McCulloch trimmer carburettor adjustment or overhaul

[aesmith]

Hi,

We have a McCulloch MT320 strimmer, not sure of the age but probably over 10 years old. The handbook gives quite clear direction on carburettor adjustments which are the normal idle speed, "L" and "H" screws as found on similar small engines. Recently it's been giving trouble, getting hard to start and not running well. I've fitted a new plug as a precaution, and cleaned the air filter before trying to adjust the carb. This is where I'm finding an issue. If I get it running nicely at tick over, it won't rev up properly. The "H" screw makes no difference at all whether fully open or fully closed. I can get it to rev properly by unscrewing the "L" screw further, but then it won't tick over so that's clearly wrong.

My question is about this "H" adjustment, which I believe controls fuel flow at high speed. I had the carb in bits and I'm pretty sure none of it's passages are blocked, so is it possible that the fault is caused by a leak elsewhere or by a stiffened or distorted diaphragm?

The machine is no longer supported by McCulloch so I don't know how easy it will be to find a compatible repair kit or replacement carburettor, so I'd like to see if I can understand the issue before searching for bits.

Thanks, Tony S

 

[Burnerman]

It's likely, Tony, that the carb has two diaphragms inside.
One is a pump diaphragm ( which is identified by two U shaped flaps) and one round metering diaphragm (often has a metal disc centre).
Due to ethanol found in fuel these days, the diaphragms don't last for ever and it's worth replacing them.Can you find any make on the carb, i.e Zama and any identification number, i.e C1Q or similar?
You need carb cleaning fluid to do these, and a compressed air supply is very handy!
One thing......did you remove the needle valve, complete with its pin and tiny spring? If you did, on the opposite end of where the pointed valve sits, is a fine gauze filter, and it's very hard to see if it's blocked.
John

 

[aesmith]

Thanks, that's correct there were two diaphragms in the carb, one under each of the side covers. I took out the needle and spring etc, but I can't remember a gauze filter so I might have missed that. I cleaned with meths (which is pretty good at removing varnish from petrol), and blew through everything.

Annoyingly I can't find the parts list now. I'm sure I had one, and that the part number given for the carb suggested it was Zama. I need to find that again, because I think I might have a chance of finding the correct kit by carburettor model. I haven't been able to find one marketed for the MT320 which as I said is long discontinued.

By the way, is it just me or are McCulloch pretty much a dead loss nowadays? I remember getting assistance from them when we first got this strimmer. In contrast last year we bought a replacement, and it never ran right - McCulloch wouldn't even provide carburettor settings or any sort of service information, and said we'd need to take it to a dealer in Dundee for them to look at. We ended up just returning it for a refund, hence soldiering on with the old one.

 

[aesmith]

Found it! Carburettor is described as "C1U-W32", and noted "Repair kits are not available for this carburettor"

 

[Burnerman]

McCulloch are very much at the lower end of the diy use scale.....more often than not, most horticultural repair people refuse to touch them.
However, for what they are, that's fine.
Look at GHS's ebay shop for carb kits (Zama is now owned by Stihl) and you may be able to recognise the carb or the spares.
The filter I'm on about will be there.....fuel passes from the pump chamber through the filter, where it's regulated by the needle valve.
So, look down the needle valve hole, then turn the carb over to see where the filter is.
It's best to use a proper carb cleaner though....it's toluene which dissolves most impurities - and it's also under pressure. Get this from a motor factor.
John

 

[Dave54]

I've got two or three bits of McCulloch kit. A chainsaw, sold cheaply from one of the discount places 30 years ago. Always has been a dead loss starting. A hedge trimmer, some years old which has been OK, although little used really And a Pro Mac brush cutter, which has actually given excellent service.
Spares for the older stuff are hopeless though.
Stihl for me next time.

 

[aesmith]

Thanks all. The old McCulloch is quite a good bit of kit, if only I could sort out this fuelling issue. They used to be one of the "Big Names" in this type of gear, but I think that nowadays they're right at the bottom of the cheap market. They like to stress the relationship with Husqvarna but in reality it appears McCulloch have changed hands a few times, and now just sit as a budget brand within the Electrolux group. (Their machines actually OEMed from MTD? or is that just in the US?)

Anyway, I'll have a look inside the carb when I get a chance. Do you think that gauze, if blocked, could have the effect I'm seeing, or do you think something else is blocked? I'd really like to try and understand the fault before buying bits, especially as finding a compatible repair kit might be challenging.

Someone pointed me to Rowena Motors (www.rowenacarbs.co.uk) as a UK Walbro and Zama specialist, any other possible sources? There seem to be parts on Ebay but only from the US, and it's quite likely their machines were different versions.

Thanks again, Tony S

 

[Burnerman]

That's definitely a Zama carb, Tony.
GHS are on 01905 333475 and are very helpful.
If you don't get anywhere there, you can get parts sent from the USA, which I have done several times without any problems.......sometimes you can get a complete carb for only a few quid more than the repair kit.
Now, I can't say if the diaphragms are goosed or not......if there are any splits of course then they are, but more often they just look a bit distorted and it's difficult to tell how good they are.
An obstructed filter will cause the same issues as blocked internal drillings in the carb - i.e not enough fuel getting through, but if the machine still seems to rev you can guarantee it will falter when under load.
Once more, to clean those drillings a good pressurised solvent is needed and compressed air a real bonus.
John

 

[aesmith]

Thanks for the tips. I might manage to get the machine on the bench this weekend and have another look inside the carb. Compressed air is available, and I'll see about sourcing some of that special carb cleaner. I'll take it from there.

Our medium term plan was to get a new Stihl next year, but if I can get the McCulloch running for not too much cash then so much the better.

Tony S

 

[Burnerman]

I hope all goes well.....just a couple of points:
It's possible to blow out a welch plug or two with over zealous use of compressed air, and the thin diaphragms have a thicker gasket attached .....keep them in the right order.
Finally, turn the mixture screws in to see how many turns you need on reassembly, and don't mix them up!
John