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

Seem most likely a fuel issue. Despite the one new carb, I would try another in a different brand.
Over nearly 60 years of repairing small engines, a new or skillfully cleaned carb is my go to solution for engines that only run on choke.
Don't be afraid of $25 carbs on Amazon. Many folks disparage them, but often they are the quickest and easiest way.
Had a tough case recently and we knew it was carb or spark. It would run a few secs maybe one attempt out of 25. New carb and she started on first pull.
 
Put the old carb back on and still won't run. Took a closer look at the ignition module and found several wires melted. Also noticed the starter is not cranking right away anymore unless I really twist the key. Wondering if the defective voltage regulator caused some damage to the switch. I cut out the damaged wires and recapped all the wires with fresh insulated spade connectors but still no change. Planning to put in a new switch tomorrow and change out the fuel lines. Seeing some dry rot cracking on the upper fuel line leading to the carb, but no leaks. Figured it'd be better to replace it now before it starts leaking. Confirmed it still starts on starter fluid. Anything else I should be checking?
 
Thanks for the update.
Replace fuel lines is always good if any doubt. When you say won't run, are you saying it starts, then quits? That's where we were after changing the regulator. When you were testing fuel flow, how long did you flow for? When you change fuel lines, drain the tank into a container, so it drains for a longer time. Crud in the tank can flow, then clog. You've replaced the fuel, where does the mower live? Could it be getting rained on?
 
I drained the tank earlier in the diagnostic process. Drained about 5gal out then refilled with new gas. And has a new fuel filter. I'll drain a significant amount again after changing the lines. Currently, it'll crank and turn over briefly on gas. It'll turn over every time on starter fluid. As I said in the last post, the key ignition is getting a bit finicky which is why I'm going to replace that now that all the charred wires have been cleaned up. In this whole process, I've identified one weak/dead coil, a couple charred wires at the ignition switch and the malfunctioning voltage regulator. Wondering if these are all related?
 
Good question. For the voltage to pull down as much as it was doing, something had to be sucking current, which can melt insulation, burn wires, and more. Check the throttle cables etc while you're doing fuel lines too. Check the choke cables, make sure they can move freely. Choke full on can kill the motor too.
 
Yes, sorry 6 mil (0.006"). After checking everything and going through the whole electrical system I'm left with just the electronic module. Anyone know how to test this? Wonder if the bad voltage regulator damaged the electronic module, which apparently may play a part in the fuel mixture. I'm much better with mechanical problems. Electrical issues are not my strength.
 
kawamoto26 said:
Yes, sorry 6 mil (0.006"). After checking everything and going through the whole electrical system I'm left with just the electronic module. Anyone know how to test this? Wonder if the bad voltage regulator damaged the electronic module, which apparently may play a part in the fuel mixture. I'm much better with mechanical problems. Electrical issues are not my strength.
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By electronic module you mean SAM unit (spark advance module), there is no testing these. Look up the symptoms online of a faulty SAM unit. I replaced mine on my Scag this year after 20 years of use. Bought a used one on EBay for $75 versus $575 new. Be sure you want it because typically no returns on electrical parts.
 
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