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Help with Stihl FS55

Arvorsteve

OPE Member
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13575
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Aug 22, 2020
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Hi guys I posted this earlier as a reply to a thread that was started last year on the subject of replacement coils for FS45, but not sure if I should start a new post.
Anyway,thanks for letting me join your forum. I think I have the same issue here with timing having tried a Chinese 1308 coil clone and would appreciate your help.
History
Bought this FS55 2007 model S/N 67573628 strimmer at a boot fair here in the UK as not working. Where I am with this now I have to assume that the outer case may not be the same age as the parts that are inside and that this maybe a 49, 59 Chevrolet, you all know the song. Therefore it might not be a 2007 model at all!
Build Spec
From the tech data posted by Fish and observation of the key-way position it's got the later 4140-030-0401 crank, so that's OK for a 2007 model, although that number does not appear on the crank itself. It's also got a flywheel with no central cast in aluminium boss and has the number 4140-400-1201A (or 12C1A hard to tell) I think it's an earlier version, hence my suspicion about it being a hybrid. Can anyone confirm to me if the 1201A or 12C1A is indeed an earlier flywheel please as I don't see it in the parts list posted earlier?
Can anyone tell me the difference between the flywheels as the later one with the raised central boss also seems to have the key way inline with the magnets. What's the difference in them that matters with the timing or are they all interchangeable with the coils?
The fitted coil is the STIHL Walbro 4140-1303A coil and it is not producing a spark. Gap is correct, flywheel Woodruff key is fine, it hasn't slipped.
Tests to date
Removed engine from casing, good compression, no scoring of bore.
Removed kill switch
Tested coil as being U/S and purchased a so called Chinese replacement coil which as everyone has confirmed so far , is now the 1308 coil. I don't see that STIHL are making two coils for this machine, although it appears they should be making a 1303A rip off?
Fitted the Chinese coil, good spark, but back firing through carb, you know the problem.
At this stage I happened on this thread but what I can't see anywhere in it is a technical explanation to what's going on here between these Chinese coils and the STIHL OEM parts. Some of you seem to be saying that fitting a STIHL coil cured their issues, but as we have read above from Boots, the STIHL coil didn't work for him.
Further Tests
From visual inspection the 1303A coil (including the Chinese clone 1308 which looks identical) aligns with the magnets on the flywheel at TDC
I remounted the discarded STIHL 1303A coil and measured the resistances with a DMM and I get 43 ohms on the low voltage side to earth and 2.7Kohms on the high voltage lead to earth, so not open circuit but maybe shorted turns. Anyone know what resistances I should be getting?
I also put the engine in my vice, set it up with a battery drill, plug out, oil down bore and scoped the output of the coil as I rotated the crank. I am getting around 1.5V peak to peak on the low voltage and 600V peak to peak on the high voltage side, so insufficient to fire the spark plug, but interestingly getting some output from the coil.
From this I determine three possibilities.
1) The coil is 'soft', maybe shorted turn on the LV side as I would expect more than 1.5V. Anyone comment on that?
2) The magnets in the flywheel are no good, but I discounted this as the Chinese coil gives a good spark.
3) The air gap between coil and magnets is too big. This has also been discounted because it seems to make no difference on coil output varying the gap between .0001" and .0020"
I baked the coil in the wife's oven (after she went out!) but that made no difference either.
Summary of my questions
Are there any key-way to magnet position variances in the different flywheels as I can't see any?
Do you reckon that a 4140-400-1201A (or 12C1A) flywheel is compatible with a 4140-030-0401 crank and a 1303A coil?
Do you reckon that a 4140-400-1201A (or 12C1A) flywheel is compatible with a 4140-030-0401 crank and a proper STIHL 1308A coil?
Is a STIHL 1308 coil supposed to cater for both the earlier and the later style of cranks and for any flywheel combination, given that they no longer make a 1303A?
If STIHL only make the one 1308 coil how do they cater for an approximate timing variance between the two cranks of 40 degrees with just the one coil if the flywheels are the same?
What's the difference between a STIHL and Chinese coil and why can't the clever Chinese make a coil that works when they seem to be able to make just about everything else in the world :)
Next Steps
I cannot get hold of a 1303A coil here and reluctant to buy one second hand even if I could anyway as there's no come back.
At this stage I am also now thinking of re-ordering a Chines 1308 coil, filing out the key-way and just playing with the timing until it fires. Drill and tap a hole in the flywheel and peg it to the crankshaft in the right place.
Or will the proper STIHL 1308 coil work do you reckon?
Thanks guys and sorry for all the questions in my first post!
ArvorSteve
 
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