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  1. #1
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    Default 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    I had written in a few weeks ago about a problem I was having with my motor. I got some great advise, rebuilt my carbs, and rebuilt the fuel pump. After that I had three or four successful outings.

    Last week I had the boat out and everything was fine. I could accelerate with no hesitation. The motor response was really great. I had to leave early so I let a friend of mine take over. Later he said that about halfway through the day the motor would bog down when he would accelerate. So he kept it slow for the rest of the day. The next day I noticed that when I had lowered the motor to store in my garage, black oil was slowly leaking from the prop. Maybe like a tablespoons worth.

    So I took the boat out today just to see for myself.. Here's what I have:

    Motor starts fine
    Idles fine
    I can raise the throttle in neutral and the RPMs go up and water circulates, no problem.
    In gear, slow speed up to about 30% is fine
    When I try to accelerate beyond that, it struggles to stay steady and the RPMs drop. I back off the throttle and everything is smooth again.
    I did notice at one point it seemed like it was struggling to keep steady at a medium slow speed.
    Brought it back tonight and saw a little more black oil from the prop.

    I have a glass inline fuel filter, and the fuel flow seemed to be normal throughout all of this.

    Any suggestions on where top start or what to look for?

    Thanks
    "For not much money, and a little bit of work, you'll be on the water in no time" - PO (June 2010)

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  2. #2
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    Sounds like you have a carb flooding or a high speed jet that's backed out causing it to run extremely rich
    When in doubt,Trim it out!!!
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  3. #3
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    Thanks for the reply. So is this something I could try to fix by adjusting screws, or should I pull the carbs out again to look for problems?
    "For not much money, and a little bit of work, you'll be on the water in no time" - PO (June 2010)

    New Project: The inevitable 1979 16’ DeckCraft restoration!

    Last Project: The inevitable 1973 15’ Reinell tri-hull restoration!

  4. #4
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    So from what I can tell, the carbs aren't flooding. At least I can squeeze the bulb tight, and no fuel escapes the carbs. That's what lead me to rebuild them in the first place.
    Also, I remember the instructions saying that the high speed jets were pre set, and not adjustable. So my plan is to get to the lake this week and adjust the screws on the carbs with the motor in water. It seems like something I shouldn't try to fix on muffs. Any other suggestions? Could carb spray help?
    "For not much money, and a little bit of work, you'll be on the water in no time" - PO (June 2010)

    New Project: The inevitable 1979 16’ DeckCraft restoration!

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  5. #5
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    Update and question:
    I went out Saturday to see if I could Adjust the carb screws and solve the problem. The water was like glass, and the lake was almost empty, go figure. For most of the day I would make an adjustment, go a hundred yards or so, make an adjustment , another hundred yards, and so on. Not much was changing. Sometimes it was too rich, others too lean. But it never would let me get past medium slow speed with out bogging down.
    Then in the last 20 minutes, we were almost back to the ramp, and my daughter asked me to do some doughnuts so we could hit the wake for fun. After a couple of figure eights, the motor picked up, and then took off like a rocket. For about five or ten minutes we were flying. Faster than I can remember going in this boat. l headed back towards the ramp, backed off the throttle, and the motor stalled. It started up again and it was time to leave. It was a blast at the end. But I don't know why it's so inconsistent.

    Any idea what this could be?
    "For not much money, and a little bit of work, you'll be on the water in no time" - PO (June 2010)

    New Project: The inevitable 1979 16’ DeckCraft restoration!

    Last Project: The inevitable 1973 15’ Reinell tri-hull restoration!

  6. #6
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    When you adjusted your carb(s), did you follow the method in the manual?

    -Turn the screw all the way clockwise until lightly seated. Then back it out counter clockwise 1 1/2 turn.
    -Then with the engine running and in forward gear, turn the screw counter clockwise in 1/8 turn increments until the engine begins to fire unevenly, due to an over rich mixture. Make sure to allow 10-15 seconds between turns to allow the engine to catch up and adjust.
    -Take note of where this position is. Then, begin turning the screw clockwise in 1/8 turn increments until the engine fires evenly and the RPMs begin to increase.
    -Continue turning clockwise until the RPMs drop off and the engine begins to misfire, due to a mixture that is too lean. Take note of where the screw is at this position.
    -Now slowly turn the screw counter clockwise where it is halfway between the Over rich and Over lean positions. Favor the over rich position. It is always better to run an engine too rich, than too lean.

    It shouldn't be too hard to find that place in between the Over rich and Over lean positions.

