Is my Stihl FS111R running hot or not?

[ueww40]

I just recently replaced the short block and piston (4180 020 1209) on my trimmer (nothing wrong with the old one, except that the valve seats were shot and can't be replaced by themselves. The engine purrs and runs like a new one (which it is) but I feel it's running hotter than before or maybe it's not and I am just paranoid. On top of that after about half hour of trimming all of a sudden the engine stops, which it never did before. My question is this. Do 4 mix engines have built in overheat protection or is it something else that cuts off the engine. Since I am still breaking her in I am taking it easy with the trimming, but it is summer time and it's hot outside. Like to hear your thoughts on this. Thanks

 

[Creaky limb tree care]

It's probably fine. Just a new engine hot smell

 

[ueww40]

It's probably fine. Just a new engine hot smell

OK, thanks

 

[hacskaroly]

My question is this. Do 4 mix engines have built in overheat protection or is it something else that cuts off the engine.

No, the 4-Mix engines do not have built in overheat protection (no sensor or anything like that). If it is not catastrophic failure like a scored piston and cylinder or an air leak, its generally a break in the fuel/air/fire cycle. I have a FS 91 and have had it occasionally cut out, but it starts right back up. When it does that, gives a quick chance to check fuel and string load anyway.

 

[ueww40]

Thanks for the info about the overheat protection or lack thereof. In that case it might be the ignition coil going flaky at a certain temperature. Not a big deal. I will keep an eye on it and do some testing when it happens

 

[ueww40]

Does anybody know what Stihl tool 4282 890 2700 is used for? And then, correct me if I am wrong, I assume that a 4 mix engine fires on every cycle, not just the compression cycle?

 

[hacskaroly]

Does anybody know what Stihl tool 4282 890 2700 is used for?

That is a piston stop, can be used to prevent the piston from cycling while trying to to get the clutch or flywheel off.

 

[hacskaroly]

And then, correct me if I am wrong, I assume that a 4 mix engine fires on every cycle, not just the compression cycle?

 

[EFSM]

I assume that a 4 mix engine fires on every cycle, not just the compression cycle?

It fires every other upstroke, just like a regular 4-cycle engine. The mix part means it takes mixed fuel. That's how they get the name.

 

[ueww40]

It fires every other upstroke, just like a regular 4-cycle engine.

Hmmm, interesting. The flywheel magneto passes the ignition coil on every revolution. What prevents it from sparking on the exhaust cycle? Ok, I got it. That's what that tool does. It locks the engine when trying to unscrew the starter cup or flywheel nuts. Thanks

 

[el33t]

It fires every other upstroke, just like a regular 4-cycle engine.

Hmmm, interesting. The flywheel magneto passes the ignition coil on every revolution. What prevents it from sparking on the exhaust cycle?

4-Mix is an example of a wasted spark ignition system. In fact, so are most four-stroke single or twin cylinder engines that use magneto.
That is, it is true that it also fires on the exhaust stroke.

Such a system also in ignition systems using a battery was also encountered in cars.

 

[el33t]

On top of that after about half hour of trimming all of a sudden the engine stops, which it never did before. Since I am still breaking her in I am taking it easy with the trimming, but it is summer time and it's hot outside.

When did you buy the fuel you use? Have you tried any other gasoline?

 

[ueww40]

The gasoline is 89 ethanol free marine gas and a week old. 45:1 mix. Interesting, I never heard of the wasted spark system, even though it had to be, it's just common sense and logic. I read up on it and it has been used on many cars and motor cycles for years. But on a magneto system there is no other way. It had to fire on every stroke. Thanks for confirming

 

[el33t]

The gasoline is 89 ethanol free marine gas and a week old. 45:1 mix.

Unfortunately, I do not know what are the parameters of the gasoline you use. A quick search did not yield any reliable data from refiners or blenders.
I asked about gasoline because you wrote that you worked when it was hot. In my part of the world, and I guess in yours as well, winter and summer gasolines differ in vapor pressure. If the vapor pressure of your gasoline is too high it can cause vapor lock, during hot weather. Although the fact that you are using gasoline recently purchased and alcohol free reduces the likelihood of this happening.
Of course, if the ratio of air to fuel is higher after an overhaul for some reason, this can cause increased temperature in and around the cylinder, that is, an increased chance of vapor lock.

Anyway, if I were you, I would try a different fuel, and if that doesn't help I would enrich the mixture - turn the H screw a little counterclockwise.

Interesting, I never heard of the wasted spark system, even though it had to be, it's just common sense and logic. I read up on it and it has been used on many cars and motor cycles for years. But on a magneto system there is no other way. It had to fire on every stroke.

There could be a way if one could distinguish between the compression stroke and the exhaust stroke. Today's ignition modules are microprocessor based, so with this information it would be rather trivial to make a correction in the code.
But this is unlikely to be worth the increased complexity in these simple engines.

 

[EFSM]

Unfortunately, I do not know what are the parameters of the gasoline you use. A quick search did not yield any reliable data from refiners or blenders.
I asked about gasoline because you wrote that you worked when it was hot. In my part of the world, and I guess in yours as well, winter and summer gasolines differ in vapor pressure. If the vapor pressure of your gasoline is too high it can cause vapor lock, during hot weather. Although the fact that you are using gasoline recently purchased and alcohol free reduces the likelihood of this happening.
Of course, if the ratio of air to fuel is higher after an overhaul for some reason, this can cause increased temperature in and around the cylinder, that is, an increased chance of vapor lock.

Anyway, if I were you, I would try a different fuel, and if that doesn't help I would enrich the mixture - turn the H screw a little counterclockwise.

I had problems with fuel boiling inside my saw when I was using it on Saturday (high of 87*). It wasn't fuel I had bought, so I'm not sure how old it was, but it could have been more of a winter blend with too much butane in it, which is what is added to make fuel volatile.

 

[el33t]

I had problems with fuel boiling inside my saw when I was using it on Saturday (high of 87*). It wasn't fuel I had bought, so I'm not sure how old it was, but it could have been more of a winter blend with too much butane in it, which is what is added to make fuel volatile.

I experienced this last year in the spring, when I still had some fuel from the winter. And the air temperature was nowhere near as high as yours. Strangely enough, it coincided with the transition here from 5% ethanol content to 10% in gasoline with a RON of 95 (AKI 90).

I've also noticed that "air-injected" chainsaws are more prone to vapor lock.

 

[David Young]

One thing I will mention if it is hot. The best thing is for it to idle. If it dies get it started asap and let it idle. If you use it continuously for a while. Let it idle 60 seconds before you shut it off. It fit is part load or light load it is not as big of an issue.

 

[lilspenny]

I just recently replaced the short block and piston (4180 020 1209) on my trimmer (nothing wrong with the old one, except that the valve seats were shot and can't be replaced by themselves. The engine purrs and runs like a new one (which it is) but I feel it's running hotter than before or maybe it's not and I am just paranoid. On top of that after about half hour of trimming all of a sudden the engine stops, which it never did before. My question is this. Do 4 mix engines have built in overheat protection or is it something else that cuts off the engine. Since I am still breaking her in I am taking it easy with the trimming, but it is summer time and it's hot outside. Like to hear your thoughts on this. Thanks

Ok, from my experience it is probably the coil. Now, also from my experience, Honda HP2 oil at 40:1 runs cooler than any oil I have used by a noticeable margin. I know, I know, but I have tried several oils because the Stihl Ultra runs dirty in my machines. Been running an fs100rx for 20 years and it's pretty much the same engine as your's.