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

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