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PostPosted: Fri Jun 10, 2016 2:41 pm 
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Hi,

My trusty Mountfield RS100 seems to be running a little rough recently. The oil level is OK, it has fuel, the fuel cap is not on too tightly, and I've just cleaned the air filter.

I dug out this service guide for the engine:

http://www.servicelink.org.uk/uploads/s ... 1b8963.pdf

And for the mower itself:

http://www.servicelink.org.uk/uploads/s ... f9a4e7.pdf

And I recorded a short video of it here. As well as sounding rough, you can see it physically move from side to side slightly.

https://youtu.be/_5pa0ZVQYBs

Has anyone had anything similar? I am perfectly willing to give any repair a go. I just can't translate the problem I have into a solution...because I don't know what's causing the rough running. It does smell a bit 'rich', I think. As in I can smell more fuel that usual when it's running. I keep the fuel in a plastic jerry can but I will admit that it is a few months old now. Could this be the reason?

Any help much appreciated.

Thanks,

John


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PostPosted: Fri Jun 10, 2016 3:17 pm 
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My "no-name" mower with a Tecumseh engine was doing the same. "Surging". Carb strip down and clean with carb cleaner and compressed air sorted it (more or less). Drain and replace fuel as well. It can be the governor mechanism, but that's rarer.


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PostPosted: Fri Jun 10, 2016 3:36 pm 
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I just drained the fuel. It had a light golden straw like tinge to it. Is that normal? I asked Google and it seemed that it varied...clear,golden,etc depending on where you got it and additives etc.

I'm more than willing to strip the carb down but no idea where to start with that. Doesn't look like it's covered in the guides. Any ideas?


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PostPosted: Fri Jun 10, 2016 3:50 pm 
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Sorry John, I don't know that engine. On mine the carb was partly covered by the air cleaner moulding. The carb itself and the air cleaner were on flanges with studs and nuts, but yours may be different. The only thing to do in the absence of any drawings etc. is to start carefully pulling things apart, that is remove the air cleaner housing etc. until you can get at the carb. Take photographs as you go especially of any linkages or cables etc. Sorry to not be of more help. Someone might come along who knows the engine.

ETA The fuel I get is a very pale straw colour. Can't hurt to change it if it's been hanging around though!


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PostPosted: Fri Jun 10, 2016 5:30 pm 
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By the carburettor there is a spring with a very long fine leg that is about 3" long that hooks on to part of the linkage bracket. This seems to govern the smooth running and stops that surge. I bent the hook end a bit so it made it a bit tauter again and this fixed the surging on my mower. This video shows a similar spring to illustrate what I am talking of. :thumbright: https://www.youtube.com/watch?v=8atnwA4D5lM

DWD


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PostPosted: Fri Jun 10, 2016 7:26 pm 
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Dave54....no need to apologise. You were more help than you might think! I popped to Halfords and got some carb cleaner. Gave that a go and it does run better for longer now. Sprayed some more in there and left for a while...and it ran even better but still went rough after a while. Not sure if this definitely indicates the carb needs a full clean or not. Unsure.

dewaltdisney...It's interesting that you mention the spring. I hadn't actually looked down there whilst the 'mower was running until after I posted the original question. You are right; there's a combination of about three springs regulating the throttle (I think). They all look OK but maybe you're right and that these have perhaps lost their original tension.

Going to put some effort in tonight on researching stripping down the carb and on the price of replacement parts. There is a local company who do 'mower servicing but I would much rather learn how to do it myself if it's possible and doesn't require specialist kit.

Thanks for all the advice...and keep it coming if anyone else can chip in!

Cheers,

John


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PostPosted: Sat Jun 11, 2016 2:52 pm 
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exploded diagram

http://www.magic-parts.co.uk/acatalog/Mountfield_SP414_RS100.PHP

Airbox off - 2 nuts and the carb slides off the studs look a similar setup to the vertical shaft B&S - take pictures as you go then you can see how to put it back together


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PostPosted: Sun Jun 19, 2016 1:35 pm 
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Thought I would update this thread...

I've taken the carb apart and, well, you know how some people are disappointed when they find the carb is really clean? Not me! There was what looked like rust in the carb bulb (if it's called that). All the gaskets were filthy or perished.

So, I', going to order a gasket kit and change all those. The question is, I've cleaned the carb bulb out but it's still pretty rusty at the bottom. I've cleaned it up as best as I can with carb cleaner etc but it's still a state. Should I also get a new one of these? I am guessing if it's rusty then some rubbish is going to end up going through the fuel system and I'll end up taking it apart again in six months time.

What do we think?


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PostPosted: Sun Jun 19, 2016 1:46 pm 
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I do not know how old the mower is but it might be worth buying a new carburretor which are around £80. I give my mowers a five year life and dump them when they start playing up. It works out at about a pound a week. I have a lawnmower fund in my accounts which I hung a fiver a month into so I have the funds there to replace it when necessary.

Keep us posted John

DWD


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PostPosted: Sun Jun 19, 2016 1:48 pm 
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Thanks for that. It's not a bad idea at all.

Looks like you can't order the replacement fuel bulb thing on the carb (not talking primer bulb here). It has to be a whole replacement.

Will keep you posted. I have taken a LOT of photos as I did this so might post a guide.


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PostPosted: Sun Jun 19, 2016 5:14 pm 
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I note on that link that Flash posted above for magic-parts that a carburettor is only £35 plus postage. At that price it is not worth messing around with the old one I guess,

DWD


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PostPosted: Mon Jun 20, 2016 3:52 pm 
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I would also give the tank a good clean as there is rust in the carb - mowers sit in the shed for half the year the fuel goes bad and you get condensate built up in the tank (unless its plastic that is)

The way I clean tanks is with a load of old nuts and clean fuel or thinners* and then give a good shake for 20 mins, clean and repeat if needed


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PostPosted: Mon Jul 11, 2016 7:25 am 
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If it's really as cheap as £35 it really does not worth messing with, unless you're in a playful mood. Like most ubiquitous tools, planned obsolescence has it's role.. The fastest way would be to replace, but if you have the time to dismantle you will surely have some fun :)


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PostPosted: Thu May 25, 2017 8:38 pm 
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Hi Just wanted to add my findings to this thread in case it helps anybody has these issues.

My Mountfield mower with the RS100 engine recently starting hunting, If the air was restricted a little by placing my thumb over the air inlet to the carb it would run fine.

After trying all the normal checks advised including checking the spark plug and by checking the fuel cap was not too tight I stripped down the carb. Initially the internals looked clean but I missed the 3 ports in the body itself.

On the second strip down I removed the jets and sprayed everything with carb cleaner, then with the straw attached to the carb cleaner I sprayed through the 2 brass coloured ports on the air filter side and the 1 brass coloured port on the engine side of the carb. (use safety glasses as it sprays back a little)

these ports I only noticed the second strip down and it runs perfectly now.

The carb is easily removed by removing the air filter cover and undoing the 2 10mm nuts. the carb is split by the 10mm bolt on the bottom of the carb bowl. Also don't get carb cleaner on the O'ring between the carb body and the bowl like i did as it stretches!

Hopefully this will help someone in the future.


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PostPosted: Fri Sep 15, 2017 9:46 pm 
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just fixed my rs100 mountfield from hunting revs remove carb
remove plastic idle screw remove black plastic plug which was located under screw and found small O ring had broken off .throw O ring fit plug without O ring .jobs a good one


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