1984 75hp L4 with unique problem? I'm baffled!

[delican]

I acquired this old motor without knowing its history but it had excellent compression and passed the leak down test with flying colors. I have jumped through all (and I mean ALL) of the hoops and troubleshooting on this old outboard and have it running fine while hooked up to water using 'earmuffs': idles great at 900 rpm and has excellent throttle response. Runs like a scalded dog...while hooked up to 'earmuffs'. Back the boat into the lake submerging the lower unit in the water (still on the trailer) and it won't idle or run. Determined to test the motor in the lake, I eventually, with a lot of starting, shifting and dying, got it into open water. At WOT it would only run a sluggish 2200 rpm. Take it out of the water, hook up earmuffs, and it would run a snappy 5800 rpm (more if I pushed it). I think of myself as a pretty fair mechanic but I am new to 2 stroke outboards and am pretty baffled by this. All I can come up with is it has a (partially) plugged exhaust and when submerged in the lake the exhaust back pressure becomes too great. Does anyone have any thoughts on this?

 

[Chris1956]

It sounds like your idle jets are set too lean. Set 'em to 1-1/2 turns open and she should idle in the water. Now on the water in gear, motor warm, adjust the idle mixture screws such that the motor will rev up. If she bogs, open the idle mixture screws one at a time, 1/8 turn and try again. You will likely need to richen them a turn or so.

This assumes the link and synch are correct.

 

[racerone]

All motors sound great running on a hose !!---Means little.-----Sounds like it is running on 2 of 4 cylinders.----Does spark jump a gap of 3/8" on each plug lead , yes or no ?---Motor needs 4 things to run.----Spark at the right time.----Correct amount of fuel.----Compression in the cylinders .----Compression in each of the 4 crankcases.----Should not be hard to figure out the issue.

 

[delican]

It sounds like your idle jets are set too lean. Set 'em to 1-1/2 turns open and she should idle in the water. Now on the water in gear, motor warm, adjust the idle mixture screws such that the motor will rev up. If she bogs, open the idle mixture screws one at a time, 1/8 turn and try again. You will likely need to richen them a turn or so.

This assumes the link and synch are correct.

Chris1956 - I appreciate the reply. I have already covered your suggestion. Link and synch are good. I started with the idle jets at 1 1/2 and they are now at 2 1/4 in order to get it idling good. The problem seems to be something else. Thanks!

 

[delican]

All motors sound great running on a hose !!---Means little.-----Sounds like it is running on 2 of 4 cylinders.----Does spark jump a gap of 3/8" on each plug lead , yes or no ?---Motor needs 4 things to run.----Spark at the right time.----Correct amount of fuel.----Compression in the cylinders .----Compression in each of the 4 crankcases.----Should not be hard to figure out the issue.

racerone - I appreciate your attempt to school me on motor basics but...Timing light shows the motor IS running on all 4 cylinders. Spark will jump 3/8"+ on each plug lead. Simple 2 stroke carbs cleaned and working. 145 PSI compression on each cylinder. Less than 10% leak down on each cylinder. Pretty hard for me to figure out the issue. What am I missing?

 

[racerone]

Good luck with your motor.----No need for further input from this desk.

 

[Mark ofs]

Be careful...manufacturers service manuals state never to run more than 1/2 throttle on muffs, or you risk damage and/or runaway engine.

 

[jimmbo]

Having spark doesn't mean the Engine is Firing. If a carb is Plugged, 2 of the Cylinders will not be contributing to the Effort, and in fact are at Risk of getting Pooched from lack of Oil.
Too bad you chased off Racer, he is one of the Best on the Board

 

[delican]

Having spark doesn't mean the Engine is Firing. If a carb is Plugged, 2 of the Cylinders will not be contributing to the Effort, and in fact are at Risk of getting Pooched from lack of Oil.
Too bad you chased off Racer, he is one of the Best on the Board

Jimmbo - Certainly wasn't trying to chase off racerone. Was simply letting him know I had already tried/checked his suggestions. I have eliminated all of typical, normal issues leaving me baffled. I thoroughly cleaned the carbs. If you're suggesting a carb may be plugged only allowing 2 cylinders to run, how would that be tested?

 

[delican]

Be careful...manufacturers service manuals state never to run more than 1/2 throttle on muffs, or you risk damage and/or runaway engine.

Thanks. I'm aware of the risks.

 

[Chris1956]

I doubt it is a plugged exhaust. What do the spark plugs look like? Any of them real clean? If so, you could have water intrusion, which puts out the fire in a cylinder or two.

Running on muffs means nothing. She will rev up on the flusher and sound real good.

What kind of boat and prop are you running? If the prop has too much pitch, the motor could run at 2200RPM at WOT, easily.

 

[delican]

I acquired this old motor without knowing its history but it had excellent compression and passed the leak down test with flying colors. I have jumped through all (and I mean ALL) of the hoops and troubleshooting on this old outboard and have it running fine while hooked up to water using 'earmuffs': idles great at 900 rpm and has excellent throttle response. Runs like a scalded dog...while hooked up to 'earmuffs'. Back the boat into the lake submerging the lower unit in the water (still on the trailer) and it won't idle or run. Determined to test the motor in the lake, I eventually, with a lot of starting, shifting and dying, got it into open water. At WOT it would only run a sluggish 2200 rpm. Take it out of the water, hook up earmuffs, and it would run a snappy 5800 rpm (more if I pushed it). I think of myself as a pretty fair mechanic but I am new to 2 stroke outboards and am pretty baffled by this. All I can come up with is it has a (partially) plugged exhaust and when submerged in the lake the exhaust back pressure becomes too great. Does anyone have any thoughts on this?

