OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Trimmers and Brushcutters McCulloch Trimmers & Brushcutters Pro Mac II starts but stalls

Hi all 2 strokers,

I will say that these small 2 stroke motors are not my specialty and thus I am asking for help.

I was given a Pro Mac II brushcutter and wanted to get it up and running as it's a fairly solid straight shaft machine and I'd prefer to use one of those opposed to a bent shaft domestic unit.

I worked it out that it was the coil that was at fault and thus this pretty much renders these machines useless, so I put it aside and waited for another similar machine with the same power plant to be listed on Gumtree.

Well I found one with a bent shaft that wasn't working properly but had a working coil and with that I felt I could make one machine out of the two.

I got the replacement machine to start but as soon as I put any pressure on the throttle it would stall. I ran a carby kit through it and it's still the same, only that it starts much more easily now with a single pull but it still stalls at the slightest pressure on the trigger.

BTW I took the Walbro carb from the original machine and rebuilt that and placed it on the second engine acquired that had a Zama carby on it but nearly identical, regardless it does the same with both carby's which tells me it's something else.

Any clues ?


Cheers,
BB.

Probably needs to be tuned properly with the carby adjustment screws.

Sorry BB can't help much, I just can't afford the time to mess with these machines, spend a million hours and still end up with the same end result. 90% of the time the carby is the fault and just because you have 2 carbys doing the same thing probably means the same thing is wrong with both carbys. Have you tried holding your thumb across the carby throat in various positions before you try opening the throttle, Cutting out as you start to open the throttle indicates an air/fuel mixture problem and varying your thumb position can prove that

Bearing in mind that I have done quite a few of these small 2 strokes, I have a couple of ideas for you.

I'm not exactly familiar with the mac II, but I presume the engine is the generic half crank Mac engine.

Try this first - Check the intake fuel line for cracks - if its sucking air, it will lean and cause the symptom you describe. Just had a Ryobi do the same thing not 2 hours ago. And check the fuel filter at the same time.

Take the carby off, and the carby adaptor (black plastic bit between the carby and engine). check the gaskets on either side line up with the small hole (which operates the metering diaphragm) and that the small hole is clear. and that the hole on the engine is also clear.



There is probably only one mixture adjustment screw on the carby (and the idle adjust). It probably will be a non tamper type. If you have an old limiting cap (from a homelite or ryobi) push it on the screw and it will allow you to turn it. Otherwise either dremel or hacksaw a flatblade head into the screw.

The last thing (after doing the above), take the exhaust off (held on by springs). Stick a piece of metal pipe (or a screwdriver) under the spring - push on both ends and lift the spring off. Don't try to use the end of the screwdriver to lever it off, slide the screwdriver through and use it as a handle - if that makes sense. Clean the carbon out of the muffler and check the exhaust port whilst you are there.

By the way, when you had the carby apart, did you remove the little metal mesh gauze filter and spray it with degreaser? they sometimes clog with invisible dirt that needs a good pressure to move it.

Best of luck - these small ones can be a bit of a bugger sometimes

Tyler

Hi BB,
Do you have a pic of the carby on it?
Definitely check the fuel filter, or remove it to test if it is the problem.
Otherwise a carby tune would be in order.
Cheers, Ted

Pls post the carburetor numbers so I know what I am discussing here. Also what did do to clean it. These cube carburetors are rather delicate to work on ie no wire probing or harsh carburetor cleaners should be used. Hopefully this not one of the rotary carburetor which can be a pain to get repaired and working right.

It sounds either engine starving for fuel or you might have a plugged spark arrester where the engine can not breathe. Both can present the same symptoms. So first either remove the spark arrester or the complete muffler and try it.

When to come carburetor there a couple initial adjustment that must done after disassembly while you are installing either a GND (gasket and diaphragm) or complete carburetor kit. First is the metering lever must set at the proper height the model carburetor in question and set the will start mixture screw(s) setting. Mixture screw setting is usually 1-1/2 turns from lightly seated.Once you start the engine then carburetor will need fine tuning.

One quick question for you AVB, with the Ryobi and Homelite carbs, how many turns out from lightly seated? I mean the ones with a very fine thread - 1.5 turns barely opens them.
I believe the ones on the McCullochs are the sometimes same.

Rotary carbs - they are a real pain

It usually is still the same for most of the cube carburetors. Don't let the thread pitch throw you off. This comes my 9 yrs of working these handheld trimmers, blowers, and chainsaws.

Now when it comes the newer air head versions I usually count my turns in before I remove the screws for cleaning. Some are as much a three turns out. I do know that Stihls are specing 1/4 turn out but that is not what I am finding on the C1Qs still using the 1-1/2 turn for the initial settings.

