Torquey Todd
ArboristSite Member
I've learned from many on here about this. I hope Fish reads this, because I've seen posts about 028s and ignitions from him.
The saw is an 028WB. Tillotson carb -- replaced all gaskets and diaphragms correctly, lever is right, passages are clear. Cylinder smooth, good compression. Good fuel, new fuel line, good pickup body, good intake boot, fuel system holds vacuum. Replaced old Stihl tank vent assembly with functional check valve, tested: no potential vapor lock. All ignition (it's a points one) parts are clean, all connections are good, the coil is properly spaced. Plug is new; saw has good spark when cold.
The symptom: saw starts easily, idles well, runs with great power for ca. 3 minutes. At 3 minutes, I'd say it like this: power becomes intermittent. No surging or racing. No "running away from its fuel." And no progressive deterioration of power. Just an abrupt arrival to a point where the saw goes-coasts-goes-goes-coasts-goes. I think I can diagnose a crankcase air leak, and this isn't like that: I can richen the mixture, and when I do, the saw can four-stroke consistently when it's going, and coast when it isn't. I can emulate the symptom by flipping the kill switch on and off. My inductive tach shows more rapidly varying RPM than what I think the RPMs are doing, by ear, when the symptom arrives. Symptom is still there if I let it cool for a minute. If I let it cool for longer, it takes longer for the symptom to return.
So I believe it's heat and ignition related.
My diagnosis is that the coil is shot because I can understand how heat could cause the coil to start grounding. I wanted to bounce this off others with experience to see if there were other diagnoses, or if anyone knows some ignition-test tricks offhand before I just swap it out. I don't have any understanding of how the points or condenser could induce a failure only when the saw is hot. Can that happen?
If I bet on my diagnosis and put a new coil on, and it isn't a remedy, then I suppose I'll have a new coil to donate to someone else as thanks. If I understand Fish's posts, then I can put an electronic-ignition coil on this saw despite the physical differences between the original and EI coils.
Thanks dudes.
The saw is an 028WB. Tillotson carb -- replaced all gaskets and diaphragms correctly, lever is right, passages are clear. Cylinder smooth, good compression. Good fuel, new fuel line, good pickup body, good intake boot, fuel system holds vacuum. Replaced old Stihl tank vent assembly with functional check valve, tested: no potential vapor lock. All ignition (it's a points one) parts are clean, all connections are good, the coil is properly spaced. Plug is new; saw has good spark when cold.
The symptom: saw starts easily, idles well, runs with great power for ca. 3 minutes. At 3 minutes, I'd say it like this: power becomes intermittent. No surging or racing. No "running away from its fuel." And no progressive deterioration of power. Just an abrupt arrival to a point where the saw goes-coasts-goes-goes-coasts-goes. I think I can diagnose a crankcase air leak, and this isn't like that: I can richen the mixture, and when I do, the saw can four-stroke consistently when it's going, and coast when it isn't. I can emulate the symptom by flipping the kill switch on and off. My inductive tach shows more rapidly varying RPM than what I think the RPMs are doing, by ear, when the symptom arrives. Symptom is still there if I let it cool for a minute. If I let it cool for longer, it takes longer for the symptom to return.
So I believe it's heat and ignition related.
My diagnosis is that the coil is shot because I can understand how heat could cause the coil to start grounding. I wanted to bounce this off others with experience to see if there were other diagnoses, or if anyone knows some ignition-test tricks offhand before I just swap it out. I don't have any understanding of how the points or condenser could induce a failure only when the saw is hot. Can that happen?
If I bet on my diagnosis and put a new coil on, and it isn't a remedy, then I suppose I'll have a new coil to donate to someone else as thanks. If I understand Fish's posts, then I can put an electronic-ignition coil on this saw despite the physical differences between the original and EI coils.
Thanks dudes.
