[john0974]

hi guys i have a 78mdl 70hp evinrude it only misfires when idling but when running doesn`t miss at all i have replaced plugs,coils,had all wiring replaced,i am running rite fuel to oil mix,had timing checked,carbies have been serviced,compression is 121 on all cylinders please help i have had it with boat mechanics and there ridiculous prices

[Dhadley]

Does it just seem to drop a cylinder and just fade away & quit?

[CharlieB]

You still need to follow the basic troubleshooting to determine the problem.

Using and inductive timing light, verify that you have spark on all cul's at an idle to rule out an ignition problem such as a broken wire at the pivot point at the timer base.

Once spark is verified then it is safe to assume that the problem is in the carbs, either not synch'd together well enough or a slight fuel or air restriction in the idle circuit, just enough to NOT fire at an idle only, yet run perfectly well from just off an idle and all the way to WOT.

Trim the motor enough to level the carb float bowls, with the air box cover off, and the motor OFF, open the throttle WOT, look down the carb throats while squeezing the primer bulb. You are looking for ANY fuel leaking into the throat of any carb, indicating a faulty needle/seat or incorrect float setting allowing that cyl to flood at an idle.

If all carbs look good, then close the throttle, add water and stir, warm up the motor and proceed.

With the air box cover off and running at an idle, stick a finger into the throat of one carb, top, then center, then bottom. Notice any difference? Try again with two fingers. If the offending cyl is too lean then the finger trick/test will act llike a choke, fattening the mixture.

One of these three test should show your problem.

This is assuming the the linkage on the carbs are sync'd.

Depending on yr, make, model, the linkage between the carbs should NOT be touching the throttle cam at an idle. If it is a model that sets idle speed by throttle position then carefully pull a pin, or bolt, such to disconnect or turn the throttle cam away from the carb linkage while you are synching the carbs. Remember how to put it back to the same position once finished. There should be a screw on two or all three carbs pivot or on the linkage at the connection to the pivot. Carefully loosen all these screws, ensure that each carb pivots freely and is allow to close completely, carefully re-tighten each screw such that once the linkage is tight then all carbs are closed AND that all carbs open in sych. Done.