1978 Mercruiser 140

[yamahafreak]

Here is the story, This motor will run fine for about 10-20 min. Then, all of a sudden the motor will just shutdown. It has been changed from points to electronic ignition. When we run just a hot wire from the battery to the coil, it does the same thing so i think we have eliminated it being in the wiring on the hot side. It has to be on the ground side. Do I need to have a condensor, or does this wiring system have one with in the wiring loom, because there isnt one inside the distributor? Any help is much appreciated!<br /><br />nate

 

[DJ]

Re: 1978 Mercruiser 140

yamaha,<br /><br />I assume you are losing spark?<br /><br />If so, I would suspect something in your "electronic system" or the coil. <br /><br />Also, you should not be running 12 volts to the coil. 12 volts will eventually overheat the coil and cause it to shut down. The power side of the coil should run about 9.5 volts. If you have 12, you need a ballast resistor in line.

 

[Trent]

Re: 1978 Mercruiser 140

Change the coil to a known good one and try it.<br /><br />For negative ground applications only)<br />To determine if your systems coil is compatible with the Ignitor, some measurements should be taken prior to installation of the Ignitor. Caution… While performing this test, never leave the ignition switch on for more than 30 seconds at a time.<br />Set your voltmeter to a 15 or 20-volt scale. Attach an 18 or 20 AWG jumper wire from the negative coil terminal to an engine ground. Attach positive (red) lead of your voltmeter to the positive side of the coil, and the negative (black) lead to an engine ground. Turn the ignition switch to the run position. Now read the voltage at the positive coil terminal. Turn the ignition switch off. If the voltage measured is approximately 12 volts, no resistance wire is present. A typical resistance wire will provide 9 - 6 volts. <br />The next step is to determine the resistance in the primary ignition. Label the wires attached to the coil terminals and note their appropriate location. Make sure that the ignition switch is off and disconnect all wires from the coil. Adjust your meter to the lowest ? ohm scale. If you are using an analog style meter make sure to zero the needle. Measure from the negative terminal to the positive terminal. Write your measurement down.<br />Now the maximum system amperage can be determined, divide your voltage measurement by your coil resistance measurement. This will give you the system current or amperage.<br />Four cylinder engines should not exceed 4 amps. Six and eight cylinder engines should not exceed 8.5 amps. If the total amperage in your system is higher than the amount recommended for your application, you should install a ballast resistor. <br />Example Voltage 12 <br /> Resistance 1.5 <br /> 12 / 1.5 = 8 <br />Total amperage 8

 

[yamahafreak]

Re: 1978 Mercruiser 140

I have changed the coil a few times. No luck. I think that the resistor may be what i am missing. I think if i remeber we tested the system while running with the wiring setup normal and it was running about 12 volts at the coil. It should be 9.5 though? Where would a guy put the ballast resistor? thanks guys

 

[petryshyn]

Re: 1978 Mercruiser 140

pretty subjective question...<br /><br />What kind of electronic ignition was it converted to?<br />>complete distributor?<br />>points removed, modular unit installed?<br />>HEI?

 

[yamahafreak]

Re: 1978 Mercruiser 140

There is a module inside of the distributor. I dont think it is HEI?? not sure...how would i tell ?

 

[yamahafreak]

Re: 1978 Mercruiser 140

does anyone have anymore input on this issue? thanks

 

[hoot]

Re: 1978 Mercruiser 140

my experience (and 0.02) with this is i have a 1978, 165 hp Merc, 250 IL6. would run 20 to 30, shut down, after sitting 15 to 20, would run awhile and repeat sequence. replaced almost everything myself; points, condensor, resistor wire, coil, fuel pump, fuel filter, check valve,...did the straight wire from the battery just like YF, still had problem. finally, replaced rotor button, distributor cap, and small, flexible, black wire coming from distributor to coil. PROBLEM SOLVED. don't know what worked for sure, but have had no more trouble. on my engine, the ballast resistor is a stainless steel wire that cuts the voltage down to the coil. the 12 volts is used as a start voltage thru the solenoid and the resistor wire drops the voltage down during running. the 12 volt circuit should drop out after the solenoid drops out. i know of 1 other identical engine that did the same thing and was fixed. let us know what you find out.

 

[MrBill]

Re: 1978 Mercruiser 140

I think 'hoot' is on the correct path. My experience has been....with cars and boats, every time an engine runs well until hot, and then is difficult or impossible to start until it cools...head for the distributor - cracked cap, damged under-side, loose/burnt wire, bad rotor, etc. I realize this isn't good diagnostic procedure, but it tends to have worked for me.

 

[hoot]

Re: 1978 Mercruiser 140

just had another thought on this. if the gas tank isn't vented to atmosphere, a vacuum can develop and shut the boat down. on some built in tanks, there is a vent close to the gas cap. a dirt dobber can fill up the vent or wax can close off the vent. i pulled my gas cap off and ran my boat while trying to find my problem.

 

[spinakersheet]

Re: 1978 Mercruiser 140

also make sure you get a coil with an internal resistor. In addition to having a resistor wire.