B&S 128M02 Won't Start

[Namestaken]

I have a Briggs and Stratton 128M02-0133-F1 on a push mower that won't start. Diagnosing this is my intro to small engines so I'm pretty green. I've done a few things to try to fix it thus far but nothing has worked. The things I've done:

• Replaced spark plug (it was pretty clean, set the new gap @ 0.030)
• Replaced oil
• Replaced gasoline
• Replaced air filter
• Tried starting with gas cap off
• Rebuilt the carburetor twice, then replaced the whole thing anyway
• Replaced a cracked intake manifold
• Replaced air vein and choke springs (mower ran poorly before it stopped working)
• Replaced rusted thermostat (aligned with air vein bracket marks)
• Replaced coil/armature/magneto (set air gap to 0.011)
• Verified magneto primary and secondary resistance
• Used ignition tester to verify spark should be occurring
• Removed cylinder head and verified it was clean inside
• Verified valves and piston open/close

Any ideas you can share are greatly appreciated!

 

[dougand3]

Sheared Flywheel key? Valve Clearance .005-.007 Int and .007-.009 Exh?

 

[bertsmobile1]

:welcome:
Now I have been nice, I will be nasty.
You have just failed myserably the first test.

Before you replace anything you diagnose the cause.
Once you start replacing things willy nilly you introduce more possabilities and obscure the original problem.

Simple diagnosis is to fit a red inline spark tester so you can actually see the spark from 3' away.
Then you put some fuel or spray carb cleaner down the plug hole and attempt a start.
If the engine fires once or twice normally then the spark timing is OK & the compression is OK
Next you do the same via the carb
IF the engine starts & runs out the fuel then the valve timing is good & you have a fuel supply problem
IF it backfires or does not fire at all you have a problem with the valves.

Both of these of course are subject to the spark tester always giving you a nice strong red flash.

IF you get no response either way the install a brand new plug & repeat.
A new plug con go bad if the engine does not fire a few times.

Next failure is you did not tell us which spark plug you fitted .
You also did not reveal where you got the parts from & what the numbers were.
There is a lot of trash out there sold by online vendors who are not mower shops & have no idea about what they are selling.

We also need a much better description of the problem with the engine before you started to replace things.
A cracked manifold ( very common ) will cause the engine to surge due to the combustion mixture being too lean to sustain full speed running.

 

[Namestaken]

Sheared Flywheel key? Valve Clearance .005-.007 Int and .007-.009 Exh?

Good suggestions, I'll check it out tonight!

:welcome:

Appreciate the feedback. My listing wasn't in any particular order. I verified with the ignition tester that I was getting a nice spark first.

I got all my parts from repairclinic.com and they all came in B&S boxes so I'd hope they were OEM. I used a part list for my model but I'll amend my original posting with the numbers as soon as I can.

The mower did nothing at all before I started replacing parts. I pulled the string and nothing happened (or even tried to happen).

I'll try your other suggestions tonight and see what I get!

 

[bertsmobile1]

Did it stop working or was it a non runner when you got it ?
Lots of silly things like a previous owner fitting a new camshaft out of time come into play.

 

[Namestaken]

Did it stop working or was it a non runner when you got it ?
Lots of silly things like a previous owner fitting a new camshaft out of time come into play.

I got it used and it never ran smoothly. I had to fiddle with the air valve (close it) to keep it running sometimes. I know the previous owners and they barely used it and definitely didn't change anything on it so nothing funny going on inside. Everything in it should be OEM.

I used my riding mower for a couple months and when I came back to this push mower it wouldn't even fire up.

 

[bertsmobile1]

All this sort of information makes daignostics a lot simplier as it eliminates some of the more left field options

You had to close down the choke to richen the mixture which compensated for the air coming in the crack in the manifold tube.

 

[Namestaken]

Sheared Flywheel key? Valve Clearance .005-.007 Int and .007-.009 Exh?

Flywheel key was solid, intake clearance was perfect, but the exhaust was @ 0.018. Would that be enough play to cause the mower not to start at all? I don't have enough knowledge to base this on but it seems like that'd prevent it from running well but not to even turn over?

 

[bertsmobile1]

In order for ,the fresh gas to get in the old gas has to get out.
With that sort of ga the exhaust will open very late, close very early and only open a very small amount.
In such cases the engine should fire with a short shot of starting fluid down the plug hole as per the previous reply.
In such a case the engine should not respond to a short shot of starter fluid down the carb as per the previous reply
The whole point of the previous reply was to allow you to diagnose the engine and hopefully get back to us with what you found.
Apparently you just did not get it.

 

[Namestaken]

Sheared Flywheel key? Valve Clearance .005-.007 Int and .007-.009 Exh?

Can't thank you enough, dougand3, your suggestions were just what I needed! I replaced the out-of-spec exhaust valve and the mower is running great again.

It's really impressive how you zeroed in on the solution immediately :thumbsup: