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I picked up a clean mower with a Briggs 104M02-0121-F1. DOM DEC 2015. The carb was taken apart and the needle was missing as well as the air filter and air filter cover. I bought a new after market carb on ebay for about $12. A new B&S are like $75. The mower started on one pull but it surged and would stall out when you pushed it. I checked all of the usual stuff like the fly wheel key, the air intake, compression is good and valves are in spec. Fresh gas and the oil is clean. It was over filled so I drained some off. As so often what is needs to happen is the jet is too lean on the aftermarket carb. I estimated it was .020 with my micro drills and I opnend to about .024 which is a good amount but it was the closest drill in that range I had. It started on one pull and idled fine. As I push it, again it begins to surge and can stall if I load it up. I let it run a bit and then pulled the plug. It looked a bit rich. That does not bother me. The Head gasket seems ok. I ran it with out the shroud for a bit and sprayed brake fluid around the head gasket and no change in idle. The compression seems fine per the bounce test. I am wondering if there is some other small part missing. I have not worked a lot on these engines as I usually work on older stuff. I looked at the B&S exploded view diagram and it seems all is there. What am I missing?

Incorrect numbers will not pull up Manuals, try looking again & include Code # xxxxxxx.
Above kit after Code Date 15030800
Above kit before code date 15030900

Sorry. 104M02-0131-F1
ser. no.: 16 1223 58 60988
DOM 12-12-2015

nbpt100 said:

I bought a new after market carb on ebay for about $12

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OEM Briggs is your best bet.

nbpt100 said:

The compression seems fine per the bounce test

Click to expand...

Please explain this? I'm interested. What is the bounce test? And who named it this? LOL

Start with a SPOTLESS fuel tank.
Fuel tank cap that VENTS proper.
Valve clearance in factory specs.
Boil clean the OEM carb.
Engine block cooling fins clean as you can get them.
Clean spark plug and reinstall.
Remove the ignition coil. Polish the mounting areas for a good coil ground.
Cold beer is your friend.

I’m betting that at .024” your surging is the result of a rich mixture. The largest I ever go is #77 drill or .018”.

Fill the hole with JB Weld. Redrill at your size.

Check the manifold
The plastic ones are prone to cracking on the cylinder side .

I solved the problem by taking out the needle from the aftermarket Chinese carb and putting it in the original Briggs Carb. It runs great now. The Chinease carb is not the correct carb for this application. The Jet tower looks different.

The bounce test is this: Lay the machine on its side. Remove the spark plug wire and spin the blade. If it firmly bounces back the compression is good enough. Not a great test, but it give a ball park estimate. If you have a compression tester you can manually spin it backward and see what kind of reading you get. By spinning it back ward you are by passing the de-compressing release. Learned that one from Steve's Small Engine Saloon.

My lesson is do not blindly trust the Aftermarket carbs. In particular the newer plastic Briggs carbs. I've Never had a problem with older aftermarket carbs.

Thanks for all of the comments and ideas.

when you lay it on the side with oil in it you fill the cylinder with oil, and you may drain gas into the oil too. Put a compression tester on it next time.

Always tip a mower carb side up and always a good idea to turn off the fuel shut off valve if present. If a shut off is not present, pinch off the gas line. There are tools specific to this but i use a small vice grips with rubber pads on the jaws to protect the line. It is Always an option to run the gas low.

Tipping a push mower on it’s side will not fill the cylinder with oil unless you let it that wat for an extended period of time. Second very little fuel will leak into the cylinder.

I teach my customers to tip their mowers handlebars to the ground .
Now the arrangements might be different down here but just about every mower sold here will stand vertical if the handle bars are tripped forward and in many cases the catcher flap will go back to stabilize it
That way the customer does not have to remember which way they should tip or confuse left standing behind with left standing in front .
I have had mowers sitting like that for months while I wait for B & S to actually get common service parts without any problems