Chainsaw Repair

How To Basics - Carb Fixes + Mods - IPL and Service Manuals => How To Basics and Fixes => Topic started by: Giles on October 11, 2016, 09:52:15 pm

Title: Chainsaw ignition timing
Post by: Giles on October 11, 2016, 09:52:15 pm
Didn't really know where to post this bet here goes-----
A friend that also works on chainsaws, asked me an interesting question. He asked me what was the position of the flywheel, in relation to the ignition module at the moment of ignition?
Like---the flywheel has two magnets and the module has two pickups. At what point of rotation does the module produce a spark?
Title: Re: Chainsaw ignition timing
Post by: RoyM on October 11, 2016, 10:31:20 pm
Just as the magnet moves away from the coil. The magnetic field around the coil collapses inducing a high voltage in the secondary winding.
Title: Re: Chainsaw ignition timing
Post by: Giles on October 14, 2016, 09:57:59 am
My friend, that asked this question, brought the Echo CS330t in question. Compression is 145# and carb is clean.
When pulling over, it will not even attempt to start, even with starting fluid. Blue spark from sparkplug with moderate pull over.
Seals and boot are good.
Out of curiosity, we hooked up an automotive timing light to determine spark. Rotated flywheel forward with cordless drill.
Spark occurred  nearly 1/4 revolution after flywheel magnets passed coil! Changed coils and both  coils presented same results.
From the markings we made on flywheel, spark is occurring after top dead center with piston rotating down app. 1/4" ?
Ignition way late.
I think both coils are bad? ?
Title: Re: Chainsaw ignition timing
Post by: RoyM on October 14, 2016, 10:20:50 pm
Flywheel key is sheared
Title: Re: Chainsaw ignition timing
Post by: Giles on October 14, 2016, 10:40:26 pm
Spark occurred  nearly 1/4 revolution after flywheel magnets passed coil!

Has nothing to do with Flywheel Key.
Title: Re: Chainsaw ignition timing
Post by: Al Smith on October 16, 2016, 04:10:15 pm
Not to sound like a smart alec but did you turn it the right direction .The spark should be 26-30 degrees before top dead center .Usually if you try to turn the flywheel with a drill motor it's turning the wrong direction for the right hand threaded nut and you unscrew the flywheel .I sent the flywheel of an old gear drive in orbit around my garage doing that and just missed my wifes Cadillac .I'd have never heard the end of that .
Title: Re: Chainsaw ignition timing
Post by: Giles on October 16, 2016, 05:10:03 pm
Al--When sparkplug is removed and the flywheel nut is torqued, I have never had one unscrew. I use a variable speed cordless drill motor and I start slow and then increase speed. When finished, I remove socket from nut while turning to prevent shock.
Also a good idea to make sure piston rings are lubricated if spun very long.
Title: Re: Chainsaw ignition timing
Post by: 3000 FPS on October 16, 2016, 09:55:20 pm
The spark should always occur after top dead center.    How much ATDC is determined by the manufacturer.   
They do not like to much advance because it can be hard to turn over.   

I have never seen a timing problem on a saw like you are talking about especially with two different coils.   If it were me I would start over and double check everything else you have done.

One of my test I like to do is to squirt some mix into the carb and then pull it over with no choke and wide open throttle.   after about 5 pulls if it does not fire at all then pull the plug and see if it is wet or if it is dry.  I do not use starter fluids on 2 cycle engines because there is no oil and the rings do need a small amount of oil to help seal for compression.

If the plug is wet then double check the spark.   Also double check the compression.   Blow the plug dry with compressed air and blow out the cylinder through the spark plug hole.   Pull the rope a couple of times and then blow it out some more.   Put the plug back in and pull it over again with wide open throttle no choke and see if it fires.   If it does not then check the plug again.

If the plug is dry then double check the crank case pressure because the fuel is not moving from the crankcase into the combustion chamber.   I would also double check the compression in this case also.
If you have fuel, compression, and spark it should fire.   
Title: Re: Chainsaw ignition timing
Post by: Giles on October 16, 2016, 10:31:48 pm
"I have never seen a timing problem on a saw like you are talking about especially with two different coils."

Neither have I --and I have worked on small engines for over 50 years.
I agree with all you say about checking, but this is wasted time when the spark apparently occurs with piston down 1/4 to 3/8"

If you have fuel, compression and spark it should fire.

But if ignition is not at the right time???

Any time I check for a solution to a problem, I always double check and sometimes triple check. Sometimes simple things cause big problems.
Have you ever heard of a person using an automotive timing light on a chainsaw?  Might be that I am putting too much confidence in timing light.
Thanks for your input.
Title: Re: Chainsaw ignition timing
Post by: 660magnum on October 16, 2016, 10:43:11 pm
Automotive timing lights have given odd readings on chainsaws before.
Title: Re: Chainsaw ignition timing
Post by: 3000 FPS on October 17, 2016, 12:18:23 am
If you think then that the timing is retarded that bad then pull the flywheel and advance it to where you think it should be.   You do not need a key in the flywheel if the nut is tight on the flywheel.  Prove to yourself if it is correct or not.    Just move the flywheel in a CCW direction to advance it.
Title: Re: Chainsaw ignition timing
Post by: Al Smith on October 17, 2016, 05:28:44 am
Some solid state coils have a built in advance which works by detecting voltage which is dependent of rotational speed .The faster the rotation the higher the voltage until coil saturation won't increase the voltage.

