Stick with the Stihl synthetic if the unit is still under warranty. There have been issues with carbon buildup in the 4-mix engines.

STIHL has a full synthetic, and a synthetic blend. I chose to go with the full synthetic even though it costs the most. I don't understand why STIHL won't lower the price of their full synthetic oil so more people would use it and reduce the need to "de-coke" as often. This would make their 4-mix engines more reliable and desirable in the market place. Charles Ranheim

I just purchased a stihl 4mix. The dealer didn't even tell me about the different oils. I got what they recommended which is the standard 2cycle stihl oil in the orange bottle.
What do you mean by "de-coke"? Is this a simple cleaning of the exhaust screen or something more complex like carb?

I went to the stihl web sight to look at the oil. The full sythetic states that it's biodegadable. Does this mean it has a short shelf life. I don't want to buy this stuff and find out I have to use it within 1 month.

Why would you recommend using the sythetic if the unit is under warranty? Why wouldn't you use it all the time if it will make the unit last longer? The dealer nor stihl recommends using sythetic oil. Is this in the manual? I'll have to read it more closely. If they don't recommend it then they should fix at least if it's under warranty. I know they recomment 89 octane or above gas.

To "decoke", means to remove the carbon from the combustion chamber. In my BR600 manual, it says under Combustion Chamber: "Decoke after 139 hrs of operation, then every 150 hours". I spoke with STIHL customer support about this. They said if I used their fully synthetic "Ultra" two cycle oil, it would burn clean with very little carbon. This carbon buildup can cause problems with the valve operation. I think if you go to the STIHL web site, and look at their oils, you will find they have three levels. The first is regular conventional 2 cycle oil. The second is a blend of regular and synthetic oil. The third is all synthetic which they say is best for their 4-Mix engines. The STIHL rep said they tested the "Ultra" oil by running it for about 500 hours and found very little carbon buildup. I have seen other posts where some STIHL dealers highly recommended using it in the 4-Mix engines. However, most professionals do not use the best oil because they don't expect to get many years of use out of their power equipment when used commercially. They feel it is not worth the additional expense to go with the Ultra oil. Being a homeowner, I can afford the additional cost of the Ultra oil and chose to go with it. Charles Ranheim

I don't think there is a shelf life problem with the synthetic prior to mixing. Once mixed you still have a shelf life of the fuel anyway.

Most of the carbon problem with regular oil has been with the larger backpack blowers. The carbon is a valve seating problem. We have seen very few problems using the regular Stihl oil in the smaller trimmer engines but the synthetic would still be better as a preventative measure. Although the synthetic may be more expensive, don't forget the 4-Mix uses less fuel then 2 stroke engines so there is something of an off-set.

Thanks for the info on the carbon issue. My dealer's counter person came off as if was a lube quality issue. Typically, I used to use Echo oil in my Stihl saw and Mantis tiller, but bought the black when I got the FS.

My BR600, with the 4-Mix engine, uses less fuel than my STIHL 420C for any given job. This is partly due to the higher output of the BR600. It also takes less time with the BR600, and I don't always have to run it at full speed like I did with the BR420C. I have used ECHO oil for years, and still use it in the BR420C because it has the standard 2-cycle engine. I plan to use the Ultra oil in my BR600 and my STIHL KM110R Power head with the 4-mix engines. The cost difference to a homeowner is not that great. I always use premium gas for my fuel mixes. Charles Ranheim

I just purchased a FS 90R today to replace my crappy 8yr old Craftsman trimmer. This new trimmer obviously has the 28.4cc 4-mix engine. Now I have read in this thread that the Stihl full synthetic 2cycle oil is the one that is most recommended. However, I was unaware of that and picked up the basic oil, the orange jug. My question to our members here that have used the FS90 4mix with the regular oil only, what results did you have with carbon deposits? Were they really that bad? To me, I do not mind having to do vavle clearance adjustments and such as I may get 150 hours in a season. So doing the work right before the season starts is not a problem.

