Rough Running 170 Mercruiser

"I have a 170 Mercruiser that&

"I have a 170 Mercruiser that's running poorly after 1hr of operation. When this happens, it acts like one of the plug wires is off. Top speed decreases by 10mph, and tach also doesn't read accurately. Exhaust turns black and smokey, and has a raw gas smell. Boat was recently tuned, with plugs, wires, points, rotor, condenser, cap, new breaker plate in dist., timed, rebuilt carb., and new stator. all this was done before it started acting up. Also when I operate the trim, the engine stalls.I think it's ignition voltage, since the tach is not reading right, and elctrical draw makes engine run even worse.Even when engine is cold and running good,normally 45mph=4500rpm,now 45mph=3500rpm. Because it has all these symtpoms only after heating up, and running about 1hr., and the tach runs off the coil, I was sure it was the coil. Wrong! New coil, same problem. I checked resistor wire, 1.5ohms. Merc. specs. are 1.8 to 2.0. Would too little resistance cause this? Doesn't sound right to me. I have a good Clymer manual, and Fluke voltmeter. everything I've checked seems good. What am I missing? Thanks guys, "Kritter""

"This is just a hunch, but if

"This is just a hunch, but if the plug wires are old one of them may be breaking down after it warms up. I would replace them all with a high quality new set. Also check and replace the plugs if questionable. What you are describing sure sounds like something flakey in the ignition system.
Rod"

"Rod, Thanks for the come back

"Rod, Thanks for the come back. I'm thinking the same as you. I just replaced all the things we talked about, but you could be right, I,m probably making it harder than it is, and over looking the obvious. I have installed new parts that were faulty before(i.e. condensers, etc.)It's a pretty basic loop on these old systems. Usually something simple. I'll let ya know what I find. Thx, "Kritter""

"Ken, It does sound like somet

"Ken, It does sound like something in the ignition system. Here are some ideas. When this happens, check the voltage at the coil. It should read LESS than battery voltage. You could remove one plug wire at a time to see if you can isolate a cylinder. Check the engine at night to see if any wires are arching. I have read some posts that the distributor shaft bushings wear easily on these engines, causing the settings on the points to change. Grab the rotor and the shaft and see if you can move it around. Guys have converted to electronic ignition with the kits, they are not affected buy a loose shaft. Check your sparkplugs, if one is different than the other it might be a sign. Food for thought. Good luck
Chris"

"I found the problem! one side

"I found the problem! one side that holds the contact on the rotor(new) was burnt off, and the contact was turned 90 degrees. I'm surprised it even ran at all! Rod, I took your advise, and did an ohms test on the plug wires, even though I just changed them 2 years ago.(Merc. wires)They checked out between 16,& 25ohms, and had a bad case of the "Greenies". New ones were 1.7 to 2.7ohms depending on length. I think this was causing the problem after the engine ran about an hour, and probably burnt up the rotor. Now I think I'll probably have to re-wheel the boat! Typically it's ran 4600rpm WOT with just me in boat w/1/2 tank of fuel, with a 23P Mercruiser prop. Yesterday I hit 5000rpm WOT(TOO MUCH!) with 2 people, and a full tank. Top speed 51 mph! Not bad for a 19 1/2' SeaRay Cuddy with a little 21 year old 170! Guys take Rod's advise, if your boats running rough, especially after warmed up, do a quick ohms test on your plug wires, no matter how new, it may surprise you! Also buy a tube of di-electric silicone, clean all your terminal connections(solinoids,starters,coils,switches,rotors,plug wires,etc.), and coat liberally to prevent the "Greenies". Thanks guys, Happy Boating! "Kritter""

"Ken; Glad to hear you nailed

"Ken; Glad to hear you nailed it.
Always good to hear a happy ending to a boat story.
Good luck,
Rod"