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

kawamoto26

kawamoto26

Member
My old Turf Tiger with Kawasaki FD750 27hp has suddenly started having a problem starting. It starts under full choke with throttle turned up to about 30-40%. I immediately start decreasing the choke and it dies. Start it again and again and it runs for a few seconds and then dies. Eventually I can get it to run for 30-45 sec at 30% throttle and 60-80% choke. Move either the choke or the throttle and it dies immediately. Have gone through the fuel system from top to bottom:
Changed fuel filter
Cleaned carb twice and then put on a new one.
Confirmed fuel solenoid is working properly.
Replaced spark plugs.
Confirmed air filter is in good condition.
Confirmed fuel pump is pumping well.
Drained the gas and replaced with fresh.

Everything has checked out in good shape and I'm at my wits end trying to solve this while my grass is getting taller by the day. I only use ethanol free gas with Seafoam. Fuel system is spotless and now with many new parts.

Also, When I can get the engine running under 40% throttle and about 60% choke, it will die immediately if I take off the parking brake to try to move it. Not sure if that means something or if it is just too underpowered in that choked state. So, I'm currently do the diagnostics in my backyard because I can't even drive it to a trailer to take it in for service. Any help will be greatly appreciated.
 
I have no experience with your equipment, so take this as general trouble-shooting advice. You may have a vacuum leak letting too much air into the combustion chamber, hence the need for choke. You can take a propane torch (unlit) and open the valve and move it around the carb connections. If the engine perks up at some point, you have found a leak. One other thought, have you checked valve clearances?
 
Hello K.,

Welcome newbie.

It always helps us understand what machine you have by providing the chassis label and engine label information. Not all machines are built the same. This also help reduce endless run on postings with a bunch of people guessing.

My observation of your problem from a troubleshooting perspective:

Fuel lines will break apart internally an create a blockage, also debris gets caught in fuel lines. The fuel filter is great for catching small particles, but some stuff that get in the tank never makes it to the fuel filter. Over years of use, the additives in fuel also built up a sticky film inside the soft fuel lines that will trap and hold sediment.

I have had to replace many aged fuel lines for this problem. Some lines can be blown clear using shop air, but the problem will keep coming back.
 
any place that air can get into the intake system, can affect mixture. That includes blown head gaskets and cracked intake manifolds. Exhausts can get plugged too. Valves can stick open and allow air in that affects mixture. Use a infrared temp gun on the exhaust to see if both sides are firing equally or if one side is cooler. It may direct you to a weaker cylinder, which in turn may lead you to the culprit. That assumes this is a dual cylinder engine. If is it a single cylinder engine then that would not be applicable. I might do a leak down test to see if the cylinder is leaking air out the head or a valve.
 
Thanks for the fast replies. The engine is a twin cylinder Kawasaki FD750. The Scag Turf tiger model STT61A-27KA, SN 7740962. Can't find the number on the engine itself. Fuel flow to the carburetor appears strong. I disconnected the fuel line at the carb to confirm the fuel pump was pumping well.

I just did a cold compression test with no air filter and full throttle. Left cylinder was at about 105psi and right at about 110psi. Adding a tsp of 10W-40 didn't change anything. Ran the engine for 3-4 min in 5-20 sec intervals since that was all I could get out of it at a time and the compression was nearly identical.
 
that would lead me to believe you have a vacuum leak or plugged jet / needle and seat. I have seen on a rare occasion depending on the ignition wiring, a coil work in the start position but fail in the run position. The symptoms are a bit different in that the engine runs until the key is released. Vibration can also be a factor where something like an oil pressure or level cut off switch works at idle but fails with some vibration from running.
 
Update: I found that one spark plug wire was not firing. Replaced both coils and confirmed both plugs are sparking. Still no change. Engine turns over immediately under full choke and can run for about 10-20 seconds before dying. Next, took another look at the fuel pump. Not sure if this is normal behavior. When I disconnect the fuel line from the carb and turn the key, the fuel pump flows strong. However, when the engine actually turns over, the flow of fuel drops to a trickle. I replaced the fuel pump and observed the same behavior. To summarize,
New fuel
New fuel filter
New fuel pump
New carb
New ignition coils
New spark plugs
Cold compression is 110 and 115 psi
Muffler doesn't appear to be blocked in any way but I haven't removed it.
Air filter is clean.
Seat switch is working properly.


I have not examined anything related to valves. I also cannot confirm that the wire going to the fuel solenoid is actually operating properly. However the fuel solenoid itself works and there is enough fuel in the carb to start initially.

Where do I go from here?
 
the voltage a spark plug gapped to .026" fires outside the cylinder in ambient air pressure is usually between 2,500 volts and 3,500 volts. At 125 PSI, the firing voltage needed is between 18,000 volts and 20,000 volts. A spark plug sparking at room pressure does not necessarily fire in the cylinder at pressure. Keep that in mind.

It appears you are having a problem supplying fuel to the engine. You can get some Berryman's B12 or Gumout Carb cleaner and spray a little into the carb throat every so many seconds as the engine needs to keep running. If the engine will run for a minute or two doing that, then the problem is fuel delivery.
 
I used an inline spark plug tester to confirm the coils were working and the plugs are new. 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. I've replaced every single component in the fuel line and don't know where to go from here. It also dies as soon as I take the parking brake off so I can't load it on a trailer easily. I checked the seat switch for continuity and it is good.
 
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