1968 evinrude 18 hp rpm questions

I finally got my motor up and running again (I'll finish the lower seal kit replacement thread a little later today probably, can't find my adapter to put my memory card from my phone in the computer right now) and I have a couple of questions now that I have my tiny tac installed. What rpm should this motor idle at, and what should the WOT rpm be?

Re: 1968 evinrude 18 hp rpm questions

from a 1965 factory service manual 18hp...full throttle operating range- 4000 to 5000 rpm...no idle rpm given...seems like it's generally 600 to 650 rpm..warm engine, in gear, pushing boat but that's kind of a subjective thing. I'd go with as slow as it'll go w/o the gears "slamming' when you shift into gear...the lower the better at idle.

Re: 1968 evinrude 18 hp rpm questions

Hi james. The rpms at idle should be around 750 rpm and it will top out around 5000-5500 rpm. My 20hp Johnson, which is virtually identicalt to your 18hp, will push me along at a little over 20 mph in my little 14' aluminum, semi v-hull skiff. Here's a vid of a nice 18hp idling in a test tank and one of me out on the water to give you an idea of what to expect.

1958 Johnson 18 HP outboard - Running - YouTube (here's a great vid of an 18hp idling in a test tank. Although, I would submerge it a couple more inches than shown in the clip.)
Wakulla Beach, FL movie.wmv - YouTube (here's a clip of me on the water. no speed demon, but the 18-20hp motors are surprisingly strong for their size.)

Re: 1968 evinrude 18 hp rpm questions

It will idle smoothly in a barrel at around 750-800, but much lower than that, and it gets a little rough. It shifts pretty smooth at that rpm, so I figured that was probably around where it should be, but my neighbor thought it should be quite a bit lower than that, so I figured I'd ask on here and see. I haven't had it out yet since all the work I've done on it, but I wanted to know what I should expect when I do get it out and to make sure the prop is right on it.

I have it on a 16 foot mod v lonestar. I was getting about 16 mph out of it the first time out (no tac at the time, just my gps) but the acceleration was horrible, and it just wasn't running very well. The trim was also off a little, but I was afraid to take that pin out on the water. Since then I've rebuilt the fuel pump, ignition, water pump, carb, and resealed the low end. Based on how bad it was running that day, and the trim being off to, I was thinking I'd probably get close to 20 mph out of it once it was running right and trimmed right. Sounds like that might not be too far off because your 14 looks to be built a little wider and deeper than my 16 so I'd say the weight isn't a whole lot different.

Re: 1968 evinrude 18 hp rpm questions

The idle from a test barrel to open water will differ. Completely different backpressure on the exhaust. Set it to about 750 in gear in reverse where it is still smooth.

The 18 will make all its power by 4500RPM at WOT.

Re: 1968 evinrude 18 hp rpm questions

I agree that you should get close to 20mph once everything is dialed in just right. Make sure the height/trim/tilt are properly adjusted. These old OMC motors have a leg length of around 18" which means they run a bit deep on a 15" transom. I built a little transom riser to raise my motor a few inches so that the anti-vent plate, found just above the prop, is level with the keel. I believe I'm on the 2-3 trim setting from the transom as well. While it's certainly no speed demon, I can now hop up on plane in about 5-6 seconds with a light load. Weight distribution will also make a big difference as well. If you run a console near the bow, you should be ok, but if you run a tiller, you may want/need an extender which will allow you to stand while underway as well as let you redistribute the weight forward as needed. I made one out of PVC and it made a huge difference. I never leave home without it.

Re: 1968 evinrude 18 hp rpm questions

I was in the second one from the transom and that was way too far in before. I've got it on the second one in from the outside and I think that will be about right. It is a tiller, but I usually have someone with me when I take it out so it leveled out ok with my 9.5 hp johnson, and this motor doesn't seem that much heavier than that one. One thing I've done is put my trolling motor battery up closer to the front of the boat and ran some heavy gauge wire to the back for the trolling motor and my switch box for my lights and bilge pump and to power my depth finder.

I noticed when I adjusted the trim that the motor was a little long. The bad part is, I just rebuilt the transom and had I known then I could have rebuilt it higher to begin with. I put a whale tail on the 9.5, and it helped a lot getting the boat on plane and got the back end up a lot higher in the water. I forget the exact numbers, but best I remember I got about another 1.5 mph out of the boat after putting it on. I know they cause some pretty heated debates around here, but it helped a lot with that small of a motor on my boat. I will be looking in to the tiller extender though after I see how it handles.

Thanks for the suggestion on getting the idle set right HighTrim.