1978 70 hp merc missfiring

[kevin]

" Help
I have a 1978 70 hp 3

" Help
I have a 1978 70 hp 3 cylinder merc that missfires under load and idles all over the place When underload it tries to run on 3 cylinders but you can feel it cutting in and out
The timming is spot on the carbs have been remove and cleaned with the idle mixture screw set between 1 and 1.5 turns out, the fuel tank has been cleaned out and new fuel added, Plugs have been replaced compression has been checked and is ok, there is spark at all 3 plugs when cranking
The engine was repaired by a mate some time ago ( eg total strip and reassemble could he have done something wrong) eg reed valve etc
The lower seals have been replaced
Any help will be greatly appriciated as im so confused on this one "

 

[tonyob]

"Kevin,

That sounds like an

"Kevin,

That sounds like an ignition problem. A bad reed valve will cause a continuous loss of power. It will not fluctuate at all.

Will the spark jump a gap of about 7/16 inch?

Rig up a spark tester that will allow you to test all three plug wires for spark at the same time. Then you will be able to see if one is misfiring now and then.

You really need to check all components of the ignition system properly to find a problem like this. Sometimes it is easier to find some used ignition parts and try replacing components until you find the problem. Buying the actual testing equipment can be more expensive than this.

"

 

[bubba]

" Sounds like good advice to m

" Sounds like good advice to me. If the stator fails on one of those black motors, it usually won't light up. The triggers rarely fail. That leaves the switchbox, ignition coils, or a bad connection somewhere. Intermittent problem: look for a poor electrical connection first. If you don't have a peak-reading volt meter, or an inductive KV meter, borrow or buy a used switchbox and try that. New: probably around one hundred dollars. Good luck. P.S.- make sure your coils have a good, clean ground. "

 

[kevin]

Hi thanks
so far i have narr

Hi thanks
so far i have narrowed it down to number 1 and 3 that have a weak and intermitant spark ( the jump test) i have swaped the conections on the swtch box to the coils and the problem moves so to me this suggests the coils are fine i have puuled the flywheel off and found two wires from the black coil where grounding so i have repaired these but still have the same prob do you think it would be the switch box or the black coil