69 Ski-Twin Power and Fuel Economy

[i386]

69 Evinrude Ski-Twin 33HP Electric Start

I've recently completed an extensive restoration on a small boat. It's powered by the 33. When I got the boat it was a mess and the motor didn't run. Until recently I had never used this motor before.

What I have done to the motor.

Compression test -> 100PSI on each cyl.
New points (gap .020)& condensers. -> Good, strong, snappy blue spark on both cyls.
New plugs J4C (gap .030)
Rebuilt carb using OMC kit -> clean as a whistle and functioning properly
Link and Sync performed per my Seloc manual.
New impeller -> pumps great, no issues with hot engine whatsoever
Fresh lower unit oil -> Shifts fine.
Initial needle settings: Low Speed 1.5 out / High-Speed .75 out
Plastic gas tank with in-line fuel filter in gas line.

Once the fuel system is fully primed the engine starts and runs with just a quick bump of the starter. Runs and idles smoothly. Doesn't smoke much. Continues to run smoothly while accelerating to WOT. With the throttle control all the way forward I have verified that the throttle plate in the carb is fully horizontal. I have adjusted the high-speed screw while on plane at WOT to the place that gives the most power. In my case it works out to be about .5 turns out. I have no way to measure RPMS at the moment. My manual says it should be making 4500RPM at WOT.

So what's the problem?

The boat is slow to plane and I feel as if the motor wants to give another 10-20%. Also fuel economy seems pretty bad to me. I burned about 4 gallons of gas this morning with about 45mins of WOT running up the river and back. I've trimmed the motor in hole #1, #2, and #3 with hole #1 being closest to the transom. #1 seems to work best though #3 has the anti-cavitation plate in a plane which is closest to parallel to the keel. Also the anti-cavitation plate is 1.75" below the keel. Keeping a 5 gallon bucket full of water up front has shown effective to stop the porpoising.

I'm not sure if I'm having an engine problem, a setup problem, or no problem at all. I thought I'd mention it here since it is using a LOT of gas.

 

[jay_merrill]

Re: 69 Ski-Twin Power and Fuel Economy

While I know that everyone uses 10% of rated hp per hour as a guidleine for WOT fuel consumption for 2 stroke outboards, I don't think that 4 gallons is hugely out of line for 45 minutes of running at WOT.

The OMC 33hp motors are crossflow designs and are a lot more thirsty that loopers. I have a pair of '58 ERude Bigtwin 35hp motors that are pretty close to being the same engine. The carbs are also close to being the same. The only time that I have ever run a boat out of gas that I can remember, is when I moved these motors from one boat to a bigger boat, and had to go down to 13" pitch props to push the boat they ended up on. I took a trip and found myself pushing the motors at WOT to get a decent speed. When I later figured out my consumption rate, it turned out to be about 6 gallons per hour, per engine.

You might want to try backing off on the throttle a bit. My guess is that you'll get much less consumption, at pretty close to WOT speed.



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[54fleetwin]

Re: 69 Ski-Twin Power and Fuel Economy

Well I would think something is not set up right.
My Fastwin 18hp accelerates my 14 footer so hard that you have to pay attention or you slide off the bench seat.

At .5 turns out on the h/s needle, I would think you are too rich on the l/s.

 

[kfa4303]

Re: 69 Ski-Twin Power and Fuel Economy

HI i386. Glad to hear you got her up and running. If it helps at all, I also have a '66 33hp and it does tend to drink up the gas, I'm afraid :/ I can easily go thru a 6 gal tank in a day on the water. Of course your consumption will vary depending on how hard you run the motor. I agree with the suggestion to run at a little less than WOT and you might be surprised how much gas you save. As far as feeling like you can/should have a more acceleration, I would think that it would be a combo of lots of things such as boat lenght, weight, design, etc...I don't know of any formula or technique for optomizing performance other than good 'ol trial and error. I can only say that my 33 hp on a 14 ft., 'glass, v-hull seems to offer more in the way of top end speed than hole shot/off the line speed. Hope that helps some. Glad you got her running. I don't here about other 33 hp very often keep us posted.

 

[cougar1985]

Re: 69 Ski-Twin Power and Fuel Economy

i also have a 70 33hp .i did not see anywheres where you said what pitch prop your using?can you raise the motor closer to even with the keel?though not necessery on my present 14 footer ive in the past used a hockey stick piece to get closer to level with the cav and keel.4 gallons a hour would not be totally unreasonable for a 33 at wot.i have a old variable pitch prop from a 56 30 hp that i used to dial in the 33 for pitch .(worked great and was simple to adjust).though i believe these props arnt really that numerous anymore.

 

[kfa4303]

Re: 69 Ski-Twin Power and Fuel Economy

I'm just using the stock prop that came on the motor in 1966. I'm not sure of its dimensions. I

 

[i386]

Re: 69 Ski-Twin Power and Fuel Economy

Thanks for the responses so far.

I did try raising the motor higher on the transom using a 1.75" high block of wood. It had the clamp screws just grabbing the top of the transom and I didn't trust it so I decided to remove it before heading out to the lake. I suppose I could rip the piece of wood down to 1" thick. Not sure if raising the motor 1" would make much difference but I suppose it's worth a try. I'm willing to buy one of those mini jack plates but only after I've exhausted everything else.

I'm not sure what pitch the prop is. I inspected it for markings but didn't find any. I didn't try to remove it nor have I ever.

I thought I'd like to have one of those TinyTach tachometers eventually. Knowing how many RPMs she's turning at WOT would be helpful in figuring out where the problem is so I think I'm justified in purchasing one.

I'd also like to take a GPS out to get some idea how fast I'm going.

Thanks again for the input. I will continue to plug away at this one and return if I have any new information.

 

[jay_merrill]

Re: 69 Ski-Twin Power and Fuel Economy

Although I don't remember the exact selection, OMC props for your motor were about 10+ inches in diameter and had pitch selections from about 11 inches to 14". The props are hard to find these days, so you may be stuck with what you have.

If the blades on your prop are a wide chord and somewhat round in shape, you probably have 14" pitch wheel. If it is more narrow in chord, it might be a 13" or 12" wheel.



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