# Stihl SH 85C Loses Power



## Blower85

I have a four-year-old Stihl SH 85C blower that has developed a problem the last three times I have used it. After running at full power for about 10 minutes, the engine speed slowly decreases until it barely runs. If I shut it down and let it cool off for about 5-10 minutes, it starts right up and will run fine for another 10 minutes or so until the problem repeats.

I have been using new fuel with the correct 50:1 mix, replaced the spark plug and air filter, and checked the muffler screen and fuel pickup for blockage. Everything seems normal, so I am wondering what could be causing this. Like I said, it runs perfectly for about 10 minutes and then starts to die for no apparent reason.

Thank you.


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## Basementgeek

Hi and welcome to TSF

When it does die on you, try starting right away, is it harder to pull start than when it has cooled down?

BG


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## Blower85

BG...thanks for your reply.

I did what you suggested and found that it is not harder to pull start. After it dies, it starts fine and idles normally, but if I engage the throttle again, it stalls out. I let it cool and everything works perfectly for another 10 minutes or so.


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## Basementgeek

Hi:

Well that is good news that it pulls the same.

When starts giving you trouble, does giving it some choke help any?

BG


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## Blower85

Hello again BG,

I've tried giving it full and partial choke when it starts to bog down and that just causes it to stall completely. I also tried slightly pressing the primer bulb, but that makes it stall also.

I've read about other Stihl blowers having a similar problem, but not after 10 minutes of normal operation. Others report that their unit starts fine and then dies as soon as they give it throttle, leading to discussions on carb rebuilds, etc. I have a Stihl chain saw and another manufacturer's string trimmer and have never encountered something like this.

Maybe I just need to have the dealer look at it, but I was really trying to figure it out myself, with the help of others in the online community. 

I'm thinking that there might be some part of the carburetor that is affected by hot engine temperatures, causing it to fail until it has cooled off. I don't know enough about the internal workings of the carb to figure out what that might be, so maybe I'm off base with that theory.

Thanks for hanging in there with me on this. :winking:


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## Basementgeek

Hi:

First I have moved this post to small engines from Yard and Garden, it is a much better fit here.

It could be that the gas tank has a clogged up air vent on the gas tank, try running it for awhile with the gas cap loose. Does that help?

Next:

Before you get into rebuilding the carb, need to check for spark immediately when it dies. If you have a good blue/blue white spark to coil should be good.
Now let cool down good, and check for spark again, does the spark about the same? Coils have been known to break down when they are warm.

BG


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## 2and4StrokeTech

Well you are in luck, I am a Stihl Silver Certified Technician! 
Believe it or not the most common reasons for the problem you have described is the fuel filter(in-tank), fuel strainer (in-carb) or the tank vent.
The fuel filter does NOT have to be clogged just restricted to cause a steady lean out.
Remove the filter inside the tank and test run the unit. if this cures the problem replace the filter. 
Tank vent....run unit until it begins to loose power then loosen the fuel cap. If the engine picks back up, replace the tank vent.
Carb fuel strainer.... remove & disassemble carb examine strainer for debris. Carefully remove strainer (do not poke a hole in it) and "feel" the stainer. If it is stiff (or hard) and you cannot see light through it replace it (rebuild carb)


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## Blower85

BG and 2and4Stroke,

Your suggestions sound great and I should have a chance to try everything this weekend. I'll post the results as soon as I can.

Just wanted to say thanks for the replies. Hope you guys had a great Thanksgiving.


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## Blower85

2and4Stroke,

I used the blower today and of course, the same problem occurred. I should point out that the engine speed decreases *very slowly* until it dies. It might take 5 minutes or so from the time it starts to slow down to the time it dies.

I removed the in-tank fuel filter and that did not solve the problem, nor did loosening the fuel cap. As for removing and disassembling the carb, I've never done that kind of thing before and am not sure if it is something that should be left to a pro repair person. Is there anything particularly tricky about this procedure and rebuilding the carb? I guess if I try it and screw it up, I can always take it to the dealer with tail between legs. :neutral:

The thing I do not understand about the carb fuel strainer is why would the unit run perfectly for 10 minutes and then start to die? If the fuel strainer was blocked, wouldn't the symptoms occur right away?

I haven't tried BG's suggestion about the spark/coil, so I'll do that next.

Thanks again.


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## 2and4StrokeTech

Well you have done all the tests I would have preformed and in my opinion you need to take it to a repair shop. In my experience your carb needs to be rebuilt but you might have another problem. As for the fuel strainer, think of it this way (the number are just examples): If the carb must deliver 1 gallon of fuel per hour but the fuel filter and/or fuel strainer are partly clogged and can only only pass 0.8gal/hr, it would take almost 10 minutes before the engine started to "lean" out. Every minute that passes it would be getting just a little bit less fuel. after a few minutes the engine can no longer run properly.


