Does any one have any experience of fixing these. Stripped mine down as it seems to be starved of fuel as it just keeps cutting out. There wasn't any noticable blockages but still won't run, any ideas anyone.
McCulloch petrol strimmer
- Thread starter dstill
- Start date
Did you notice two rubber diaphragms when you delved into the carburettor?
Its best to replace them as they can leak or lose their flexibility when the machine isn’t used for a while.
John
Its best to replace them as they can leak or lose their flexibility when the machine isn’t used for a while.
John
Hi John,
Thanks for the reply.
I didn't notice any rubber diaphragm but did notice a sort of plastic gasket with what looked like a diaphragm built into the middle and another sort of foil circle on the carburettor that looked quite flimsy.
Thanks for the reply.
I didn't notice any rubber diaphragm but did notice a sort of plastic gasket with what looked like a diaphragm built into the middle and another sort of foil circle on the carburettor that looked quite flimsy.
The carburettor will have one or two of these very thin diaphragms......one is called a pump diaphragm and one is called the metering diaphragm.
All carbs have a pump diaphragm but if yours has a round throttle slide that turns up to 90 degrees then it won't have a metering one.
As you realise, the drillings inside these carbs are miniscule - you rarely see any dirt - but they can only really be cleaned with an aerosol of carb cleaner and/or careful use of compressed air.
Have a look at GHS Ltd for parts.....often a replacement non original carb is cheaper than a gasket set.
Failing that, if you could set a few photos up?
John
All carbs have a pump diaphragm but if yours has a round throttle slide that turns up to 90 degrees then it won't have a metering one.
As you realise, the drillings inside these carbs are miniscule - you rarely see any dirt - but they can only really be cleaned with an aerosol of carb cleaner and/or careful use of compressed air.
Have a look at GHS Ltd for parts.....often a replacement non original carb is cheaper than a gasket set.
Failing that, if you could set a few photos up?
John
Excellent pics - many thanks! That looks like a Zama (owned by Stihl) C1 series carb......see if you can find the full numbering should you need it again.
Point - Get all of the external crap away before you strip down, you don't want anything going inside!
Pic 1, (L to R) The pump diaphragm which looks like its seen better days! The U shaped flaps are valves that allow fuel to pump - they flap open and closed. On the right is a fine gauze filter that is directly behind the needle valve and allows fuel to it.
Pic 3. That's the needle valve assembly. Removing the philips screw releases the rod that holds the needle....don't lose the fine spring that holds the needle shut!
Pic 4. That's the metering diaphragm controlling fuel from the primer bulb into the carb. It looks reasonable but a bit moth eaten around the circumference.
Please note! Each diaphragm has a gasket with it - when you split them its easy to forget which way round they went so take care.....the diaphragm actually sits onto the alloy carb face in the vast majority of cases.
So....an aerosol of carb cleaner or brake cleaner is essential. Remove every bit of the carb you can and lose nowt
. Squirt the cleaner down each and every orifice both ways. If it gets you in the eye its worth it 
Any adjusting screws, count the turns out from fully in.
Replace the diaphragms and gaskets as a set.....genuine Stihl are around £15 but pattern bits are available.
Check out Garden Hire Spares (GHS Ltd) for what you need....sometimes a non genuine carb is cheaper than the genuine gasket set!
Be lucky
John
Point - Get all of the external crap away before you strip down, you don't want anything going inside!
Pic 1, (L to R) The pump diaphragm which looks like its seen better days! The U shaped flaps are valves that allow fuel to pump - they flap open and closed. On the right is a fine gauze filter that is directly behind the needle valve and allows fuel to it.
Pic 3. That's the needle valve assembly. Removing the philips screw releases the rod that holds the needle....don't lose the fine spring that holds the needle shut!
Pic 4. That's the metering diaphragm controlling fuel from the primer bulb into the carb. It looks reasonable but a bit moth eaten around the circumference.
Please note! Each diaphragm has a gasket with it - when you split them its easy to forget which way round they went so take care.....the diaphragm actually sits onto the alloy carb face in the vast majority of cases.
So....an aerosol of carb cleaner or brake cleaner is essential. Remove every bit of the carb you can and lose nowt
. Squirt the cleaner down each and every orifice both ways. If it gets you in the eye its worth it Any adjusting screws, count the turns out from fully in.Replace the diaphragms and gaskets as a set.....genuine Stihl are around £15 but pattern bits are available.
Check out Garden Hire Spares (GHS Ltd) for what you need....sometimes a non genuine carb is cheaper than the genuine gasket set!
Be lucky
John
Hi John,
That's unbelievable, how do you know so much just looking at the photos, I'm gobsmacked, thank you.
I've looked at replacement carburettors but they seem to be around the £40 mark. A gasket/diaphragm kit and carb cleaner from GHS is around £15. My only concern would be I don't have compressed air so after buying the parts I'm not confident I can clean out all of the channels sufficiently, what do you think?
That's unbelievable, how do you know so much just looking at the photos, I'm gobsmacked, thank you.
I've looked at replacement carburettors but they seem to be around the £40 mark. A gasket/diaphragm kit and carb cleaner from GHS is around £15. My only concern would be I don't have compressed air so after buying the parts I'm not confident I can clean out all of the channels sufficiently, what do you think?
Compressed air is a great thing to have but it can bite you back......just looking back to picture 3 of the needle valve assembly, you can see one round and one rectangular shape.
These are called welch plugs, and they cap off drillings that are impossible to machine, so they are formed and then plugged afterwards. Compressed air can cheerfully blow these things away without you knowing
However carb cleaner - which is a vicious solvent called toluene - should do the same job and I wouldn’t go near a carb without some.
I so think replacing those diaphragms and gaskets is necessary in this case.
Good luck with the project - its a learning curve if nothing else!
John
These are called welch plugs, and they cap off drillings that are impossible to machine, so they are formed and then plugged afterwards. Compressed air can cheerfully blow these things away without you knowing
However carb cleaner - which is a vicious solvent called toluene - should do the same job and I wouldn’t go near a carb without some.
I so think replacing those diaphragms and gaskets is necessary in this case.
Good luck with the project - its a learning curve if nothing else!
John
Brilliant, thanks John, I'll give it a go and let you know how I get on
Thanks for all your help, I really appreciate it.
Best regards
Dave
Thanks for all your help, I really appreciate it.
Best regards
Dave
No worries Dave, these things have been known to drive me nuts too 
John
John
Hi John, I managed to get hold of a kit and fit it and thoroughly cleaned the carb out with carb cleaner. There was one gasket and one diaphragm extra in the kit which I'm guessing is for another similar model. The only part I don't think is fitted correctly is a sort of steel flat piece which sits near the float chamber, had to prise the old one out but can't seem to get the new one to stay firmly in place.
The strimmer starts quite easily but it won't run without my finger on full throttle and even then doesn't seem to be able to get to full speed and eventually dies. I've tried adjusting the mixture screw from nearly closed to fully open but still no luck!
The strimmer starts quite easily but it won't run without my finger on full throttle and even then doesn't seem to be able to get to full speed and eventually dies. I've tried adjusting the mixture screw from nearly closed to fully open but still no luck!
Can you give me a pic of the suspect steel flat piece please? If its slightly domed it could be a 'welch plug', and they are critical.
John
John
Yep, they are welch plugs all right......used when the drill has to turn odd angles and machining is the only way.
You need to replace them I'm afraid...you'll see they are domed and to fit them, its dome side out then punch it down ever so gently to force it tightly into its housing.
Try GHS Ltd for carb repair kits - hope you get lucky! Failing that its a replacement carb, unfortunately.
John
You need to replace them I'm afraid...you'll see they are domed and to fit them, its dome side out then punch it down ever so gently to force it tightly into its housing.
Try GHS Ltd for carb repair kits - hope you get lucky! Failing that its a replacement carb, unfortunately.
John
I could only find one to replace on my carb John, the one on the left, does that sound about right?
I will try fitting it again but apply more pressure. Do you think this could be the reason its struggling to run?
I will try fitting it again but apply more pressure. Do you think this could be the reason its struggling to run?