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View Full Version : Chain saws a question ?



J. Dillon
11-28-2002, 09:56 AM
Have a balky chain saw, a McCullah. It starts up all right but when stopped for any reason, it won't restart unless left alone for 5 or more minutes. Is the mixture too lean, rich or what? It does seem to like to run at full throttle. When left to idle it will cut out unless the throttle is "goosed". What should I look for ? :confused:

ishmael
11-28-2002, 09:58 AM
Gettin' ready to carve the turkey?
;)

Gary Bergman
11-28-2002, 10:06 AM
Sounds likea spec of crud or shellac in one of the little passages in the carb. If you are handy, split the little carb apart being careful of the diaphram and clean it real good.

B. Darrah Thomas
11-28-2002, 11:21 AM
I had a Mac, they fly from trees suprisingly well when you're frustrated. Mine had enough hang time to resemble a yellow POS falling to it's death. :D
Buy a Stihl or "Husky", no more Mac problems.

Gary Bergman
11-28-2002, 07:28 PM
I actually traded off my Stihl years ago for Poulan....Like em better for very small but important 48 below nites in the Plains reasons....Frostbite and big ash trees, ah, Christmas!

Don W
11-28-2002, 07:39 PM
I would look for someone else to cut that that tree. It's damn cold out.

Mrleft8
11-28-2002, 10:56 PM
I think your chainsaw is just plain old tired and needs a beer every so often.

WFK
11-28-2002, 11:05 PM
I'm with B. Darrah Thomas. My first saw was one of those yellow things and to me it looked real good in the steel bin at the local land fil :D

Joe (SoCal)
11-29-2002, 12:58 AM
Stihl smile.gif

Shang
11-29-2002, 07:57 AM
...And don't forget to wear ear-protection...

...I SAID, EAR-PROTECTION...!

(...Damnit, why don't they answer...)

Wayne Jeffers
11-29-2002, 08:26 AM
Jack,

It sounds to me like your mixture might be a little too rich. A clogged air cleaner can be the cause.

First, make sure the air cleaner is clean enough to allow air to flow to the carb without undue restriction. It's also possible that dirt in the carb is part of the problem, but if it idles okay when cold, this is less likely the problem.

Background: Small two-strokes have two mixture adjusting screws, low-speed and high speed. You will need to identify which is which. The biggest danger to your two-stroke motor is running too lean, especially at high speed, because a leaner mixture burns hotter to begin with and the mix also carries your lube oil. Too lean can burn up your engine. Too rich only makes too little power.

With the engine running at idle, adjust the idle mix screw to a leaner setting. You should hear the engine idle a little faster. Find the maximum RPM by sound using only the mixture screw. Finally, back out the idle mixture screw (richer setting) 1/4 turn.

Repeat the process with the high speed screw while holding the throttle at full open. Remember to back it out the quarter turn at the end.

Your dealer can do this in about 10 minutes and he has a small tach to read the results of adjusting the mix more precisely than your ear.

I second the recommendation for Stihl.

Wayne

ahp
11-29-2002, 12:32 PM
I don't know how much cutting you are doing or where you are doing it. Consider an electric, if feasable. They are not as fast, but I have cut 12 in diameter live oak with mine. Always starts.

J. Dillon
11-29-2002, 09:09 PM
Thanks for the trouble shooting tips.

Wayne, before ripping apart the saw I noticed that if I kept the throttle a little squeezed it kept on running when left alone. Usually it stalled out. So I set the throttle idling speed up a notch or so. This seems to work for now.

Had to finish taking down a tree that broke under the weight of snow. Don't know what species but was told it was a golden rain tree ( no snickers please) It does put out in the fall a kind of golden minature Chinese lantern.

Hated to loose that tree.

JD

capt jake
11-29-2002, 09:16 PM
Shouldn't set the idle any higher than needed. If it starts to move the chain, it is set too high! Just barely trying to move the chain. That chain should be moveble be hand (when not running). No great drag should be felt. smile.gif

Wayne Jeffers
11-30-2002, 09:51 AM
Jack,

I second what Capt. Jake said regarding idle. The chain should not move (i.e., the centrifugal clutch should be disengaged) at idle.

