Engine bogging down

[mark1090]

Hi All, I need some help in diagnosing a problem. My Z121,48", zero turn has a problem with the engine being pulled down in rpm as I mow. With out the PTO engaged the Kohler 21hp engine runs great but when I engage the PTO to mow the engine rpm fluctuates, It seems the mower is pulling the rpm down. I did find a idler pulley with a dry bearing and replaced it but it did not fix the problem. All the spindles are quiet and are smooth, as well as the other idler pullies. Has anyone had a similar problem, could you point me in a direction to fix this problem. Does anyone have an idea where to look or trouble shoot this problem.

 

[h1ck0ry.b]

Since the engine runs fine with the PTO disengaged, it's most likely an issue with the mower deck drive system, particularly the PTO clutch. When it's failing or worn, it can put an excessive load on the engine when it's engaged, causing the RPMs to fluctuate. The clutch itself could be the source of the resistance. Other than that, though less common, it can also be an issue with a spindle bearing that only shows up under the load of the moving deck belt. Even if a spindle seems smooth when you turn it by hand, it could be binding once the engine is trying to spin it at high speed.

 

[graceanderik79]

My neighbor and I both run Kubota Z121s, mine's a 2019 model with the same 48" deck and Kohler 21hp. He had the same issue last season where the engine was bogging when the PTO kicked in. Turned out one cylinder wasn't firing under load due to a loose intake rocker arm. Try checking spark and valve clearance. Also, double-check fuel flow too, his filter was clean but the pump was weak. Hope that helps!