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How do I adjust my Chainsaw to RUN properly?

Posted by fcivish Zone 6 Utah (My Page) on
Mon, Jun 18, 07 at 9:26

Hi. I have a McCulloch MAC-130 2.0 chainsaw, but it isn't running very well. I can get it to run briefly, but not for long enough to use it.

There are 3 screws on the top that adjust the engine. Those 3 screws are labeled: IDLE, LOW and HIGH.

Does anyone have any idea how to adjust these to help my engine run properly? I have already cleaned the filter and have the proper fuel/2 cycle oil mixture, etc.

I can get the engine running, either with the trigger locked in the 'Starting' Acceleration position, or, once it is warm, I can get it running in idle. But in either case, it stops running after 30 seconds to a minute.

Which of these screws should I adjust first?

I can presume that the IDLE screw will adjust my Idle speed. How do I determine the proper Idle speed?

What do the High and Low screws adjust? Do I tighten them all the way down, then back them out? How far? How can I tell?

Can anyone even tell me what all these screws do, or what the labels on them mean? Thanks.


Follow-Up Postings:

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RE: How do I adjust my Chainsaw to RUN properly?

The LOW and HIGH are the mixture needles for low and high speed operation. I don't know about your engine/carburetor, but a rough setting could be 1 1/2 to 2 turns out to start with. NEVER tightly screw the needles into their seats when resetting the needles, this will damage the needles and/or seats and make accurate adjustments and good operation possibly impossible. Just gently snug the needles and back them out. Get the engine started, raise the idle speed if you have to to keep things running for the time being.

Set the high speed jet by accelerating the engine to full throttle and holding it there. Slowly screw the HIGH needle in unitl the engine stumbles, this is the mixture starting to go lean - running/cutting here can overheat the engine and burn pistons, etc. Slowly unscrew the HIGH needle, the engine RPM will peak and as you continue to unscrew the engine will begin to stumble again - this is the mixture going rich. Running here will cause carbon to build up in the exhaust ports and/or foul spark plugs sooner. Just screw the HIGH needle in a little until the stumble goes away and the RPM picks up. This is on the rich side of peak RPM and is a good place to run - just a hair rich is good for power and cooling, IMO.

Now let the engine RPM drop down to idle. The LOW mixture screw only works with the throttle closed to the idle position, so slow the idle speed down with the IDLE speed screw if you need to. Its OK during adjustment to overslow the idle a little. Now turn the LOW screw in until the engine stumbles a little (lean side) and then back it out until it stumbles a little (rich side) and set the LOW mixture 1/2 way between these 2 stumble settings. Now reset the IDLE speed to make sure the engine isn't too slow that it kills but slow enough that it idles and disengages the chain clutch after reving.

Repeat the whole process if necessary to get things tuned in. Hope this helps.


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RE: How do I adjust my Chainsaw to RUN properly?

Thank you very much for the help. It's running much better now, though I'm still trying to adjust it a bit.

Please read the following and tell me if I'm doing it right.

First, I took the advice in some other threads, and turned the HIGH and LOW adjustment screws all the way down until they were closed, then I opened them (counter-clockwise) by one full turn each. It immediately ran better. I tried the 1 1/2 to 2 turns open that you had recommended, but that was too close to my previous settings and it didn't work that well.

Then, I started it and let it run while warm, and tried to rev it up. I could only get it to keep running, at high speed, while keeping the choke part way closed. From other threads, I gathered that this 'Partial Choke' makes the engine run richer. So, I turned the HIGH adjustment a bit MORE open (counter-clockwise or richer) until it would run at high speed with the choke fully open.

Then, I adjusted the HIGH adjustment in the way that you mentioned above, and then I did the same for the LOW adjustment.

I'm still not sure how to adjust the IDLE, and how to determine the best setting there.

I also notice that it now runs much hotter (and faster) at high speed than it used to. It was running so hot, it started melting all types of oily crud (sawdust and dirt) that had been hiding in the housing, where I wasn't aware of it, and all that oily stuff started falling out of the little vent holes on the bottom. MUCH hotter than it has run for years. Now I wonder if it is running TOO hot.

Does this sound right, or have I over-adjusted it in some way?

Again, thanks for your help.


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RE: How do I adjust my Chainsaw to RUN properly?

Hot is a relative term. Internal combustion engines are heat engines, so they will get 'hot'. The things we can do to keep them from getting too hot are
- keep the cooling fins and fan clean of debris (sawdust, leaves, mouse nests, excess oil, etc.
- keep the engine rpm's up so the cooling fan can move enough air
- run at the right mixture setting (too lean will overheat the engine and burn the internals, and
- don't abuse the engine by burying it into a monster tree and expecting the little engine to chug on through.

If the high speed mixture is just on the rich side of high rpm, you should be OK there. Just clean the engine fins and fan up and you should be OK. As far as the idle speed, obviously you want it slow enough that the clutch will disengage the blade. I like to keep mine fast enough, though, that I can set it down and it will just purr away waiting if I have something to do/prep for a moment or walk between trees. That's also the reason I center the idle mixture between lean and rich - less tendency while idling to fowl the plug being rich or stall because its lean.


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RE: How do I adjust my Chainsaw to RUN properly?

"I also notice that it now runs much hotter (and faster) at high speed than it used to. It was running so hot, it started melting all types of oily crud (sawdust and dirt) that had been hiding in the housing, where I wasn't aware of it, and all that oily stuff started falling out of the little vent holes on the bottom. MUCH hotter than it has run for years. Now I wonder if it is running TOO hot."

If it is over-revving, then you are either set too lean or have an air leak. Figure out which it is before going any further, or you'll burn the saw up and make adjusting it a moot point. You may have scored the piston and/or cylinder already if you ran it like that for any period of time.

Myself, I'd take it to a dealer for a vac and pressure test and go from there.


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Regarding the hi speed setting.

One thing with the hi speed adjustment is this:
You can run the engine too fast by turning that needle in too much.
The fuel mixture lubricates and cools the engine.
A 2 stroke will run hotter and faster than it should if the mixture is too lean.
Too much of that, will damage your engine.
You need to back the high needle out while running to make the engine "four-cycle" a little. A burbly rasp that cuts in and out while running at high speed.
If you're not sure of yourself; have someone who knows show you for the first time, just to be on the safe side.
Good luck with it.
-br


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