Why does my lawn mower engine keep dying?

I am in a night class called Design for Industry. In that class we design, make, and test a mini indycar.  We are using the same car from last year, but the previous teacher didn't exactly car about the car, or so it seems. We had to fix the wiring, key ignition, solenoid and starter motor. After we got the wiring and electrical all fixed, we decided to take the car out and try to run it. It did run, but it was rough. If we wanted it to idle while just sitting it wouldn't do it,however. If I (the driver)  slowing opened the choke and gave it gas, the car would start moving and go pretty well. It would go around the track for maybe 100ft. and then it would just cut off. I tried not taking my foot off the gas, but that still didn't work(only caused me to spin out around the corner lol). So we took it inside and put it on jack stands. We took off the carb, cleaned both jets, and sprayed the whole thing with carb clean. We put the carb back on but still died. We can get it to "idle" but it is at a really high RPM. When we try to adjust the idle screw it either revs up more or dies. The previous class and teacher, choked the engine to kill it. So when we took out the plugs they were dark, or rich. We replaced them and the engine still died. NOTE: there was a centrifugal clutch on the car, but it was rusted together so we ended up taking it off. My classmates and I were thinking that the engine doesn't have enough suction, for the fuel pump, at low RPMs.     Model: 303777    /    Type: 1162-E1        /    Code: 03011511

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I had a similar troubles with a 5 horse B&S.

It needed a carburetor rebuild.

ben2000 (author)  Josehf Murchison1 year ago

Well I thought that too but then we switched the whole carburetor with another, and it did the same thing. So maybe the mixture screw is off?

If it is a B & S changing the carburetor won't work, in fact that can make the problem worse. The bleeders are different between manufacturing runs on B & S engines.

You said you sprayed carburetor cleaner on the carburetor, did you run carburetor cleaner in the fuel to make sure the internal parts of the carburetor are clean.

ben2000 (author)  Josehf Murchison1 year ago

Alright, we got it to run and idle consistently so that's a plus. But my next question is about the charging circuit. We have a 5 to 9 amp alternator. One black wire coming from the alternator, there is also a voltage reculator/ rectifier which has a yellow and red wire coming from it. The brown wire is coming from the fuel shut off solenoid. I'm am not sure where if the wire coming from the regulater is supposed to go straight to the battery or to the key switch and then to the battery. Also where should the fuel solenoid wire go.


Fuel solenoid or pump, should be off when the motor is off or it will flood the engine and drain the battery, so it should go to the ignition switch.

Is that a starter that doubles as an alternator, or just an alternator?

Both should should be shut off by the ignition switch or they can drain the battery, but the wiring is different.

That looks like the starter and the starter solenoid in the pic.

ben2000 (author)  Josehf Murchison1 year ago

So I need to place the fuel solenoid wire somewhere it gets 12 volts when the key is in the start/run position? Or is it not needed as we kill the engine by grounding the plugs? The small bolt with the 2 red wires in the right head corner is the grounding pin.

That is just the starter the alternator is under the flywheel. The solenoid is the one on the frame with all the wires going to and from it - yes. But would it be correct if I took the red lead coming from the regulator (small 1'' x 1" box under the grounding pin) and went straight to the positive battery terminal ?

The fuel should be on when the motor is running and off when the motor is not running.

The alternator regulator should be switched off with the motor so it isn't draining the battery while the car is sitting.

ben2000 (author)  Josehf Murchison12 months ago

When you say "the fuel should be on when the motor is running and off when the motor is not running." you basically mean that the fuel solenoid needs 12 volts when the engine is on and needs to be grounded when the engine is off?

You can put the fuel solenoid to the battery but it should have at least a switch to turn the solenoid off when the motor is off.

First rule of engine tuning: Never assume, always check and tune ;)
Might be a good idea to take the motor with the carby to a small engine tuning guy for the setup.
He could have a decent look at the carby, set it to manufacturers defaults and go from there.
But only too often the culprit in such projects can be far simpler.
Cinsidering everything was unused for a long time and the fuel might not have been removed from the system either:
Do a full clean/replacemt of the fuel system, this includes a NEW fuel filter and making sure all lines are still flexible.
Last thing you want is a fuel line letting air in.
Inside the carby you should find another fine filter screen - they love to goo up from old fuel and block.
A look from the outside makes them appear fine, take the screen out and you often see a wax like layer covering it all - replace or clean with acetone if possible.
Last but not least all moving and rubber/gasket parts in the carby.
I know from long games with liner trimmers and lawn movers that a cleaned carby (outside) appears to be totally fine until you compare the parts with the rebuilt kit....
Hard gaskets are sometimes fine after soaking them for a few hours in fresh fuel, rubber parts not so much.
The needle valves often use a rubber seat and when hard and malformed from being unused the "dint" causes all sorts of problems from overfueling at idle to producing big drops instead of fine vapor at high revs.
Wax build up around nozzles can do the same or even starve the carby for fuel.

Only real way to get going IMHO would be to clean everything that can be cleaned, replace the rest and to consider a rebuild of at least one of the carbies.
If with that the thing runs again give it a proper tune up and once done playing flush the carby with an alcohol sewing machine oil mix for long term storage so the fuel is out.
The alcohol will evaporate over time and leaves a fine film of oil on all vital parts.
Of course the best would be to remove all moving parts with rubber on them or to make sure they stay in an open position.

ben2000 (author)  Downunder35m1 year ago

Thank you for the response. We will probably empty out the
gas tank, clean any debris in it, and fill it with fresh gas with some Seafoam
mixed in. I found out that that the valve cover is cracked on the side with the
fuel pump. I’m not sure on how or where it cracked, but I’m sure this may be
part of the problem. I will know as of Monday night.

rickharris1 year ago

It if a 2 stroke make sure the mix is correct and that you checked the needles valves in the carby as well.
If it has a diaphragm it will need replacing due to the long time with no use - they go hard.

ben2000 (author)  Downunder35m1 year ago

It's a four stroke but I'm sure old gas would cause a problem too. Would too much gas coming through the carb, while it's at a low idle cause it to die?

Of course and it floods the cylinder, wets the park plug...

Black plugs indicate your stoichiometric air fuel ratio is way off.

Perhaps your fuel pump is missbehaving.