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, 5 months ago.

Viewing 15 posts - 31 through 45 (of 63 total)
  • Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65857
    quote BillW:

    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.

    yea I knew I would get flack over reving it in neutral. I just wanted to show that it can do it with no load, which even though it doesn’t prove anything, it does show that the motor can spin that fast. Also ignore the high idle it had in that clip, I adjusted it back down to 800 later. I was messing with the low speed mixture, and it throws the idle off everytime you do that.

    You comment about the 25hp motor: I actually have a 1989 Mariner 25hp motor sitting in the garage. It’s got a water intrusion problem I’m trying to solve (already replaced the exhaust plate gaskets and there is no head on these motors) I’m sure once I fix the water intrusion on the 25hp motor and run it on my boat I’ll forget all about the 18hp. I just don’t want this 18hp motor to beat me. I keep reading about how great they are, and it is about 30lbs lighter than the 25hp motor. Since I stripped the 18hp motor all the way down, sandblasted and painted it, it looks nice and I have at ton of time in it. I’ve replaced quite a few more parts on the 18hp motor than I’ve mentioned, so in additional to time, I have quite a bit of money into this thing, so I’m reluctant to give up.

    frankr
    frankr
    US Member - 1 Year
    Replies: 3930
    Topics: 43
    #65861

    I have a reading problem—don’t like to do it. So, with out going through this long saga, maybe I’m asking something already brought up.
    Fuel pump diaphragm ruptured?? That’ll dump extra gas into the crankcase, making it run rich.

    Ignorance is simply a lack of education. Ignorance can be cured. There is no cure for Stupid.

    Avatar
    billw
    US Member - 1 Year
    Replies: 989
    Topics: 33
    #65866

    The Mariner may be faster but the 18 will idle better. The Mariners (and their Merc cousins) idle like a muffled dirt bike. I am a self-proclaimed "Merc guy," but I never liked those 25s.

    Avatar
    fleetwin
    US Member - 2 Years
    Replies: 2509
    Topics: 29
    #65868

    OK, so your boat performed nicely with a newer OMC 15hp on it, and your Dad’s rig performed sluggishly with your engine. This would seem to rule out hull problems. The 9" pitch prop is the smallest available, so the RPMs should be up there….Just review with us what tach you are using to determine you are only getting 3900-4000RPM. All in all, I think the carb swap is the best next thing to try….

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65886
    quote fleetwin:

    OK, so your boat performed nicely with a newer OMC 15hp on it, and your Dad’s rig performed sluggishly with your engine. This would seem to rule out hull problems. The 9″ pitch prop is the smallest available, so the RPMs should be up there….Just review with us what tach you are using to determine you are only getting 3900-4000RPM. All in all, I think the carb swap is the best next thing to try….

    I’m using a "tiny tach" you can see it in the video links. There is a 9.25×7 prop out there… but I don’t think my boat is near heavy enough to warrant that pitch, the 9×9 should be more than enough. I’ve ordered the dual knob carb..

    Avatar
    fleetwin
    US Member - 2 Years
    Replies: 2509
    Topics: 29
    #65905

    Yes, I agree, having to under prop with a 7" prop is just hiding a big problem with a band aid. OK, you were using a "tiny tach", have you tried it on any other engines? I am assssuming you used a GPS to get the speed info. Certainly seems like something is holding the old girl back, hopefully the different carb will answer some questions with positive results….

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #65908

    Haven’t tried the tiny tach on other engines but it seems dead accurate to me. And for speed I"m using GPS.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #66130

    I’ve got a carb with the adjustable high speed circuit. Hope to test tomorrow.

    Avatar
    billw
    US Member - 1 Year
    Replies: 989
    Topics: 33
    #66132

    I count four carbs total so my hat is off to you. You are really going after this thing!

    As a word of encouragement, I will tell the story of a Mark 5 Mercury I have. Of course, it has a high speed mixture adjustment. One day, I put it on a 12 ft aluminum boat and dialed it in nicely, planing that boat off. Awhile later, that same day, I put the same motor on an 8′ Tiny Titan homemade hydro. It would barely plane the hydro off, it was so rich for that boat change. Leaning it out more for that boat made a HUGE difference. It planed off and went like an entirely different machine. My point here is that fixed jets are great but cannot possibly be right for every boat. Good luck.

    Avatar
    garry-in-tampa
    Lifetime Member
    Replies: 2955
    Topics: 29
    #66157

    The fixed jets were supposed to keep the constantly fiddling ignorant motor owner from constantly mis-adjusting his motor. I believe they introduced almost as many problems as they fixed. You could no longer adjust for humidity, boat load or altitude. I have put an adjustable needle valve in every fixed jet motor I intended to run. A tiny bit of crud can lean out a fixed jet enough to cause damage under load. Likewise overly aggressive cleaning (using a small file to clean out an oriface) can cause a loss of power and an increase in smoke and fuel use. Then again, that’s just my personal opinion . . . 😆

    Avatar
    rjoynt
    US Member - 1 Year
    Replies: 138
    Topics: 25
    #66158

    FrankR suggested checking out the fuel pump. I had an 18hp that had the small square fuel pump on it and didn’t run properly. Changed it for the bigger pump and it had a huge difference in performance.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #66169

    21mph!! 5,050 RPM. 21mph was my top speed, I’m running 19mph most consistently at 4,950RPM

    Avatar
    david-bartlett

    Replies: 1228
    Topics: 105
    #66170

    Stephen,

    You are moving in the right direction. Glad to see progress. Can you remind me what you have for a boat? Just curious, as I have a 14′ MirroCraft Deep Fisherman that I run 18’s on occasionally and I got 24MPH with a 1960 Johnson 18 that I fixed for a friend this summer.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #66171

    This was posted on another forum.

    I had read that they changed from 24:1 to 50:1 around 1959 when they went from clamshell to needle bearings. However, this document shows 1964 as being the change from 24:1 to 50:1. That would explain why the fixed jet carbs went from a #64 main to a #57 main.

    Avatar
    stephenspann27

    Replies: 86
    Topics: 2
    #66172
    quote david bartlett:

    Stephen,

    You are moving in the right direction. Glad to see progress. Can you remind me what you have for a boat? Just curious, as I have a 14′ MirroCraft Deep Fisherman that I run 18’s on occasionally and I got 24MPH with a 1960 Johnson 18 that I fixed for a friend this summer.

    It’s a 1964 14′ lone star v bottom boat.

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