Two examples of failures I've encountered on this design .Briggs 18 HP twin.It would fail on full advance and fire so early you could not get it started until it cooled off .042 Stihl that would not advance,I doubt it would turn much over 6000 RPM under this condition .

I'm not familiar with the coil in question weather it's an advancing type or not .Usually that kind either has three coil stater  pieces or three magnets on the flywheel or at least the ones I've seen do .What you have I don't have the foggiest idea .--more --
Title: Re: Chainsaw ignition timing
Post by: Al Smith on October 17, 2016, 05:38:29 am
--now then .Coil voltage is dependent on three things,rotational speed,lines of magnet flux cut and air gap of the coil to fly wheel ,faster means more,closer means more ,position the coil fires at in relationship to the magnets.

If the coil fires on the leading or trail edge of the magnets the voltage will be less,dead center magnetically will be the highest .

The reason you need advance is to "light the fire" so the max rate of expansion occurs at slightly past top dead center .It could be almost top dead center on a slow running engine like a john Deere ,or say 35 degrees on a two cycle running say 17-18000 RPM .It's not that the piston speed is out running the spark it's you can't get the expansion to get going on the fuel .
Title: Re: Chainsaw ignition timing
Post by: Al Smith on October 17, 2016, 05:52:27 am
Another thing which is only the theory but it bears an explaination . An magneto  ignition coil generates AC voltage but since it is only on the positive side of ground it's called pulsating DC .

It has the same sinosoidal wave form as AC .The trigger device fires the coil when it detects a negative transition in the wave form by an assortment of transistors ,capacitors and a bunch of electronic gooblede goo .

Nice to know info maybe but for all intents you can't do much with a solid state coil other than replace it if it doesn't work .
Title: Re: Chainsaw ignition timing
Post by: 3000 FPS on October 17, 2016, 11:28:02 am
"I have never seen a timing problem on a saw like you are talking about especially with two different coils."

Neither have I --and I have worked on small engines for over 50 years.


Over 50 years.    I would say the problem is something else other than the coils.   
Title: Re: Chainsaw ignition timing
Post by: RoyM on October 17, 2016, 12:36:07 pm
Hey, I resemble that remark. >:( :D :D :D I had a Remington Mitey Mite (yeah I know ::)) give me fits, no spark. Changed the points, nada. Changed the coil, nada. Installed the ignition from a running saw, still no joy. Installed ignition from dead saw on other unit, worked fine.
Threw it on the shelf until the customer started complaining.
For some reason I had the bar and chain off, I was pulling the rope and noticed the clutch was turning BACKWARDS. Somebody had assembled the F-M rewind incorrectly.
Title: Re: Chainsaw ignition timing
Post by: 3000 FPS on October 17, 2016, 12:38:59 pm
Hey, I resemble that remark. >:( :D :D :D I had a Remington Mitey Mite (yeah I know ::)) give me fits, no spark. Changed the points, nada. Changed the coil, nada. Installed the ignition from a running saw, still no joy. Installed ignition from dead saw on other unit, worked fine.
Threw it on the shelf until the customer started complaining.
For some reason I had the bar and chain off, I was pulling the rope and noticed the clutch was turning BACKWARDS. Somebody had assembled the F-M rewind incorrectly.

Now that is funny.
Title: Re: Chainsaw ignition timing
Post by: dannyupsolate on October 17, 2016, 01:05:48 pm
great thread ;D  other day I was reading a mastermind post where he filed of 20 thousandths off the key to advance the timing on his hotrod saw. my customers ms250 is still breaking pull ropes and damaging starter fingers. if I file 10 thousandths off other side of key to retard timing a little reckon it would tame this saw a little.
Title: Re: Chainsaw ignition timing
Post by: 3000 FPS on October 17, 2016, 01:28:43 pm
great thread ;D  other day I was reading a mastermind post where he filed of 20 thousandths off the key to advance the timing on his hotrod saw. my customers ms250 is still breaking pull ropes and damaging starter fingers. if I file 10 thousandths off other side of key to retard timing a little reckon it would tame this saw a little.

Yep it would but just put it back to stock.     I have a few saws also that I have advanced the timing on and you really have to pull them with conviction or else it pulls the handle right out of your hand.   But at the higher RPM's they sure do run.
Title: Re: Chainsaw ignition timing
Post by: dannyupsolate on October 17, 2016, 02:36:49 pm
this saw is stock but I have put 3 ropes and 2 starters on it this year. it belongs to a homeowner that only uses it once a month or so. it runs great but to rough on starters.
Title: Re: Chainsaw ignition timing
Post by: Al Smith on October 17, 2016, 03:38:42 pm
You might be able to move the coil out some which would in theory lower the voltage some what .I have no idea if a Stihl 250 has an advance coil or not,never worked on one have seen very few . If it kicks that bad put a D-handle on it .

Every mule kicker I own has a D-handle,2100 Husky,038 Stihl mag,084 Stihl which btw broke a bone in my hand once .I have a few with elasto start handles .
Title: Re: Chainsaw ignition timing
Post by: dannyupsolate on October 17, 2016, 08:19:31 pm
ms 250 uses the small rope. it also killing the little flip out fingers in starter and broke a spool. I am sure if it had the bigger stronger starter would not be killing as many parts.