18HP saga continues. Can’t get past 14mph and 3,900 RPM.

Home Forum Ask A Member 18HP saga continues. Can’t get past 14mph and 3,900 RPM.

This topic contains 62 replies, has 14 voices, and was last updated by Avatar stephenspann27 1 year, 8 months ago.

Viewing 15 posts - 16 through 30 (of 63 total)
  • Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65832

    Something I was just thinking about. If 18hp is at 4,500 RPM, then 4,500 RPM should be my goal. I hit 4,000 RPM a few times today for a short while when messing with things. At 4,000 RPM I was only going 14mph. How much faster would I be going at 4,500 RPM?

    Tubs
    Tubs
    US Member - 1 Year
    Replies: 2225
    Topics: 127
    #65833


    Have you checked that the whole exhaust is clear of carbon?
    Partial blockage somewhere could cause this issue.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65835
    quote Tubs:


    Have you checked that the whole exhaust is clear of carbon?
    Partial blockage somewhere could cause this issue.

    yes I tore the midsection and everything down, sandblasted it and painted it.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65836

    I checked my main jet and it’s a #57. The carb the motor came with has a #64 main jet. Not sure why they changed the main jets that drastically for different year models.

    http://forums.iboats.com/forum/engine-r … 7351789626

    Mumbles
    Mumbles
    Canada Member - 2 Years
    Replies: 4037
    Topics: 416
    #65838

    If the same problem is on all three powerheads, are you using the same mag on all three? Is the mag plate worn and wobbly causing erratic ignition timing? New coils, points condensers. What about new plugs and plug wires? A new set of NGK B8S might help along with new copper core wires. If the wires seem good, corrosion can build up under the spark plug boot causing all kinds of electrical problems. Maybe pull the wire and terminal out of the boot and then trim a half inch or so off the end of the wire before reinstalling the terminal. It seems strange the issue follows all three motors.

    The crank in your ’61 motor has needle and roller bearings throughout and originally used 24:1 mix. ’64 was the first year OMC called for 50:1 in their motors, using proper outboard oil.

    How heavy is the boat and would the prop have too much pitch for the load causing the motor to lug? An eighteen should push a light boat around 25 MPH and a 15 about the same.

    https://www.youtube.com/watch?v=wTK67n5P9DE

    Avatar
    billw
    US Member - 1 Year
    Replies: 1019
    Topics: 33
    #65839

    I know you have tried three power heads. However I still would like to know the compression. 18s are unstoppable and get used and abused until the cows come home. Thus, they are often found with high cylinder wear. They will still run like a fine Swiss watch at lower speed, even with this wear. If you haven’t got 115 psi or better, you may not ever make it to 4500. Many times, you see them with 90, even 80 psi compression and they are smooth as silk; but the power just isn’t there.

    At this point, you have been whipping this thing forever; so it is more of a matter of pride than money, that you get it to work. If I were you, I would rig it up with an adjustable high speed jet carb bowl or whole carb, at least as an experiment and see if that helps.

    Avatar
    kerry
    US Member - 1 Year
    Replies: 323
    Topics: 14
    #65841

    Since I have an early 60’s 18 on the back shelf for later, I may as well ask- will an earlier carb with an adjustable high speed fit ? Unless I get lucky at the dealer, and he has a kit #278623 in stock, I might as well keep my options open.

    Avatar
    fleetwin
    US Member - 2 Years
    Replies: 2615
    Topics: 31
    #65843
    quote BillW:

    I know you have tried three power heads. However I still would like to know the compression. 18s are unstoppable and get used and abused until the cows come home. Thus, they are often found with high cylinder wear. They will still run like a fine Swiss watch at lower speed, even with this wear. If you haven’t got 115 psi or better, you may not ever make it to 4500. Many times, you see them with 90, even 80 psi compression and they are smooth as silk; but the power just isn’t there.

    At this point, you have been whipping this thing forever; so it is more of a matter of pride than money, that you get it to work. If I were you, I would rig it up with an adjustable high speed jet carb bowl or whole carb, at least as an experiment and see if that helps.

    I like this idea…..The high speed needle might be too long and interfere with the lower pan though…
    I have quickly reread this thread, but I don’t see anything about the prop style/pitch, pictures of the boat, or a description of the tachometer you are using….15MPH does seem way too slow for an 18hp though, have you tried it on a different boat?

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65846

    Mumbles, The missfire at idle i was chasing was either form a worn mag plate, or a bad plug wire. I bought a mag plate off ebay with the coils, points, wires, condensers etc already installed. I double checked the points gap and put it on my motor and the low RPM missfire went away. I’m not sure what my boat ways. i’m running a 14′ v bottom aluminum lone star boat, with a little bit of wood, bow mount trolling motor and battery. I’m willing to change from 40:1 to 50:1 but I dont’ see that small of a change making a big difference at WOT. I can see it making a difference with fouling plugs at idle etc. Since I know now I only have two main jet size choices, and I was already running the smallest available I think I’m going to try to get the older style carb with the adjustable high speed circuit.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65847
    quote BillW:

    I know you have tried three power heads. However I still would like to know the compression. 18s are unstoppable and get used and abused until the cows come home. Thus, they are often found with high cylinder wear. They will still run like a fine Swiss watch at lower speed, even with this wear. If you haven’t got 115 psi or better, you may not ever make it to 4500. Many times, you see them with 90, even 80 psi compression and they are smooth as silk; but the power just isn’t there.

