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Small engine repair - any ideas? CONCLUSION

AJB Temple

AJB Temple

Sequoia
Joined
Apr 15, 2019
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Name
Adrian
Anyone used to small engines (this one is 35cc) have any ideas? Old gentleman in the lane has brought his 7 year old chainsaw to me and asked me to have a look at it as he has seen me use my similar saw. (Using vs fix are 2 different things). He uses it to trim his apple trees. He can’t start it. It’s a Stihl MS211, without the easy start and tool-less chain tensioner (ie the base model). The local fixer he uses has quoted him £300 to repair and service the saw :eek:, which is uneconomic as a new 14” basic one is about £340.

I dumped the fuel (which upset him) and put in a new plug gapped at 20 thou, fuel line filter and air filter: cheapo clone ones, not Stihl branded and some pre.mix ethanol free fuel. Engine burped on full choke on a few pulls, and fired up first pull on choke off.

However, it runs like a three legged piglet. Bogs down and hesitates a second or two before throttling up. Chain is not too tight and is on the sprocket. Chain brake works. Muffler and spark guard looks clear. Fuel line looks OK (not rigid). Viewing piston and cylinder through exhaust port looks OK - no sign of scoring or much carbon build up on the piston.

I don’t have a vacuum pump or compression tester that will fit this motor, but my instinct on pulling it over is that the problem lies in fuelling. He’s been putting cheapest grade petrol in his garden machines, and doing his own 2 stroke mix with the bottle method and is a bit confused about what ratio he used. Left with fuel in over winter. My guess is the 2 carb diaphragms are knackered by ethanol.

I’ve never stripped a carb off a small motor and never replaced the diaphragms. It’s all rather small and fiddly from a quick look on YT. Before I get him to purchase new rubbers and a proper NGK plug and new fuel line, does anyone have any other ideas as to what could cause loss of power and poor throttle response. Nice old couple (I bake for them) and I will help if I can, but have warned them that I am just an amateur. Spent £9.99 so far on clone bits (fine for air filters) but good quality NGK plug and the carb kit and and new fuel line kit will run to at least £60 ish delivered I guess for proper parts. Worth a punt for him?
 
A clone carb will set you back about £20 and is probably the way to go. The diaphragm in these carbs can harden over time leading to poor running. From past experience it's easier to swap the carb than mess about with a kit
 
It could be that the diaphragm has hardened after fuel has evaporated locally. I've Chickanic get them running and keep revving up and down for quite sometime to soften the diaphragm. In extremis, a carb kit might do the trick.
 
Hi Tris. Thanks. Yes I've seen the PRC made clone carbs and it is amazing how they make and ship them for that price. I try to avoid Chinese stuff personally but it's not my saw and I will ask him what he wants to do. A Stihl replacement genuine carb is around £85 plus the fuel line and plug I think. Will try to establish exactly what part us fitted. I have a Stihl MS211BC as one of my saws and the carb on that is visually different to his. I've also realised whilst running it again just now and fiddling with the mixture, that his half choke setting doesn't appear to work and the top trigger paddle is gummed up.

9F - yes I've seen chickanic and the Canadian lady "married with small engines" who is very methodical and does proper tests. Hence arrived at ethanol knackering diaphragm's. There's not much to these engines but without being able to test compression and air leaks it is just a guess and I don't want this to be a time pit for me. If he's happy with a clone then I will get him to buy one and I shall stick that on as cheaper than stripping the Stihl carb and replacing diaphragms and needle and a lot quicker for me.

Thanks for the links. Super helpful.
 
PS I tried chicanics run them up and down trick for 20 mins or so, but I can't get rid of the throttle lag, which is disconcerting in use, and it never seems to get to full chat for long - it bogs down under almost any load and I've resharpened the chain so it's not that.
 
I can’t offer any advice to Adrian but I am interested in the idea that ethanol mix can have a adverse effect on the carb/engine.
I’ve been running a Stihl Kombi system since we came here, 15 years or more. It has a Stihl 4-mix engine, a 4 stroke engine running on a petrol/oil mix. I’ve been using 10 % ethanol unleaded petrol and (tempting fate) have had no issue with the motor after the usual winter lay up. I do not empty it of fuel either. I’ve not even changed nor cleaned the plug since new either. I am not advocating this as best practice. Have I just been lucky?

This is the current model of what I have with strimmer, brush cutter and long reach hedge trimmers.

KM 111 R Petrol KombiEngine | STIHL

Powerful STIHL 4-MIX® engine with 1.05 kW ✓ Use this professional power tool together with our KombiTools!
www.stihl.co.uk www.stihl.co.uk
 
Yep, it's a known problem Andy, Roger is right. Ethanol degrades and gums up the rubber components such as carb diaphragms, flexible fuel lines from inside the tank through to the carb inlet, and gums the filter in the fuel tank. It's much better to use ethanol free fuel made for the purpose in 2 stroke motors. I know yours is a 4 stroke but lubricated by oil in the fuel, so maybe your tool is less susceptible.

Pre mixed ethanol free costs me about £19 for 5 litres currently. So not that cheap, but to put that in context, 2 litres has been enough in my own chain saw to take down a few medium sized trees and cut them into 8" logs which close stacked is at least 3 cubic metres in our back log store, which we did in the past few days. I've got about the same amount of wood again to go which is longer term log burner stock. Then two much larger trees which I may not risk taking down myself as I'm a scaredy cat.
 
