Scag mower suddenly will only run at 40% throttle and 60% choke

kawamoto26 said:
New update: confirmed spark plugs are working by taking them out and watching for spark on both plugs. Both are firing just fine. Next confirmed 12V being delivered to the fuel pump. After a suggestion from my father-in-law, I checked the voltage across the battery and found it at 13.5v before starting. After starting the mower and running on 80% choke the voltage across the battery was 5-7V. Does this mean I have a bad voltage regulator? Could that mean I'm not getting 12V to the fuel solenoid? The wire on the solenoid is difficult to get to without removing the entire carburetor. I've also gone through batteries every 1-2 years, which would suggest the voltage regulator may have been frying them. Thoughts?
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Average life of a mower battery without extra trickle charging is about 3 years. If your stator or regulator/rectifier is bad, you would never get 1 to 2 years out of a battery. Still think it is fuel flow or carburetor issue. Fuel pumps are almost almost pulse pumps on mowers, and choking it is fuel/air issue, not electric issue.
 
kawamoto26 said:
I used an inline spark plug tester to confirm the coils were working and the plugs are new
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How large of a gap did the spark jump in LOW light conditions? New plug doesn't mean it works. New doesn't mean new these days. Recommend non resistor plugs for small engines. Coils are normally so weak, barely enough spark to run compared to car/truck ignitions for sure.
kawamoto26 said:
The symptoms have continually pointed to a fuel issue which is why I removed and cleaned the old carb twice (it was immaculate to begin with) and then replaced with a new aftermarket carb and tested the fuel solenoid on the bench top.
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If the engine dies it wants more fuel, spark, air or compression. Spray some 2 stroke mixed fuel into the carbs throat. Keep doing it to make it run for say 2-5 seconds. If it will stay running with continual squirts then she wants more fuel.

Spraying fuel into the carb and she runs a bit then dies confirms two things. One is you have good spark. Second is she wants more fuel.

kawamoto26 said:
I removed and cleaned the old carb twice (it was immaculate to begin with)
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Did you remove the emulsion tube? Guessing not. Did you check float height? Guessing not. Most people spray brake cleaner in the bowl area and call it good. Sometimes it takes 2-6 cleanings to make one work. Did you clean the emulsion tube? Most likely it has a clogged pilot circuit.

When I see people like you firing the parts canon at it, everything new on it except the driver, tells me something got missed and new parts are not new anymore. You need to TEST what you have BEFORE getting the credit card out buying more UNKNOWN parts to make it run. All new parts WILL REQUIRE TESTING to verify they work. All OLD PARTS the same.
 
I did check and clean the emulsion tube on the old carburetor. I dismantled the entire thing and cleaned every single component. None of them had any visible residue at all. I've been using ethanol-free gas with Seafoam for the last 4 years and it has kept the carbs on all my small engines virtually spotless. After thorough cleaning and reassembly twice and it still didn't run, that's when I bought a new aftermarket carb, thinking I had missed something. New one and spotless old one behave exactly the same.

Fuel pump is electric (pulse style) which is why I checked that I was getting 12v supplied to it. However, I ran out of time/daylight last night to check the voltage to the pump once the engine is running. It was the voltage across the battery that dropped from 13.5V down to 6V once engine was running that has me concerned. Then I drained the gas tank and when I run the pump (w/o engine running) the fuel flow is strong so I don't suspect there is any substantial debris blocking the line. And, I did change out the clear in-line fuel filter at the very beginning when I was getting this "fuel starved" behavior.
 
But the pump works well, fuel flows well with engine not running, then voltage down with motor running. But something is drawing it down...

Check voltage with pump running, then with engine running.
 
Something is not making sense here. What is the pump amp draw engine on and off? Getting crazy values engine on. DC amp clamp time.
 
So, switched out the voltage regulator and now I'm getting a consistent 12-13V on the battery with engine off or on. No longer seeing the drop when the engine is running. Next, confirmed the 12V at the fuel pump, and now the flow does not decrease when the engine is cranking. So the new voltage regulator has at least fixed those weird symptoms. However, the engine still won't run longer than 5-20 seconds.

Tonight, I was able to get the multimeter on the wire to the fuel solenoid and confirmed 12V to the solenoid when the key is on. Wiggles the connection in all directions and no loss of voltage. I can also hear the solenoid activating when the key is turned on.

I also ran a smoke machine into the air intake and did not see any visible air leaks at any of the carb gaskets or anywhere in the intake manifold.

I pulled the relay connected to the ignition. Relay seems to be in good condition. 71 ohms on the 85/86 resistor and the 30/87 switch activates just fine. Just for grins and giggles, I checked the valve clearances and fine tuned them to 6 mil. Still won't run.

So, the engine starts under full choke and will run for a few seconds. Also starts on starter fluid. Acts like there's either not enough fuel or too much air. Yet, fuel is good. Flow is strong to the carb. Solenoid is working properly. Carb is new and clean. No air leaks around the carb or intake manifold. Compression is good, valves are good, coils are new and plugs are firing. What am I missing?
 
Good news, glad the regulator fixed the voltage drop problem. Try the original carb. It really sounds like a fuel problem at this point.
 
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