Mcculloch ProMac 3600 Brush Cutter Problem

[Dave54]

I've got a brush cutter as above. Some years old now but has been running fine. Stupidly, I left fuel in it for some time while not using it, and inevitably it gave problems.
Started fine, and ran OK after a bit of spluttering. Started using it, fine, then after about 20 minutes the revs would die. Would tick over OK, but when the throttle was opened it would rev and then die back to tickover.
Letting the unit cool seems to "fix" it for a while. Until the next time.
Took the carb (Zama C1Q) apart and treated it with carb cleaner and compressed air.
Lines / filters / (newish)spark plug seems OK. Cap vent seems OK. Engine is nice and quiet mechanically as far as I can tell.
Tried it today. It's still the same.
Any thoughts before I start replacing stuff.
Thanks in advance.

 

[Burnerman]

Did the two diaphragms seem in reasonable nick, Dave? Ethanol in the fuel is giving them a hard time.
Next, did you unscrew the two adjustable running jets, marked L and H usually - and squirt carb cleaner in there?
John

 

[Dave54]

Diaphragms seemed OK I had a good look. Hadn't thought about the ethanol eating them.

Aha! No I didn't unscrew the screws and clean them out!
Thanks John! I'll try that. Be tomorrow now.

If that doesn't work I'll try a new fuel filter and a carb. They seem cheap enough on eBay.

 

[Burnerman]

Be reasonable with the compressed air, you can blow two welch plugs out
Obviously check how many turns the needles are set at - they aren't interchangeable.
Regards
John

 

[Dave54]

Useful thoughts John. Thanks again! I always go a bit easy with the air!

 

[Burnerman]

Apologies Dave, I should have added there's a very fine gauze filter just before the fuel is fed into the needle jet chamber....easy to miss.
If you remove the needle jet,its swivel pin is held by one philips screw and the assembly lifts clear. There's a spring holding the jet lifter fork under tension and that is easy to lose!
John

 

[DaveHerns]

What about blocked vent in fuel tank filler? That would explain why it runs for 20 mins then plays up.

 

[Burnerman]

Hell, for 20 minutes I read 20 seconds
Its getting hot and bothered so do try a new plug - although it could be fuel vapourisation. When it misbehaves, does a little choke help?
John

 

[Dave54]

What about blocked vent in fuel tank filler? That would explain why it runs for 20 mins then plays up.

Thanks Dave. Already checked. See original post!

Hell, for 20 minutes I read 20 seconds
Its getting hot and bothered so do try a new plug - although it could be fuel vapourisation. When it misbehaves, does a little choke help?
John

I nearly said (I should have) it seems as if it's a bit hotter than usual when it plays up. Although it seems to run hottish anyway. It's a bit like being strapped to a moped!

I'll try another plug. I didn't try choke yesterday, but when it originally happened, before the (admittedly quick) carb clean I did, and it picked up a bit. It feels like fuel starvation. It just suddenly won't run at any revs but will start /tickover fine. Let it cool and it's OK again for a while.

As you say John fuel vapourisation? Why so suddenly though?

As I said it's an oldish machine. Had little done to it, grease in the head, and a sparkplug, air filter clean occasionally for the engine.

 

[Burnerman]

Just wracking my brains (whats left of them after visiting Ibiza for a week )

These machines don't use much fuel, so any tiny blockage may allow the engine to perform ok for a few minutes but during full bore use it will start to fail - although on tickover it may run for ages.
If the mixture is weakened for any reason, the engine temperature increases at the same time which could have something to do with it.
One of my tricks is to clamp the machine down, and with the engine running, increase the tickover speed with the throttle stop screw.....if it starts to falter, unscrew the H needle a tiny bit and see if the thing picks up.
John

 

[Dave54]

I think I'll give the carb another, more thorough clean. It's weird how it really will do quite a bit of work before the problems start. It really is about 20 minutes before it begins to fade.
Possible I suppose that some muck in there is moving about, causing it to run weak etc. as you say.
I'll try that running it on the bench. Thanks again John.

 

[Dave54]

Pulled the carb apart again and did as John suggested. Cleaner and air. Exactly same result.
I cleaned a bit of crud out of the cooling system as well, it wasn't blocked, but there was grass debris in there.
Engine runs OK for a few minutes, and then starts to cut out when revved. Choke helps as suggested, so it seems like fuel.
Think I'm going for a carb rebuild kit, and a new fuel filter (although the existing one seems OK)
I'd get a carb, but it's a Zama C1Q M31, and I can't see one of those for sale. (Don't want to spend too much as a new Stihl beckons if this one keeps playing up!)
I'll pull the thing apart again to check I'm getting the right bits.

 

[Burnerman]

You need to get every little piece out of the carb Dave, which means that it will be a cube of aluminium by then!
A few scratches with a scriber could help the orientation of some of the bits come reassembly time.
Keep an eye out for that gauze filter I've mentioned, and when you are in there, the spring loaded lever that pulls the needle valve open should be parallel with the aluminium face.....some of them get bent downwards.
John

 

[Dave54]

I'll pull it right apart then, or as much as I can.
What about the welch plugs John? Do they need to come out? They're generally a drill job to get out aren't they?

Now where's the club hammer. . .

The annoying part is that it all looks remarkably clean in there. None of the shellac / gummy type carp that you usually get in carbs that have had fuel left in them.

It's got to be worth an hour or two and a fiver to see if it's fixable. A new comparable Stihl is pretty pricey, although I must say i had the loan of one of the lighter ones a while back, and it was fine. Still getting on for £300 though.

 

[Burnerman]

You'll see the welch plugs, one round and one rectangular with rounded ends.....just leave them be unless compressed air has shot them across the garage
I use a carb cleaner aerosol with a plastic tube, and direct the air very carefully.
I agree, the filtering is so good you can't imagine there being any issues really!
If it restarts, increase the idle speed so the thing runs comfortably and then tweak the H screw slightly. Hopefully the motor will respond!
John