Boosting the idle speed may be fine as a temporary get-around, but it's not a good long-term solution. I'm sure you will still have the problem re-starting when hot because you're running too rich.

In the adjustments I described, you should not have to "rip apart" anything, really. There should be a cover on the top rear of the saw which is removed by turning a single knob. This allows you to get at the air cleaner to remove the sawdust which collects there. If one is using a chainsaw a lot, as for cutting large quantities of firewood, this is a maintenance job which needs to be done daily.

I would not recommend you take apart the carburetor until you've eliminated a number of other possible problems, especially if you've never taken apart one of these little guys before.

I don't know the origin of the name, but I've seen the "golden rain tree" in the seed/nursery catalogs. I won't snicker. ;)

Wayne

J. Dillon
11-30-2002, 10:22 AM
Wayne,

I did take the carbureator apart a while ago and cleaned out the air filter. It all seemed ok. Didn't use the saw much since. This saw has a clutch on it where the chain is dis engaged. I do keep the adjustment for the chain tension running free and am very conscientious about all lubricants. However cleaning the air filter again might be a good idea.

Thanks Wayne for reminding me.

PS glad some one else heard of that kind of tree. ;)

JD

PeterSibley
11-30-2002, 02:39 PM
I had a similar problem with an old Homelite once,similar ,but not the same.Thing wouldn't restart once it was hot,I'd get maybe 20 mins use then STOP and nothing.Nearly drove me mad trying to work it out........turned out to be a fine wire off the coil,it had a break in it and when she finally got up to the critical temperature the wire expanded enough to part...and when she finally cooled down and the wire contracted she ran like a dream and of course she worked beautifully in the repair shop cos they only start em,never get really hot!That one was really fun !

Mrleft8
11-30-2002, 09:34 PM
William MacClure introduced "golden rain" trees to New Harmony Indiana in the 1820's. They're also known as "pride of China" trees. Not found widely pretty much anywhere else in the USA except around Monticello. T. Jefferson is suspected to be the initial importer of these trees....

Syd MacDonald
11-30-2002, 09:47 PM
Check for a rubber throat connection between the carburator and the cylinder head. A small hole or tear in the rubber can lean out the mixture and cause erratic idling.

Sam F
12-02-2002, 09:26 AM
Originally posted by Wayne Jeffers:
Jack,

It sounds to me like your mixture might be a little too rich. A clogged air cleaner can be the cause.
WayneAll good advice Wayne... perhaps I can add a few things for J. Dillon.


Originally posted by Wayne Jeffers:

Background: Small two-strokes have two mixture adjusting screws, low-speed and high speed. You will need to identify which is which.

In every engine I have ever seen, the idle adjustment screw is always the one closest to the engine.


Originally posted by Wayne Jeffers:
The biggest danger to your two-stroke motor is running too lean, especially at high speed, because a leaner mixture burns hotter to begin with and the mix also carries your lube oil. Too lean can burn up your engine. Too rich only makes too little power.
Too rich a mixture also causes carbon build-up on the spark plug, exhaust ports and spark arrestor in the muffler.
That spark arrestor is often a small mesh screen (often stainless steel). If it starts to clog, the engine will run poorly especially at high speed. The easiest way to clean it is to remove it (usually it's held on by only one or two screws on outside of the muffler), hold it in some pliers you don't particularly care about and burn off the carbon with a propane torch.


Originally posted by Wayne Jeffers:
With the engine running at idle, adjust the idle mix screw to a leaner setting. You should hear the engine idle a little faster. Find the maximum RPM by sound using only the mixture screw. Finally, back out the idle mixture screw (richer setting) 1/4 turn.

Repeat the process with the high speed screw while holding the throttle at full open. Remember to back it out the quarter turn at the end.

