Mcculloch ProMac 3600 Brush Cutter Problem

[Dave54]

Back to the brush cutter. A couple of uses, and it started doing it again. I've got a repair kit for the carb, new fuel pipes and filter, and a new fuel bulb on order. The fuel bulb seemed to be pumping up very frothily.
Couple of questions. One is the old pump diaphragm was stuff that looked like film. Clear and quite stiff. The new one is quite soft rubber. Any known problems there?
Where is that little gauze filter mentioned by John in a previous post. I can't see it in the fuel feed, although there is a spare in the kit. is it lurking under the welch plug in the cover where the fuel comes in?
Cheers chaps.

 

[Burnerman]

The diaphragms are usually very thin and stiff - the nearest description is the celluloid from old camera film?
The diaphragms always go towards the carb itself, the gasket on top.
If you can access the needle valve which is held by one screw locating its spindle, the tiny filter is on the other side of that, i.e fuel passes through the filter before hitting the needle so you look at the carb's other side.
There's nothing to be gained by removing the welch plugs, they just hide drilling passages......but if one is missing, the engine won't run.
Are you able to post pics? (Certainly I can't help you there - no idea!)
John

 

[Dave54]

Thanks John. It's a pattern gasket set of "Far Eastern" origin, so I was wondering about the "soft" replacement diaphragm. The original seems OK, so I may well keep that. As you say it's (the original) made out of something like camera film.
Ah! That's where that filter is! I'll have a proper look. (Which is what I should have done in the first place of course!)
I'll leave the welch plugs alone.
I'll leave posting pics for the moment. If I can't sort it out I'll post some.
I'm a bit p'd off about it TBH. I thought it was fixed. I can still feel that new Stihl calling me from afar!

 

[lostinthelight]

I believe the shiny stuff is called mylar?
litl

 

[lostinthelight]

Couple of questions. One is the old pump diaphragm was stuff that looked like film. Clear and quite stiff. The new one is quite soft rubber. Any known problems there?.

There can be problems when its done the other way round ie myler used where the original was rubbery.
The mating surface on the carb body can be quite roughly finished on the carbs that originally had a rubber pump diaphragm and when its replaced with myler it does not seal but the carb has to be pressure tested to diagnose this....BTW, have you pressure tested your carb, it can save hours of frustration?
litl

 

[Dave54]

Thanks. No I haven't pressure tested it. The pressure testing kits look a bit expensive for a one off to be honest.

 

[Burnerman]

I use my pressure testing kit to see at what pressure the needle valve will come off its seat......it has shown up on the odd occasion a slight leak under the metering diaphragm cover - the one with the single screw.
Have you looked at GHS Ltd for a replacement carb copy, Dave? Ridiculously cheap at around £15 and I have used them before.
John

 

[Dave54]

Good grief John! You can't expect me to pay £15 for a complete carb when I've just spent nearly a fiver on a gasket set!

Seriously though. Thanks. I might well go down that route if I can't get the thing working.

 

[Dave54]

I've put the carb back together with the new diaphragms, and a new fuel filter in the tank.
Found the little filter in the carb. Looking in the wrong place. Anyway a squirt of cleaner and it's clear.
Put everything back together as cleanly as possible. Fuel in, pumped the bulb. Seemed to "pump up" better than before. Second pull it fired. Shut the choke, pull again, and away she goes as clean as a whistle. Let it warm up a bit. Rev it, and it picks up cleanly. There's been a tiny bit of hesitation between low and high for a long time now.
Seems OK now. Running better than before. Cleaner pick up etc. somehow.

When I took the one diaphragm out of the bag, it was that springy mylar film. It looked like rubber in the bag!

So it look promising. Wet here today, so I'll try it properly on a drier day.

Thanks again chaps, all help has been very much appreciated!

 

[Burnerman]

Nice one - long may it continue!
I'm pleased to hear the diaphragms were the traditional material - you had lost me with the rubber comment!
If you do intend to replace the machine at any time, the Honda 4 stroke one is worth considering.....they always start with no choke juggling so they are brilliant to hire out.
John

 

[Dave54]

Cheers John. I never take stuff out of the packet until I need it, at least not if I can see it's the right thing, so that was the reason.
I'd looked at the Honda. Looks a good machine for the price. Honda generally seem to make good gear as well.
Hopefully though this one will last another year or two. It looks a bit scruffy now, but it hasn't really had that hard a life. It really does run beautifully when it's going properly.