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I think I have a bad coil

cuinrearview

cuinrearview

Red saw lover
Joined
Oct 24, 2014
Messages
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43
Location
Hillsdale, MI
It's no secret I like the 5100s and 5105 Dolmars. As many as I've had on my bench, I've never experienced the bad coils I hear talk of. I got the latest 5105 going tonight. Compression is good, good fuel, good spark. She tuned right in and responds beautifully. 2800 at idle, revs as quick as these do to 13.5k. Slapped a bar and chain on and went to the test log. As soon, and I mean as soon as it touches wood it loses everything. I lift it out of the "cut" and it returns to a smooth idle. Over and over. I tried it from cold and got maybe a second of cutting. It tried to run backwards a little and died. Fired back up and same.

The flywheel is tight and on key. I only have one other coil, it's a red PS460 one though and looks pretty crusty. I got nothing from that.

So, fellow Dolmar honks, is this what a bad coil acts like? If so, anybody got a spare black coil?
 
NIP Group
cuinrearview

cuinrearview

Red saw lover
Joined
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Messages
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Age
43
Location
Hillsdale, MI
No reason to believe the carpet doesn't match the drapes on this one. I thought of a couple sources for coils I can grab. The wiring all looks right. The coil is out right now so I can scrutinize the insulation later in the week. It's an easy start model so feeling too much advance is an impossibility, but running un-loaded everything feels spot on. Thanks for chiming in fellas. Sometimes you get into something and ignore possible solutions.
 
Old2stroke

Old2stroke

Never too many toys
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Jan 24, 2016
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Ottawa, Canada
It's no secret I like the 5100s and 5105 Dolmars. As many as I've had on my bench, I've never experienced the bad coils I hear talk of. I got the latest 5105 going tonight. Compression is good, good fuel, good spark. She tuned right in and responds beautifully. 2800 at idle, revs as quick as these do to 13.5k. Slapped a bar and chain on and went to the test log. As soon, and I mean as soon as it touches wood it loses everything. I lift it out of the "cut" and it returns to a smooth idle. Over and over. I tried it from cold and got maybe a second of cutting. It tried to run backwards a little and died. Fired back up and same.

The flywheel is tight and on key. I only have one other coil, it's a red PS460 one though and looks pretty crusty. I got nothing from that.

So, fellow Dolmar honks, is this what a bad coil acts like? If so, anybody got a spare black coil?
Are you saying the engine actually ran backwards and fast enough to make the chain run backwards?
 
cuinrearview

cuinrearview

Red saw lover
Joined
Oct 24, 2014
Messages
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Age
43
Location
Hillsdale, MI
Are you saying the engine actually ran backwards and fast enough to make the chain run backwards?
It returned to idle, the chain stopped, then moved a couple of links backwards. The chain is tight so it wasn't slack. I re-created it to make sure I actually saw what I thought I saw.
 
Old2stroke

Old2stroke

Never too many toys
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Messages
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With normal ignition advance, an engine will not run backwards so if it does, something has changed to give no advance and the spark must be occureing at or near TDC. Modern coils are an ignition module with semiconductors that determine the advance, so that would be best guess for culprit. Last engine I saw that would actually run backwards was a Bultaco spanish trials bike, if it kicked back when starting chances are it was running backwards and you had to be real careful letting out the clutch. Ignition advance was purposly set close to TDC for low end power.
 
lone wolf

lone wolf

MS 200T King
Joined
Oct 5, 2009
Messages
55,571
Location
Prowling The Pine Barrens
It's no secret I like the 5100s and 5105 Dolmars. As many as I've had on my bench, I've never experienced the bad coils I hear talk of. I got the latest 5105 going tonight. Compression is good, good fuel, good spark. She tuned right in and responds beautifully. 2800 at idle, revs as quick as these do to 13.5k. Slapped a bar and chain on and went to the test log. As soon, and I mean as soon as it touches wood it loses everything. I lift it out of the "cut" and it returns to a smooth idle. Over and over. I tried it from cold and got maybe a second of cutting. It tried to run backwards a little and died. Fired back up and same.

The flywheel is tight and on key. I only have one other coil, it's a red PS460 one though and looks pretty crusty. I got nothing from that.

So, fellow Dolmar honks, is this what a bad coil acts like? If so, anybody got a spare black coil?
If it runs at real low power I would say coil possibly. Swap it!
 
cuinrearview

cuinrearview

Red saw lover
Joined
Oct 24, 2014
Messages
8,949
Age
43
Location
Hillsdale, MI
Thanks for confirming my thoughts gentlemen. I remembered this morning I could probably grab my Dad's 5105 or open the box on my Makita to get a coil to try before i put up a want ad.
 
Old2stroke

Old2stroke

Never too many toys
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Before blaming the coil, are you absolutely sure you don't have a fuel flow problem from the main jet? Restricted flow could let an engine rev up under no load but bog big time as soon as it's loaded. Wouldn't make an engine run backwards though.
 
cuinrearview

cuinrearview

Red saw lover
Joined
Oct 24, 2014
Messages
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Age
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Location
Hillsdale, MI
Before blaming the coil, are you absolutely sure you don't have a fuel flow problem from the main jet? Restricted flow could let an engine rev up under no load but bog big time as soon as it's loaded. Wouldn't make an engine run backwards though.
Carb appeared fine when I had it apart yesterday. I have another to try if needed. I've experienced fuel related issues before, this seemed different and was what led me in the coil direction. The transition from running perfectly to absolute dogshit is so immediate.
 
cuinrearview

cuinrearview

Red saw lover
Joined
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Messages
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Update: I threw another used carb at it and a coil from my Dad's runner. Same outcome. I tested compression after that, so it was warm. I stopped pulling once it passed 155. I got my re-build kits from Definitive Dave yesterday, so I'll try re-building one of the carbs. I'm going to try a 5100s muffler after that in case the cat is plugged. Then a different fuel tank/line that I have laying around. After that I'll pull the clutch and see if there's anything in there.
 
lone wolf

lone wolf

MS 200T King
Joined
Oct 5, 2009
Messages
55,571
Location
Prowling The Pine Barrens
Update: I threw another used carb at it and a coil from my Dad's runner. Same outcome. I tested compression after that, so it was warm. I stopped pulling once it passed 155. I got my re-build kits from Definitive Dave yesterday, so I'll try re-building one of the carbs. I'm going to try a 5100s muffler after that in case the cat is plugged. Then a different fuel tank/line that I have laying around. After that I'll pull the clutch and see if there's anything in there.
Well you put a known running coil on it right? Why don't you vac-press test the saw in the meanwhile. all you have to do is put the suspect carb on the other saw also!
 
cuinrearview

cuinrearview

Red saw lover
Joined
Oct 24, 2014
Messages
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Hillsdale, MI
It has already passed p+v. I just assume people assume that that is done. I've got a fair amount of spares for this so swapping parts is easy. I also ordered the kits for both carbs. At $5 a kit I'll just do them both and have a spare ready to run. I just bought another junk one of these yesterday so I'm sure I'll use it.:)
 
Colt Marlington

Colt Marlington

ArboristSite Guru
Joined
Oct 6, 2018
Messages
856
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Texas
Thought I had a bad coil on the 009L.
It would start and run. But acted like it was getting hot real quick and wouldn't rev, then die and wouldn't start for a few minutes.
Took the saw half way apart to pull the muffler and check the piston and cylinder, but all looked well.
Caught CAD and went and got a bunch of other saws, and put the 009 on the shelf for several months.......

Then finally decided to try a new spark plug to replace the one that looked perfectly fine...and now the saw runs perfect again.
 
lone wolf

lone wolf

MS 200T King
Joined
Oct 5, 2009
Messages
55,571
Location
Prowling The Pine Barrens
It has already passed p+v. I just assume people assume that that is done. I've got a fair amount of spares for this so swapping parts is easy. I also ordered the kits for both carbs. At $5 a kit I'll just do them both and have a spare ready to run. I just bought another junk one of these yesterday so I'm sure I'll use it.:)
Use a known tested runner!
 
cuinrearview

cuinrearview

Red saw lover
Joined
Oct 24, 2014
Messages
8,949
Age
43
Location
Hillsdale, MI
Thought I had a bad coil on the 009L.
It would start and run. But acted like it was getting hot real quick and wouldn't rev, then die and wouldn't start for a few minutes.
Took the saw half way apart to pull the muffler and check the piston and cylinder, but all looked well.
Caught CAD and went and got a bunch of other saws, and put the 009 on the shelf for several months.......

Then finally decided to try a new spark plug to replace the one that looked perfectly fine...and now the saw runs perfect again.
A Denso W22 was the first thing I did...
 
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