Your dealer can do this in about 10 minutes and he has a small tach to read the results of adjusting the mix more precisely than your ear.

I second the recommendation for Stihl.

WayneNothing wrong with that advice but I would do it a bit differently. I'd check the muffler/spark arrestor first and clean it if necessary. Then start the saw and let it warm up. Run it at full throttle and slowly screw in the high-speed mixture screw (farthest from the engine).
If it's not set too lean already, the engine will speed up and then as you continue to tighten the screw it'll start to drop speed. (If the speed drops from the outset you're set too lean) Stop tightening the screw and start loosening it. Find the highest rpm spot and back the screw a tiny bit off from that. Maybe an 1/8 of a turn. You want to be barely aware of an rpm drop.
Consider that a provisional setting.

Let the saw return to idle. It may not want to, so keep a finger on the trigger.
(Here I'm going on the assumption that your saw isn't very far out of adjustment since it works reasonably well...).
It may idle, but assuming it doesn't, follow the same procedure as with the high-speed mixture jet. Screw it in until the speed drops and then back it off. Wayne may be right to back it off a 1/4 turn but every saw is peculiar. It may be only and 1/8 or it may be 1/2.
Here's how to find out for sure. Once the saw is idling OK repeat the high-speed adjustment. (The reason changing the idle makes a difference is that that idle jet bubbles a bit of fuel even at high speed so it's adjustment always changes the high end a little.)
Once the high speed mix it finalized let it drop to idle again and do one final test. (What you're trying to simulate is normal use.)

Let it idle a while and then at once bring it to full throttle. If the saw stalls, your idle is too lean and the engine starves for fuel as you open the throttle (that's why cars w/ carbs have accelerator pumps. Chain saws don’t and you have to set the idle a bit richer than the ideal because of it). Screw out the idle mixture adjustment a bit. What you want is to turn the idle mixture screw out just enough to keep the engine from stalling when you suddenly squeeze the trigger but not too much.
It may be that the idle speed drops too much and the saw stalls so use the idle speed screw (usually bigger and off to the side) to open the throttle plate enough to make it idle. If the saw isn’t very far out of adjustment, only a tiny tweak is enough for that.
Some things to look out for:
Small two-strokes are very susceptible to air leaks. Even a small leak will cause poor idling. It it’s intermittent, the symptoms are that the saw initially idles OK but then speeds up and stalls.
The cracked rubber intake is a possibility as already mentioned, but also watch out for leaking seals around the crankshaft. This is especially likely at the sprocket end. Vines, wood fibers and bark can wrap around the sprocket shaft and ruin the seal. (You’ll probably need to pay a mechanic to fix that) The screws that bolt down the carb can work loose as well and allow an air leak past the carb base gasket.

Disclaimers:
1. I’ve seen carbs so out of adjustment that at high speed they were partly running on a much too rich idle adjustment. Someone had compensated by turning in the idle speed all the way. If that’s the case, it would be best to start from scratch by screwing in both idle and high speed mixture screws all the way (Gently!) and backing them both out 1 & or 1& turns. Then do the adjustments outlined above.
2. Before doing any carb adjustment first make sure that the spark plug is in good condition and gapped correctly, the air filter is clean and the muffler is free of obstruction. Any air leaks will make a correct carb adjustment impossible.
3. Some saws have problems so obscure that they are very difficult to diagnose. I’ve seen all sorts of weird stuff from porous castings, to a Mac, which would almost stall in a log, cough, and then run backwards (that turned out to be a bizarre electronic ignition module problem). So your mileage may vary!

Alan D. Hyde
12-02-2002, 09:50 AM
The point about the spark arrestor is a good one. If it's sufficiently clogged, it will kill the engine, since the combusted gases will be unable to exhaust.

I usually punch about a half-dozen extra (and larger) holes in the arrestor screen to prevent the problem.

Often you can get a badly running two-cycle weed wacker at a garage sale for next to nothing, punch these holes, and it will run well...

Alan