    At this point, you have been whipping this thing forever; so it is more of a matter of pride than money, that you get it to work. If I were you, I would rig it up with an adjustable high speed jet carb bowl or whole carb, at least as an experiment and see if that helps.

    Bill, I’ve got another thread on here from a January when I was talking about not being able to reach max RPM. Someone said the exact same thing you are telling me, and that’s what lead to me buying the 3rd powerhead.

    viewtopic.php?f=2&t=10394

    I completely tore down two previous powerheads after removign them. The original one (which I never ran on a boat) was indeed worn out, all the cross hatching was gone, the crank was purple from being hot, corrosion had worn away a huge amount of material in the water jackets. I think it had about 95 PSi of compression. The second powerhead (off ebay) which I did run, the ebay add I think sayid 125 PSi, my gauge showed 105PSI. That’s the powerhead I was running when I started the post in January. When I took that motor apart I found that it wasn’t worn out but it had been hot, the pistons were scored, the bore was scored,and the crank journals were purple. The third powerhead which I’m running came from a AOMCI member, and is supposed to my "known good" powerhead. I think it has around 125PSi of compression as well. I’ll have to double check. I sort of put this project aside for the summer, and I’m working on it again, so some things are fuzzy .

    A agree with you on the adustable jet carb. That is my action plan.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65848
    quote fleetwin:

    quote BillW:

    I know you have tried three power heads. However I still would like to know the compression. 18s are unstoppable and get used and abused until the cows come home. Thus, they are often found with high cylinder wear. They will still run like a fine Swiss watch at lower speed, even with this wear. If you haven’t got 115 psi or better, you may not ever make it to 4500. Many times, you see them with 90, even 80 psi compression and they are smooth as silk; but the power just isn’t there.

    At this point, you have been whipping this thing forever; so it is more of a matter of pride than money, that you get it to work. If I were you, I would rig it up with an adjustable high speed jet carb bowl or whole carb, at least as an experiment and see if that helps.

    I like this idea…..The high speed needle might be too long and interfere with the lower pan though…
    I have quickly reread this thread, but I don’t see anything about the prop style/pitch, pictures of the boat, or a description of the tachometer you are using….15MPH does seem way too slow for an 18hp though, have you tried it on a different boat?

    Around January last year I had this rig out, along with my father in law’s 14′ flat bottom, which is bare bones, and his 1990 15hp evinrude. I did swap the motors and boats around at that point. I think my boat ran 19-20MPH with his 15hp. I don’t remember for 100% sure what my 18hp pushed his 14′ flat bottom to, I think it was around 15mph. He is boat has less drag for sure, and is probably lighter.

    As far as the adjustable carb fitting my lower plan. There is already a hole in the pan I think the bass shaft can go through. I’ll enlarge it if i need to. I’m determined to make the older style carb work.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65849

    Oh, prop pitch. I was running the stock 9×10, but I installed a 9×9 yesterday. No change at all in speed or RPM.

    Here are some vidoes of the motor idle, reving to max (I know this is bad to do), accelerating, and starting.

    https://www.dropbox.com/s/vowb22skkj0n5 … 0.mov?dl=0

    https://www.dropbox.com/s/3i59j6msbwjj8 … 2.mov?dl=0

    https://www.dropbox.com/s/69o36l1n0ny1c … 1.mov?dl=0

    Avatar
    billw
    US Member - 1 Year
    Replies: 1019
    Topics: 33
    #65851

    Yeah, revving in neutral doesn’t really prove anything. It takes very little effort to make it WAY past 4500 when no-loaded. It would probably do it with one plug wire off. Very bad form. 😉

    As for the adjustable jet thing, I was thinking that even if it didn’t fit in the lower pan, you could cut one short, file a slot in the end and tune it with a screw driver, or something of that nature, just to prove or disprove a point.

    By the way, 18s do use a ton of fuel. Even good ones. Maybe it’s the relatively small carb bore of the earlier ones. (Pre-’69) I don’t know why; but they do. I don’t want to start a crap storm but Mark 25s are way better on fuel, in my experience.

    Steve D
    Steve D
    US Member - 1 Year
    Replies: 465
    Topics: 60
    #65854

    on Ebay right now…
    http://www.ebay.com/itm/Johnson-Evinrud … Mx&vxp=mtr

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65856
    quote Steve D:

    on Ebay right now…
    http://www.ebay.com/itm/Johnson-Evinrud … Mx&vxp=mtr

    Crap I didn’t see that. I already ordered a 1964 model carb that’s the dual knob style. It was only $40 shipped, so I didn’t pay a ton more than the needle kit.

Viewing 15 posts - 16 through 30 (of 63 total)

You must be logged in to reply to this topic.