Re: Merc Thunderbolt not firing on all cylinders...
depends on the cause, here is what I suggest:
* remove #4 spark plug cap, measure inside the cap to ground, should be around 950 ohms. No resistance, bad wire or bad coil
* if no resistance, remove #4 plug wire from the coil and measure to ground, should be around 950 ohms
* measure the resistance of the coil wire, should not be open. These can break from removing plug wire cap from the spark plug
* measure from coil trigger wire to ground, there should be resistance similar to other cylinders
* check coil #4 ground
* remove your #4 coil and inspect it for burns or a hole burnt thru it.
if none of these are the problem, you probably have a bad switchbox or pickup that is under the flywheel, but start the above first.
I had this problem with my '77 Thundebolt 50hp, I had a broken plug wire which causes a coil to melt down, that energy needs to go somewhere. I needed a new coil for about $38.
