18 hp Johnson performance

[pfolmar]

I have a 1963 18 HP (determined through research on the model number the cowling is from a 25) Johnson that I recently purchased. It pull starts easily and seems to run well. I have put it on my 16 foot Polar Kraft jon boat. The boat is the wide (66") version and is quite heavy as it has a casting deck built in the front . With me, a trolling motor, battery, etc., I would think that I should be able to get this boat on plane with this engine. So far I have not been able to do so. I have tried moving the pin into different holes and the next to last one out from the transom seems to perform best, but still not quite planing. Any suggestions would be appreciated. I am quite the novice as this is my first small outboard project.

 

[Chinewalker]

Re: 18 hp Johnson performance

Hi PDF,<br /> Have you tried a different prop? Those old 18s like to top out a bit above 4500 revs for best performance. If the motor was originally propped for a lightweight tin can then it may be overpropped for your loaded down PK... something akin to driving up a steep grade, towing a trailer and trying to keep it in fifth gear. Assuming the motor is running well otherwise, that might be the way to go. For instance, if you're running an 11-inch pitch prop now, you might want to find a 10 or 9. If you're already running a lower pitch prop and the RPMs are good, then you may have other issues. It's also quite possible that the 18 simply doesn't have enough juice to kick the loaded down 16-footer over the top...<br />- Scott

 

[CATransplant]

Re: 18 hp Johnson performance

Well, my best guess is that your 18hp just doesn't have enough beans to plane your wide jonboat. It wouldn't surprise me at all.<br /><br />What's the max hp for that boat, from the manufacturer's plate? I'll bet it's well over twice that 18hp, probably more like 50 hp. <br /><br />Sounds like it's time to start saving up for something with a little more under the hood.

 

[CATransplant]

Re: 18 hp Johnson performance

Well, I stand corrected. If this is the Dakota model, maximum hp is just 25 hp. So you should plane with the 18.

 

[pfolmar]

Re: 18 hp Johnson performance

Hey thanks for the replies. The boat has no manufacturer's plate, it is missing. However, it had an old (read not working) 50 HP Mercury on the back of it when I purchased it. From what I have been able to research the Max HP for this boat from the manufacturer was a 40 HP. Then I added weight with the casting deck. You are probably correct and the 18 just does not have the UMMPH to get it up to plane. GPS shows max speed (into a head wind) of 10.3 MPH. I will look for a good deal on a short shaft tiller steer 30 - 40 HP. Good luck to me. At least I am not out much I only paid 150 for the 18 HP and have an old 14 footer I can put it on. Thanks again

 

[pfolmar]

Re: 18 hp Johnson performance

Me again, I am still not absolutely sure what engine I have. The Model number on the transom bracket identifies the engine as a 1963 18 HP. However the cowling says that it is a 25 HP and appears to be from the early 70's. Is there a number somewhere on the powerhead that I can find so I can be for sure what I have? I want to know if the cowling is right or if the transom bracket is right. I am betting on the bracket. The fuel supply is from a standard tank with standard Johnson fittings not from a pressure tank. If it is the 63 18 HP should I be running 24:1 or 50:1 mix? If it is the 25 I know that I have some issues with performance as I would think that a 25 would plane this boat. Thanks again,

 

[steelespike]

Re: 18 hp Johnson performance

While it is surely under powered I think that carefully loaded and trimmed it should plane While the casting deck adds weight it is in the right spot to get you over the hump.And that nice wide bottom should help get up on the water.<br /> I once planed a 15 ft 6 ft wide plywood runabout<br />with 5 adults aboard.Ran at an indicated 21 mph may have been closer to actual 18-20 but it planed and stayed that way for a 20 minute trip.<br /> You surely wont plane easily but I think it should be routine with a sound motor, balance and trim.Of course the need is for a bigger motor.

 

[Paul Moir]

Re: 18 hp Johnson performance

What colour is the engine? If it's white it's almost definitely an 18hp.

 

[pfolmar]

Re: 18 hp Johnson performance

The engine is white from the power head down. The cowling is white and says 25 hp. The part around the powerhead (where the powerhead meets the shaft) is green.

 

[pfolmar]

Re: 18 hp Johnson performance

Hey guys. I got it figured out. The engine had a blown head gasket. Running on only one cylinder because water was getting into the bottom cylinder. Changed gasket, plugs and wires and hope to get much better performance now.