Stihl 4 Mix, I want to de carbonize, Need help with when to pour it in the cylinder Valve/Piston position.

[JOE.G]

Hi, I have Pole saws, Weed whackers and Blowers that have the 4 Mix motor, I have a bottle of the Stihl branded De Carbonizer and it says to pour in when both Valves are open, Is there a way to figure this out with out pulling valve cover? Would piston be at TDC or BDC I am guessing at TDC I could tell if there was no pressure pushing finger off of plug hole that both would be open?

Do you fill it at TDC?

Thanks

PS I have always ran Non Ethanol 90 Plus octane with Stihl Ultra oil. I am not noticing any issues but figured most of these are over 10 Years old with some having a lot of HRS on them and would just do it for Maintenance purposes.

 

[Vintage Engine Repairs]

The end of the exhaust stroke and beginning of the intake is when they’ll both be open, but I have no idea how long the overlap actually is and how far they’re held open. Take off the valve cover and feel when they’re both under compression.

 

[JOE.G]

Yeah I may just wait if that is the only way until I get ready to adjust the valves. Is Seafoam the same thing or similar as the Stihl stuff is no longer made and is expensive when you do find it.

 

[Vintage Engine Repairs]

If it was me, I’d leave well alone. The more you fiddle, especially when it’s not needed, the more you increase your risk of causing an issue. Leave the bottle on your shelf until you need it. At that point though, you’re better to go inside the engine and manually remove it, check the cam, followers, bearings, rod, rings, piston, valves etc.

 

[HansFranz]

PS I have always ran Non Ethanol 90 Plus octane with Stihl Ultra oil. I am not noticing any issues but figured most of these are over 10 Years old with some having a lot of HRS on them and would just do it for Maintenance purposes.

Is this something Stihl recommends as part of routine maintenance?

Only reason I ask is, I just this summer bought my first "4-mix" engine, on a Stihl FS91R string trimmer.
I was really surprised how much power/torque it has...

In general, I would agree with Tom above: If it ain't fixed, don't break it!

 

[cscltd]

They don’t really carbon up anymore since they updated engine many years ago.
Unless your running overly heavy mix, poor oil, or no guard with 2 feet of string out.

 

[JOE.G]

From what I read these motors tended to have Carbon issues when left to idle to long or poor quality oil was used. The pole saw and weed whacker are getting harder to start when hot but I would not say its bad yet I just notice a difference, Figured I'd De Carbon and check the valves this season.

 

[bwalker]

They don’t really carbon up anymore since they updated engine many years ago.
Unless your running overly heavy mix, poor oil, or no guard with 2 feet of string out.

They still carbon up and removing the guard to run longe line if anything would make them burn cleaner.

 

[HansFranz]

removing the guard to run longe line if anything would make them burn cleaner.

FWIW, the Stihl dealer where I bought my FS91R said that removing the guard to run longer line "is the first thing the local grass cutter guys do" ... and that it voids the warranty as soon as they do, for whatever reason (wasn't really clear to me)...seems like running longer line would bog the motor and make it work harder, since it would be cutting more grass at any given instant...made no sense to me why this would void the warranty...

 

[bwalker]

FWIW, the Stihl dealer where I bought my FS91R said that removing the guard to run longer line "is the first thing the local grass cutter guys do" ... and that it voids the warranty as soon as they do, for whatever reason (wasn't really clear to me)...seems like running longer line would bog the motor and make it work harder, since it would be cutting more grass at any given instant...made no sense to me why this would void the warranty...

It does make the motor work harder.
Redmax sells "hi torque" trimmers with a lower gear head ratio to run guard less. It's the only way to fly IMO and I have done it for years. I also tend to run a larger trimmer than most. The old Stihk FS250 was a beast. Running a Redmax 3050T now.
The void warranty thing is a joke IMO. One could simply place the guard back on for warranty work.

 

[JOE.G]

I sprayed Seafoam into the carbs , two Identical BR 600 Back pack blowers one with low HRS and one with a bit more, the amount of smoke from the higher HRS one was crazy, I did all of my small engine stuff some Chains saws smokes a lot also, Not sure if this really means anything or not. I'll soak the cylinders when i do the valves on the 4 Mix stuff.

