[chrismahon]

We have a small 2 stroke Still strimmer FS45- about 20 years old now. After the starter ring disintegrated taking the pull start mechanism with it we waited 5 weeks for the parts to arrive, so are a bit behind with work. For some unknown reason sometimes it just won't rev. Cleaned the plug but it made no difference yesterday, but tried again this morning and it didn't miss a beat. Now yesterday it was 28C and this morning 20C- perhaps there is an operating temperature limit, although there is nothing in the manual. We had a warm spell just before it broke and had problems then. Running with 95 octane fuel and Still 1:50 oil mix.


Has anyone else had this problem with a 2 stoke engine?

[Maysie]

Could it be some crud in the carb? 

My Stihl Kombi has never suffered from what you describe and has been used in some VERY warm weather, but my old Husky chainsaw used to run like an old dog until I gave it a thorough service and carb clean.  It is good as gold now. 

[cloddopper]

Sounds like the highly volatile oils in the petrol evaporated around the reed valves in the carb & thus made a glue of thicket than normal 2 stroke oil that either blocked a jet or restricted the reed valve movement 
.
 Always running the machine out of fuel at the end of a session ,  pouring  out any extra fuel into clean a store can with a lid if needs be before running it dry  helps tremendously .
It also helps stop thicker evaporated mix being present to clog the little filter on the end of the fuel line that is in the tank .

 When it next comes to filling if you haven't already used th stored mix else where  pour a little fresh made fuel in the can and fill up with that.

Do you used a " Fuelsaver"  , an additive you put into modern petrols & 2 stroke mixes  to stop it oxidizing and otherwise deteriorating ? Old petrol 2 months or more is not so good to make up a 2 stroke mix with , especially in warm climes.

[chrismahon]

I'm using the red Still oil at the moment, but I have their long-life stuff which is supposed to be for keeping the mix fresh during longer storage. I always empty the fuel tanks on everything, just because of the fire hazard in an attached garage. The petrol for the mix I bought on Saturday, so it is as fresh as possible, but it is E10, so 10% ethanol and I wonder if that's a factor?


You are probably right about a good service Maysie- it's still on the original spark plug and air filter.

[Zyg]

Sounds like fuel starvation - have you tried cleaning the air filter

[chrismahon]

Think you are right about fuel starvation Zyg because the more I open the throttle the worse it gets and it will actually cut out completely. The air filter looks clean and I have blown it backwards with compressed air to be sure. I wonder if it is some kind of vapour lock in the carb caused by heat?

[cloddopper]

I'm using the red Still oil at the moment, but I have their long-life stuff which is supposed to be for keeping the mix fresh during longer storage. I always empty the fuel tanks on everything, just because of the fire hazard in an attached garage. The petrol for the mix I bought on Saturday, so it is as fresh as possible, but it is E10, so 10% ethanol and I wonder if that's a factor?


 Ethanol kills most soft pliable rubber,  so if you have an old machine it might be worth checking  ….the lift tube is perhaps a buttery coloured orange , if so it might have a tiny crack in it ..the latest replacement genuine Sthil I fitted in Aug last year were a  milky yellow supposed to work with ethanol in  fuel.

 That also makes me think about checking the diaphragm in the carb it should not be wrinkly .if so get a repair kit and fit a mew one
.

  Photograph every step you take dismantling it .. the neck top computer is a lot more fallible them most people think . 
 

You are probably right about a good service Maysie- it's still on the original spark plug and air filter.

[chrismahon]

They sell petrol here in 4L containers, specifically for garden machinery. Next time I'm in the shop I'll look at the labelling, because perhaps that is ethanol free to avoid the rubber deterioration problems? Still don't know why it works well one day and not the next though?

[chrismahon]

It's 25C and I've just been strimming under the electric fence. Once again I had a period where it wouldn't rev fully. The fuel level in the tank is above the feed pipe assembly and I could see bubbles forming above it. I know that one pipe supplies fuel to the carb and the other is some kind of return, so is it returning vapour to the tank?

[cloddopper]

The only way I know that you can get pressure back through th carb and possibly the fuel lift tube is because of crank case compression . You say two tubes  into the tank...  interesting .   One may be a return but that's not common on a two stroke as there is often a tiny needle valve in the carb .
 Though that said when the diaphragm  pushed one way it may also push on the return stroke so could well be a return line to stop the diaphragm  rupturing under back pressures if added ethanol fuel is used .

I also wondered if it is a breather tube for the fuel tank, though most fuel tanks have a breather in the filler cap .. they do get blocked sometimes and cause a form of fuel starvation as the fuel in the tank is used  a slight vacuum starts to develop in the tank . Which  makes them almost impossible to start till the vacuum is broken ( undo the fuel cap and then re tighten it ..if it cures the problem look carefully at the fuel cap for a 0.04  mm breather hole .. they're often found in the wings of the cap , use a strand of copper wire from some electrical flex to "  proggle"  it clear . Then wash the cap in clean petrol before you put it back on as you don't want the crud going in the fuel tank .

  Does your machine make a ( Wait for it  ,wait for it ) wow wow wow wow wow ....................sort of up and down noise when it's running on full throttle rather than a deep throaty continuous roar ?
 If it is rising and falling it indicates crank case compression ?
 
 If so you have a worn barrel & piston & only a repair will sort it out .
 You can check it out by removing the silencer and looking in the port to see the side of the piston , it should be clean slightly oily & shiny not matt or scratched .

[chrismahon]

The bubble on the carb is full, so no problem drawing fuel Cloddopper and it used to rev fine, but the last time I used it the temperature was cool and after a few minutes it stopped revving completely, so worse than before. Waited for it to cool down and tried again, but still wouldn't rev. YouTube features someone who claims to have stripped a carb down and refurbished it, but only showed it coming apart. and I suspect they couldn't re-assemble it. Clearly servicing one is going to be a nightmare so I bought a cheap replacement from China for €13, complete with gaskets, plug, fuel pipe and air filter. Not a perfect fit but a few minor mods to the air filter housing and it all went together and now runs fine.


I've bought a cheap Chinese carb before for a Stihl MM55 tiller which wouldn't pump fuel up (stuck non-return valve). Had to modify the original air filter for that as well, but the carb works fine and has done for 2 years. So if anyone has problems like me I'd say don't waste time messing with it, just fit a new carb.

[Jonah by nature]

Hi, had a similar problem with two strimmers that the step son offloaded to me. He had been using his stale pre mixed "wonder fuel"?In them. Emptied the tanks of this crap and put fresh mixed fuel and gave them a good dose of carb and choke cleaner. After a few very smokey starts they now run like a dream!... a good clean does wonders!

[cloddopper]

 Nice one Jonah .
Sounds like the fuel in the carb evaporated  and left thick oil in the reed valves which then slowly evaporated  & stuck them to the operating faces ..

 That's why it's so important to run the 2 stroke machines dry before you put them away for much more than a week even if you do have the fuel saver additives in the fuel .

The running dry of the machine sucks out all of the fuel mix in the carb & a new charge of the correct mix ratio is usually enough to unstick anything that has stuck due to the micro thin film of oil left behind .