Older 12HP Briggs cut out problems... a puzzler.

So I just repowered a Craftsman rider with an older 12 hp Briggs that came off of an old mower with no deck. Wiring on the older mower was "inspired" with starter button to bypass starter switch and to kill the motor, the old fuel cutoff valve. No problem, as it seemed to run fine when I got it last year. With all the weird wiring, and as the newer rider had a different wiring harness (several generations between the two), I just bypassed the kill wire (unplugged) , cleaned and serviced the flojet carb, new oil, fuel filter, replaced the gas, checked for fuel flow (including fuel cap) and gave it a go. Here's what happens:

I can start the mower with my hand as a choke (possibly need to replace the choke plate as it is a bit pitted) and it will run for 30 seconds to 2 minutes and then dies. I've removed and recleaned the carb, made sure presets are correct on all the needle valves, made sure float is set correctly and not leaking etc.

When the motor starts to die, I can keep it running for a bit by choking it again with my hand to restrict flow, but then it will die again.

So: It doesn't appear to be electrical, fuel flow seems fine and when it runs long enough to adjust the carb, it runs great...but then it dies again for no apparent reason. It is however hard to start with just the normal choke.

Any thoughts?

Check air filter for replacement, engine needs to breathe.

No air filter through any of this. Open carb. It actually runs better with restricted flow from using my hand to modulate flow as a improvised auto choke.

As I recall that carburetor has a intake manifold that's a elbow that fastens to the engine block. Just make sure you don't have any vacuum leaks at that point. Then also you might want to go through a good carburetor cleaning because it sounds like a fuel problem and maybe something plugged in the carb.

Thanks for all the quick responses!

Carb is spotless as it's been cleaned three or four times through this process. There is also a new gasket on the intake that is seated well.

My next step is to pull the head to check for a sticky exhaust valve as it seems to have a little back pressure when it starts to quit. I'll update the thread when I get that done...

RSass said:

Thanks for all the quick responses!

Carb is spotless as it's been cleaned three or four times through this process. There is also a new gasket on the intake that is seated well.

My next step is to pull the head to check for a sticky exhaust valve as it seems to have a little back pressure when it starts to quit. I'll update the thread when I get that done...

Click to expand...

:welcome:

Cleaned how ?
Fully dissasembled and all perts ultrasonically cleaned in carb cleaner ?

Checking the valves won't hurt but unless you have a sticky deposit it will also not create you symptoms,
You are not getting enough fuel into the engine for the amount of air.
O you have an air leak or a fuel starvation problem.
As it runs fine for a few minutes I would go for a hanging float needle.
Is is a new needle ?
did you put a kit through the carb ?
How is the throttle butterfly pivots ? flogged out oval and sucking air ?

Cleaned how ?
Fully dissasembled and all perts ultrasonically cleaned in carb cleaner ?

Not quite. Soaked in carb cleaner and blown out with compressed air. Ultrasonic cleaning...interesting thought[/I]

Checking the valves won't hurt but unless you have a sticky deposit it will also not create you symptoms,
You are not getting enough fuel into the engine for the amount of air.

I may have had a small leak in the gasket and cleaning off the carbon didn't hurt anything. Valves looked good. New gasket tomorrow.

O you have an air leak or a fuel starvation problem.
As it runs fine for a few minutes I would go for a hanging float needle.
Is is a new needle ?

Yes, All new Briggs parts and special attention to cleaning the internals...may do it one more time though.

did you put a kit through the carb ?

I'm not sure what this means? It was the Briggs carb kit. I didn't replace the Welch plug as it seems to be (no new float).

How is the throttle butterfly pivots ? flogged out oval and sucking air ?

They were, but I attempted to tighten them up. I will pick up that kit tomorrow when I get the new gasket. As I didn't have that part at my local store, I wanted to see if I could do some backyard persuading on the flanges. They tightened up and I thought would seal enough...possibly not... Would some rust on the choke butterfly cause these symptoms?

Thanks

Choke does not matter as it is before the fuel venturi.
Worst it will do is allow dust into the engine and not open/close fully.
If you post the model and type numbers some of thoes in the trade might do a double check that you got the right rebuild kit.
In the mean time slip the float bowl off, put a container under the carb, turn the fuel on and work the float up & down to check that it both lets in fuel & cuts off the fuel supply when in the level position.


Now we get to silly things.
Is the fuel tank and lines clean ?
Do you have the correct fuel filter ? The gravity feed filters are different to the pump filters and fitting a too fine filter can slow down the fuel just enough to starve the engine.
does it have a fuel pump ?
did the original engine have a fuel pump?

bertsmobile1 said:

Choke does not matter as it is before the fuel venturi.
Worst it will do is allow dust into the engine and not open/close fully.
If you post the model and type numbers some of thoes in the trade might do a double check that you got the right rebuild kit.
In the mean time slip the float bowl off, put a container under the carb, turn the fuel on and work the float up & down to check that it both lets in fuel & cuts off the fuel supply when in the level position.


Now we get to silly things.
Is the fuel tank and lines clean ?
Do you have the correct fuel filter ? The gravity feed filters are different to the pump filters and fitting a too fine filter can slow down the fuel just enough to starve the engine.
does it have a fuel pump ?
did the original engine have a fuel pump?

Click to expand...

Thanks for this. I did have the correct rebuild kit. I don't know if the fuel filter is correct for the 12 hp. It may not be gravity feed any longer based on the tank location and I will do some checking on the fuel pump issue. The 12 hp engine replaced a 21 hp briggs that had broken connecting rods. I've yet to crack that (21 hp) engine open to see if it could be salvageable, but that's another day. I would imagine it had a fuel pump as the tank is under the seat on the new mower, but I will have to check.

Model number is 281707 0200 01

As for the tank lines, I did clear them when I replaced the fuel and the tank looked spotless. It's not a big deal to change out the fuel lines as I have extra hose, and when I do I'll try connecting it directly to the carb to test if there is a restriction in the shut off valve or new filter.

My thinking is that I have too much air leaking in around the throttle shaft and that is leaning out the air fuel mix. There doesn't appear to be any foam washers on it, it may have perished and I'll pick up a kit today that may fix it.

Is the gas tank mounted higher then the carburetor? If it isn't you will need a fuel pump.