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I have a yard machines push mower with a 5.75 briggs engine that will not start. I have cleaned the carb and put a new kit in. I have repaired tons of these with this same problem. It is usually the main jet. There is plenty of fuel flow to carb and carb bowl is full. Primer bulb works and will push fuel into the carb venturi. But mower will not start. Yes I would say spark. But when you spray carb cleaner into the carb it will fire up and die after it burns that off. Then I thought maybe weak spark.
The two differences I see are with the fuel from the primer bulb isnt going as far into the carb and then the difference of ignition point of gasoline versus carb cleaner. any help would be appreciated. Sorry for rambling.

What type of carb is fitted to you're engine.

Model type code
12H802-3306-B3

Coils dont usually give weak spark, only spark or no spark.
Is the spark plug working under compression conditions?
Is the engine sucking the fuel in. Try little oil down plug hole.
Usually its always carb on a model 12.
Manifold cracked?
Valves not seating?

It acts like it isn't sucking the fuel in. That is the best way to explain it. But wasn't sure how to verify that or what would cause it.

Usually bad starting on a primer is the gasket not sealing properly which dosent push fuel into carb but as you say you spray carb cleaner then it rules out primer as it runs then cuts out.

Could just be that when you spray cleaner through carb its going into cylinder and running which then could be rings or valve not closing all the way.

It has 110 psi compression. I put some oil in spark plug hole and it did not help with pulling fuel into cylinder. It will run as long as I spray carb cleaner into carb. It is just not getting fuel for some reason but plenty of fuel in bowl, bowl vent is clear, main jet (bowl screw) is clear, and emulsion tube all spotless.

Was the photo taken before or after cleaning, looks if it could do with a clean again..!
Have you got good compression..?
Check to see you have good seal on gasket between carb and engine, you maybe down on vacuum..!
If you have spark and compression it has to be fuel..! (not being pulled in)
Do you have item 163 upside down (just a thought) it can happen and are the hole's in the gasket free (vacuum hole)

When the carb is fitted does it flex up and down as if the manifold is cracked? it could be drawing air instead of fuel

The compression test shows rings are sealing but im still thinking valves.
With plug fitted and blade fitted does the engine turn freely like no compression or is there a bounce to it.
This kind of checks valves seating or head gasket.
This is where a leakdown test comes into its own and you get a result in minutes.

yeah I wish I had the setup for a leakdown test.
When rope is pulled it has good bounce.
Carb is mounted good. Gasket is good. No movement.

gasket is right side up.

Could a partially sheared flywheel key cause the valves to open at the wrong time not allowing a good vacuum in the cylinder to draw the fuel in?

The owner told me that last year he bent a new blade and had to replace it. So I am going to check it and rule it out. But just wondering if that could happen. Thanks.

Yes its worth a look, will open too soon or too late..!

bwestbrook said:

Could a partially sheared flywheel key cause the valves to open at the wrong time not allowing a good vacuum in the cylinder to draw the fuel in?

The owner told me that last year he bent a new blade and had to replace it. So I am going to check it and rule it out. But just wondering if that could happen. Thanks.

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Could be the problem as timing is all out so valves are opening and closing at the wrong time and the spark is at the wrong time.

The thing that throws me off on that is that when I spray carb cleaner in the carb it fires and runs good just like it should and don't run like timing is off. But I didnt know if it could be off just enough to lose vacuum in cylinder but still run good when spraying something.
Thanks for replies. I will be able to check it tomorrow.

bwestbrook said:

The thing that throws me off on that is that when I spray carb cleaner in the carb it fires and runs good just like it should and don't run like timing is off. But I didnt know if it could be off just enough to lose vacuum in cylinder but still run good when spraying something.
Thanks for replies. I will be able to check it tomorrow.

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Bad gas maybe? Try squirting some gas in the carb throat or cylinder and see if it fires? :confused3::confused3:

I can spray gas or anything like it and it fires up. The difference is when I spray it it goes further into the carb throat. If I just prime it it doesn't go past the front of the carb. Which is why I think it is the vacuum in the cylinder.

Don't want to sound stupid, but have you checked your fuel for water. I had the same problem this spring and the tank was half full of water.

Doesn't sound stupid to me. That is the stuff we overlook most. But yes this fuel is fresh.

Can you have someone pull the engine over while you hold your hand tightly in front of the carb. You should feel suction and after four good pulls your spark plug should be very wet.

Yes I will try that tomorrow. It is at the fire station. So we will check.

I just wanted to add that I had one of these and it would not start unless I pushed the primer at least six or seven times to get enough fuel for a proper start. However, your problem is that it will not run even if started with carburetor cleaner so what is preventing fuel from leaving the carburetor and entering the intake manifold?????

Problem solved. I had good suction with hand over carb while rope was pulled so I checked flywheel key. Of course it was sheared and off about 3/8 inch.
Thanks for all the replies.

So it was the flywheel key......well done, and job as they say:thumbsup: