checked compression and got around 70psi cold. After adding some SAE 30 engine oil, the compression increased to about 130psi (the mower still wouldn’t start after this.)

As the compression increased with adding oil, I believe this should point to an issue with the piston rings or cylinder walls, and not incorrect valve adjustment/sealing.

My conclusion would be that this engine has too low of compression to run because of the piston rings or cylinder walls, and needs to be rebuilt or replaced.

Do any of you know the minimum compression this engine would need to start, and does my diagnosis and conclusion seem sound to you all?

I would like to get some additional opinions prior to doing a large and expensive repair.

 

[hlw49]

Service says compression should 71 psi or higher. Inline spark tester is not a good spark tester to test spark. You need a good mechanical tester that the spark has to jump at least 1/4 inch gap. You cannot tell if a spark plug is good or bad by looking at.

 

[StarTech]

Service says compression should 71 psi or higher. Inline spark tester is not a good spark tester to test spark. You need a good mechanical tester that the spark has to jump at least 1/4 inch gap. You cannot tell if a spark plug is good or bad by looking at.

Well 1/4 [0.250] inch is a little much as the Briggs tester is at 0.166 inch. But just a spark is seen in one of neon testers doesn't mean the plug is good as it can be shorted out. Also even plugs showing spark outside the compression zone can not spark under compression. Even new plugs don't mean they are good. This is why you need to be using a known good plug for testing.