Speaking of limited coils or not ? Four Mix Engines

[ZeroJunk]

Maybe Stihltech or somebody can tell me how to tune a four stroke Stihl weedeater. The FS90 and FS91 sound to me like they are skipping.
What I have been doing is wind out a typical working amount of line and lean the high screw until I hear the engine slow a little and then turn it out one turn.
This is completely arbitrary. You can turn it out 1, 2, 3 ,4, turns it never seems to change. Just sounds like it is skipping. And, I have worked on hundreds of them. They are all that way.

So, anybody know the proper method ?

 

[pdqdl]

I'm not sure how finely tuned your ear is, but of course it's skipping!

That's why they call 'em a four stroke.

 

[ZeroJunk]

I'm not sure how finely tuned your ear is, but of course it's skipping!

That's why they call 'em a four stroke.

LOL. I'm not talking about that.

 

[SteveSr]

Maybe Stihltech or somebody can tell me how to tune a four stroke Stihl weedeater. The FS90 and FS91 sound to me like they are skipping.
What I have been doing is wind out a typical working amount of line and lean the high screw until I hear the engine slow a little and then turn it out one turn.
This is completely arbitrary. You can turn it out 1, 2, 3 ,4, turns it never seems to change. Just sounds like it is skipping. And, I have worked on hundreds of them. They are all that way.

So, anybody know the proper method ?

I don't work on many of these but what I do with any of these (either 2 or 4-stroke) with a limited coil is to mount a spare string head and remove the string cutter/ deflector. Next extend the line in small increments to get the engine off the limiter and then tune the "H" on the carb for best power (4-mix) or slightly rich of max power for 2-stroke. YMMV

 

[Vintage Engine Repairs]

This is something I spoke about with a local stihl tech.

He uses half a tank of fuel and extends the trimmer line half way out and will set the H screw to be just on the limiter.

When the line is long it will be under it
When the line is short it will be bouncing off the limiter constantly.

He said that although he has been doing it this way for many years and that he’s old school, he said if you ask the newer techs, that they may have very different methods.

Edit:

I recently got an EDT 9 for exactly this reason now that I repair both commercial and residential garden power equipment and after (a friend and bad influence ) trains encouraged it. They aren’t cheap at $180 Australian, but that’s nothing compared to the cost of peoples commercial equipment I work on.

The Oppama tach is better in every way, apart from that it won’t read limited coils like the stihl one does.

 

[ballisticdoughnut]

After warming up the engine for a minute I’ll tap the head on the ground until the string is cut by the guard. I then hold at WOT and turn the high speed in (lean) until I just hear the RPM start to drop. After that I’ll turn the high speed out one to one and a half turns or so. I’ve noticed on the FS and KM 131 you can turn the high speed out quite a bit and it will still hit the limiter.

 

[ZeroJunk]

After warming up the engine for a minute I’ll tap the head on the ground until the string is cut by the guard. I then hold at WOT and turn the high speed in (lean) until I just hear the RPM start to drop. After that I’ll turn the high speed out one to one and a half turns or so. I’ve noticed on the FS and KM 131 you can turn the high speed out quite a bit and it will still hit the limiter.

That is what I have been doing. But like you say, it seems like once you hit a certain point it will deliver no more fuel regardless .

I'm just think, if you lean off the limiter that can't be good.

 

[ballisticdoughnut]

That is what I have been doing. But like you say, it seems like once you hit a certain point it will deliver no more fuel regardless .

I'm just think, if you lean off the limiter that can't be good.

I have a KM131 myself and have tuned a few hundred of them that way and never had one come back burned up.

 

[SteveSr]

After warming up the engine for a minute I’ll tap the head on the ground until the string is cut by the guard. I then hold at WOT and turn the high speed in (lean) until I just hear the RPM start to drop. After that I’ll turn the high speed out one to one and a half turns or so. I’ve noticed on the FS and KM 131 you can turn the high speed out quite a bit and it will still hit the limiter.

The 131 is a higher power engine and needs more load (i.e. line length) to get it off the limiter.

 

[SteveSr]

I have a KM131 myself and have tuned a few hundred of them that way and never had one come back burned up.

I believe that the 131 is a 4-mix aka four-stroke engine. These fire every other revolution so won't melt down like a 2-stroke. They just won't produce max power if set too lean. It would be interesting id Stihl had a method for tuning 4-mix engines.

 

[Vintage Engine Repairs]

I believe that the 131 is a 4-mix aka four-stroke engine. These fire every other revolution so won't melt down like a 2-stroke. They just won't produce max power if set too lean. It would be interesting id Stihl had a method for tuning 4-mix engines.

The can melt Steve, they use the fuel & oil mix to to lubricate the crankshaft, valves, bearings and cylinder walls too.

 

[sawfan123]

You need the original settings.
That depends on the carb. They vary. A lot.

All primer equipped 4 mix are semi fixed jets.
They dont adjust like a saw. The mixture screws only fine tune.
Like for acceleration and smooth idle.
They shouldn’t stall during a rollout test.
Limiter activates approximately 9500.
Best idle speed is approximately 3000.
They have a tendency to stumble at the recommended
Lean running, very lean will cause wrist pin bushing seizures first.

 

[ZeroJunk]

I have seen a lot of BR600 and BR700 blowers seized on the wrist pin. But, not a single weedeater. Wonder why ?

 

[sawfan123]

The handheld engine have been very reliable. The rest of the tool wears out first. -0- seizures.
Blower life is on par with everyone elses. Not impressive. But I have seen a fair amount seize.
Seems at about 100 hrs. About 1/3 season for landscapers around here.
Never denied a warranty Never told why. But thats Stihl.

 

[pdqdl]

I have seen a lot of BR600 and BR700 blowers seized on the wrist pin. But, not a single weedeater. Wonder why ?

Blower engines get run wide open with no remorse. Most guys don't run a weed eater wide open except in spurts.

 

[ZeroJunk]

Blower engines get run wide open with no remorse. Most guys don't run a weed eater wide open except in spurts.

That's true.

The BR600 BR700 etc. just have a brass or whatever alloy bushing for the wrist pin. One would not think it could seize to the point of locking the whole thing up but it can and does.

I'm thinking that was a really curious engineering decision.

I should add that there is no scoring on the piston at all usually. Just the end of the connecting rod is blue.
We keep a couple of complete engines as well as a different back plate since the new motors mount a little different.

 

[stihltech]

Blowers do not like over revs. A short blower tube will do it. Take one off or hold it against a wall and you will see the difference.