If any small engine gurus want to help, Kawasaki FH721V bogging when engaging blades on a hot engine
Posted: 5/30/2026 9:15:48 PM EDT
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Good evening, if anyone has some advise on this I'd love to hear it. It is a Ferris zero-turn mower that sat in a non-insulated barn w/ only 25 hours on it from roughly 2003-2023 before I got it. I knew going into it that it would be possible for the mower to have issues being what its history was, but I rolled the dice. I paid very little for it but have since had it to a Ferris mechanic twice that never resulted in much success despite having sunk a couple thousand dollars into at this point. Both times I got it back from them it ran great for a couple mows and then went back to not starting again. Said mechanic had a list of things they said they did, but it became fairly clear that I may be getting the runaround from them. My father in law is a farmer and has worked on engines his whole life, he offered to take a look at it as a project since he is semi-retired and harvest was over for the year (this was winter 2024). He did a complete overhaul on the thing and replaced a lot of parts and apparently had a good time doing it. The man is incredibly thorough, I can't even imagine how many hours he had to have spent on it and he only wanted reimbursed for parts which is an incredible deal. He brought it back and this thing ran like an absolute raped ape for a solid year. I ran it once every two weeks even throughout winter because I was so happy to have this thing back in running shape. He sent me a pic of the gas he pulled out of the tanks compared to normal gasoline. The original mechanic said he drained all the old gas and replaced the fuel lines, but it appeared that was not the case. The insides of the engine were basically covered with varnish. Fast-forward to a couple months ago and it started bogging down again when blades are engaged and will kill the engine. I did some basic troubleshooting via youtube and all signs pointed to left side coil being dead. I replaced the coil with an Amazon coil (yeah, I know) that had great reviews, timed it based on available instructions, and torqued everything back to spec based on Kawasaki's manual. Success... it was back to running like normal again for a couple weeks before it started bogging down again when the blades were engaged (but only after it had been running for a while). Add to this, the engine started surging up and down when just sitting there running full-throttle. I just ran it for a ~30 min w/ some seafoam in the tank after hitting the air intake w/ some carb cleaner. The surging seems to have abated but it still bogs down when engaging the blades. I'm not sure where to go from here. I did read that some cheaper coils have heat issues which MAY explain it? TL/DR summary: *Ferris zero-turn w/ Kawasaki FH721V with 25 hours on it that sat unused for ~20 years in a non-insulated barn (midwest, wild temp swings) *Problem is engine bogging down when engaging the blades (accompanied by belching black smoke out of the exhaust). When I first engage the blades on a relatively cold engine it does fine, but after running a while and doing the same it bogs down). Engine was mildly surging but it seems some seafoam and carb cleaner has taken care of that for now. *Left side ignition coil was recently replaced by me with Amazon coil since the left side had failed *When I take the spark plug boot off of either side while running, the engine loses a little power but doesn't shut off (this is in contrast to what it was doing when the left coil had failed) If anyone has any ideas, PLEASE feel free to share. I am not an engine mechanic by any stretch of the imagination. Attached File |
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Check the choke. Manual or automatic choke? Black smoke is a sure sign of excess fuel. A non working choke (excess fuel and not enough air) will cause your symptoms. Dirty carbs usually run lean. But, I can’t rule out a dirty carb causing your issues. If choke is good, an intermittent coil could also cause those symptoms. Coils are usually work/no work but occasionally they can be intermittent. I’d use OE coils whenever possible. Yes they are more expensive. |
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lack smoke , its getting too much fuel. or not enough air. Mud daubers get into every thing here. Try it with air cleaner off, then have a look at the carb float. If it has gas in the float ti will flood like that. |
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| Carb has more crud in it that was pulled thru the system. After that many years of sitting it's absolutely certain there is varnish and crap everywhere in the tank, any old lines and on any surface gas has sat against. Does it have a GOOD in line fuel filter on it? If one wasn't installed, it needs one and not some little thing, the biggest one you can get that will fit. The surging is almost always lean and jetting related due to it being constricted. |
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I'm getting down to the last box, the others have all been destroyed...
I'm getting down to the last box, the others have all been destroyed...
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Verify both cylinders are getting spark. Pull one side spark plug boot and try to run it. If no change that side isnt doing anything. If it wont start that side is doing the heavy lifting. See whats wrong with the other side. Another way to check is spray water from a squirt bottle on the exhaust headers on the running engine. Steam= working fine, water sitting on the header = cold not producing power. |
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Black smoke, sounds like a crudy carb issue, make it run rich, that can cause some surging, also fouls plugs. Open carb, clean, make sure float isn't sticking or pin holed & clean or replace plugs. Then fire it up. Before you do above. you said it bogs when blades engaged & eventualy lugs enough to kill engine, check the pulleys and jack shafts see if they need to be greased or if they're seizing a bit. |
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Can't be an engine problem Kindasucky is = to our better than honda according to the internet. My experience based on actually working on small engines for the last 20 years or so. Fuel system issue. Ignition system issue. There isn't that much to either system and if you've spent more that a couple hundred bucks you are definitely being jerked around by incompetence. |
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Good suggestions so far. I'd add that the engine probably has a governor on it that may be out of adjustment or it has a spring that may have gotten weak/broken. The other counter-intuitive aspect of bogging at clutch engagement is that many OEMs recommend engaging and disengaging the clutch under conditions exactly the opposite of what most people think - you engage at full throttle, disengage at idle, instead of the reverse. Gravely/Ariens puts it right in the manual. The larger lawn service guys all do it that way. Are you engaging at low throttle settings? |
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Surging and bogging down, almost always starving for fuel. Your engine wants 12:1 to 13:1 fuel air ratio to be happy. I have ethanol fuel out here and over the winter all my gasoline engines eat fuel filters and damaged diaphragms in fuel pumps from the ethanol in fuel. Also, ethanol attracts water into the fuel, again contaminants. Never ending maintenance. Welcome to 2026 and environmentally friendly gasoline.
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Originally Posted By Shadyman: Surging and bogging down, almost always starving for fuel. Your engine wants 12:1 to 13:1 fuel air ratio to be happy. I have ethanol fuel out here and over the winter all my gasoline engines eat fuel filters and damaged diaphragms in fuel pumps from the ethanol in fuel. Also, ethanol attracts water into the fuel, again contaminants. Never ending maintenance. Welcome to 2026 and environmentally friendly gasoline. |
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Replace the carb with a new OEM. Replace the fuel lines, filter, and anything that flows fuel. Drain 100% of existing fuel and replace with fresh from the pump. Replace spark plug(s) and verify spark with a tester. It sounds like fuel varnished or rusted from water. This looks like a fuel supply issue and it’s repeating because there’s a buildup somewhere. |
| Both my mowers with Kawasaki engines started bogging down just like you described. I replaced the carb on one and just cleaned the carb on the other and the problems went away. |
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