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Results 1 to 11 of 11
  1. #1

    Default 1968 650 merc issues

    Hey i have a 1968 merc 650 on a 14ft run-about, my problem is that this thing should haul ass and it doesnt, last year changed the old beat aluminum 13pitch prop for a 11 pitch pirahna, with the 13 it was boggy when towing the tube or with with four ppl, but empty i could only get about 25mph out of it, now with the 11 pitch it is quick out of the hole, tows awesome, and uses about half the fuel but still only get 21 mph @ 5600 rpm. The compression was 120psi, we were thinking that mabe the timing may be out. any ideas or tips?

  2. #2
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,662

    Default Re: 1968 650 merc issues

    You need to check things out.---------------There is more going on than prop selection.-----------------Check to make sure that you have spark at each plug wire that jumps a gap of 5/16" or better with a snap you can hear.-------------Check reed valves.---------Could be a carburetor flooding or a jet partially plugged.------------It sounds like it is only running on 3 cylinders.

  3. #3
    Join Date
    Dec 2009
    Location
    Yuma, AZ, USA
    Posts
    107

    Default Re: 1968 650 merc issues

    Well number 1 and dont get me wrong but your not going to be breaking any speed records.From what I understand in your post the boat was running a 13 pitch prop which would have given you approx. 5200rpm excellent.By dropping your prop down 2 pitch sizes you increased your rpm approx.400 rpm. 1 increase or decrease in pitch = 200rpm.Your spec's from mercury on your motor recommend if I am not mistaken 4800 to 5300 rpm.So you are over by 300 rpm (not good).For towing and weight wise I would maybe have selected a 13 pitch 4 bladed prop.As for 120 psi not bad if all cylinders are within 10% of each other.Selecting the correct prop size is a pretty tricky job every boat,horsepower,displacement,weight are all factors that will affect performance and speed.There are folks here on this forum that will get you the best advise possible,they got me on the right track.
    Happy boating and good luck.......
    [FONT=Impact]Best Regards[/FONT]
    [FONT=Impact]Steve[/FONT] ;)

  4. #4

    Default Re: 1968 650 merc issues

    the spark plugs in it are the flat ones with no adjustable gap, the 13 pitch prop seemed to work fine when there was no load on the motor but performance went down as soon as you started towing or had the boat full of ppl, and it almost sounded like it was pinging with the 13 pitch as well, but the sound went away with the 11, i replaced all the plugs and caps and that did help but still not alot. I may have the rpm wrong havent run it in awhile, but i know that while towing it is in the right rpm its only when the boat is lightly loaded that it goes over the rpm range by a couple hundred
    Last edited by knightracer_08; 02-19-2011 at 09:25 AM.

  5. #5
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,662

    Default Re: 1968 650 merc issues

    Check to see if the engine is running on all cylinders.---Check to see what gap the ignition system will fire !!----------Have you cleaned out the inside of the distributor cap ??--------------The decals on the motor say 65 HP-----------A shop with a marine dynamometer should check your motor to see if it is actually producing the power.--A little money well spent.
    Last edited by racerone; 02-19-2011 at 09:37 AM.

  6. #6
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    12,505

    Default Re: 1968 650 merc issues

    ..."Check to see if the engine is running on all cylinders.---"

    Come on! It planes of, doesn't it? Sheesh.

    You might be off a bit on timing and jetting. To find out the top cowling has to come off, along with the top carb's throttle arm (one screw). Fire her up and hold it wide open (not a problem) and set the timing with the light 'til it's spot on.

    Next comes jetting. Temporarily replace the surface gap plugs with Autolite 303s (that you can read). Go boating and run the motor HARD for several minutes at WOT. Then shut her down fast as you can without a problem. Let the plugs cool a bit (to keep from burning yourself) then remove them all and take a look at the colors: All 4 should be a light tan or brown color. If any are stark white (too lean) or dark colored (too rich) you'll need to rejet.

    Have fun!

    Jeff

  7. #7
    Join Date
    May 2007
    Location
    Ireland,the only country in the world you can go North to go South
    Posts
    1,374

    Default Re: 1968 650 merc issues

    And you'll only be off the timing if someone has been playing with it

  8. #8

    Default Re: 1968 650 merc issues

    ya im not sure what was done to the boat before i got it, it was a trade in at a dealer that never sold and i picked it up for 200 bucks for boat, motor, and trailer cause they just wanted it out of the yard hadnt been run in two years, i have changed the points and cleaned the distributor, it is running on all cylinders. the dealership i got it from said they could set the timing for about 100 bucks and seeing as what i paid for it i can probably spend a bit lol, and there are some markings on the flywheel that indicate that someone has worked on it b4
    Last edited by knightracer_08; 02-19-2011 at 07:04 PM.

  9. #9
    Join Date
    May 2007
    Location
    Ireland,the only country in the world you can go North to go South
    Posts
    1,374

    Default Re: 1968 650 merc issues

    On top of the distributor there is a pulley.Pull the cap and you'll see an arrow on the pulley. There are three dots on the flywheel,turn the flywheel until the dots and arrow line up. Pull # 1 plug and check if the piston is TDC if it is then your timing should be ok.
    She should fire at 7-9 BTDC at idle. Oh and you need to have this engine in a lake or a very big tank to set max advance timing,has to be running WOT.Maybe that's why they want a $100

  10. #10
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,662

    Default Re: 1968 650 merc issues

    On a 14' boat that motor should be running a prop with 15" / 17" pitch---------------Something wrong with the motor or the boat.

  11. #11
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    12,505

    Default Re: 1968 650 merc issues

    Set the timing the way I described above. If you changed the points, the timing may well be off. And while the top cowl is off, see if the carbs open all the way as the throttle is advanced. That setup was famous for sticking so you only achieved full spark advance, without the carbs opening all the way. That alone might be your problem.

    Jeff

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