Husqvarna lc121p with B&S 725EXi engine low power.

W

WaltDisney

Member
Helping someone with their mower. Wouldn't run after a winter due to restricted main jet in carburetor. Cleaned that out, replaced the old dirty air filter and fouled spark plug, oil was where it needed to be and was in good condition. Now upon running it, it's super low on power. I've cleaned areas around the govner arm/shaft and spring, nothing. The flywheel brake is retracting all the way so there's no pressure on the flywheel, compression was reading 95psi on multiple compression tests.

However, in my investigations I had the air filter housing/assembly removed to make sure the thermostatic autochoke and govner were working as intended - which they were, and I noticed the engine had power back, so I went to reassemble the only missing parts, the air filter group, and then the power dropped back again. Continuing further, I would place just the air filter back plate over the carburetor intake side and then RPMs would instantly drop. Upon removing the plate, RPMs would pick right back up. The filter housing plate is clean and the tube the air goes in from is unobstructed. This tells me the engine is running rich. Fuel delivery is totally fine, there are no mixture adjusting screws on this carburetor either, the govner is working as intended as stated above. The air filter plate can't possibly be my problem, I've searched online and haven't seen anything describing what my exact air filter plate issue is causing. I am stumped.
 
Did you change the oil, or just look at the level and top off. Could be a case where this is gas in the oil and by installing the air cleaner base the engine is pulling in gas fumes from the breather hose connected to the back of the air cleaner cover. Or something else isn't lining up like the atmospheric vent hole that allows air into the carb bowl. If that hole is getting obstructed will cause a siphoning effect from the fuel bowl.
 
I didn't do anything with the oil, I checked it's level and it was topped off and in good condition. I do understand and follow your reasoning for why it would run rich with the breather tube attached to the plate. I did some more quick investigating before work and I think that your suggestion of the plate is blocking off the carberetor's fuel bowl vent is the issue. When I place the plate in alignment over the carburetor, the plate totally covers up the vent hole's intake side. I will drill a hole through that area in the air filter plate when I get back home and update the thread with my findings. Thank you for the help.
 
I agree with above. You did so much but you didn’t change the oil? It couldn’t hurt to go and change it now.

It seems you have narrowed down a possible linkage issue? Is something retarding the throttle?
 
It also can be the intake o-ring at carburetor are not seated since the op had the carb off. I know when I remove these carbs sometime the retainer and o-ring becomes dislodged.
 
andrewslarge said:
Is something retarding the throttle?
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I don't think so, when I had the A.F. off, I was watching the govner move the throttle and shaft like normal, I even cleaned the dirt and gunk off of where the govner arm comes out of the C.C. just to be sure it was clear of obstructions and lubricated it. The throttle linkage arm didn't appear to be bent either, nor did the choke linkage.

StarTech said:
intake o-ring at carburetor
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Are you referring to the ring on the outside air side? Or the engine intake side? Both of those o-rings are accounted for, I'll double check that they're completely seated when I get home. I'll also go for an oil change on it too.
 
Update: Drilling holes in the air filter plate and reinstalling it didn't fix the low power, I drilled out a big hole to open up the fuel bowl vent passageway and the mower is still bogging down. Yet running it without that plate the mower works totally fine. Furthermore, that same plate didn't even have an outlet hole for the C.C. breather tube to vent back into the air filter housing.
 
Further update: I took a deeper look and found that the flywheel key wasn't installed correctly, it was on the incorrect edge and was damaged, so I replaced it. That sorta fixed the problem with the rewind starter jerking back in your hand during starting, it will still occasionally do that. I would like to think that replacing the flywheel key and retiming the engine would have fixed the power loss, but it still hasn't and running without the airfilter plate is still giving the most power. I'm honestly at a loss for what is causing this mower to act like this. I checked the valves and they're both in place and not loose. I am at wits end.
 
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