MORE DETAILED INFO: All input is valued and appreciated. Let me tell you in greater detail what I've done to this motor before I discovered the current issue: I either ohmed or checked direct voltage on the coils, trigger, stator and switchbox. Replaced one coil and the switchbox. Checked the entire ignition system including wires and plugs; new plugs. Checked compression and leak down. Replaced the fuel lines and put a new diaphragm in the fuel pump. Removed and thoroughly cleaned the carbs. After all of this, it started, idled and ran fine after some additional idle jet adjustment. The problem only occurs when the lower unit is submerged which creates much more exhaust back pressure than running it with a water hose. I know that the expansion chambers (exhaust) on 2 stroke motorcycles sometimes get plugged if the fuel/oil mixture is too rich resulting in excessive back pressure. I also know that if your catalytic converter gets plugged, it can create excessive back pressure. Although not common, either will result in sluggish engine performance. That is what I believe is happening with this motor but I am reaching out to folks with infinitely more experience with Mercury outboards than I have. Your additional thoughts on this would be welcome. Thanks!

 

[Chris1956]

Outboards of that era do not have much in their exhaust passages. They are mostly wide open. If you drop the gearcase, you can shine a flashlight up the exhaust passage and see most of what there is to see.

Did you say what your spark plugs look like?

 

[delican]

I doubt it is a plugged exhaust. What do the spark plugs look like? Any of them real clean? If so, you could have water intrusion, which puts out the fire in a cylinder or two.

Running on muffs means nothing. She will rev up on the flusher and sound real good.

What kind of boat and prop are you running? If the prop has too much pitch, the motor could run at 2200RPM at WOT, easily.

Chris 1956 - New plugs. The old plugs all looked the same; none stood out as different. The motor came off of an 18' fiberglass bass boat MFG unknown. I put it on my 18' Landau pontoon boat. The prop is a 13 1/4 x 19. If the thing would start and run when the lower unit is submerged, it would be easier to determine if it was a prop issue. I know that the expansion chambers (exhaust) on 2 stroke motorcycles sometimes get plugged if the fuel/oil mixture is too rich resulting in excessive back pressure. I also know that if your catalytic converter gets plugged, it can create excessive back pressure. Although not common, either will result in sluggish engine performance. Not knowing the history of the motor, if the fuel/oil mixture was too rich and/or the motor was idled for extended periods of time, like while trolling, could that not soot up the exhaust? Exhaust being plugged may be a reach but I'm out of other reasonable possibilities. Help!

 

[delican]

Outboards of that era do not have much in their exhaust passages. They are mostly wide open. If you drop the gearcase, you can shine a flashlight up the exhaust passage and see most of what there is to see.

Did you say what your spark plugs look like?

Nothing different about any of the plugs. They all looked the same. I replaced them but probably didn't need to. Dropping the gearcase to have a look was my next step unless I got some info that sent me in another direction.

 

[Chris1956]

That prop is likely too much pitch for that 'toon. However, you first need to get it to idle when in the water. Your timing light says all are firing?

Not sure what else to suggest.

 

[jimmbo]

Chris1956 - I appreciate the reply. I have already covered your suggestion. Link and synch are good. I started with the idle jets at 1 1/2 and they are now at 2 1/4 in order to get it idling good. The problem seems to be something else. Thanks!

In 99+% of the time 1 1/2 turns is too Rich. Are the Floats set correctly? I like to spray a bit of Premix into the carb to see how the engine responds to extra fuel, others will say to choke it momentarily.
Your timing light will reveal if Voltage is getting to the Plug, but that is no guarantee that the Plug is firing, as a crack in the Porcelain or Carbon Track can short the Current to Ground.
Your motor, certainly doesn't have a Catalytic Converter, and as already mentioned the Exhaust Passage doesn't have any restrictive areas.
Before spending more time on tinkering, or any money on Parts, do a Compression Test and post the Results

 

[delican]

In 99+% of the time 1 1/2 turns is too Rich. Are the Floats set correctly? I like to spray a bit of Premix into the carb to see how the engine responds to extra fuel, others will say to choke it momentarily.
Your timing light will reveal if Voltage is getting to the Plug, but that is no guarantee that the Plug is firing, as a crack in the Porcelain or Carbon Track can short the Current to Ground.
Your motor, certainly doesn't have a Catalytic Converter, and as already mentioned the Exhaust Passage doesn't have any restrictive areas.
Before spending more time on tinkering, or any money on Parts, do a Compression Test and post the Results

Jimmbo - I sprayed premix into the carbs at an idle when I was trying to decide if the idle needed to be richened up. At 1 1/2 turns, the motor liked the extra fuel. I'm at 2 1/4 and a shot of premix will load it up so I stayed at 2 1/4. I set the floats to spec when I had the carbs apart. I'll do another compression test in the next day or two and post the results. Thanks.

 

[delican]

That prop is likely too much pitch for that 'toon. However, you first need to get it to idle when in the water. Your timing light says all are firing?

Not sure what else to suggest.

Timing light says all are firing.

 

[jimmbo]

Can the Spark jump a 3/8" Gap? If it can't then it is weak. As for the Timing Light, that doesn't prove the Plug is Firing, all it proves is that the Current is getting to Ground, perhaps through the Plugs, perhaps not.