Briggs 6HP Surging on Walk Behind String Trimmer. Lots of Troubleshooting. Running out of ideas. Video Inside.

[cao-2000]

I am trying to revive a Briggs 6HP Walk Behind String Trimmer for my elderly neighbor.

I have tried a lot of things one by one, but now at a point where I need some guidance before I spend any more money or time on this thing. It was sitting for a long time, so I started with the following:

-Fresh Non-Ethanol Fuel with Sta-Bil
-New Carb & Gasket/O-Ring (Chinese Clone)
-New Fuel Line from Tank to Carb
-New Air Filter
-New OE Spark Plug Gapped at .030"

The engine starts and runs, but surges, so I checked the following:

-Fuel flow is very good from tank to carb, with or without the filter/shutoff.

-Took apart the brand new carb, probed every orifice including main jet thinking maybe there was debris. I used my welding torch cleaner and lots of carb cleaner. Everything seems clear.

-Governor linkage appears in working order and is well lubricated.

-Opened Gas Cap to eliminate venting issue.

-Disconnected pulley to eliminate any potential load issues.

-Replaced Manifold pipe & gasket.

-Lightly Sanded the flywheel and coil surfaces as they were very rusty, and spaced them using a business card.

-While it is surging, I sprayed carb cleaner around both sides of the carb, as well as the other end of the manifold pipe, and nothing changes, so I think all three gaskets are sealed well. Also did this along the pipe, no change.

-Hooked up my spark plug tester inline and the strength of the spark definitely coincides with the surge, but I'm not sure that is telling me the flywheel or coil is faulty, or that something else is the root cause. I also had to make the gap really narrow (almost touching). I made a video below.

Not sure if this engine has adjustable valves, but I suppose that could be a thing, and is new territory for me.

The other thing is the primer bulb, which doesn't seem to prime very well, but not sure if a leaky primer bulb could be related.

At this point, I am thinking it could be:

-Faulty Carb/Jet
-Bad Coil
-Valves out of adjustment
-Leaky Primer Bulb

FYI - we are at 4500 feet elevation, but never had an issue with surging because of elevation on other machines, just reduced power/RPMs.

I couldn't really find a true "service manual", here are the details of the machine and engine:

Craftsman Walk Behind String Trimmer
Model#: 917.773707

Briggs & Stratton Engine
Model# 128H02-0515-E1
Code# 05061756

Thanks,

Chris

 

[sgkent]

leaky primer can cause all sorts of surging and odd running. Start with that as your first fix.

 

[bertsmobile1]

I advise this so often I can type it in my sleep
Fix the old carb before you think of buying a clone carb from ScAmazon or Evilpay
If you decide to get a new carb buy it from or through a real mower parts retailler as those ones are the good ones that passed quality control . The cheap ones form "Lucky You" , "Cheap Cheap Now" that are undrranded and very cheap are the ones that failed quality control and were originally sold as scrap.
A quality part will rither be branded or come in branded packaging
If it comes in a plain cardboard box or resealable plastic bag, keep the bag / box for use and throw the contents away.
On top of that if it is a modern plastic carb they come in a dozen variations and getting the correct one for your engine can be a problem .
They all fit but only the right ones will work properly.

Surging is 99.999% of the time an air fuel ratio that is too lean so either the carb is not puting enough fuel into the air or air is is entering the cylinder after the carb has dispensed the correct amount of fuel