According to a reply in 2022 on a Stihl UK blog post https://blog.stihl.co.uk/how-will-e10-petrol-affect-your-stihl-chainsaw/
Stihl changed the gaskets and fuel lines on their engines 18 years previously (2004) to make them resistant to E10 fuels.
They still of course recommend not storing E10 for too long and emptying fuel tanks when not being used for extended periods.
I guess I will have a problem sooner or later. My Honda mower has also been run on E10 but that has had a couple of oil changes and new plugs since new,
 
^ Their school report should say "could do a LOT better".
 
My 13-year old Viking mower has a Briggs & Stratton motor and is approved for E10 fuel. I change the oil and air filter every year, and can't remember the last time I changed the spark plug. It still starts on the first pull every time.

My new Stihl RL 540 scarifier has a Kohler motor, and is approved for E10 fuel.
 
I have fitted a cheap clone carb to something, I can't remember what, and it runs fine. That was after fitting a replacement diaphragm kit which made no difference.
 
There is an excellent youtube channel, which specialises in Stihl chainsaws: "Married with small engines".

They're a Canadian couple, who own a business selling and servicing chainsaws (Stihl service centre, I think), lawn tractors and other small-engine machines. Mrs. mostly repairs and services the chainsaws. Useful points from watching Erica work on these saws:

0. Ethanol mix does stuff the rubber bits, including both the fuel pump and diaphragm sides of the carb. Replacement of those is ridiculously easy.
1. Stihl do carb service kits, considered to be a better option than 3rd party carb replacement (after proper cleaning!).
2. Also at risk are the fuel hose and the air inlet for the fuel tank. Again, both are easily replaced.
3. If it's stood unused for a while, particularly with Ethanol fuel in it, it's worth rinsing out the tank and changing the fuel filter (lives inside the tank on the end of the fuel line).
4. Pretty much everything you need to do on the saws is accessible with either a T27 Torx or M8 nut spinner (latter for the carb and inlet stuff).

I used to play with small 2-strokes (didn't we all?), and frankly, Stihl saws look ridiculously easy to work on. The Canadians, who I think are an authorised service centre, use a specialist combined vacuum and pressure gauge for testing the fuel system and the crankcase seals, but you could improvise with a manometer (Plastic tubing in a U shape) for vacuum, and probably any sort of low-reading pressure gauge (e.g. I have several rescued from old boilers (with tubing) that read well between 1 & 3 bar).

If they have a video on this specific saw, I'd watch it before coming to any conclusions, as there are various common problems with specific models. I have to say, that the service/support from Stihl for them seems to be really good. I know, Canada is the land of the chainsaw, and it may not be so good here, but they do seem thoughtfully made, and you can see design improvements incorporated into later generations (not just cost reduction for manufacture!).

Hope that helps.
 
Thanks Eric. Very good links. Yes, I mentioned her in a post above as I came across he a few days ago. Erica is the best chainsaw person on YT in my opinion - far more to the point and methodical than chickanic, who is also good. She does have a 211 on her site - I found the one you linked to last night :rolleyes: . The 211 is a homeowner saw and as she says for pro shops the labour cost of repairs is not worth it - gets too close to a new saw cost. This is exactly what my neighbour has found. They are a great little saw if looked after properly.

I know people say E10 fuel is totally fine on their tools. Good for them I say and who am I to argue. But that is not the experience of the pro repair shops. Even Stihl on their current website say that E10 fuel should not be kept for more than 30 days as alcohol separation occurs and even shaking the container well does not remix it properly. It also encourages water apparently. In reality guys like my neighbour buy a 5 litre can of fuel from the pump and it will last him for a year or more. I use more than that (quite a bit more) and I find ethanol free fuel is far superior = no starting and running hassles. In my case (and my neighbours) I will do a shed load of sawing for a week or so, and then not touch the saw again for six months.

Anyway I've picked up a diaphragm kit from a Stihl dealer, Godfrey's in Sevenoaks, a genuine filter set and an NGK plug, plus a new chain and bought him 5 litres of ethanol free high quality fuel from the local mower guy / gun dealer :eek:. Hopefully I will be fixing it this afternoon after dealing with the bees. He's happy to pay for the bits and I'm being paid handsomely for my time as his wife (who is 84! and still keeps chickens and turkeys) has made me a ginormous steak and kidney pudding and a chicken and leek pie :cool:
 
Carb stripped and both diaphragms / gaskets replaced along with needle, new fuel line and filter, new oil filter, new chain (turned teh bar over as well as deburred the slots), new fuel cap (found gasket damage to old one) and new plug. Diaphragm was knackered - stiff and felt a bit like baking parchment. Took me about 1 hour and 40 mins to strip, clean and re-assemble the carb, following Eric's demo on her YT web site and do all the other bits. Trickiest part was replacing the fuel line in the tank, as I don't have the special gripper tool. Reset carb from fully shut (screws all the way in) to settings that ran well. Now runs and accelerated fine, does not stall or bog down and idles fine.

I was going to post snaps, but actually I highly recommend the site Eric linked to above.

De-burring the bar and turning it over periodically is a good idea I now know. This one was really quite worn and my neighbour had also filed the teeth unevenly on his chain, so one side was noticeably shorter than the other. Almost certainly the reason for the saw bogging down in the cuts and not cutting straight.
 
Good to hear it's running again, sounds like you have an excellent system of exchange going there. Seems that the sweet smell of success is a steak and kidney pudding 😋
 
Ha! Yes. Will be eaten tonight.

Had to fire up my biggish hedge cutter as well: Stihl HS 56. Wouldn't fire, even with fresh fuel. Checked the air filter and got rid of dead bits of hedge, then squirted in the secret weapon "easy start" spray with filter off. Fired instantly. Put filter back on and good to go. Easy start is like magic.
 
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