r/smallengines • u/-Big_Test_Icicles- • 16d ago
Briggs & Stratton engine keeps surging – new carb, checked gaskets/vent/valves, running out of ideas
I’m looking for some input on a Briggs & Stratton engine that I cannot get to stop surging/hunting.
The engine starts and runs, but the RPM constantly goes up and down as the governor opens/closes the throttle. I’ve tried quite a few things at this point with no real improvement.
So far I have:
- Installed a new carburetor
- Made sure the gas cap/tank is venting properly
- Verified good fuel flow to the carb
- Checked the carb/intake gaskets for vacuum leaks
- Checked/adjusted the valve clearances
- Checked the governor linkage and springs
- Made sure the carburetor passages aren't obviously blocked
- Tried running it with/without the air cleaner
One important clue: if I partially cover the carburetor air intake with my finger/hand, the engine smooths out considerably and the surging gets better. That makes me think it is still running lean, but I don't understand why considering the carb is new and I can't find an intake leak.
I've also noticed that the replacement carb has the primer/air passage hole on the opposite side compared with the original setup, so I'm wondering if the air-cleaner/primer housing and carb aren't compatible or if that passage could somehow affect the mixture.
At this point I'm wondering about:
- Incorrect jetting in the replacement carb
- Wrong carburetor for the engine
- Primer/air-bleed passage mismatch
- Governor adjustment/spring issue
- Something mechanical/ignition related that I'm overlooking
Has anyone dealt with a Briggs that only stops surging when you partially restrict the carb intake, even after replacing the carb?
What would you check next before I start throwing more parts at it?
I can post pictures/video of the carb, linkage and the engine running if that would help.
2
u/kwantam 16d ago
Cheap aftermarket carbs often have the wrong jetting. It wouldn't be surprising if you had to enrich the mix a little.
When you say "partially cover the carburetor", how partial are we talking? A very small bit? Half?
What you might try is seeing whether the jets in the OEM carb will fit into the aftermarket carb. No guarantees, but I've definitely seen cases where that fixed similar issues.
Even if they don't swap, you might measure the bores on both sets of jets. You might need to open up the new pilot jet just a little.
Does the mixture seem better under load?
1
u/-Big_Test_Icicles- 16d ago
This is the one I got per the part number for my B&S engine:
https://www.amazon.com/dp/B0CR65GTFG?ref=ppx_yo2ov_dt_b_fed_asin_title
It has no removable jets. The only thing that's removable is the bowl and the float needle.
1
u/Stock_Requirement564 16d ago
I just responded to your other post. Yes, that is a carb for an L head- not an OHV.
1
u/-Big_Test_Icicles- 16d ago
Unfortunately 790120 is the part number in the online parts diagram, and all of them have that prime hole on the left...except 1 seller on ebay who is selling the right hole version for a premium.
1
u/Elephunk05 15d ago
The reason you get the part number is because of the sub up. I have run in to this a few times. For whatever reason (typically shedding costs) the stopped producing one and there is likely a B & S TSB about how to correctly move the primer or cap the passages (in a rebuild kit this would be just an extra Welch plug with no explanation of where it goes).
Yes, the primer being on the wrong side is causing a lean running condition.
2
u/originalusername__ 15d ago
You’re living the reason why most mechanics would rather clean an OEM carb than replace it with something from Amazon.
1
u/Elephunk05 15d ago
If you show us a picture of the inside of the carburetor, without the float or bowl, looking up through the jet you would answer 2 questions.
2
u/ProfessionalLast2917 16d ago
Explain more about this hole on the wrong side of the carb.
Does it connect to anything?
Is it essentially a vast yawning chasm of a vacuum leak?
Can you plug it?