Re: 25 HP Mercury has me stumped
Hi Wildbill,<br />I have this same engine and recently had to troubleshoot a combination of carb and ignition problems so it's all fresh in my head.<br /><br />First you need to get some spark out of that thing. There are really only five points of failure for getting spark on this system: ignition stator, trigger stator, switchbox, coil (2 each), and spark plug (2 each). Not counting wires to/from each component.<br /><br />If you have elec start, (I do), you have another auxillary stator under the flywheel but this has absolutely nothing to do with ignition - just charges battery or runs nav light.<br /><br />Take it from the top and ohm everything out that has a factory recommended reading available. Maker sure you disconnect each component from switch box, etc. when testing.<br /><br />Stator:<br />High Speed coil [Blk/Wht to ground] 120 - 180 ohms.<br />Low Speed coil [Blk/Yel to ground] 3200 - 3800 ohms.<br />If any dead shorts or open circuits, stator is bad - new one about $100 to $120. If resistance is substantially lower than readings above you will have a weak spark and might want to replace.<br /><br />Trigger:<br />Brn/Wht to Brn/Yel - 650 to 850 ohms.<br />If this is shorted or open you need to replace. This is your version of a points cam and your system won't know when to spark. about $100? not sure.<br /><br />Coil:<br />Each coil has +/- terminal. Should be 0 ohms across these, (closed circuit).<br />Coil tower to + or - terminal: 850 to 950 ohms.<br />Same story, if coil has open circuit or tower has dead short or low resistance, you need to replace.<br /><br />Switchbox:<br />I don't have any test data for this guy. If all three above pass and you've checked your spark plug, plug wires and other wiring, then you probably have a bad switch box.<br /><br />Get a $9 spark tester if you don't have one. Makes spark check much easier.<br /><br />Good Luck.<br /><br />P.S. My ignition problem was weak spark due to partially bad stator, (low resistance reading on LS coil, approx 1700 ohms). Sympton was cold nature /prone to stall at idle /delicate mixture-idle setting.<br /><br />Carb problem was bad check valve in primer pump. Sympton: flooding, won't idle and a bunch of other wierd fuel flow stuff.
