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  1. #1
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    Default 1980 Merc 90 HP inline 6 starting problems

    Recently retired military experienced mechanic here but, new to boating. Just bought a boat with an inline 6 w/startability problems. As I understand it, these motors are dependable but can be hard to start. Ths thing runs great when it is running and starts well when warm.
    It is getting good fire on all 6 and has compression on all 6 from 120-123.
    It seems that a lot of fuel comes from the carbs when tilted also.
    At first my friends kept telling me I was flooding it out but they came over to help. 1 person on the key, 1 on the primer bulb , and 1 on the manual choke. It took 10 minutes to get it started. (Electiric chokr does not work. Not getting voltage) seperate issue.
    I am leaning towards rebuilding the carbs. they look fairly simple compared to a Quadrajet but, as with anything new. I need to ask a few questions and get some tips before wasting any money.
    I already put a fuel pump kit in it. It did not look bad. Minimal rust and corrosion in pockets diaphram was not damaged and check valves functioning properly. Whether they are fully seating or not I am not sure. I guess I could empty the fuel pump and blow through it and pull a vacuum on it or something to check that.
    Also a couple of other problems but I think I am on the right track here. It does not charge and the tach does not work. I am going to test the rectifier tomorrow.
    Any tips suggestions or links would be very much appreciated.

    Thanks in advance,
    Tom

  2. #2
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    Wink Re: 1980 Merc 90 HP inline 6 starting problems

    Tom, That motor should have a manual choke knob. You can try it and see if the motor starts quicker, until you fix your electric choke problem. Those carbs tend to spill fuel when you tilt them up.

    make sure the electrical connects are clean and tight. The starter needs to spin fast to do a good job choking and to create enough spark energy.

  3. #3
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    Default Re: 1980 Merc 90 HP inline 6 starting problems

    Full carbs 'puke' when tilted. Period.

    Stacked 6's are notorious for wanting fuel to start, REQUIRE MAJOR amount of choking.

    Nothing runs like a well tuned 'Tower of Power'.

  4. #4

    Default Re: 1980 Merc 90 HP inline 6 starting problems

    Quote Originally Posted by tc86gn View Post
    Recently retired military experienced mechanic here but, new to boating. Just bought a boat with an inline 6 w/startability problems. As I understand it, these motors are dependable but can be hard to start.

    Hi Tom, I run and have run a number of old towers lots of hours since 1986, I have never had any starting problems with any of them, (contrary to the frequent comments here on this forum). It maybe just the fact that I have the starting proceedure down, or that I take real good care of my girls. As to being "dependable" mine are but I run them in a clean lake, if you are in a salt water envirornment they may not be as good, (so I have heard anyhooooo). They have weak fuel systems, (the 1980 is better then the older girls), weak cooling systems, but if ya keep yer eyes on the weak spots they run real hard for a lot of years.

    Ths thing runs great when it is running and starts well when warm.
    It is getting good fire on all 6 and has compression on all 6 from 120-123.
    It seems that a lot of fuel comes from the carbs when tilted also.

    As long as they don't overflow while pumping a firm bulb when the engine is upright: yer OK, (IMHO).

    At first my friends kept telling me I was flooding it out but they came over to help. 1 person on the key, 1 on the primer bulb , and 1 on the manual choke. It took 10 minutes to get it started. (Electiric chokr does not work. Not getting voltage) seperate issue.
    I am leaning towards rebuilding the carbs.

    Nope, you said it runs good, (I don't ever fix somethin' that's not broke) Scaaty will likely disagree. I'm leanin' towards an electrical issue producin' weak spark when startin' cold.

    they look fairly simple compared to a Quadrajet but, as with anything new. I need to ask a few questions and get some tips before wasting any money.
    I already put a fuel pump kit in it. It did not look bad. Minimal rust and corrosion in pockets diaphram was not damaged and check valves functioning properly. Whether they are fully seating or not I am not sure. I guess I could empty the fuel pump and blow through it and pull a vacuum on it or something to check that.

    That effects how the engine runs after the start. If ya pump the bulb firm the old girl has the juice ta fire, a weak pump will starve out when the hungry old girl starts ta drink.

    Also a couple of other problems but I think I am on the right track here. It does not charge and the tach does not work.

    That is why I'm suspectin' yer electrical system.

    I am going to test the rectifier tomorrow.
    Any tips suggestions or links would be very much appreciated.

    Thanks in advance,
    Tom
    Questions

    #1 How do ya know it is a 1980?

    #2 Does it have a distributor or six individual coils?

    #3 Do you have a fairly new, large starting battery that has a full charge prior to hitting the key? (VERY IMPORTANT TO HAVE SOME REAL FIRE WHEN YA HIT THAT OL' KEY)!

    I have a 1979 115 with a distributor, and my son has a 1981 115 (with the six coils). All my other towers are older with distributers, (which I prefer due to the familier nature of 'em, and they all explode within a few turns when ya have a firm bulb of fresh mid grade pre mix, [no alcohol] and a real hot battery). If yers has the six coils a weak battery/charging system can be a real issue when starting it! Faulty grounds, internal wiring can also be an issue. My overpriced $.02 JR
    A proud n' rare Independent Cornservative from the Seattle Metro area, who runs the finest old black boat anchors money can buy!

