OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Chainsaws and Concrete Cutters McCulloch Chainsaws MacC CS420T fuel flow problems?

Greetings folks!

I got this saw in 2015, but in terms of regular work in real time, you could say it's only seen about 3 months of use, so not old and should be in prime condition still.
I had in the past a lot of trouble starting and running it, and it stalling in the middle of a cut, and wasted money on trying to get a dealer to service it but with no result from that quarter.

So I bit the bullet and removed the Zama C1M-W26C carburettor myself and took it apart. AFAICT all the gaskets and diaphragm, checked carefully under magnification, are in tip-top condition, so I sprayed carb cleaner copiously down the mixture screw holes and other accessible passages to make sure all they were clear, particularly through to the three idle ports in the venturi which lie under the welch plug. I then re-installed the gaskets, diaphragm and reservoir retaining plates. The only other work I had to do was Bondic weld the black plastic cam lever for the actuator for the auxiliary air intake valve (It had split slightly on its mounting post), and replace the primer bulb fuel line from the upper reservoir (some very light seepage around the bulb housing).

After this work the saw ran very well; when warm I could stop it and start it with the softest of pulls. But it's performance has been variable. I now try to start and run it briefly each day. Sometimes it starts first pull, sometime it takes a few more. Usually I have to initially screw in the idle speed screw about two turns and I find starting without choke is the best. Once going I can then screw it out again until the chain stops. But careful observation in bright sunlight shows that during running I'm getting bubbles of air forming in both the main tank filter to carb lower reservoir line, and also in the priming circuit. However there are now no signs of any fuel seepage or leaks anywhere - the floor of the housing above the tank is completely clean and dry. So unless it's a tank-not-being-full problem, how is this air getting into the lines?

I suspect that air in the system is causing sudden fuel starvation after idling nicely along for a while, or upon opening the throttle quickly (normally acceleration response is excellent). Sometimes I can force it to die by simply tilting the saw over to the right, although this is not all the time. I suspect that this tilt causes air in the carb upper reservoir to rise to the left, at the highest point, right near the opening into the idle circuit passages and so the fuel flow stops.

I tweak the idle mixture screw to optimum each time I run it. The performance inconsistency is intermittent; I have had certain days of beautiful and responsive running, and others where it's a beast to get going or keep going.

Hi Merry_mynd

What's the in tank fuel filter like ? and is it sitting flat in the tank bottom ?

Cheerswce

G'day,

Thanks!

I've just come in from having a start and run test and shining a torch down inside the fuel filling point (with cap and retainer clip removed).

I hadn't ever removed the tank filter line assembly as, due to the low usage of the saw and good 'fuel hygiene' practices and high quality 2-stroke oil I now use, I didn't think it was warranted. It certainly looks clean enough on the outer surface of the sintered plastic cylinder and there are no leakages around the grommet.

However, it seems that you came up with a stroke of genius! Having been used to seeing filter units flop around inside the translucent tanks of line and hedge trimmers, I had assumed the same for this, despite the black opaque plastic that the tank is made from obscuring the view. If you look inside from the right hand side of the saw with the torch, the grommet is up on the right ceiling of the tank (i.e. toward the back of the saw) and its internal tube sweeps down and toward the left (the front of the saw). Over to the left of the tank ceiling is the primer circuit return line 'spigot' which protrudes slightly into the tank also. The filter had become wedged in behind this somehow, and although nominally level, it probably was sitting about 4-5 mm above the 'floor' of the tank, which isn't completely flat anyway, but has a sloping section on that side..It was obvious the filter had become stuck as I couldn't shake it loose by gyrating the saw, but had to use long nose pliers to free it up.

Hmm. that suggests poor design strategy, but also the inner Tygon tube segment might need to be a bit shorter. Anyway, due to your insight I now know how to fix this if I see it happen again. Meanwhile, the priming action has removed most of the air pockets, and after starting and idling, I notice no bubbles now in any of the fuel lines.

I believe I do have another residual problem with the release of trivial amounts of 'scale' deposited inside the carburettor (engine version of kidney stones 'dissolving' - I can tell by listening to the engine tone while slowly revving the idle circuit after tuning the mixture,, but I've been working on this issue for a month or so, and finding the problem has decreased exponentially with regular running and paying attention to using clean, very fresh fuel. So for now anyway, all is looking good!

Thanks heaps for your expert evaluation!

Pure guess work Merry_mynd and glad it was of assistance. Those filter screens are very fine, if it's easy enough to get at might be worth a clean and check if you can easily blow through it, if not, I find putting them in a small container of CLR clear overnight works wonders.

wce

As silly as this will sound, run down to bunnings (hopefully you aren't in Vic) and get a bottle of this https://www.bunnings.com.au/mechanic-in-a-bottle-120ml_p0169259

Sounds stupid - mechanic in a bottle. I hate snake oils as much as the next person, but once I find one that actually works, I recommend it.
I have soaked diaphragms in it neat and neither metering or fuel pump diaphragms hardened or were damaged (old ones became slightly more plyable but not perfect)

I had a badly gummed up talon trimmer carby - cleaned as well as I could. Would then run on choke, but not off (even idle - non adjustable carby). Tried some sort of liqui Moly carby clean additive - did nothing.

Drained fuel and put in this stuff and within 5 minutes it would run off choke and rev up reasonably (bit boggy still)

Just today, I suggested a friend get a bottle for his sanli which would only run well on choke, and would stutter and spontaneously stall at idle or part throttle.

He added a bit more than prescribed ml per L to the tank, shook back and forth - I listened over the phone as he started it and it started surging badly - told him to be patient. 3 minutes later it suddenly came to life - idle went smoother and up (he put idle up before to stop stalling). Would accept throttle fine.

Will this fix a carby that has sat for 10 years full of stale fuel - No. Will never replace a carby teardown, but it get rid of the stuff you can't reach


Peter Brock reckoned his energy polariser made a 'shi*house car good' - this additive makes a Shi*thouse carby passable, and reasonable ones better

Unfortunately, it won't help my 'new' ryobi chainsaw. Guy brought it over and straight up told me he was an idiot and grabbed the wrong fuel can. Piston not awful, but its not so good.


Edit - should also say I have no vested interests in the manufacturer (B3C fuel or something like that) or GA spares (who distribute it)