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## Blower85

Thanks again 2&4. I was looking at a lot of YouTube videos on Stihl carb rebuilds and it actually looks pretty easy, plus the rebuild kits are relatively inexpensive. Like I said, it is worth a shot and the worst that can happen is that I have to take it to the dealer if it still doesn't work. I am a little surprised that the unit would need all of this after only fours years. For home owner applications (< 1/2 acre of property), do you find this to be a normal interval for a rebuild? I follow all the recommendations for storing the unit during the winter.

*BG: *If you're still following this, isn't the coil on the side of the unit with the pull starter? If so how can I check the spark at the coil with the unit running? (I would imagine you shouldn't remove the cover while it's running.) How would I restart it after it cools if the cover is off?

Thanks again for your help and advice.


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## Basementgeek

Hi:

Stick you spark plug wrench in your pocket 

When it dies, remove the spark plug, ground the outside to the plug plugged into the wire, to a metal part of the engine, pull the recoil starter rope.
No need to to remove cover.

Also, 2 cyc oil and gas does not store well, goes bad quick. I never keep it on hand pre mixed, I mix it as I need it.

BG


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## Blower85

Ok guys...here's the update. I was at the Stihl dealer the other day to pickup some parts for my chainsaw and one of manufacturer's reps was there. I mentioned the problem I was having and what I did to try to diagnose it. He immediately said it is probably the coil, so it looks like *BG* was right on target. Even though my unit is four years old, the rep said it would be covered under warranty.

I took the blower to the dealer today, told the tech what the problem was and he said he'd check it out and let me know if it would be covered under warranty. I brought some notes explaining what I had tried already, but he didn't want them. He just gave me a receipt for my blower and all it says on it is "Do heat sink test, will not run for long". Not a terribly friendly guy, but he probably knows what he's doing and just didn't want to get into it with me.

I've done a bunch of web searches for heat sink tests and find no reference related to small engines. Can any one explain what this is?


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## Adamd85

Hello blower85 

I'm also a silver level tech from a dealer. Not really sure what a heat sink test is but kinda sounds like u r losing spark. Which as long as u r the orginal owner is lifetime warranty. Now stihl has a tool to test ign module but unfortantly i dunno how to do this without. As long as unit runs great leading up to this. then just quits id put my chips on coil. How long can u run unit before it dies? If within 10-15 minutes id replace ign module


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## Blower85

Adamd85: Thanks for your reply. I am feeling a little unsteady about the dealer that has my unit. The guy wasn't very friendly and seemed like he didn't want to bother talking to me about the problem. They are a big Stihl dealer and have had the blower for over a week. I just called and they said they have 15 other people ahead of me, so I might pick it up and take it somewhere else.

My unit runs perfectly for 10-15 minutes and then slowly dies. I let it cool off for about 5-10 minutes and then it starts right away, runs perfectly for another 10-15 minutes and then slowly dies again. You and BG seem to be in agreement about the coil, so I hope I can get that replaced under warranty.


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## kbowley

Hi,
This is what I understand from your description; The blower starts fine (good compression, ignition, fuel) and after working it at full throttle the engine will slowly lose power as if it is not getting fuel and eventually will not run at full throttle. The engine will start back up but only run at idle unless you wait for it to cool down. 

With that understanding, 4+2 stroke is on the right track and you have eliminated the first things to check on the trouble shooting list.

The next logical steps are carburetion, exhaust, cooling and ports. I suggest first that you remove the muffler and ensure the screen is clean, remove shrouds and ensure the cooling fins are clean and airflow is not obstructed. Next I would purchase a rebuild kit for the carburetor, Make sure that after removing all parts and disassembling that you spray carb cleaner through every orifice you can find...do not used compressed air as you may damage the unit. Ensure gaskets and rubber are in good shape with no cracks and are properly sealed. If the engine has reed valves, replace them. I believe that engine uses a Zama RB carburetor. Ensure that the pulse port is clean and is aligned with the carb, ensure the primer bulb is in good shape and not sucking air, same with fuel lines. Also, you can purchase a new carburetor for a cost of around 35 dollars.

Ensure that you have the correct heat range spark plug, If i recall that engine uses an NGK bpr6, I may be off on the letters but I am quite sure the heat range is 6.

Hope this helps


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## K-B

Blower85, do you have any updates on this unit? I am interested in finding out what the dealer decided. My first guess would have been tank vent, but since you eliminated that, I would also guess it to be the coil. Nowadays, coils don't always outright fail or lose spark, there's a lot more technology being used in coils (i.e. digital controllers) and when they fail they can create some pretty weird symptoms.


