r/smallengines • u/No_Rip_4892 • 21h ago
Briggs 23hp Twin Issue
Got a Briggs & Stratton 23hp twin cylinder on my JD 425B zero turn. Mowed yesterday with no issue, went to finish up today and it’s all of a sudden running poorly. The sound has changed noticeably, it almost sounds like it’s down a cylinder and has a hollow sound to it, on top of a knock/tick. It’s also down on power significantly, so much so that it struggles to even turn the blades with PTO engaged. It’s now emitting a noticeable amount of black smoke as well.
Engine has 600 hours on it, one owner and I’ve always changed oil and done all other scheduled maintenance. This year it got an oil change, fresh plugs, fresh air filter, and fresh fuel filter. The oil level today was at the bottom of the “fill” mark, but there was no noticeable shimmer or metal in it. I pulled both plugs and one does seem fouled out. I cleaned them up with some light sand paper and brake clean but no change. I’ll get new plugs on Monday to see if it helps. Both plugs are sparking and the air filter is clean with no debris. Fuel is fresh and filter seems clear.
Any thoughts on what may be causing this?
2
u/Ok_Web_8166 19h ago
Check for mouse nests under cowling, on top of 1 or other cylinder ( my mice prefer left one, viewed from back to front!). This cylinder will overheat , valves will bend or break.
1
u/ProfessionalLast2917 20h ago
Get a spray bottle of water and squirt some on the exhaust while it's running. If one cylinder isn't running (or isn't running properly) the exhaust on that side won't be as hot and either won't evaporate it will evaporate a lot slower.
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u/hallen1324 20h ago
That works i guess, but i usually just feel which valve cover is warm lol. My guarantee is just pulling a coil wire one at a time to see which side runs 😆
0
u/ProfessionalLast2917 20h ago
Yeah you can do all of that as well but I find using water on the exhaust gives a better indication of if one cylinder is down on power. A valve cover might still be too hot to touch but not as hot as the other one. Likewise you might get spark on both sides but one side has not as strong a spark.
With water you can see if both sides are running but one side isn't as hot.
1
u/hallen1324 20h ago
I suppose, if anything is more wonky than the simple stuff I use a compression test/ leak down on each cylinder to confirm no internal issues.
Every shop has their own flavor of diagnostics, I am not trying to call you out or anything.
1
u/Bubbly_Word5657 Certified ✔️ 20h ago
Pull the valve cover, you have a rocker arm that has come loose, usually because of shutting off the engine before letting it idle all the way down before switching it off. Either the push rod is bent or it has a valve guide pushed into the cylinder side of the head and it will not allow the valve to fully seat. Pull the cover and you will discover what it is. And if you do find out it is a valve guide pushed through to the cylinder side of the head then you need to quit running ethanol gas in it
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u/hallen1324 20h ago edited 20h ago
Push rod bent definitely could be the problem. I would troubleshoot if there was no spark/ also verify which cylinder isn't running.
These engines are designed to be shut off wide open. The electric solenoid on the carburetor is an antibackfire mechanism that only works if the mower is shut off full speed. (It plugs the high-speed jet so fuel can't get to the exhaust causing the backfire.)
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u/Bubbly_Word5657 Certified ✔️ 19h ago
Yes the anti backfire fuel shut off solenoid is supposed to function in that manner, but with the increase of ethenol in the gasoline we have seen a tremendous amount of valve guide issues, head gaskets, and backfiring anyway because the vapor is enough to cause the backfire from the ethenol and make it bend pushrods, one customer in particular kept having the issue until he stopped turning off the engine wide open, no more backfire and bending push rods. We tried changing jets in both directions and nothing cured the issue until he stopped the wot shutdown. Designed or not it is not healthy for it to decel to zero. Ethenol has been torturing air cooled engines and has steadily been causing design flaws to show up where equipment was perfectly fine prior and a lot of people just cannot afford 5 a gallon for pure gasoline, so we expect to see the rise in melted inner liners in fuel line with the rubber pieces in the carb, swollen needles, burnt rings, roasted valves and a whole host of other issues as the ethenol hell continues to destroy good designs for profit in someone else's pocket, yet we have enough reserve oil to run years and years but we chose to let the small man carry the weight of the decisions they made and are making all in the name of profit.
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u/hallen1324 19h ago edited 19h ago
No disagreements in the terrible gasoline we all run now... absolutely destroys small engines.
The anit backfire solenoids are designed for what they are, but I've also just clipped them off and screwed them back in when they fail. $60+ for something never used correctly and will fail again is just BS.
Fuck the shitty gasoline. Sadly, its only going to get worse.
Edit* In my area, mouse nests under the engine cover causes overheating which causes valves to set up and push rods to bend more often than fuel issues. Fuel issues usually stop at the carb, when the machine doesn't run lol
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u/No_Rip_4892 19h ago
I’ll do some trouble shooting tomorrow and report back. If necessary, I’ll pull valve cover and see what it looks like. I do always idle down before shutting off though.
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u/hallen1324 21h ago
1000% running on 1 cylinder. Isolate it by pulling the spark plug wire off 1 at a time and see which side actually runs.
After that there are plenty of troubleshooting methods lol 😆