The lawn care shop I purchased my trimmer at is right on the way to work but probably a bit too far to make it there and back on my 20min lunch break. I could always exhange the orange bottle stuff for the white. But for those that have used ONLY the petroleum based orange bottle stuff in the FS90 or similar 4mix trimmers, were carbon deposits REALLY that bad? Because if they are not that bad after the recommended 150hr intervals, then I will just use the orange bottle oil and do the maintenance every spring.?

That is what we supply for the 4-mix engines as the synthetic is not yet available here and we have not (to this point) had any issues.

The valve adjustment procedure and the "De-coking" are two separate issues. The STIHL Ultra oil mix is supposed to burn much cleaner, preventing a frequent buildup of carbon. This carbon build-up can cause valves to stick or not close properly. Even if the carbon build-up is low, you may still need a valve adjustment due to normal wear in the valve train. The ultra oil is supposed to give the best lubrication as well, and may reduce the need for valve adjustments. I plan on checking my valve clearance at an estimated 140 hours to see where I stand with that. I am not sure if and when I will go through the "De-coking" procedure. From what I have read on this, and other forums, it sounds like if the engine is not running smooth, it may indicate a carbon buildup. Hopefully, using the Ultra oil will prevent the carbon build-up as well as reducing wear on the valve train. Charles Ranheim

Charles,

With regards to valve clearance and whatnot, it probably is similar to adjusting valve lash on an automotive engine with solid lifters. With the right feeler gauges and any other needed tools, valve lash was a simple procedure to do. However, this engine is a wee bit smaller than, say, a Big Block Chevrolet engine. :) Is obtaining access to the valvetrain a big deal on a FS90? Also, I am assuming the 4Mix engine is a flathead engine much like my 25yr old Briggs lawn mower engine where the valves are inside the block and not the head. Is this correct?

If the FS90 4Mix engine is indeed a flathead design, then I assume getting access to the combustion chamber is very simple. Just unbolt the head, clean head, cylinder walls, and piston top, correct? If it is indeed that easy, any good recommendations on cleaners to "de-coke" the needed surfaces? I have disassembled many old Briggs engines and they are so simple for those with average mechanical aptitude.

I should note that I did decide to exhange the 6pk of orange bottle oil for the white bottle full synthetic 6pk. After all, it was only 3 dollars, or 50¢ per bottle, more for a 2.6oz 6pk. Surely one less trip to McDonalds or Sheetz will pay for that 2 times over. And since it does indeed reduce carbon buildup, is 50¢ per bottle really that much to pay for simple peace of mind? I do not think so. I just wish Stihl would indicate the full synthetic oil recommendation on their website listings for all their 4Mix products. It would be of great value to a Stihl newbie such as myself. :)

donaldjr1969
While I have never de-coked or adjusted valves myself because my machines have only a few hours on them, I have done some research and spoken with some dealers about the procedures. I also looked at a picture of the valve adjustment procedure. The 4-Mix engines I have are not like a normal 4 stroke engine. From what I see, the valves are in the "head", but the "head", according to my dealer, cannot be removed. It is all part of the engine block. The valves are adjusted via the rocker arms as you would in a normal valve in head engine by first removing the rocker arm cover. A special fluid is used to de-coke the engine. This fluid, (STIHL # 0781 313 8019), is poured into the combustion chamber via the spark plug hole. After an hour it is removed. STIHL recommends this de-coking procedure be done by a dealer because of the cleaning fluid used. I'm sure you have to use some common sense so you do not get sprayed in the eyes with this fluid. Again I have not done any of these procedures yet myself. I am just passing on what little information was given to me. I will probably order a shop manual before doing it myself. Perhaps someone on this forum who has done it can be more specific. So far, I have found little information because the engines are new, and the need to do these procedures have not been needed that much yet. I'm sure as time goes by, more information will be available. Take care, Charles Ranheim.

Charles,

First off, Feel free to call me Don. :) Anyway, that one piece block/head combo is really a good idea in terms of total block strength. But how one would install the valves is a question to me. I could only see this being feasible in an overhead valve engine. One would insert the valve from the bottom of the cylinder valve stem upwards. Push the valve through the top, attach your spring and retainer. Now if it is a flathead design, like in the image below, it would seem nearly impossible to install a valve w/o a removeable head.