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## kbowley

"_Not really sure what a heat sink test is"_

A heat sink test is talk down for simply running the engine at full load/full throttle until it reaches a temperature that no longer increases under full load, it remains constant, its as hot as it will get. If the engine begins operating erraticly before or when the temp peaks, the cause is trouble shooted dependant on the symtoms.


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## Blower85

*KB:* I finally have an update for everyone and am kind of annoyed at the dealer. He left a voice message for me indicating that I had so much dirt in the carburetor, he decided to install a new one without calling me first to approve the repair. He said that everything else checked out ok. The total cost is $95.00. This leaves a bad taste as the unit cost me around $250 four years ago and now it needs an expensive repair like this??? I am finding it hard to believe the carb could be that bad, as the unit does not see heavy use and I am very good about changing air filters, making sure not to let any dirt get into the carb when I change them. I'll be picking up the unit next week and giving it a test run. I'll let you know if the repair did the trick or not.


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## kbowley

It is not uncommon for Zama carbs to go bad, I have replaced several of them. They are far more intricate than a standard carb as they must work in all positions, even upside down. The internals have check valves that are not serviceable, tiny balls with springs inside the galleys. Hence the reason I suggested to not use compressed air to clean it.

The price is reasonable, sounds like he charged an hours labor plus the cost of the carburetor. The Zama RB costs around 40.00 retail.'

I am confident that your problem is resolved based on your description of the symptoms.

It matters not how the unit was used, they simply fail once in a while as does anything. I strongly suggest using Marvel Mystery Oil mixed at one ounce to one gallon to avoid future problems with the Zama unit.


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## Adamd85

Alot of the carb issues we see is due to ethonal in fuel. Simply ruining the diaphrams in carbs research star tron fuel treatment it helps with ethanol issues. And drain fuel out of unit and run dry if gonna sit for a long while. This is we preach to our customers and fairly see rhem return with similar issues.


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## kbowley

The failures of Zama carbs is rarely related to the diaphragms. Rather, it caused from lake of lubrication of the galleys. As the check valve balls move back and forth with each revolution, it wears the galleys out so that the balls either fail to seal or bind. The general consensus is that using the OEM oil greatly reduces wear compared to using generic 2-stroke oil. When using a generic, one should mix 3 ounces to one gallon rather than the 50:1 2.6 ounces. The use of Marvel Mystery Oil virtually eliminates failures as well as being an outstanding fuel stabilizer and water dispersant.

In summary, failures of the Zama units is eliminated if one uses both the Stihl pre-measured 2.6 ounce containers and one ounce of Marvel Mystery Oil per gallon of fuel. No other additives should be used as it is counter productive.


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## Blower85

Hello and Happy New Year,

I picked-up my blower and the tech gave me the old Zama carburetor to show me how dirty it was, especially the fuel strainer screen. It was almost completed clogged, so it looks like 2and4StrokeTech was right on the money with his assessment. I'm a bit stumped as to how the carb could get that dirty. I've never had that problem with any of my other equipment. The ignition module tested fine. I tried the unit today and it ran perfectly...problem solved. I was hasty in expressing dissatisfaction about the tech, as he charged me $30 for the new OEM Zama carb and $60 for labor...seems reasonable enough. In retrospect, I should have pulled the carb myself, but based on feedback I was getting, I was leaning toward the ignition module, especially since it would be covered under warranty. Oh well...the unit works great which is the main thing.

Finally, is the use of Marvel Mystery Oil recommended by most/all of you? Should I use it with my other equipment (chainsaw & trimmer)?

Thanks to all who participated in the discussion. I learned a lot and appreciated the help.


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## bozodog

With today's ethanol fuels, you should use some sort of stabilizer. We use Sta-Bil Marine for all our stored gas, 2 and 4 cycle.


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## nutwiss

I know this post is 5 years old, but I had a similar issue: Stihl 85 dying slowly after 10 minutes, often in a smoky mess.... also hard starting.

Adjusting the original ignition coil fixed the hard starting (gapped to 0.2mm / 0.008in - the only suitable shim I could find), but did not fix the dying issue.

A cheap aftermarket ignition coil, again gapped to 0.2mm, has fixed the problem. I've just run 3 tank-fulls through, non-stop without a hint of issues!


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## AVB

A common heat related problem with coils. Slow dying is only one the ways these fail due heat overly sensitive electronics.

I had air blown off and set a fire. Long flames out the exhaust. Engines idling only. Engine running fine at idle and full but accel and decel problems.


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