No fire on 1 plug

I have a 1975 85 horse mercury with no fire on bottom plug someone told me to check the fire that was the reason it was not taking gas in the water. Can anyone tell me what might be the problem and what to look for this thing is killing me Thanks

Re: No fire on 1 plug

Does that motor have 4 coils or one? If one coil, check for the spark plug wire shorting to ground somewhere. If four coils, run the troubleshooting test on the CDIElectronics web site.

Re: No fire on 1 plug

It only has one coil and the wire seems fine are there anything in the dist that would cause this? Thanks

Re: No fire on 1 plug

So no spark on the bottom cylinder of a distributor ign motor....Remove the dist cap and look for dirt or grease on the contacts. The rotor is non-removeable, is very fragile and cost $200+ to replace, so be careful.

Check for carbon tracks to ground on the dist. cap. Check for plug wire arcing to ground. Check to make sure spark plug base is grounded.

Re: No fire on 1 plug

I just put that little carbon thing in the cap that rides on rotor button and cleaned the dist cap and all contacts and new ends on plug wires. Any other suggestions I am up for anything. Thanks for the reply

Re: No fire on 1 plug

Measure the resistance on the spark plug wires. You will need to unscrew them from the cap. Resistance should be zero.

Re: No fire on 1 plug

How did you determine the lack of fire in #4 cylinder? (BTW, #1 is at the top).

Did you check for spark while everything was connected (with a timing light or "firing indicator"), or did you stick a spark plug in the boot and ground it while cranking over? Are you sure the spark plug is good?

Very unusual for a distributor-model CDI ignition to lose spark on only one cylinder. It can only be a bad distributor cap, bad spark plug wire/boot, or bad spark plug.

If you run the motor at night, a spark plug wire with a bad spot that's arcing to ground will look like fireworks!

If you end up pulling the cap again, see if you can post a good picture of the insides.

One last thought, many times I've seen bad ends on the spark plug leads; the screw-on brass ferrules are crimped onto the wire, and unsuspecting mechanics try to pull them out of the dist cap instead of unscrewing. You might want to pull the #4 wire out of the cap and test it with a meter. It's solid-core wire, so the resistance should be very close to Zero Ohms.

HTH & let us know what you find.........ed

Re: No fire on 1 plug

I checked the plug wires and cap and just put new ends on the screw in part because I did not know they screwed out in the begining so got everything back together and have all plugs firing now. When you put it in a water tank it will idle and take gas put it in the lake as soon as it goes in gear it dies. Don't know what I did but the fire is there now. Now if I can get it to run in the water I will be set I think. Thanks again for the replys have helped me a bunch. Buford