175 Black Max Won't Run When Cold

[mulliganmagic]

My fishing partner's 175 merc black max will not run when it it cold. By cold I mean not only engine temperature but also air temperature. Early in the morning when it is first started you can press the gas to the floor (hot foot) and it boggs like it is blown up and will not plain off. Sometimes it takes some reving in nutral and then suddenly it will give a little jerk and plain off and run fine for usually the rest of the day even if it sits for a while. It's already been to the mechanic (well respected mechanic in the area) and he said that the wire covers over the trigger wires had become hard over the years and were not allowing the spark advance to advance all of the way. He said when the engine would become warm it would also warm the wire covers and allow the spark advance to have full movement. So he removed the old cover which was some kind of tubing and taped the wires. This fixed the problem for only one or two times out and then the head gaskets went out. Those have since been fixed but the bogging problem still exists when the engine is cold. It sounds like an electrical problem to me stator, rectifer, power pack, or trigger, but I do not know which one. Does anyone have any idea as to what could be the problem or how to test them? Also while idling on the ramp we pulled the top and bottom plug wires and it did not affect the engine, but when the middle wire was pulled it did have an affect. This is the same for both sides of the engine. I take it that this could mean that the engine could be idling on the two middle cylinders. I'm confused where to go from here. I appriciate all responses.

 

[CharlieB]

Re: 175 Black Max Won't Run When Cold

First thing, get your hands on a compression tester, verify all cylinders adequate for the engine to run well.

Second, a spark tester or inductive timing light, to verify that the ignition is firing all cylinders.

These two tests are absolute basics and should be done. Period.

If both return satisfactory results, THEN move on to the fuel system.

Verify fuel pump flow, volume and pressure, only after all this is it time to look into carbs.

 

[MJ76]

Re: 175 Black Max Won't Run When Cold

I agree w/ CharlieB. You should start out with a compression & spark test to find out what the problem actually is. Otherwise you're guessing, and replacing parts this way can get expensive.

The trigger needs to be able to move easily through the full range of spark advance. If it doesn't, find out what's restricting it and fix it, or replace it.

If you have one or more cylinders that don't have spark, swap the coil primary wires from one that has spark with one that doesn't. That will tell you if the coil's bad. You can do the same thing with the stator and trigger inputs to the power pack. Switch them around and see if the problem stays on the same cylinder or moves when you swap wires. This will tell you which component is faulty. The rectifier is part of the charging system and has nothing to do with the ignition.

Oh, and do have a service manual handy to identify the wiring, otherwise you'll likely do more harm than good. A good service manual will detail this troubleshooting procedure as well.