{{gwi:311173}}

See how the valves are beside the cylinder? Just looking at it, I cannot see how a flathead design can work without a removeable head. Note that this is an OLD Ford Inline 6 engine, but fundamentally, my Briggs engine is the same.

That sounds like a very easy procedure to decoke the 4mix. I was thinking that with a removeable head in a flathead style engine, one only needed to pull the head, and soak everything in Gumout or equivalent. Then if there are any deposits on the stem side, aka the seat, or sealing, side of the valve, the user could merely slighty rotate the starter cord to get the valves open. Any idea how the 4mix would get the sealing side of the valve decoked? Probably NOT a good idea to add the fluid, install the spark plug, and give the starter cord a few pulls! :)

I thought the actual 4mix itself has been around a couple years. Are they just new to the string trimmers? Whatever the case, I love my new trimmer and feel it was worth every cent of the $300 I paid for it. I just like to know any little bit of maintenance information as it is something I can easily do myself. When I am NOT trimming the yard of mine, a friend, or a relative, I will just tear down that Craftsman piece of junk and see if it could use a rebuild. Get any parts I need, re-ring the piston if need be, and maybe even take a cylinder hone to the cylinder walls if it warrants it. In the meantime, I have some trimming to do for my aunt. :)

Donaldjr1969 writes:
Probably NOT a good idea to add the fluid, install the spark plug, and give the starter cord a few pulls! :)

I should clarify that statement. I DO NOT mean to connect the plug wire to fire the engine. I was referring to merely installing the plug to seal the combustion chanber yet leave the plug wire disconnected. However, too much fluid would hydrolock the engine and possibly even bend the valves if one pulled the cord forcefully enough. Try explaining THAT one to the Stihl techs! ;)

Don

donaldjr1969
I hope you understand the 4-Mix engine is completely different from the picture you posted. As stated in my earlier post, the valves are in the head and are driven by rocker arms in the head. I'm sure everyone understands you cannot turn over an engine with the cylinder full of fluid. The de-coking fluid has to be completely drained before restarting the engine. I still find it interesting that nobody has come forward yet explaining in detail how they de-coked their engine. I have to believe it is because the engines are new, and most have not needed it yet. From what I read in other posts, the professionals are not going to do it until the engine is not running right. At that point in time, they may even decide they got enough use out of the tool to justify simply replacing it when it fails. My BR600 and KM110R start and run great. I want to maintain them so they last a long time. Charles Ranheim

Charles,

Yeah, I understand that the 4Mix is indeed a totally new engine. And after reading your last post, I can see where it is an OHV enging just from the shrouds. And I would never turn over an engine full of fluid. I've seen photos of hydrolocked engine damage and it is NOT pretty! I was mererly inquiring if it were maybe possible to put in a tiny bit of fluid and give it a gentle pull or two. Certainly nowhere near enough fluid to fill the combustion chamber when the piston is at TDC. Then, after decoking, the plug is removed and all fluid is expelled from giving the starter a good yank or two. I have done this on a 2stroke snowthrower that I "degunked" years ago. But then Charles, while not impossible, it would be harder to hydrolock a 2stroke as most of the excess water would be blown out the exhaust port during the upswing of the cylinder. But once again, I did not fill the entire chamber with gumout. I poured in a little bit while the cylinder was at TDC. And the excess went out the exhaust port during cranking with the wire off. If anything, restarting the engine with ether when I was all finished was probably the most dangerous thing I did. ;-)

As for the old engine, I posted it so that other readers of this thread that may not understand what I mean by a flathead engine can see for themselves. Most lawn mower engines, and I am guessing other 4stroke trimmer engines, are of the flathead design. A very simple design to work on unless you need to replace a valve. Yeah, I understand my engines. :) While I have never done a full rebuild of an automotive engine, I have helped with head gasket replacements and cam changes on SB Chevy engines. Surely if one can do that, anything I may need to do to my 4mix in the future would be a simple task.

