Oh look! It's another broken small gas engine. This one is special, well, sort of. It's a Briggs and Stratton Platinum Engine. It has overhead valves, which is still kinda new to me for an American made engine. Briggs and Stratton held on to the flat head design in the small lawn mower market right up until the turn of the Century, 1997. So, lets Rip into it...
Usually when I start debugging a small engine I want to try to start it. Then I can evaluate things like the ignition, and fuel system condition. On this one the Start Handle was missing. So, I guess we are going to trouble shoot the Starting System first. I removed the engine faring, exposing the Recoil Starter. Then I removed a few more screws, and the Recoil Starter is removed from the engine.
I disassembled the Recoil Starter assembly, and found damage. There is a keeper that holds the end of the recoil spring which is damaged, and it not holding the spring. This means the starter is not working, in multiple ways. The Pull Cord has been cut, the Pull Handle is missing, and the recoil spring keeper is damaged. This is an indication of long use. I was stuck here for a time looking for a part. But then realized that I have another mower with a very similar engine, and I could borrow the starter off the good working mower to test the broken mower. How fortuitous...
After swapping the Recoil Starter Assembly from the good working mower to the broken mower I was able to do a little more troubleshooting. The engine cranks, and I am reasonably sure that it is getting compression, and spark. When you pull the cord you can feel the compression stroke of the motor because the cord gets a little harder to pull periodically. You can hear the carburetor "whine" on the intake stroke, and you can hear the exhaust exhale on the exhaust stroke. This tells me that the rings, and bore are generally doing their job by making compression, and that the valves are sealing. I tested the spark by spraying some Carburetor Cleaner in the intake of the Carburetor, and cranking the engine. This resulted in the engine briefly running, and then shutting off.
With the testing I have done so far it looks like my Briggs and Stratton small engine has a fuel system problem. The fuel is not being metered properly into the engine, and the engine will not run because of this. The engine is not getting enough fuel to run. This mean we have to dis-assemble the carburetor, and see if we can find a problem. First there are two linkages we must disconnect from the carburetor.
Modern Mowers have automatic features that make the operation of the mower easier. In the picture above is a feature that engages the choke on the carburetor when the mower is cold. It is a thermal actuator connected to the muffler. Its function is to push the choke open when the mower warms up. For what we are doing here we need to remove this linkage from the carburetor so that we can remove the carburetor. So we need to remove three screws from the muffler heat shield, and two screws from the muffler, and then we can disconnect the linkage.
While you have these exterior pieces removed from the engine you can clean those spaces you would not normally have access to. Maintenance is largely about cleaning stuff, like the air filter, the fuel filter, and the little bits that get grimy...
The Air Cleaner Assembly holds the carburetor in place in conjunction with the bracket you see around the carburetor. The flow connection of the carburetor is a tube, with an O-Ring. I am beginning to like the simplicity here. Also, it is an all plastic carburetor, and is the first plastic carburetor I have worked on. It's kinda weird, but also makes sense to my engineering brain.
OK, now that the carburetor is detached we can get on with the task of determining our fuel system problem. At this point I have to note that I have not found a discrete fuel filter. Usually with lawn equipment you need a fuel filter because of the excessive amount of debris associated with lawn service. This mower does have a large, pleated air filter, but not a fuel filter. Maybe there is an inlet screen on the carburetor fuel inlet.
Small engines used for lawn mowers run at a single speed, and the carburetor can be simplified because of this. This carburetor is very basic. I think it still has idle, and run circuits, but I have to admit it is still a little mysterious to me. There are two screws to remove the float bowl, and then inside I found what appears to be a Jet Module which contains the metering jets for the idle, and run circuits.
There are three tubes on the Jet Module, and I think that two of them are the idle, and run circuits. Then I am assuming that the third is a vent, but I am not sure at this point. So, I am going to clean the Jet Module, and the Carburetor in general, and then re-assemble everything, and give it a Krank...
All the bits, and pieces go back together in reverse order, just like in the old car manuals, LOL! Easy right? No, really this is a very simple engine, and I think if we can get the Gas to the Bang Box itz gonna Krank...
I want to note some other features that I found on this fuel system. It has a Positive Crankcase Ventilation tube that runs from the crankcase to the air cleaner. There is a Gas Tank Vent that runs from the top of the gas tank to the air cleaner. Compared to other small engines I have worked on, this one has an oversized muffler. I wear headphones when mowing, so having less external noise is good.
I also want to note some things that are not on this engine. There is no Primer Bulb, which was once used to circulate fresh gas into the carburetor. Also this engine does not have a user accessible throttle adjustment. Old mowers had a throttle, and I guess these newer mowers use the governor to regulate engine speed. New mowers are more powerful, and less troublesome, I guess...
Well, looka there... I inspected the Jet Module carefully before I cleaned it, and it looks like the actual problem was there. Two of the jets looked open, but the one in the middle seemed clogged. After I sprayed with with the carburetor cleaner the "jet" in the middle opened up, and I got that warm, fuzzy feeling of success. The two jets on the ends have metal inserts, like they are used to precisely meter fuel, while the one in the middle was just plastic, and kinda seems to be a vent tube. I have to look into Briggs and Stratton Plastic Carburetors a little more. OK, Happy Mowing...
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