Here there are several different EPA tamper resistance caps and each one has a different way of making the adjustment on count of them. Some are easily removed, some are cut off, and other are just spring loaded and the adjustment depress the lock to unlock them.And of there is the Pacman (both large and small), 21 spline, 7 spline, single D (both large, and small), Double D, small flat blade, and a 4mm Hex. These are just a few of the adjustment tools now.

What's stupid about these locked out adjustment screws just to meet arbitrary laws, is they're ask perfectly set from the box with zero hours use when they inevitably require adjustments down the track after minute changes occur, such as filter no longer being pristine, affecting air flow, loosening up of the engine etc.
It's putting the factory's requirements to satisfy the bureaucrats well before the end user.

No they not set perfectly out of the box. I do a lot re-cals as a customer brings in new machine that are stalling right of the box. Since I am not a dealer I charge a small fee for the tuning but most dealers charge the customer too for those that are brought at the big box stores. I even seen metering levers set incorrectly on brand new ones too. Probably why I have the Zama Z , Walbro W, and the latest Walbro W meter lever setting tools.

As the limiters they do serve a purpose other than being a pain for the end users. They prevent customers from leaning out the 2 cycles which when they do they trash the PNCs Stihls and ECho both like to run a little richer than normal but it better for the life of the engine. So that they don't run at max rpms so what, it is better to have long life then run at top and a short life.They also serve a purpose as some trimmers will vibrate out the mixture screws. Yes I have seen where the caps were remove and a mixture screw is lost.

These limiter caps don't bother me as at least I can adjust the carburetor unlike those without any adjustments. The problem is learning how disable log enough to make the adjustments.

Hi AVB, MF, Tyler and Mod BigTed,

I'm really stoked by all your indepth responses. I shall take some photos today and post them up here so you all can understand which model engine and carby I'm playing with here.

The thing that makes me feel that the spark arrester is blocked is that with both carby's the symptoms are exactly the same and that makes me feel that something else bar the carby is amiss. I do feel that the carburettor requires a full fine tuning as I would expect that is the case after a full disassembly.

I'll go through it all again armed with the info many of you have provided and report back at the end of the day along with the images.

Once again I can't thank you guys enough.

Cheers,
BB.


PS. For AVB,

I used throttle body cleaner with the long tube attached to the spray can to blow through all the holes and there were a few gelled up spots which are all clean and clear now.

This is why putting your thumb over the carby inlet helps work out if it is an airflow problem through a gasket failure, always try the simple things first

This why I use block offs and a pressure/vacuum tester. If tester indicates a pressurization and vacuum is just a negative pressure then I positive pressurize and use soapy water to find the leakage. This does however loosening the carburetor and muffler to insert the block offs and re-tightening them to seal. Some seals leak only if a vacuum is pull or if only positive pressure is applied.

But if you work on these enough you get a set initial test procedures that work for you. Things like will it fire fuel prime. It usually takes me about 15 minutes using none invasive tests to determine what is most likely wrong with a two cycle engine. It take more time of course as you got to get the test and hand tools out and back up. None invasion tests save a lot unnecessary tear downs.

My life philosophy is to avoid dismantling where possible, even if it means putting up with a problem I can tolerate, rather than risk making it worse or starting another.

That is the whole purpose of non invasive tests. Yes the test equipment costs upfront but if you are in the repair business and not just a DYIer it pays for itself fairly quickly in just the time savings alone, not mentioning the gaskets that get torn during invasive inspections. I even looking buying an inspection camera too but so far I have manage to avoid it as I need one that will fit through the 10mm spark plug holes. Not many of those out there and ones that are is too expensive for small shop.

Now the time save by not disassembling unrepairable equipment is time I can spend on repairable ones.

I have gone from taking all day to troubleshoot and repair a string trimmer 9 yrs ago to under an hour for most repair jobs. And sometime less than 15 min as with the Stihl trimmer I did a few minutes ago. The customer explained to me his problem as we walked to his truck and Knew from the description of the problem with the Stihl hedger that he most likely had a plugged spark arrester. When I pull it, it was fully clogged. Cleaned and reinstall for the customer and he off to use it again.

Hi Guys.

I decided to have another look at it this morning and took all the above posted advice onboard and checked the carby settings and exhaust.

Wow those spring loaded retainers on the muffler are a bit of a bugger to get off aren't they ?

There is no spark arrestor fitted but I soaked the muffler in petrol and kept flowing it through, the massive amount of muck that came out is amazing, meanwhile I started the machine without the muffler fitted and it started and ran really well, no more stalling whatsoever, thus this told me it was a clogged exhaust box.