  5. #5
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    Default Re: 1980 Merc 90 HP inline 6 starting problems

    Quote Originally Posted by OldMercsRule View Post
    Questions

    #1 How do ya know it is a 1980?

    #2 Does it have a distributor or six individual coils?

    #3 Do you have a fairly new, large starting battery that has a full charge prior to hitting the key? (VERY IMPORTANT TO HAVE SOME REAL FIRE WHEN YA HIT THAT OL' KEY)!

    I have a 1979 115 with a distributor, and my son has a 1981 115 (with the six coils). All my other towers are older with distributers, (which I prefer due to the familier nature of 'em, and they all explode within a few turns when ya have a firm bulb of fresh mid grade pre mix, [no alcohol] and a real hot battery). If yers has the six coils a weak battery/charging system can be a real issue when starting it! Faulty grounds, internal wiring can also be an issue. My overpriced $.02 JR


    Wow! lots of good info here. I appreciate the responses.
    I only know it is a 1980 as that is what I was told SN#5911625 It has 6 coil packs and 2 modules. 1 for each 3 coil packs. He was the second owner but a little cloudy on what year it was.
    I need to get a better battery for cranking. It does not seem to be real strong. I will do that tonight and test the rectifier.
    Fuel does run out of the carbs when tilted and when I prime it. Although it may be coming out of the throttle shaft bushings when pumped up.
    I can't wait to get this initial starting problem fixed.
    For now I will concentrate on getting my electrical issues lined out first. Then go from there.
    I feel that I have a strong engine that just has a few bugs.

    Thanks for the guidance.

  6. #6
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    Default Re: 1980 Merc 90 HP inline 6 starting problems

    It is normal for the carbs to puke some fuel when tilted up.

    It is NOT normal for fuel to leak while pumping the primer when the engine is straight up/vertical. You may have to clean carbs or replace needles/seats. Then again, how hard are you squeezing the primer, you CAN over-pressure the small floats.

  7. #7
    Vice Admiral Scaaty's Avatar
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    Default Re: 1980 Merc 90 HP inline 6 starting problems

    Ya might want to try a simple test. Do what ya been doing, but after a bit of decent cranking, yank the plugs...Wet?..or Dry? Yep on the check for a good spark too, as mentioned...

  8. #8
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    Default Re: 1980 Merc 90 HP inline 6 starting problems

    Before you take the carbs off, take the covers off of the bowls, you might just have a stuck float. That would have saved me rebuilding a perfectly fine carb.

  9. #9
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    Default Re: 1980 Merc 90 HP inline 6 starting problems

    Thanks for the input all. The carbs upon closer examination seem a little abused to me. Rounded off plugs etc.. also the owner told me it was jetted to a 115 which I think may be too much for this boat. I see a few leaks from a few plugs. I am sure there are a few bad gaskets in the bunch. I was wondering if it would be wise to go back to the proper size jets for this motor.

    Also I tested the rectifier and it failed virtually every test in the book. After closer examination I noticed that 2 yellow-red stripe wires going to the rectifier from the stator were missing. I found them broken off at the stator. Hope fully there is enough wire left to solder and heat shrink a couple of wires here. Hard to tell until I pull some more covers.

    Should be good to go after this rat chasing.

  10. #10

    Default Re: 1980 Merc 90 HP inline 6 starting problems

    Your engine looks to be a 1981 from the serial number, (I would verify that the serial number on the block [where it bolts together with the crankcase] is the same as the one on the mid section). I would be very carefull with the jets, (I admit this subject is over my head so Chris1956 or some of the other ace techs need to advise you here). If you jet the engine too lean you can burn pistons, if it really is a 1981 90 horse power I would jet it exactly as the factory specifies for the planned elevation you will be running. The 1982 90 was exactly like the 1981 115 as that is when Mercs changed to prop rated engines on the inline 6 from the crank ratings 1981 and back so verifying exactly what you have is important. (BTW: the 1982 115 was the same as a 140 in 1981). Sounds like the electrical and weak battery is suspect, as the stator condition is very important to the proper operation of those six coil engines. My overpriced $.02 JR
    A proud n' rare Independent Cornservative from the Seattle Metro area, who runs the finest old black boat anchors money can buy!

  11. #11
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    Default Re: 1980 Merc 90 HP inline 6 starting problems

    Quote Originally Posted by OldMercsRule View Post
    Your engine looks to be a 1981 from the serial number, (I would verify that the serial number on the block [where it bolts together with the crankcase] is the same as the one on the mid section). I would be very carefull with the jets, (I admit this subject is over my head so Chris1956 or some of the other ace techs need to advise you here). If you jet the engine too lean you can burn pistons, if it really is a 1981 90 horse power I would jet it exactly as the factory specifies for the planned elevation you will be running. The 1982 90 was exactly like the 1981 115 as that is when Mercs changed to prop rated engines on the inline 6 from the crank ratings 1981 and back so verifying exactly what you have is important. (BTW: the 1982 115 was the same as a 140 in 1981). Sounds like the electrical and weak battery is suspect, as the stator condition is very important to the proper operation of those six coil engines. My overpriced $.02 JR


    Thank you. That is sound advice that I will be taking. Thank you for the positive engine ID.

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