Marine Technician Here- Need help

[Nobama]

Hey guys. I'm a marine tech working on a personal motor. Finished school not too long ago, so I am a fairly new tech, not new to motors. Got a problem that I need help with so I'll give it a try here.....

1993 Johnson 40 (tracker) (converted to premix)

Compression: 140, 140
Good blue spark
Both carbs recently cleaned, properly adjusted
new fuel lines
new tank
new ign coil (tested bad)
sync and linked
VRO2 was bypassed with new fuel pump/ lines

problem: Engine will not idle

What I have done: Set idle mix screws all the way from very lean to very rich, while advancing timing through all ranges. i even maxed out the idle timing stop screw. Idle timing can be fully advanced as far as screw will allow and idle mix screws through all ranges will not idle engine.

Engine will idle with warm up lever almost fully up. All adjustments have been made on the stand with muffs with no back pressure so this is even more confusing because it should idle even higher with no back pressure.

Let me know what ya got. This is not the first time I have seen issues with the 40, 48, 50 idling, but never to this degree....

 

[multimech]

Re: Marine Technician Here- Need help

What I have not heard is what does it do instead of idling? Does it sneeze? Does it seem to load up? Does it die out slowly? Are both cylinders taking a load? Have you tried putting extra fuel into the carbs? What was the result?

My experience with these engines is that everything must be correct or it will not idle correctly.

Here is a simple starting point, because it seems everything else is perfect...have you done a leak down on the cylinders? A compression test tells you that the cylinders are able to pump a gauge to where it should be. A leak down will tell you the state of those cylinders. Sometimes it can be a glaringly obvious thing if you look in the right places.

Every engine of this sort I was able to get near idle and then work down to a steady idle with little or no problem. Not that it didn't take time and patience.

 

[Faztbullet]

Re: Marine Technician Here- Need help

These engines are bad about scored cylinders on exhaust side. Yes they can be scored and have good compression but will not idle at all. Pull the plugs and inspect cylinder walls.....

 

[seahorse5]

Re: Marine Technician Here- Need help

Pull the flywheel and make sure the inner timer magnet is not loose.

Test the cylinderhead temp with a laser infrared gun, it has to be above 125 and ideally around 145 to idle well.

If the motor was run without a thermostat, very often the bottom piston is scored even though the compression looks to be OK and that affects the crankcase compression and vacuum which in turn affects the idle quality. Like the man said in an earlier post, get a leakdown test done on both cylinders.

 

[Rc3469]

Re: Marine Technician Here- Need help

Had the same problem and finally checked the fuel line fittings, Replaced the fittings to tank and motor. solved problem.

 

[boobie]

Re: Marine Technician Here- Need help

Does the motor smoke a lot when it's idling ??