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

 

[Stump Shot]

 

[cao-2000]

I saw that video, and I do have some micro drill bits on order.

It will be interesting to see what the real fix ends up being…

 

[huskihl]

They surge from running lean. Richen the carburetor up if it has adjustable jets or clean the fixed jets inside.

 

[cao-2000]

They surge from running lean. Richen the carburetor up if it has adjustable jets or clean the fixed jets inside.

Sadly there are no adjustable jets, and I have cleaned all the passageways, even though it is a new carb.

 

[huskihl]

Sadly there are no adjustable jets, and I have cleaned all the passageways, even though it is a new carb.

May have to drill them out if they’re clean. I’d try a heavy dose of Seafoam before I drilled anything though

 

[mainer_in_ak]

Adjust the governor properly and install new governor springs. This surging is all in the governor setting. I'm almost certain this should fix the issue. The governor arm can slip from its initial setting and the governor springs weaken over time, very common problem. Simple/cheap fix.

 

[cao-2000]

Adjust the governor properly and install new governor springs. This surging is all in the governor setting. I'm almost certain this should fix the issue. The governor arm can slip from its initial setting and the governor springs weaken over time, very common problem. Simple/cheap fix.

There is only one governor spring that I know of, and I already replaced it with a new one.

I’ll need to find instructions on how to adjust the governor properly.

 

[JB-PlantHeirloom]

If you have the vane type plastic governor, make sure it freely moves and it is not beat up and shredded. They can wear at the bottom, some just get frayed along the leading edge. As mainer_in_ak posted, I have fixed that issue too with springs, though on some (Honda) I just bend the linkage. As others have posted, going rich does not hurt too. I have had some where it was fixed enough to live with, especially those B&S that have the carb inside the tank, unlike yours.

I can't visualize your model by the number, but, on some I just bend the linkage to permanently put it at a much higher idle and that solves it for the most part.

 

[huskihl]

It’s not the governor. The jet is either too small from epa restrictions or it’s got crud in it. The engine loses speed because or poor air/fuel ratio and then the governor kicks in to bring rpm back up. Repeat. Repeat

 

[Shane¹]

I believe @huskihl is right even if the carb is new it doesn't take much to plug up a jet. Also as the engine older things wear and sometimes more fuel is required. Fixed jet carbs are the stupidest thing they ever made.

 

[cao-2000]

Thanks for the info guys. I am confident that neither my old jet or the new Chinese one have any debris, as I have soaked them both in acetone and spent a lot of time with welding cleaners.

However, I have ordered some micro drill bits and will be trying to open them up a little.

I am going to go all the way with this project, as this is the perfect learning project.

 

[backhoelover]

Need to check your idle air control jet

 

[backhoelover]

It feed air to your emulsion tube. Also spray some carb clean around the carb, intake and head gaskets make sure you don’t have a leak

 

[cao-2000]

Have sprayed carb cleaner around both sides of the carb, intake tube, and intake tube gasket, and no leaks. Also have a new intake tube and gasket. I do have a primer bulb on order for the heck of it.

 

[Al Smith]

I've got a pressure washer that did that .It's relatively new for a Briggs with an OHV head ..Rather than fiddle with it I used gasoline with about a triple dose of "Stabil " which must have cleaned out the jets .Danged things rest for about 8-9 months a year and only run for a short time the entire year .Comes as no surprise they get clogged up .Danged pump locked up too which also is no surprise .That said it's a good one when it decides to run .

 

[mainer_in_ak]

Any updates? The mechanical bllsht that us working class folks have to deal with, in the name of saving money, is excessive. Least we can do is support one another I suppose, we're all in the same boat.....

 

[cao-2000]

Any updates? The mechanical bllsht that us working class folks have to deal with, in the name of saving money, is excessive. Least we can do is support one another I suppose, we're all in the same boat.....

It will take awhile for parts to come in, but I’ll update once I have some progress…