1985 Mercury 60 hp running with cowling on/off

[Todd 1669]

Greetings.
The above motor has ran great all season until the last few trips out. She starts up and idles fine. In neutral the motor will rev up like a champ. Will run at low throttle ok but when increasing throttle it just bogs down like its suffocating. If you hold at increased throttle it will hiccup like it wants to surge and take off. The first few times it did it I was able to play with the throttle and get it to clear up and run normally. The last few times I couldnt get it to clear up so I took off the cowling and then it ran fine. Im not able to get the cowling back on while were on the water so I dont know what would have happened if it covered up again but it ran like a raped ape with it off.
From what Ive read one of the causes could be an exhaust leak into the cowling but wouldnt that affect the motor at idle and low throttle as well? Especially since the motor will rev at high rpm in neutral with the cowling on.
Ive checked to see if the cowling might be rubbing ignition wires but dont see anything obvious.

 

[racerone]

Serial # of the motor.-----Plug wires are suspect.----I am not there to do some simple testing.

 

[Todd 1669]

Serial# is 6461967.
1985 Merc 60 Hp 3 cylinder.

 

[racerone]

With cowling off start the motor on a hose.-----Squirt extra oil into carburetors.-----Heavy smoke may make it easier to spot an exhaust leak.-----Perhaps run in the dark ( depends on the neighbors ) to see if there is spark arcing anywhere.----Push plug wires around with a stick to see if wires are the problem.----Perhaps replace rubber boots on the plugs.----It will rev up in neutral running on 2 of the 3 cylinders.-----Water pump maintained ?-----Compression test results are ?-----Any water leaking around the top sparkplug ?

 

[Todd 1669]

I will do compression/smoke and check for arc and plug boots tonight.
It does have a new water pump/impeller and pisses like it should.
It does have a teardrop leak out of the water jacket gasket on top but its been there for a while.
Ive been putting off replacing the gasket because the fishing has been so good. I guess its time to get around to fixing it.

 

[jimmbo]

The gasket may not be the problem, those water jacket covers are famous for warping.
Based on your description of the symptoms, sounds like dirty carbs, or ignition cut out. Adding to what racer suggested... inspect the wires going to the trigger plate for damaged insulation and/or signs of grounding

 

[Todd 1669]

Compression in all 3 cylinders was within a few lbs of 105 on my chepo compression gauge.
I found some spots on the spark plug wires that had some rubbing but not real deep. I wrapped and taped them real well until I can get some replacement wires. The rest of the motors wiring looked fine. Ran it in the dark and didnt see arching anywhere.
I have one of those wrap around the plug wire tachometers and found top and bottom cylinder idle at around 1650 RPM. Middle cylinder showed around 2400 RPM. Does that cause concern? Spark was real sharp at each cylinder and drop test was fine.
Is it possible to recreate the running conditions that would create an exhaust leak in the driveway on a hose or does the motor need the backpressure of being in the water and in gear? At idle and low speed is the exhaust directed to the same exits as it is at full throttle?
That would explain why it wont bog down in neutral and low speed but does at half to WOT.
I tried the smoke suggestion in the driveway on the hose but really couldnt tell what was going on. It created quite a fog though.

 

[Todd 1669]

I got out last night and ran the motor hard. No bogging. Ran great.
I cant narrow down what the cause or the solution was but I appreciate all the advice given.

 

[jimmbo]

Was the cowl on it last night?

" have one of those wrap around the plug wire tachometers and found top and bottom cylinder idle at around 1650 RPM. Middle cylinder showed around 2400 RPM."

what are you trying to say here? Is there a lack of spark at those rpms?

 

[Todd 1669]

Cowling was on and it ran fine.
As far as the rpm differences I was really just wondering what might explain how that might happens.

 

[w2much]

That would be your problem. Read again jimmbo's post. There is no difference in RPM, only spark.