I reset the mixture settings at both idle and also high speed running at this time but realised that I would have to readjust them once the muffler and shaft assembly was re-fitted.

Once I got the muffler pretty clean I cleaned the outside on the wire wheel and repainted it in flameproof black paint and after it was dry re-fitted it and the shaft back to how it should be. Again it started and ran how it should and then repeated the carby mixture adjustments, thus now it's running perfectly.

Once again guys, thank you so very much as 2 strokes are just not my forte' and some of these little things catch me out at times.


Cheers,
BB.

Well Done Bonnar_Bloke

Those spring retainers are real buggers. Although embarrassing, I will tell you what occurred the first time I tried to remove these springs (on a Mac TM 251sst, which is the same as a Weedeater featherlite, and GMC trimmer).

Warning - this story is 100% stupid and 100% true

I had the motor partly disassembled (still attached to the curved shaft of the trimmer though), but couldn't get the springs off.
In a rising state of anger, I put it on the floor, used my feet like a vice, and tried to remove the springs (highly stupid with hindsight).
Well, the screwdriver slipped, I lost balance, fell over the engine (trying not to step on it), tripped over the curved shaft and ended up at a good jogging pace as I tried to get my feet back under me.

About 3m away (on brick paving) I finally fell over, took a little chunk out of my right knee, elbow, and a knock on the head. A bit of Mercurochrome later, I eventually got the springs off.

That's how i learnt not to use the end of the screwdriver to pry the spring off, but slide it through and use it like a handle

Regards
Tyler

Hi Tyler, Bonnar_Bloke and all,
many will benefit from this thread, not just the original poster. We can all learn from terrible tales of mishaps such as Tyler's. Sudden give ways trying to get things off are one of the most common hazards of working on stuff. All too many are not mindful of that, leading to anguish from skinned knuckles or worse, particularly from use of the wrong tool for the job or wrong application of the tool.

Spring hooks tools are easy to make. I make them all the time.

Hi all,

Well I've dug out my other McCulloch which is a Titan 2030 bent shaft machine which oddly now has the same issue happening as the original machine that I posted here for.

I have gone through all the basics and then removed the carby which had jelled up fuel throughout it. I've cleaned it all out with pressurised throttle body cleaner and blown it all out with compressed air and all the gaskets were fine as it had a carby kit put through it about 3 years ago and then hung up and put out of the way.

Now all this blastard thing does is fire up instantly and as soon as I put any pressure on the trigger it just stalls instantly. There's absolutely no restriction in the exhaust, matter of fact I've started the machine without the exhaust box fitted and it still does the same thing where as the previous machine ran perfectly without the exhaust and thus I soaked it and flushed it out. From there the original machine ran and just required the carby idle and high speeds adjusted carefully and now that ones a beauty while this bent shaft machine has got me stumped.

The only thing I can think of is the diaphragm might require replacement, apart from that ........................ donuts

Cheers,
BB.

Bonnar_Bloke

There are a few things you should be looking at:-
Check the carby again - in case more jellied fuel has flowed in since you last checked. Really closely check the wire gauze filter as they sometimes have invisible muck.
Then it's check the fuel line for cracks, the main filter for clogging.

One other thing you can try, is screw the idle screw in progressively as far as it will go - see if it will still run, or if the motor stalls at the 'shock' of a small throttle opening. Could be the main fuel circuit is blocked, but the idle circuit is ok.

Best of luck
Tyler

Metering diaphragms do go bad just sitting without use once they have been exposed to fuel. Basically they get stiff.

But first seat both mixture screws counting the turns as you go in so you can reset to the same setting. This may unplug the mixture port holes as I have seen them still plug even after cleaning. If this doesn't help try adjusting the L mixture a little richer ie 1/8 turn CCW and see if it stalls less as the unit may simply be running too lean at idle. This something I have seen several times after installing a new kit as the parts settle in, the adjustments change. It is one of the reasons I keep an unit for a few days after a carburetor rebuild.

Question. Is this Titan using a Zama or Walbro carburetor? The metering lever is set differently between the Zama and Walbro which why I have Z metering tool for the Zama and both W metering tools for the Walbro.

Hi AVB and Tyler,

I'll take all this fabulous info on board and give it another strip down on all sections of the fuel system and while I'm at it I shall add a new diaphragm, this way all will be completely replaced.

I push lots of throttle body cleaner through all orifices and it certainly had a few blockages which were cleared at the time, but who knows until you try again.

Thanks again and I'll report back with the findings and results.

Cheers,
BB.

Hi All,

Well after a few weeks of me sorting out the instant stalling issues with the first "Pro Mac HD II" machine that I refurbished it has resorted back to doing the same thing and starts perfectly but as soon as you attempt to squeeze the throttle it will stall.

