out board wont reach max rpm

[jsiedentopf]

Hi I have a 1970 johnson 40 hp. It is on a 16 foot lone Star with a closed bow. I can not get this boat on plane, even at max throttle it wont reach max rpm. I have very little in the boat. It IS rated for a 75 hp according to the sticker. but my local boat guy says it should get up and go with motor. Can anyone help me with some suggestions to try before I bring it to the shop or give up on it.

 

[iwombat]

Re: out board wont reach max rpm

Easily a majority of these kinds of cases are from one cylinder dropping out from either spark or fuel issues. Put it in idle in gear and tie up to the dock (or put a little load on some other way). With a pair of insulated pliers start removing plug wires one at a time. You should hear a change, or the motor might even just die. If there's no change, that's your bad cylinder.

 

[ezeke]

Re: out board wont reach max rpm

You might find reading the thread at the following link helpful in getting the boat to perform better: http://forums.iboats.com/showthread.php?t=235855

Remember that the motors' HP ratings were changed due to a change in the rating system in 1985, so that compared to the current motors, your horsepower is closer to 35.

 

[kenmyfam]

Re: out board wont reach max rpm

The set up of your outfit is a huge factor in getting the best performance. Just one inch in motor height gave me an extra 5mph on my outfit. Trim, load, prop are also to be considered. Remember you are just a little over 50% of your rated horse power so you may have to "prop down" in pitch considerably to get it up on plane and with that ratio you need to have a near perfect set up. Look at this before spending money on a motor that may well be O.K.
Good Luck and let us all know.

 

[iwombat]

Re: out board wont reach max rpm

These guys are right on. Could be a simple case of configuring the motor for the boat.

 

[jsiedentopf]

Re: out board wont reach max rpm

The set up of your outfit is a huge factor in getting the best performance. Just one inch in motor height gave me an extra 5mph on my outfit. Trim, load, prop are also to be considered. Remember you are just a little over 50% of your rated horse power so you may have to "prop down" in pitch considerably to get it up on plane and with that ratio yo need to have a near perfect set up. Look at this before spending money on a motor that may well be O.K.
Good Luck and let us all know.

so are you talking about what slot the motor is trimed to, cause I have tried that, or do you mean having the motor sit higher up on the transom? Ill will have to try the spark plug thing. that might be why it wont idle for long before it floods out and dies?

 

[kenmyfam]

Re: out board wont reach max rpm

I am talking about the entire motor set up. Trim is just one factor of it. The pitch of your prop could be too much for the power you have available and the push your hull needs to get it to plane. Lowering pitch is like changing down a gear in your car. More pushing power but less top end. Motor height and how much of it is in the water is huge as well. Long shaft motor on a short shaft transom will give you a huge amount of extra drag. The cavitation plate should be roughly in line with the bottom of your hull. You can then start fine tuning from there. Post a photo or 2 from different angles and we can go from there.

Compression check, carb clean and new plugs never hurt either. Compression check will tell you a lot about your motors health.

 

[jsiedentopf]

Re: out board wont reach max rpm

compression is not bad around 100 if I remember right, I will have to try moving the motor up, but I will take some pictures when it light tommarrow. I am just getting to my wits end that I cant get up to speed. I got guys in 12 foot aluminum boats with 9 hp motors passing me on the water.

 

[kenmyfam]

Re: out board wont reach max rpm

compression is not bad around 100 if I remember right, I will have to try moving the motor up, but I will take some pictures when it light tommarrow. I am just getting to my wits end that I cant get up to speed. I got guys in 12 foot aluminum boats with 9 hp motors passing me on the water.

Yep, you can make those 12ft tinnies skip along pretty good with a 9/9hp and a good set up. Let us know what diameter and pitch prop you are using as well. Photos are worth a million words.

 

[jsiedentopf]

Re: out board wont reach max rpm

Ok, I warmed up the motor today. Then I unplugged to top plug wire and the motor died. Then I repeted the test on the bottom plug wire and the motor stayed running with little to no change. So I think this is my problem. But I then used one of those slug plug gap testers that makes it so you can see the spark jump a gap. I got nice blue spark to jump about a 1/4". so anyone have any ideas to get me closer to a resalotion? I hope I dont have to replace this motor. Also I took some pictures.

 

[iwombat]

Re: out board wont reach max rpm

How does the bottom plug look? Could be as simple as time for new plugs. You might try running it at night to see if your plug wires are arcing.



You could also stand to raise that motor up on the transom some too. Probably as much as a couple of inches, even.

 

[jsiedentopf]

Re: out board wont reach max rpm

How does the bottom plug look? Could be as simple as time for new plugs. You might try running it at night to see if your plug wires are arcing.



You could also stand to raise that motor up on the transom some too. Probably as much as a couple of inches, even.

The plugs are new. I will try moving the motor up, do you think that would keep the motor from reaching a higher rpm? I am new to this if you couldn't tell. Thanks again for your help.

 

[steelespike]

Re: out board wont reach max rpm

Spark should jump about 7/16 in open air tester.Harder to jump under pressure.Could be a bad wire or even a defective plug.
Could be a weak coil If it hasen't been replaced it is probably weak.

 

[jsiedentopf]

Re: out board wont reach max rpm

hmmmm, I will have to try again tomorrow and make the gap bigger to see if it will still jump. I did replace the coils but not the wires. I guess I will do that next. I remember that it was sort of strange how the coil ends of the wires just push into the coil without some kind of connector. god it would be nice to get this boat running good. Then people will stop telling the best thing about owning a boat is buying it and selling it.

 

[jsiedentopf]

Re: out board wont reach max rpm

One more question, for spark plug wires, would the normal automotive grade wires work for an outboard, I have plenty that have never been used.

 

[iwombat]

Re: out board wont reach max rpm

No, those are carbon core wires. Marine wires are solid core.

 

[jsiedentopf]

Re: out board wont reach max rpm

oh ok, I guess I will be making a trip to west marine then.

 

[kenmyfam]

Re: out board wont reach max rpm

Seems like great advice. Only one thing to add. Did we get a compression check yet ???

Edit, I see we got the "around 100lb" earlier in the thread.

 

[iwombat]

Re: out board wont reach max rpm

My local NAPA stocks marine wires. You could try there if it's closer.

 

[jsiedentopf]

Re: out board wont reach max rpm

ok guys, I have an update. I went ahead and replaced the spark plug on the bottom cylinder. I also did a comp test for good measure. Compression in at 155 psi. So I figured ok let see if it will fire up. Sure did, idles smooth, even runs on that cylinder alone when it would not before. So I think I got the problem licked. I wont know for sure till I can get back out on the water. Also a few of you mentioned moving to motor up on the transom. Do you think I should start with an inch or just move it up a full two right off the bat? Again thank you for all help. I dont know how much more I could have take.