Suggestions on improving performance on 115HP O/B Johnny...

[kshelly]

Any quick tips on improving engine performance on a 1974 115HP Johnson O/B? I usually live by the motto, "If it ain't broke, don't fix it", but if anyone knows a few quick performance enhancing secrets, I'd like to give them a try!

 

[DHPMARINE]

Re: Suggestions on improving performance on 115HP O/B Johnny...

I like your motto.<br />A few this come to mind tho:<br /> Pull the motor cover off,with motor NOT running,turn fully to starboard.Put into forward (rotate the prop if needed).Now push remote control to full throttle.Watch the throttle linkage move.It should pivot as fast as you move the control.If it doesn't,the pivot needs a cleaning and relubing.<br /> Get the factory manual and perform a sync and link (co-ordinates the carbs to each other and to the timing advance.<br /> Keep stock,and remember it's thirty years old.<br />DHP

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

Thanks DHP. I have the manual on it's way. I'll check linkage & sync as you suggested. I really have no complaints about the engine. Top speed is 31MPH. Plenty fast for skiing & pleasure boating. I always have the age thing in mind. I don't want to push this 30 year old "grandpa" engine too far! Runs like a champ. If I get too crazy and fix what ain't broken, it might end up broken!!!!

 

[DJ]

Re: Suggestions on improving performance on 115HP O/B Johnny...

erau85,<br /><br />31 MPH! What is it pushing?<br /><br />Perhaps you are overpropped? The engine should be able to turn 5500 RPM's with a light to moderate load.

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

Geeze, now you have me wondering if my boats a dog on the water! Boat weighs about 3000 lbs., will that make a difference? I thought 31 MPH was good!

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

If I recall, RPM's about 4500 at WOT. Too low? The seller gave me 2 extra props with the boat. He (the seller) told me the one currently installed was new. It was recommended because the boat didn't get up on plane quick enough. How do you know if a boat is over propped??? How do you determine which prop to use?

 

[DJ]

Re: Suggestions on improving performance on 115HP O/B Johnny...

erau85,<br /><br />3K is no lightweight, that's for sure. <br /><br />However, I just wonder, like DHPMARINE, if you're getting everything out of it that you can.<br /><br />A tach is probably the most useful instrument you can have on a boat. I would invest in one. Or, look at these.<br /><br />Yes, 4500 is too slow. You are lugging the engine. NOT GOOD. Prop to run at 5500 with light/mod. load.<br /><br /> http://www.tinytach.com/origgas.html

 

[DJ]

Re: Suggestions on improving performance on 115HP O/B Johnny...

I would also do this.<br /><br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=36;t=000006

 

[DJ]

Re: Suggestions on improving performance on 115HP O/B Johnny...

erau85,<br /><br />I would just about bet that you are running a 13-1/4" by 17" or 19" prop. Maybe more (21").<br /><br />Each inch in prop pitch (the last number) is equal to about 200 RPM's, up or down.<br /><br />Look for numbers stamped on the hub of the prop and let us know what you have.

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

OK, since when is boating suppose to get so complicated? If the engine is in fine working order, is it just the prop that influences engine RPM?

 

[DJ]

Re: Suggestions on improving performance on 115HP O/B Johnny...

Pretty much. As long as the engine is in good tune.<br /><br />The decarb procedure I mentioned is preventive maintenance. It helps to make sure you do not stick a ring or destroy it through detonation.<br /><br />Since your engine has been lugging, I'll bet it's pretty coked up.

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

OK, maybe I'm a little dense, but WHY is my engine lugging? Prop? Carbon build-up? Out of adjustment? :{

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

DJ,<br />Ok, each inch on prop changes RPM by 200. The higher the number decreases or increases RPM? I'll be taking my boat out in about an hour, I'll get the prop numbers. Assuming I'm about 1000 RPM low, I would need a prop 5 increments lower, or higher???

 

[DJ]

Re: Suggestions on improving performance on 115HP O/B Johnny...

Most likely, the prop. <br /><br />Look at it this way. A prop is like the final gear in your car. If it is too high. Your engine lugs. It's sort of like trying to climb a steep hill in overdrive.<br /><br />The other items are routine maintenance items and tuning checks.<br /><br />With a manual, you can do all that PM stuff in a leasurely afternoon.

 

[DJ]

Re: Suggestions on improving performance on 115HP O/B Johnny...

A decrease in pitch (lower number) would make your RPM's go up.<br /><br />Your thoughts may be right. However, we need to know what you have now. Let's say it's a 21". By going to a 17", you'll be right where you need to be. And, it will "rocket" out of the hole.

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

It amazes me that I thought I had a great running motor only to find out it was cranking out low RPM's. I think my boat is good too, unless it sinks! I'll do the seafoam thing. Have to wait for the manual to make adjustments. I could screw around with my 2 extra props too. Thanks!

 

[DJ]

Re: Suggestions on improving performance on 115HP O/B Johnny...

I'll bet it does run great. That's a great engine. It just may be able to run even better. <br /><br />In the meantime, dump some Sea Foam in the fuel tank. Let it work for you, while you're out playing.<br /><br />NAPA has it.

 

[Dhadley]

Re: Suggestions on improving performance on 115HP O/B Johnny...

If I recall, RPM's about 4500 at WOT.

Is 4500 accurate or is that a guess? We need to know the actual top rpm's with an average load.<br /><br />You dont want to lug a 2 stroke outboard. The increased combustion temp will promote coking and eventually a ring will break.<br /><br />Take a test run with your average load and see what the rpm's are. Its a very good excuse to go boating. If the rpm's are that low maybe we can get you closer with a prop change and a set up improvement.<br /><br />Good luck!

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

Dhadley;<br /><br />My boat does have a tach. 4500 at WOT. I'll verify that shortly, since I'll be taking the boat out in about an hour or so.<br />Thanks!

 

[kshelly]

Re: Suggestions on improving performance on 115HP O/B Johnny...

4600 RPM at WOT. I'll de-carb the thing and experiment with the two extra props I have.