Merc 85 starting problem

[Don McCrory]

Bought an older boat with Mercury 850 OB (SN3619164) knowing it had erratic starting problems. First time I tried starting it, it lit right off! Several stops & restarts went fine. Seems to run great when it does start. Tried again later and---Nothing. Appears there is no spark. Doesn't seem to be related to temperature or anything else that is common. Am guessing ignition module but no way to test- (has "Thunderbolt Ignition". This is a CD system is it not?) Any ideas??

 

[Chris1956]

Re: Merc 85 starting problem

Don, Those Thunderbolt ignitions are CDI. They have a switchbox mounted on the engine support, in front of the carb intakes. They also have a green wire from the switchbox to an ignition coil, and then the high voltage lead to the distributor. On the switchbox there is a red battery wire (always hot) and a white wire that runs from the ignition switch to the switchbox, for supplying voltage to the switchbox, when the ignition is on. I would check these two wires for +12 volts when you motor is a no start. Check them while cranking the motor with the key. I suspect you will find low or no voltage due to a intermittent ignition switch or enough corrosion in the ignition circuit to cause low voltage.

 

[Don McCrory]

Re: Merc 85 starting problem

Thanks Chris--<br />Checked voltages with starter engaged-drops a couple of volts throughout the circuit-still won't run. Tried jumpering from battery to different places but makes no difference. Got me puzzled???

 

[Chris1956]

Re: Merc 85 starting problem

Don, I think you should check for spark when you have the no start condition. To make a spark tester, take an old conventional (with a gap)spark plug and cut off the strap electrode. Then put the spark plug lead on the spark tester and hold it against the engine block and crank the motor. If the spark jumps to the threaded part of the spark plug, it is good. <br /><br />If the spark is good, the fuel system is the next suspect. Spray fuel into the carb throat and crank it. If should start and run for a few seconds. If this occurs, then the carbs may need to be cleaned.