can't start "Mac" chainsaw

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  #1  
Old 03-12-07, 06:13 PM
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can't start "Mac" chainsaw

A 38 cc McCulloch ( MS1838AV, model CS38EM) quit when I was cutting overhead, and would not restart. No spark, so I checked magneto module. Mounting screws appeared tight, but flywheel edges scored slightly, and short leg of module was away from flywheel a considerable distance - say .5 inch. I reset the clearances between flywheel and BOTH legs to about .010. I now have (some) spark at the plug upon cranking, but no obvious firing. Cranking with priming/choking eventually results in a wet plug - and that's all!



(1) What are the correct procedures and specifications for adjusting the magneto assembly? Do these include special timing adjustments, or is setting the gap (s?) sufficient?

(2) If I remove and dry the plug, dip the plug in gasoline or add a bit of gas through the the plug hole, quickly re-insert the plug and crank the engine a few times, should the engine not fire or run a bit (even if there is also a fuel delivery problem)???

Any help or constructive criticism would be appreciated!

- Wile
 
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  #2  
Old 03-12-07, 10:57 PM
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The engine should run with gas in the plug hole. I wonder if the flywheel key has sheared since you described scoring marks on the flywheel. If so, what caused the scoring in the first place? Could the coil have moved and contacted the flywheel?
 
  #3  
Old 03-13-07, 10:52 AM
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can't start "Mac" chainsaw

Thanks, Cheese. My thought was that the coil might have moved - i.e. been kicked back by the flywheel rotation. IF so, could the impact have impaired some related function other than basic firing of the plug?? However, part of my problem is that I'm not certain how the assembly SHOULD be correctly placed. Maybe my gaping efforts are not correct, or not precise enough from a timing perspective. (The key is OK as I pulled the flywheel to check. The scoring mentioned is minor but obvious. Given the relatively light-duty nature of the saw, I wonder if too much pressure could have flexed the engine components and moved the flywheel slightly off center. I think that only likely if the flywheel/coil clearances were set relatively close.)

If I have spark, does that guarantee that the coil is OK? The coil presumably could control other functions, such as timing and advance for high speed. So even if the coil sparks the plug, is that adequate? The spark is thin, but very definite and will jump an eighth inch or so to ground. Should the spark have real snap, like some larger engines I have owned??

Are these small engines so easily flooded, that a faulty carb could keep the plug too wet for engine starting? The owner of the saw thinks that something in the carb may be stuck (he had it loose) and that it is dumping gas directly into the engine. However, I don't see any evidence of that when I remove the plug and spin or tilt the engine.


The adjustment valves for the carb were factory set and are locked in place by plastic retainers, so they should not have vibrated out of adjustment. When running, the saw was always somewhat tricky to start - it really only started if you manually held the throttle wide open.

Any additional thoughts would be appreciated.

- Wile
 
  #4  
Old 03-13-07, 12:08 PM
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You are setting the coil airgap clearance correctly. There is no timing setting related to the coil. The correct air gap is .012". The flywheel key is the timing setting...if it's not sheared then the ignition timing is presumed correct. I recommend using blue Loctite on the coil mounting threads as they are notorious for loosening up. Before you go any further though, I'd pull the muffler off and have a good look at the condition of the piston, ring and cylinder. This may be your trouble with scoring of such. If all looks good and you have the correct coil airgap then the best way to check for sufficient ignition strangth is to hook up an inline spark tester between the spark plug and coil wire and then check for spark. At this point I don't think you need to worry about any possible carburetor troubles, from the sound of things.
 
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