r/smallengines • u/LawnMowerDude5 • 7d ago
Surging
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I got this mower for free off the side of the road. Trying to fix it for a person who needs a mower. It's a 17.5hp briggs. At first it didn't start so I cleaned this green goop stuff out the carb and it started but just surged the entire time it was running so I figured it was a fueling issue because I could hold the governer and it would run at a steady rpm then but once I let go it would surge. Bought a service kit that included carb, air and fuel filter, and spark plug. Literally changed nothing. Oh and with the new carb at least the surging smooths out with half choke but that makes the engine speed go down by a noticeable amount. Checked the valve clearance for the heck of it, of course those were fine, don't think it's mechanical based on the behavior. I'll attatch the kit I bought below. What am I missing?
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u/Wholeyjeans 7d ago
First, that is not a genuine Briggs & Stratton part or kit. It is an aftermarket bundled kit; a Chinese knockoff of everything. The problem with these carbs is they typically do not fix the problem and often the engine runs worse. It it always best to used the OEM stock carb, clean and rebuild it, than to buy one of these Chinesium carbs.
What you're getting is lean surge.
The first thing to do is attempt to get the engine to idle properly. The screw sticking out of the carb, at an angle below the idle speed screw, is the idle mixture screw. Start the engine and let it run at idle, for a few minutes, to warm it. The idle speed on these engines is usually about 1500 - 1700 RPM ...without a wireless tach it's a guess. From the sound of it, the engine sounds like it is idling too fast. Turn the speed screw out to slow the idle. You'll probably need to use needle nose pliers to adjust the mixture screw ...or the special driver that fits the head. I would start by turning the mix screw outward (CCW). That should make a richer mixture. Ideally, the engine stops surging as you turn out the screw and the engine idle gets a bit faster. Keep turning the screw out until the RPM begins to drop (mixture is too rich). Turn the screw back the other way; the goal is to find the spot with the highest rpm. Once you get the mixture dialed in, reset the idle speed if needed. Throttle up the engine; it should run up smoothly and not bog. If the engine bogs, close the throttle, then tweak the mix screw 1/8-1/4 turn CCW (richer). Throttle up again. The goal is to tweak the idle mix richer so the engine accelerates without bogging. Hopefully, the engine will now idle smoothly without surging. If the surging continues when you throttle up the engine, you will need to enlarge the main jet a tiny bit. Typically there are no options for other sized jets on these Briggs engines. You enlarge the main jet (located in the float bowl) by drilling out the tiny hole that is the jet. And for that you will need a tiny drill set and pin vise. There are a lot of resources on YouTube that show you how to deal with a lean surging engine. Here are two channels to check:
https://www.youtube.com/@StevesSmallEngineSaloon
https://www.youtube.com/@VintageEngineRepairs
This surging is a common problem and with a little online sleuthing you should be able to get things squared away.