    It is hard to say what is going on based on the inconsistency of the running reports. Honestly, it sounds fuel related to me. I say this because my 79 mercury 402 40hp did very similar things when I was rebuilding it. I would trace up your fuel line. It may have some issues in the fuel pump, and they are so easy and cheap to rebuild, I'd do it just because. It may have a weak diaphragm which is causing it to struggle to pick up enough fuel when the motor is under load. A fuel pump needing rebuild will generally idle great, and even do alright at low speeds, but it will start to give out when you increase the throttle. These old motors are touchy. It could be any number of things. But in my opinion, fuel systems are easier to work on than electrical issues. Good luck, let us know what you find out.

  7. #7
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    Thanks for the input. I also agree that it's probably fuel related. I did use the above method. But with little change. So I would try a little leaner, and a little richer. Neither seemed to help.

    I have only been out four times since rebuilding the carbs and fuel pump. at the same time, I also replaced the two fuel lines inside the motor, as well as the fuel line from the tank.
    The first time out was perfect. It ran all day. halfway through the second day is when the trouble started.
    Since this was a rebuild of a 1974 boat, and I am using the original tank, I am suspicious that the tank may have had crap in it that I didn't get out when I cleaned it. I have an inline filter in the line from the tank. I noticed last time out that when I started the motor, there was a little crud in the filter. So I'm going to take the carbs apart again, and clean them out. Then I will try a run on a smaller portable tank and see if I have the same problem.
    "For not much money, and a little bit of work, you'll be on the water in no time" - PO (June 2010)

    New Project: The inevitable 1979 16’ DeckCraft restoration!

    Last Project: The inevitable 1973 15’ Reinell tri-hull restoration!

  8. #8
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    That is a good place to start. I would definitely get a new tank though. Also, when you do the carbs I would recommend buying a gallon of Chem-Dip, and soaking the disassembled carb bodies and all the pieces for a few hours. I did this to mine, and it came out looking like a brand new carb. Just make sure you remove any plastic parts and don't put them in the Chem-Dip. That stuff is stout and it will dissolve plastic. But I'd really recommend soaking them then rinsing with water, and drying before reassembly.

  9. #9
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    bad link. will try later.
    "For not much money, and a little bit of work, you'll be on the water in no time" - PO (June 2010)

    New Project: The inevitable 1979 16’ DeckCraft restoration!

    Last Project: The inevitable 1973 15’ Reinell tri-hull restoration!

  10. #10
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    We'll try this again -
    Here’s a quick follow up to this thread:

    I did end up removing the older tank, and replaced with a 6 gallon portable.

    I removed and disassembled the carbs. I soaked the parts in chem-dip for about 4 hours (each carb). After I reassembled, they still weren’t running correctly. I removed them for a third time and discovered that I had misplaced a washer on each carb. I only mention this because even with a misplaced washer, the motor would idle fine, I just couldn’t get any thrust. While I had them apart this time, I sprayed each part with carb spray, and blew out everything with compressed air.

    I’m pretty sure the problem was with the gunk left in the old gas tank. It apparently doesn’t take much to foul a carburetor. My advice is to rebuild with the manual handy. And take your time to clean out each part and blow through with compressed air. It makes all the difference.

    This video is from yesterday. It was very windy, but I was able to at least test the motor.

    WARNING: it may be loud!

    "For not much money, and a little bit of work, you'll be on the water in no time" - PO (June 2010)

    New Project: The inevitable 1979 16’ DeckCraft restoration!

    Last Project: The inevitable 1973 15’ Reinell tri-hull restoration!

  11. #11
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    Ok have you adjusted your floats correctly(critical).Also check your needle and seats are not getting jammed.Check floats are not taking in fuel(like a sponge).
    Also when i first read thread i thought lower gear seal.Just to make sure drain some gear oil out of leg to make sure no water inside(milky/creamy colour)....good luck.
    "I hear and I forget. I see and I remember. I do and I understand."

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  12. #12
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    Default Re: 1974 Merc. 50hp - Black oil from prop - Poor acceleration

    Quote Originally Posted by aussieflash View Post
    Ok have you adjusted your floats correctly(critical).Also check your needle and seats are not getting jammed.Check floats are not taking in fuel(like a sponge).
    Also when i first read thread i thought lower gear seal.Just to make sure drain some gear oil out of leg to make sure no water inside(milky/creamy colour)....good luck.
    Yes, thanks for your input. The last time they were apart, I went through everything very slowly. I adjusted the levers, and made sure that the needles were dropping freely.

    I dropped the lower unit to check the impeller and to see if anything looked out of place, all is well. I also drained the gear lube. It was dark from use, but not milky or creamy.

    As you can see from the video, I'm getting much better acceleration now. I think all that's left is to make minor adjustments and call it done.
    "For not much money, and a little bit of work, you'll be on the water in no time" - PO (June 2010)

    New Project: The inevitable 1979 16’ DeckCraft restoration!

    Last Project: The inevitable 1973 15’ Reinell tri-hull restoration!

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