 

[Chevboy0167]

https://www.arboristsite.com/threads/stihl-engine-de-carbonizer.42116/

 

[SteveSr]

They still carbon up and removing the guard to run longe line if anything would make them burn cleaner.

One of our trail guys had an FS110 4-mix and somewhere found .100" SQUARE (.141" diagonal) line for it. The next thing you know the engine was so carboned up that it wouldn't run! Had to tear the motor completely apart to remove all of the carbon.

This trimmer still had the guard on it so the line was the correct length but the line size and square profile overloaded the powerhead which led to overheating and carbon buildup.

 

[bwalker]

I sprayed Seafoam into the carbs , two Identical BR 600 Back pack blowers one with low HRS and one with a bit more, the amount of smoke from the higher HRS one was crazy, I did all of my small engine stuff some Chains saws smokes a lot also, Not sure if this really means anything or not. I'll soak the cylinders when i do the valves on the 4 Mix stuff.

Spraying a light oil into your running motor will make it smoke.... but it won't remove carbon very much if at all.

 

[bwalker]

One of our trail guys had an FS110 4-mix and somewhere found .100" SQUARE (.141" diagonal) line for it. The next thing you know the engine was so carboned up that it wouldn't run! Had to tear the motor completely apart to remove all of the carbon.

This trimmer still had the guard on it so the line was the correct length but the line size and square profile overloaded the powerhead which led to overheating and carbon buildup.

I'm not buying it. I've ran square line like that for a very long time without issue and with no guard.
When you load a motor more it burns cleaner. Regardless a four stroke motor that uses mixed gas is just a stupid design.

 

[ZeroJunk]

Running long line just wears the gear head out and rounds off the shaft faster. That is the trade off for being able to cover more area.

I think if you want to clean the valves and seats you need to take the thing apart. I have two back pack blowers that I got free that way.

There are some you tube videos that show you how to get them back together in time.

But simply, there are arrows on the cam that line up with marks on the crankcase at top dead center. All there is to it.

 

[lone wolf]

I'm not buying it. I've ran square line like that for a very long time without issue and with no guard.
When you load a motor more it burns cleaner. Regardless a four stroke motor that uses mixed gas is just a stupid design.

Thank the stupid EPA mandates for the stupid 4 Mix!

 

[SteveSr]

I'm not buying it. I've ran square line like that for a very long time without issue and with no guard.
When you load a motor more it burns cleaner. Regardless a four stroke motor that uses mixed gas is just a stupid design.

Well, I am just the one doing the autopsy and not the primary user. As I recall most of the carbon was on the back of the exhaust valve causing compression loss and the exhaust port clogging. I think that I had to use a drill bit in the exhaust port with the valve removed to clean out the carbon it was so bad.

BTW, How does Stihl Ultra behave if subjected to high operating temperatures. Does it form gum and hard carbon?

From my limited research carboned up 4-mixes seem to have been an issue causing them to be scrapped due to the labor required to clean them up.

BTW, Stihl stopped selling their "decarbonizer" at least a decade ago. I suspect because it didn't work very good.

 

[bwalker]

Running long line just wears the gear head out and rounds off the shaft faster. That is the trade off for being able to cover more area.

I think if you want to clean the valves and seats you need to take the thing apart. I have two back pack blowers that I got free that way.

There are some you tube videos that show you how to get them back together in time.

But simply, there are arrows on the cam that line up with marks on the crankcase at top dead center. All there is to it.

That's just not been my expiereance in regards to head wear, Zero.
I have a friend that's been doing lawncare since the 90's. He hasn't noticed it either.
I've only had one head ever fail and it was on a Stihl FS85 that was
ran with a brush disk extensively By hired hands. Brush disks are much harder on the head as your constantly banging the disk into saplings at full throttle.

 

[bwalker]

Thank the stupid EPA mandates for the stupid 4 Mix!

Most other company's didn't go that route. Thank Stihls inability to engineer a compliant product.