Yeah, I know I may seem pessimistic at times, Charles. But keep in mind I am a DIY person with regards to small engines. I have the tools and would only need to buy feeler gauges to work on the 4mix. As it is only new with 3 hours use, I know decoking is a long ways off. However, since this is an engine totally new to me, I may ask questions that may seem redundant. But they are for the purpose of gathering information. As you said earlier, maybe somebody has already done a decoke and valve adjust. And I can easily apply their experiences for my own use. While my Stihl dealer is a first class operation and is Gold Level certified, if I can do the maintenance/adjustment work myself, that is more money in my pocket. After all, if my car needs a brake job or, say a tie rod end, I do the work myself. So therefore I would do the decoke/adjust myself. That gives me more money for future accessories! :-D

Don

Not sure how much this helps, but if you have the regular stihl oil not the synethetic, why not mix a little seafoam into the gas can when your mixing your oil. Seafoam used through the fuel system is going to remove any deposits in the fuel system and it also removes carbon deposits from the valves and piston.

I have never used Seafoam, but I have heard many good things about it. However, I would worry about mixing it in 2 cycle oil where it is not only used to treat the fuel, but must lubricate the bearings correctly. The STIHL Ultra oil not only burns cleaner, it is supposed to be a superior lubricant. I, myself, would not take the chance it might not provide the necessary lubrication needed for the working parts of my expensive power equipment. I still have some ECHO 2-cycle oil, which is a synthetic blend. I plan on using up this oil in my ECHO equipment, and making a different mix for my 4-Mix engines using STIHL Ultra oil. I have to admit mixing Seafoam with the lower quality 2-cycle oil may be better addressed by someone else. Charles Ranheim

den69rs96
I finally purchased a can of SeaFoam for the first time. I could only get it in the spray can from PeP Boys. It says on the can it will remove carbon deposits. However, I still worry about the Seafoam passing through the bearings on the way to the cylinder intake valve. I thought of spraying it in the spark plug hole, but it would not get on the valves that way because the valves are in the head. What I could spray in the spark plug hole would only land on the top of the piston. Why don't you call or write to STIHL about using Seafoam in their regular 2-cycle oil to help prevent carbon buildup. My guess is that they will not make any recommendations or comments about someone else's product. Good Luck. Charles Ranheim

In another older thread there is a post by Nevada Walrus, he is very experienced, and I must agree with his advice, use the ultra! These little four strokes are not like any other engine I, or you, have probably ever had apart before, and they have thier own set of problems, best listen to the guys that work on them all the time, and until you and I become real familiar with them, leave the decoking to them. Why? Some of the carbon eaters will also destroy seals if you get it in the wrong place for very long, I know this from experience, and in order to do the job the Stihl stuff is quite potent, not something you want to mess with casually.
As for Seafoam, it is non-destructive in two strokes and doesn't harm the carb parts, incredible, great in outboards that sit for long periods in the off season. I have no idea if it is effective in Stihl's 4-mix motors.
I have another reson to use the Ultra, I've become more and more sensitive to chemicals, and I seem to be affected less by the Ultra, than by any other oil I've used.
My conclusion is this; I repair all kinds of engines for a living, I'd rather not have to fix my own more than absolutly necessary, so I use the best oils available - they really make a difference! I worked at a camp that used "green death" , hey it was donated and there wasn't much money to go around. They eventually threw it out and bought good oil, that stuff ruined more weed-eaters! And I got tired of fixing them!
As far as what Stihl is going to admit, suggest, advise, require, just understand the implications if they say anything. Just use the good oil, and enjoy your equipment - they try hard and are not perfect, only one perfect, (and I'm not him)
R

I just purchased a stihl FS70RC-e I am noticing oil coming from the exhaust in the form of small spots. within 30 minutes time of running it was enough to cover the inside of the surrounding case. I was wondering if this is normal possibly burnoff due to being brand new or if this is a bad sign.