New Johnson 115HP 4-stroke missing at hi-RPM?

[tony_cliffy]

Hello!<br /><br />I am at that part of my break-in period for my new 2004 Johnson 115HP 4-stroke where I was advised to go to full throttle for brief periods of time. But as I increased my speed towards full throttle the engine started missing or possibly modulating the output via rev limiter. I do not have a tach installed yet (I will soon) so I do not know my RPM's, but the boat speed was only 33 MPH and everyone says this boat should do far more than that with the 115HP engine.<br /><br />It does *not* sound like cavitation. Do you think I am hitting a rev limiter or some other RPM-limiting feature? My little engine book does not describe a rev limiter feature. No alarms are sounding, the engine has plenty of oil and waterflow and otherwise works fine. But I feel the engine is preventing me from reaching max speed.<br /><br />I have varied the degree of trim and it appears to happen regardless of trim setting. Also, I am approaching full throttle by slowly increasing speed, not by performing a "hole shot." For example, I am cruising at 25 MPH with good stability and flat water and begin to give it more throttle until the engine starts to make this noise like a rev limiter is kicking in. I immediately back out of it.<br /><br />I do not know what prop is on it other than my reputable dealer completely installed this engine and chose the prop based upon his knowledge of my boat.<br /><br />The engine is mounted to a 20' Crestliner aluminum center console which is a light boat (@850 lbs) with only a moderate V in front and fairly flat along most of the bottom.<br /><br />Thanks!

 

[JB]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

You really need a tach, Tony.<br /><br />My guess is that you are underpropped for your light boat and are hitting the rev limiter, but that cannot be proven without the tach.<br /><br />Good luck.

 

[DJ]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

I agree with JB. You're kind of in the dark without a tach.<br /><br />You may, in the meantime, want to make sure the prop hub isn't slipping.<br /><br />Mark a line on the hub and another on the propshaft. THey should stay aligned. If not, the hub is slipping.<br /><br />I've seen "new" props slip.

 

[tony_cliffy]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

JB - do you know for sure if my engine has a rev limiter? I can not confirm this with any documentation I have available to me. I will ask my dealer, but nothing beats confirming things like this yourself.<br /><br />Are automotive tachs suitable (mechanically) for testing? I am sure a marine tach is better to resist water, but do they function with the same electrical signals? For example, could I use a name brand automotive tach short term to see where my problem was occuring? I have an automotive tach but am still shopping for a marine tach that will fit on my console properly.<br /><br />Thanks!

 

[Cricket Too]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

tony....Check out tinytach.com, they have some tachs there that are small and may fit on your console. Good Luck.

 

[DJ]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

tony,<br /><br />Since that is a four stroke, an automotive tach. will work fine.<br /><br />The easiest to use would be an inductive type. Just clamp the lead on a plug wire.

 

[tony_cliffy]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

JB - I think you are right on track. I didn't suspect it because I don't know much about props other than proper selection impacts performance. <br /><br />I trust this dealer but recall him emphasizing how he chose this prop for my boat. Short of taking it somewhere else - is there any 'rule of thumb' calculation or guides so I can determine if this prop is appropriate? It is a light boat (~850 lbs. before gear, passengers) without much of a vee, so it doesn't offer as much moving resistance as similarly sized 20' boats (assumption on my part). The engine is the max available for that boat and weighs around 475.<br /><br />Thanks!

 

[clanton]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

One limiter limits rpm to about 3000 unless in farward gear, not sure about top rpm, but I think has a limiter. Has the alarm system been checked and working? If the engine was installed with new wiring harness, just buy the Johnson tach and plug it in.

 

[Solittle]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

tonycliffy - Until you get a tack anything on props or missing at WOT would be pure speculation. Also is the speed measured by a GPS?

 

[tony_cliffy]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

Yes, speed is measured by a Lowrance GPS unit....?

 

[jim dozier]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

Get a tach. In the meantime, go get six big friends, a big cooler of beer, and fill up the gas tank and bait well and see if it still does it. If it doesn't with the load you are overpropped for a light load.

 

[tony_cliffy]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

My dealer confirmed it was underpropped. He said he installed the factory recommended 19" prop but was going to order a 23" replacement today.<br /><br />I read the iboat help info regarding props so I understand the physical difference of going from a 19 to 23 inch prop as far as "forward movement" is concerned. But does this mean I will decrease my fuel economy as well?<br /><br />Thanks!!!

 

[Javelin 353]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

If it,s a four stroke fuel consumption should be the least of your worries

 

[jim dozier]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

Bouncing off your rev limiter is probably not giving you your max fuel mileage. My guess is that it will improve a tad particularly at the high end. You have essentially been stuck in low gear.

 

[clanton]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

Rpms limited to 6500. If you exceed 6500, rpms will be reduced to 3000 rpms, to reset reduce engine speed below 3000 rpms for about 1 second and ecm will reset.

 

[tony_cliffy]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

Clanton,<br /><br />I did not experience what you describe. RPM's were not dropped to 3000, the limiter simply started skipping a cylinder or something else. The RPM's stayed fairly high, although I backed them down myself immediately.<br /><br />The drop to 3000 is supposed to occur if the engine overheats, runs low on oil, or something like that. At least that is what the manual says. But this engine didn't drop down to 3000 when the rev limiter kicked in. It just holds the RPM's steady at the limit setting.

 

[DJ]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

Tony,<br /><br />Going from a 19" to a 23" is a HUGE jump. You can figure that each inch in pitch is worth 200 RPM's.<br /><br />Lugging your engine can be just as detrimental to it as over revving it. It just takes longer to kill it.<br /><br />I think, before I accepted that 23" prop, I would ask the dealer the following.<br /><br />1. What RPM is is supposed to turn at WOT?<br /><br />2. What is it going to turn at WOT with the 23"?<br /><br />Don't let them "yes" you out the door.

 

[jegervais]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

A 22 or 23P prop is probably going to put him right close to the ball park. WOT for the 115 is 5000-6000 rpm and with a lite load, should favor closer to 6000 rpms. Rev-limiter hit in around 6300. <br /><br />I'll bet his dealer hasn't prepped many 4-strokes and propped it "with the recommended 19P prop" using the old 2-stroke knowledge book. Problem is, the 2-strokes typically use 2.0:1 ratio, while the 115 4-banger is a 2.59:1 ratio.<br /><br />-John

 

[tony_cliffy]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

Awesome advice and a great education! Thanks to you all.<br /><br />Today I removed the 13x19 prop and replaced it with the 14x23. I am also installing a tach although struggling to find the signal wire for this engine (I posted another thread about this). I hope to have this problem resolved by this weekend.

 

[Cricket Too]

Re: New Johnson 115HP 4-stroke missing at hi-RPM?

Just want to snake a quick question in here. Are the 4 strokes rated for and able to maintain higher rpm's (5000-6000), for extended periods of time like 2 strokes can? Just something I always wondered about. The shop I used to work in seemed like we always had alot more I/O's getting taken apart than OB's, and I always thought that 2 strokes just made more sense for marine applications.