Re: '82 Evinrude 35hp idling one cylinder only
We have a spark that will not jump the spark plug gap at idle speeds.
As electrical potential builds across plug gap, just before it discharges through the gap giving the spark, there is always a leak current through the surface of the porcelain insulator on the plug, which slightly discharges potential built up and counteracts on spark strength.
Do the following test to ensure misfiring due to leakage current on spark plug.
- remove boot from lower lead and just touch lead end to spark plug tip.
- start engine, warm up and run at idle
- as soon as misfiring starts, pull lead end (with a pair of insulated pliers) and introduce an auxiliary gap between lead end and spark plug tip of say about 0.03"
Does it help idling ?
This auxiliary gap restricts leakage current on plug and spark jumps both gaps at max strength.
On the fuel side, this leakage current increases with wet fuel oil or dry carbon deposits on porcelain. Spark plugs are constructed so as to sustain a rated heat flow through the porcelain and self clean carbon deposits. Limitations are that a too warm spark plug will cause pre ignition top end rpm and a too cold will cause misfiring at idle.
On the electrics side, any part may be faulting.
To rule out coil faulting, swap primary connectors on coils and boots on spark plugs to see if problem moves to top cylinder.
Also test your sparks simply by unplugging and removing the boots off the lead and holding the end (with a pair of insulated pliers) about 1/2" away from head to observe a crispy blue strong spark, while keeping your engine running on one cylinder as low rpm as possible.
On the fuel side, make sure your mixture is not more than 50:1 oil (use TCW III mixture oil). Adjust a lean mixture at idle and sustain idle by advancing idle timing.
Compression is a major issue, get and post readings. Normal engine wear has such symptoms.
Decarbing your motor, hotter plugs and higher octane gas may resolve problems for quite some time !!!
