Fuel mix 20:1 or 50:1

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LabLover

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I picked up a used, near new condition Stihl HT-131 (4-mix engine) pole saw for $200, yesterday. I was discussing fuel mix with the previous owner and he was telling me that he likes to use 20:1. Stihl specifies 50:1 and it goes against my grain to deviate from manufacturer's engineering specifications.

I checked the plug in the machine and there is no fouling.

My (probably stupid) question is; aside from fouling, what are the downsides of running such an oil rich mixture? I expect the engine would not achieve peak performance and that starting would be more difficult? Having never deviated from fuel mix specs, I would appreciate comments from anyone who has first hand experience with this type of situation.

Thanks.
 
20:1 is a leaner mix than 50:1 - more oil, less fuel. Downside is more smoke, carbon build up and maybe power loss.
Have you pulled the muffler to check the P&C for carbon build up? Whatever you run, be sure to tune the carb for that mix.
There are MANY threads on fuel mix here, try searching and prepare to do a lot of reading.
 
I have a Stihl FS91, it is either the first or 2nd year production for the 4-mix engine. Countless tanks of fuel and more hours than I care to think about on that little machine. Nothing but Stihl Ultra through it at 50:1 and it is still going strong. If it is down on power, check the valve lash, that will wake it right back up.

In my experience running too much oil only causes problems. Lessons learned the hard way.
 
Many thanks to all who replied, I have certainly learned something. I will continue using 50:1, might switch over to synthetic and will adjust the carb on the new pole saw. Does anyone have a recommendation for a good synthetic that gives the best bang for the $$. Also, does anyone have a recommendation for an inexpensive tachometer for occasional use with chainsaws?
 
Many thanks to all who replied, I have certainly learned something. I will continue using 50:1, might switch over to synthetic and will adjust the carb on the new pole saw. Does anyone have a recommendation for a good synthetic that gives the best bang for the $$. Also, does anyone have a recommendation for an inexpensive tachometer for occasional use with chainsaws?
Stihl Ultra HP is technically a "synthetic", by definition...It is an ester, which is a Group V oil. In the USA, Group III, IV, and V are labeled synthetics. If I remember correctly, Ultra HP was specifically designed for use in the 4 mix engines.
 
Many thanks to all who replied, I have certainly learned something. I will continue using 50:1, might switch over to synthetic and will adjust the carb on the new pole saw. Does anyone have a recommendation for a good synthetic that gives the best bang for the $$. Also, does anyone have a recommendation for an inexpensive tachometer for occasional use with chainsaws?
Been using Woodland Pro synthetic from Baileys - site sponsor, for several years - no problems.
 
STIHL 4-MIX technology works using a four-stroke rotation principle while lubricating all the internal parts via the two-stroke fuel mix, hence the name 4-MIX
 
STIHL 4-MIX technology works using a four-stroke rotation principle while lubricating all the internal parts via the two-stroke fuel mix, hence the name 4-MIX
So basically the same thing as a 2-stroke, but with only half as many power strokes?
Or...all the bloat of a 4-stroke, but with the added benefit of aerosolizing the drain oil into the air?
200.webp
 
So basically the same thing as a 2-stroke, but with only half as many power strokes?
Or...all the advantages of a 4-stroke, but with the added benefit of aerosolizing the drain oil into the air?
Winning!
Stihl some how managed to produce an engine with none of the benefits of a two stroke and a few less than a traditional four stroke..
And I about spit out my coffee when the mentioned light weight in that marketing piece.
 
Sounds like it's a full on 4 stroke that uses the premixed oil/fuel to lubricate the engine. Like BWalker mentioned non of the 2 stroke advantages .
 
So basically the same thing as a 2-stroke, but with only half as many power strokes?
Or...all the bloat of a 4-stroke, but with the added benefit of aerosolizing the drain oil into the air?
200.webp
I would say None of the above.

4-Mix is a 4-stroke engine. It somehow routes the intake air/fuel charge through the crankcase so that the oil lubricates the internals before the charge makes it to the cylinder. No oil sump required. Exhaust temperatures are much higher on a 4 stroke engine vs. a 2 stroke, resulting in more complete combustion of the oil along with the fuel for reduced smoke and probably emissions.

I have 2 FS 91 trimmers with the 4 mix engines... they are wonderful. More torque than 2 stroke... they don't bog down as easily. They sound like they are running slower, but they're not.

And they are lightweight. I can't discern that the FS91's are noticeably heavier than my older FS80 two stroke. The difference to me is negligible.
 
What would be the advantages of a 2 stroke that the 4-mix engine would not have?
Two strokes are considerably lighter. A four stroke only fires and makes power every other rotation, therefore it takes a motor with nearly twice the displacement to produce the same power as a two-stroke
 
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