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

[Faztbullet]

Check exhaust also as I have seen 2 motors with dirt dabbers blocking exhaust. One was a 60Hp Johnrude and other was a 115 TOP

 

[delican]

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.

Spark will jump a 3/8"+ gap. I will check spark gap again at the time I'm doing another compression test and post the results. May be able to get it done today...if I get the time.

 

[delican]

Check exhaust also as I have seen 2 motors with dirt dabbers blocking exhaust. One was a 60Hp Johnrude and other was a 115 TOP

Thanks Fazbullet. Dirt dobbers had entered my mind as a possibility. Removing the gear case to get a look up the exhaust is next on my list...after all other possibilities have been looked into. Thanks again!

 

[delican]

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.

Jimmbo - Just completed another compression test and a spark gap test. I wasn't set up to start and warm up the motor for the compression test so the results are for a cold motor. When I checked compression earlier with a warm motor, there was 145 psi on all cylinders. The cold results were #1-125 psi, #2-125 psi, #3-120 psi and #4-120 psi. I have my spark gap tester set at 1/2". All four leads jumped that gap. What now?

 

[jimmbo]

The Cylinders that are not firing, what do the plugs look like, Wet or Dry? Inspect the Insulators for Cracks
With the Engine running, on Muffs as it will not too bad with only 2 cylinders firing, check that air is entering the Carb, and also check for air-fuel being pushed back, out of the carb.
Also examine the Seam of where the crankcase halves are mated, looking for oil leaking.
Both carbs fully closed at Idle, and remaining Closed even after a substantial amount of Timing Advance?

 

[delican]

The Cylinders that are not firing, what do the plugs look like, Wet or Dry? Inspect the Insulators for Cracks
With the Engine running, on Muffs as it will not too bad with only 2 cylinders firing, check that air is entering the Carb, and also check for air-fuel being pushed back, out of the carb.
Also examine the Seam of where the crankcase halves are mated, looking for oil leaking.
Both carbs fully closed at Idle, and remaining Closed even after a substantial amount of Timing Advance?

All cylinders seems to be firing. I inspected the plugs when I had them out yesterday. I didn't see any defects. Earlier when I was working on getting the motor running properly, I choked out the carbs with my hand over each. There was plenty of air entering then. Didn't notice any air/fuel being pushed back. There is a small amount of oil underneath the powerhead; a very small amount. I have looked around that area with the motor running but have not seen any leakage. I have NOT checked to see if the carbs remained closed while advancing the timing. I'm not sure exactly how to accomplish that task.

 

[Chris1956]

Do any of the spark plugs look real clean? If so, you could have water in the cylinders.

 

[delican]

Do any of the spark plugs look real clean? If so, you could have water in the cylinders.

They all look pretty clean but the plugs are new/low hours. I've not seen any indication of water while cranking the motor with plugs removed.

 

[jimmbo]

They all look pretty clean but the plugs are new/low hours. I've not seen any indication of water while cranking the motor with plugs removed.

You won't as the Water Jackets are Empty, and there is no Exhaust Reverberation to push water into the Cylinder

 

[airshot]

Jimmbo - Just completed another compression test and a spark gap test. I wasn't set up to start and warm up the motor for the compression test so the results are for a cold motor. When I checked compression earlier with a warm motor, there was 145 psi on all cylinders. The cold results were #1-125 psi, #2-125 psi, #3-120 psi and #4-120 psi. I have my spark gap tester set at 1/2". All four leads jumped that gap. What now?

I am no mechanic but have serviced different types of engines for many, many years. The cold vs warm compression numbers don' t look right....in all my years of engine work, never saw a gain of 25 lbs from cold to running. If the engine hasn't run in months then mabey but a 25 lb jump in compression over a 24/48 hr time period is sonething I haven't saw before. Are you sure your comp gage is working properly ??

 

[electricjohn]

Do those carbs have fixed jets that may be plugged?

 

[airshot]

All cylinders seems to be firing. I inspected the plugs when I had them out yesterday. I didn't see any defects. Earlier when I was working on getting the motor running properly, I choked out the carbs with my hand over each. There was plenty of air entering then. Didn't notice any air/fuel being pushed back. There is a small amount of oil underneath the powerhead; a very small amount. I have looked around that area with the motor running but have not seen any leakage. I have NOT checked to see if the carbs remained closed while advancing the timing. I'm not sure exactly how to accomplish that task.

"All cylinders seem to be firing" is not accurate enough to make any diagnosis.....each one needs a spark jump gage to know for sure. If your only going to guess, we can't help. When making tests for diagnosis, these test must be done right and accurate or ???????

 

[delican]

This post is a FYI to all of you who attempted to help me resolve the issue with my old 75 HP Mercury. I want to say thanks for your time but I had already tried 99% of the suggestions without success. My final guess that the exhaust could be at least partially plugged was wrong as well. I finally found a local Mercury guru who solved the problem. He first confirmed all of the tests that I had performed and actions I had taken to be satisfied he could move forward from there. What he discovered was that the 3 BLEED RESTRICTORS in the block needed replaced. He replaced them and adjusted the idle screws accordingly and the old motor runs like a top. For what its worth, I wanted to share this information. Thanks again for your time.

 

[delican]

"All cylinders seem to be firing" is not accurate enough to make any diagnosis.....each one needs a spark jump gage to know for sure. If your only going to guess, we can't help. When making tests for diagnosis, these test must be done right and accurate or ???????

BTW: If you would have read any of the earlier posts you would know I tested each plug wire with a 'jump gauge' resulting in a spark that easily jumped 1/2". All of the tests and actions I took were done right and were accurate. Thanks.

 

[racerone]

I install 8-32 stainless set screws with a 0.040" hole.----Replaces those nylon restrictors with an improved version that will not fall out.

 

[Chris1956]

Gee, I hope that works for you. I never found the bleed restrictors to do anything but lower the quality of the idle. Maybe your motor is different than my IL6 and Il4 motors.

 

[racerone]

The inline 6 has 5 of these bleed restrictors.---The 4 cylinder used 3 of the cheap nylon inserts.----I usually find 3 of them missing on an inline 6.

 

[airshot]

BTW: If you would have read any of the earlier posts you would know I tested each plug wire with a 'jump gauge' resulting in a spark that easily jumped 1/2". All of the tests and actions I took were done right and were accurate. Thanks.

Sorry if I missed that post....

 

[delican]

Gee, I hope that works for you. I never found the bleed restrictors to do anything but lower the quality of the idle. Maybe your motor is different than my IL6 and Il4 motors.

Well, at least at this writing, replacing the bleed restrictors seems to have solved the problem on my L4. Thanks!