I'm starting to think that these pump diaphragms that are supplied in the Chinese Walbro kits are garbage and that the actual rubber material that it's made out of is not very fuel resistant and suffers from distortion by having fuel on its surface for an extended period of time.

My question here is, does anyone else feel that these cheapy kits are substandard ? (approx $3.00 per kit delivered to your door)

Cheers,
BB.

I have used some these kits without problems especially the metering diaphragms which I was getting from an US source for 0.94 usd. Now I do do got resource them as that vendor has closed up shop after 20 yrs.

I would not think related to the fuel pump (usually Mylar) or metering diaphragms since it will idle and this where these at minimal operation due to engine impulses are less active. Now I have seen a few kits (even OEM) where I need to re-adjust the carburetor after a few days as things settle in. It does sounds like fuel balance problem. Since I not there to put my hands on it; it is a lot harder to explain how these mixture settings interact with each other. Usually you tune low speed first and then the high speed but some adjustments are fine tuned in to get the best acceleration response.

I 100% agree with AVB - even if it was the crappiest and unsuitable bit of material, it should have lasted more than the few weeks.

A Homelite I was doing last week had fuel lines that looked good on the outside, but gummy/disintegrating internally. The gum would block then clear then block the jets in a cycle.
As AVB said, often after installing the carby kits you need to do the fine tune, but I can't see the change being so great that it won't rev at all.

Try the fuel lines (and the gauze filter again). I have found that screwing the idle screw in a fair bit will get it running on both the primary jet and idle circuit.
And then put the choke on a bit - sometimes this added suction clears the main jet.

The better way is to remove the mixture screws and clean it out properly, but sometimes the quick fix works just as well - as long as it wont suck any more muck in from gummy fuel lines or filter.

I tried everything on that homelite, and in the end it was the bloody Tygon fuel line.
Regards
Tyler

Usually when I am rebuilding a cube carb I go ahead and install new lines and even a new pickup filter if I suspect water has been in the fuel tank. Die cast tend to corrode once exposed to water sending a grey mud thru the carb's internals. Many times the internal filter screen catches it but not always. There is also filter screen so tight on filtering they are clogged and not cleanable. These are the ones you can not see through in the first place.

Yes Tygon has its problems but sometimes that all that will fit in certain applications. Even though those lines claiming 100% Ethanol resistance are problem causers. I had one case on my personal saw where I installed new F-4040 lines and a week later the lines literally were melting in the fuel mix. I replaced with the same line (off the same roll) with the same fuel mix and didn't have any further problems. I can't to this day explain what caused the problem.

Stens came out with their Tru Blue line that claims 100% Ethanol but it fail internal tests here. Wasn't to harden or shrink which it did both. Harding leads to fuel pickup not moving around as it should and the shrinkage causing leakage at thru holes. Even the LP1200 line cause problems when its inner liner fails. It collapses causing fuel starvation.

Whenever I can I use Nitrile rubber lines which seems to superior to the plasticize lines as a replacement line but there fittings smaller than the 2.5 id line I have or in the case Poulan saws just too large OD to fit through the tank through holes. I am considering try to enlarge these pass through holes but just don't want to it on a customer before I get to try it on parts unit.

I more often than not replace the lines and the filter as well. Especially if the machine has lived near the ocean (one tip over here is about 1km from the ocean and the suburbs near it are the same or closer).
The cooling fins are usually corroded, and the accompaniment of a pitted die cast filter and sludge.

The internal gauze was ruined on that Homelite - simply not cleanable. I seem to recall some of the Zama's (C1U-H49 from memory - the one without a choke plate) have those super fine gauze filters.
they are very strong metal as well - hard to remove, harder to check for cleanliness.

Hi all,

Well it then looks like another complete strip down and thorough cleaning out. The fuel lines were all replaced with new stock and the filter was reverse cleaned with throttle body cleaner. Tank was totally flushed out and chemically super clean when finished. the only thing I didn't replace was that gauze filter but it was blasted through with throttle body cleaner under pressure also.

Anyway I'll try again and see how we go.

Thanks to both of you for all the input. Every little bit helps and gives oneself a different perspective of the job at hand.

Cheers,
BB.

I reverse flush spray the filters out as well(except i use cheap degreaser) - the metal bodied walbro ones are especially prone to having the outside look fine, but the bits of metal under the filter medium tend to flake off - really powdery metal.




The gauze filters don't take much to clog - even if there was some dust in the fuel line you used, it could do it after it gets flooded with fuel and flattens out over the filter. The fuel filters can also disintegrate on the inside and clog the filter with 'hair' - so definitely check it again