4-cycle brush cutters... anyone work on these much?

[J D]

I've been given a couple of these to look at by a friend. 1 is almost new & given what he's tried & what it's doing I suspect it's not getting impulse to the carb.
Do these things work the same way a 2-stroke does for impulse? Anything in particular I should be aware of working on these things?
Cheers

 

[2-StrokeDude]

I've been given a couple of these to look at by a friend. 1 is almost new & given what he's tried & what it's doing I suspect it's not getting impulse to the carb.
Do these things work the same way a 2-stroke does for impulse? Anything in particular I should be aware of working on these things?
Cheers

One thing I would check on those four stroke trimmers/brush cutters is the valve lash. Since they are very small four strokes with tiny valves, they seem to not stay in spec for a super long time and need adjustments frequently. I had one of those 4 stroke Troy-Bilt trimmers I was working on once and after replacing the carb and trying a bunch of tuning it would still not run right. So I pulled the valve cover off and checked the valves and sure enough they were way too loose, so I tightened them up to spec and then it ran fine.

Also some else correct me on this if I am wrong but I don’t believe those four carbs on small engines like that work off of an impulse like a two stroke. In fact, I believe there’s a spring actually that is pushing up against the metering diaphragm that helps to act like an impulse generator.

 

[2-StrokeDude]

I've been given a couple of these to look at by a friend. 1 is almost new & given what he's tried & what it's doing I suspect it's not getting impulse to the carb.
Do these things work the same way a 2-stroke does for impulse? Anything in particular I should be aware of working on these things?
Cheers

Ok looks like I was right, there is a spring in the carb that pushes up on the pumping diaphragm, not the metering diaphragm, but that is what helps pump the fuel to the carb because there is not much of an impulse coming from the engine. Here is a picture of where the spring sits in the carb, I pulled to image for a YouTube video…

 

[J D]

Ok looks like I was right, there is a spring in the carb that pushes up on the pumping diaphragm, not the metering diaphragm, but that is what helps pump the fuel to the carb because there is not much of an impulse coming from the engine. Here is a picture of where the spring sits in the carb, I pulled to image for a YouTube video…

Thanks for your input, that spring looks like a real good way to shorten the life of that diaphragm.
Do you remember what the valve clearance was set to on the one you worked on? So far I haven't been able to find any service info on these things

 

[2-StrokeDude]

What model of brush cutter do you have? I cannot exactly remember the clearance for the valves, might be around 0.05 mm, if you can send me the model I can see if I can find info for it.

 

[gggGary]

Plus 1 on valve clearance
Stihl 4mix use .10mm or .004"
Saw an (offbrand) 4 stroke one laying by the curb. talked too the owner; hard start they bought battery.
Set valves vroom vroom.
I have 4 of the stihl 4mix and yes setting valves every so often part of life owning one.
PS I really like weed whipping with them. Pole saw is a good unit too. I've run that bar fully buried in hardwood many times.

 

[J D]

Thanks guys.
They are a KNK branded KM4080312.
Older unit I believe lost power & got hard to start so will check valves on that for sure. Newer unit (probably less than 20hrs on it) runs on prime but won't draw fuel up. Swapping carbs didn’t change the fault on either unit hence my suspicion of the impulse or something similar being the issue

 

[J D]

Update...
Newer trimmer carb had a leaky metering valve & a coil that wouldn't produce spark under compression. Older trimmer I suspect needed the valves adjusted but I never made it that far as I robbed it of a coil & a carb to fix the newer one.
Whats the best way to tune these things? Sticker on the side says 6500RPM. I had it tuned by ear to about 7200RPM. If I wind it down below 7k it just sounds rich & lazy (but then I'm not used to 4 strokes so I don't really know). Would 6500RPM be a max no load or a working RPM?
Cheers

 

[sean donato]

Update...
Newer trimmer carb had a leaky metering valve & a coil that wouldn't produce spark under compression. Older trimmer I suspect needed the valves adjusted but I never made it that far as I robbed it of a coil & a carb to fix the newer one.
Whats the best way to tune these things? Sticker on the side says 6500RPM. I had it tuned by ear to about 7200RPM. If I wind it down below 7k it just sounds rich & lazy (but then I'm not used to 4 strokes so I don't really know). Would 6500RPM be a max no load or a working RPM?
Cheers

Max no load would be my bet, they don't usually tell you max loaded rpm.