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Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4728
Experience:  42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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Hi I own a 1978 Mercury 140 outboard I have an ignition problems

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Hi I own a 1978 Mercury 140 outboard I have an ignition problems once the motor gets to running temperature it cuts out completely and will only start again once cools down.When it is running it runs perfect have reconditioned carbys and done everything on the fuel side so it isnt on that side of things.
The easiest way to diagnose an ingition problem on that engine is to use a timing light on one of the spark plug wires and see if the ignition fails when running. When the engine quits so should the ignition.
There are a couple different ignition components that can and do fail from getting hot.
The trigger, mounted inside the distributor.
The ignition coil and the switchbox.
To check the switchbox, check for 12v on both the white wire from the key switch to the switchbox, check on the switchbox, and the red wire. Check while cranking. You must have at least 9 1/2 VDC on BOTH terminals. I usually recommend 10 VDC. If less than 9 1/2 VDC you can damage the switchbox. The red wire gets battery voltage from the battery and the white wire gets it's voltage through the key switch.
Check the the brown wire on the switchbox, while cranking. You will need at least 9 VDC here.
To do a static test, use a spark tester or spark plug that is grounded, in a spark plug wire going to the coil, disconnect from the dist. connect a jumper wire between the red and white wires. You will need 12VDC here. Connect a jumper between the brown and white wires on the trigger side. Disconnect the trigger wires, Now touch the black wire on the trigger side to ground. You should get spark . if not the coil or switchbox is bad.
The other voltage tests need a specialized meter, a peak reading voltmeter. A normal volt meter will not work.
However if you have the voltage going through the switchbox and voltage on the brown wire chances are the ignition coil is bad. You can put an OHM meter on the coil and then using a hair blow dryer warm the coil and look for changes in the resistance.
Do these test when the engine is in failure mode and post back what you find if you need further assistance.
Please note; the trigger, rotor and other parts in the distributor need a complete disassembly to replace. The rotor does not pull off like a car engine and the spark plug wires unscrew from the dist cap.

Edited by JeffG on 2/5/2010 at 2:31 PM EST
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Customer: replied 7 years ago.
thanks for that i can do those tests with equipment i have as i am a motor mechanic by trade. i would like to change the switch box and the trigger also the coil as i am preparing to keep the boat a while and mechanically it seems in good nick great compression and the engine sounds really smooth when its going. I have already reconditioned the starter motor ,fuel pump and carbys and made a huge difference to how the motor starts and runs. I am having trouble chasing parts as alot have become obsolete. I have found a store that can supply some parts but they have different part numbers and i was wondering how i find out if i can use any compatible parts to do the same job or is second hand the only way to go because im worried there that im just going to buy more worn out parts. I have serial numbers and complete schematic drawings and parts for the engine itself is it possible for you to acquire any info on my options
Luckily engine parts are not that hard to come by. The ignition parts are available from CDI Electronics,
Many electrical and ignition parts are available aftermarket as well.
Basically almost all of the parts from 1973 through the early 1980's will work, basically any of the distributor engines.
The 140 you have is actually a 150! Lucky you. In 1978 Mercury came out with a V-6 Mercury and since they couldn't have two 150's they renamed the inline a 140. You also have the low dome pistons which are good for prevention of detonation.
Any other questions on this engine just ask, I know the inlines well.
Try in addition to CDI Electronics
Sierra Marine for parts. I believe they have distributors overseas.
I would very much recommend getting the Mercury OEM service manual, particularly if you rebuild the dist. They are still available from Mercury.
Customer: replied 7 years ago.
thanks for the help with your answers i have chased up nearly everything i need to fix the problem the only part i cant seem to find is the coil is their an aftermarket coil i can buy or only second hand optional if so wher can i buy the coil thanks.
(NNN) NNN-NNNN 2 is the part and is still available from Mercury.