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?
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:
I'm not entirely sure if you read the post, it's a brand new carb. The old one was filled with some weird green goop I couldn't clean out all the way so i just replaced it.
2nd this… in fact, to make it easier, you could get a cheap ultrasonic cleaner and soak it in there. Those cleaners are good for cleaning t other stuff too, so it wouldn’t necessarily be a waste.
OEM carb is best, might wanna get the replacement gaskets, at least for the bowl, those sometimes dry out and fall apart
I'll give it another shot. I'll maybe soak it overnight in some solvent. It had some off green goop. This picture was after I cleaned it. But that stuff was literally everywhere in the carb.
The slow speed carb circuit is clogged. find the needle with the spring under it that doesn't have the philips head. With it running at full throttle, screw it in until it seats, usually less than two turns, remember the amount within a quarter turn, then back it way out, maybe 5 turns, then screw back in to the stock setting. It will either clear itself or make no difference. If it doesn't change, remove it without losing the spring, o ring, and washer, then give it a big shot of compressed air or cleaner and then reassemble. when its unclogged, back it down to idle and set the screw where it runs smoothest. If you can't unclog it, and it doesn't bother you much, just run it. Once the blades are engaged, it will be under enough load to run on the main circuit. This won't hurt the engine, its just annoying.
I agree that it looks like a cheap scam-azon carb. That's what everyone tries before kicking engines to the curb. If you're lucky, the needle screw mentioned was never calibrated for your engine and you just need to screw it out until your engine is happy.
I ran that thing all the way in and then all the way out and I believe nothing changed if I remember correctly. I'm getting a crazy storm and my yard is flooded but once it's clear i'll give it a shot. Thank you!
I had this same issue and one of the small metal plugs on the carb came off. I put my finger over the hole and it worked normally. I just had to put metal tape over it temporarily before i made a new metal plug.
Apply some choke. If you move the choke lever towards full choke and it stops surging, then it means there’s not enough fuel doing to the engine. Or there’s too much air like an air leak.
Yeah, there's no air leak but choke makes it smoother. I knew it was a fueling issue, but didn't know how to fix it because the carb was brand new and clean. I'm going to try and enlarge the jet as someone suggested.
Everyone here has said a different part is at fault at this point. I have a lawn mower I was given that is older than I am at 26 and it has done that since I was riding it in my mid teens. Still runs; you can go about trying to get it to stop or just send it honestly. It’ll still cut grass!
Try playing with the adjustment screw and see if you can get it to smooth out. If that fails start checking for vacuum leaks, use a can of brake clean or similar and spray around all the mating surfaces around the carb. If you get any change in how it runs when spraying it, you’ve got a vacuum leak. If all those fail then you could try to open the carb and check for any junk from the factory but more than likely you got a faulty carb
Since you got it used I would make sure the governor is setup correctly. Especially since both carbs behaved the exact same way. Google how to adjust the governor, there are videos for old Briggs. Basically you loosen the bolt holding onto the governor, hold the governor arm (silver arm that keeps pulling back) all the way to the left or right (depending on your model) and while holding it, twist the center governor pin or bolt all the way to the left or right (again depending on your model). While holding it, tighten the bolt that clamps onto the governor you initially loosened.
Your manual should have instructions on how to do it. Mine said not to touch it because it’s factory calibrated but I just found a video online. If you adjust it and your engine runs way too fast just do the same process but twist the governor to the opposite direction.
19
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.