Engine hesitation??

[7lazy77]

I don't mean to clog up the thread, but I had this listed under the Johnson forum & so far no responses with 117 views, so I thought that I would try here......
Please excuse my lack of knowledge & terminology when I try to explain my problem, since I am still pretty new with boats. I have a 1994 115hp Johnson that is on the back of a 24' pontoon. I purchased this boat last year & have been trying to work out a few kinks. At the time, I noticed when I would accelerate to WOT (around 4000-4500 rpm's), the engine would suddenly "jar" & feel like it would hold back without gaining anymore speed. It is hard to explain, but when the engine hits a certain point at higher rpm's there is a that jolt & it almost feels like something is restricting the flow of fuel in the carbs?? Took the boat in when I had it winterized the end of last year & had the carbs cleaned & gone thru (thinking that this would fix the problem), but when I took it out the first time this year, it STILL does it. The funny thing about it is that the jolt & hesitation seems to only happen when the motor is trimmed down (with the bow pointed up). When it is vice versa, it seems to do just fine, but the problem doing it this way is with my bow down in the water more, I am really plowing thru water & I only get 17 mph at 4500 rpms! Motor runs great at lower rpm's (regardless of how the trim is set) so I am very confused at what causes the motor to jar & holds back on me before I can get it to WOT??? I am hoping to pull my kids on a tube this summer, so it would be nice to resolve. Any help is greatly appreciated!

 

[lncoop]

Re: Engine hesitation??

Welcome back lazy. Sorry you're having probs with the new to you barge. What prop are you using? I ask because you mentioned you're only having problems at certain trim angles, and your max RPM strikes me as a tad low. Post the deets on your prop and the recommended RPM band for your motor.

 

[7lazy77]

Re: Engine hesitation??

Incoop, thanks for your response! Motor came withh 13 pitch steel prop, but I swapped it with an 11 pitch Solas aluminum. Sorry, but where would I find the RPM band rating???

 

[lncoop]

Re: Engine hesitation??

Well, so much for my theory. I was hoping you were propped at a high number like seventeen, which would decrease your RPMs, but if you're down at eleven RPMs should be too high, not too low. Something is definitely not right. How many RPMs do you get at the trim angle where it seems to be running better? By the way, the RPM band can be found in your manual, but I'd guess 5k to 6k.

 

[rogerwa]

Re: Engine hesitation??

I would do a compression check just to ensure that we have good compression. I had a 25' with a 90hp that would spin a 11p prop at 5500RPM. If you bought the boat with a 13p prop, which is what is should turn, either they ran it lugging (not good) or something changed that caused a lack of power.

I would also look for possible restriction or airleaks in the fuel system.

Your main goal here is not to increase speed, it is to get the engine running well in the correct RPM. The speed is what it is..

 

[rogerwa]

Re: Engine hesitation??

It also occurred to me that the jolt may be ventilation of the prop, which would explain the speed as the prop doesn't have a good bite.

 

[lncoop]

Re: Engine hesitation??

The elephant in the room is the 4k to 4500 RPMs. Time for some troubleshooting.

 

[Silvertip]

Re: Engine hesitation??

You need to explain this "jar" a little bit better. A jar is momentary thing. Imagine hitting an undewater obstruction like a stump or rock. That would be momentary. If the jar is prolonged, such as the engine note (its sound) changes, the rpm may drop, and stay that way for a bit (several seconds to a minute or more) before the engine picks up speed again may inidicate an engine problem. As you found out, guessing at problems and telling a service agency what YOU want them to do is not the way to get something fixed. Tell the shop what happens and let THEM figure it out.

The manufacturers receommended WOT rpm band for your motor is 4500 - 5500 rpm but you want to be closer to or at the 5500 rpm number rather than the 4500. So look at the tach and see what it does when the "jar" happens. If the rpm jumps up then you have ventilation or blow out which happens when the prop cannot get a good bite on the water. You may simply be trimming up to high. If the tach does not rise when the jar occurs, then you may have an engine problem. It is also possible that the shift linkage needs adjusting as the clutch dogs in the lower unit may be worn and skipping under load. That definitely results in a jerk that feels like you hit something. An ignition problem is also possible.

 

[7lazy77]

Re: Engine hesitation??

Before I can better answer some of your questions, I need to get the boat out again to pay closer attention to the tach. I will report back with my findings. As far as the "jar", when I am starting to accelerate towards WOT, when it happens the jar is enough to throw you a little forward, but not off your feet. If I remember correctly, the rpm's will remain the same from the moment the "jar" happened. The engine will then sound a little muffled, but the rpm's do not change (increase nor decrease). I haven't ran it very long while it is doing this, I usually immediately decrease the throttle to where it runs decent.

 

[7lazy77]

Re: Engine hesitation??

Got the boat out this weekend & ran it a little more. After playing with the throttle a little & trying different trim angles I found that I can get the rpm's up to the 4500 - 5500 range. The "jar" would sometimes happen after I ran it a while (18 mph @ 5500 rpm) or even sometimes a little less rpm's or sometimes if I tried to throttle up faster instead of slower. It was a little windy this weekend & I also noticed that when I was going into the wind, the "jar" would occur less or even not at all. When the "jar" would happen, there was a jump in rpm's. The rpm's would stay higher until I backed off the throttle. Appearing from the feedback, it appears that it may be a ventilation problem? Does this sound correct? If this is the problem, what can be done to fix it?

 

[rogerwa]

Re: Engine hesitation??

You may also have a slippage in the prop hub. The easy way to tell is to put aligning marks on the prop housing and the hub. After you have the problem, check the orientation of the previously aligned marks. If they are off then you have slippage in your prop hub.

 

[7lazy77]

Re: Engine hesitation??

Wanted to add a few pics of my motor & the mount to show where the water line is in correlation to the top of the lower unit. Is this where it should be? I am on the last hole (to lower motor) on the engine mount, so I don't think I can drop the motor any further. ANY OTHER HELP is greatly appreciated to figure out how to fix my problem! View attachment 148041View attachment 148040

 

[lncoop]

Re: Engine hesitation??

Looks like the motor is low enough, and the symptoms don't really sound like a spun hub to me, as a spun hub shouldn't result in lower RPMs, just loss of bite. Sorry lazy, but I'm stumped. Sounds to me like whatever is going on is mechanical vs hydrodynamic (may be misusing that term, but I think you get the point). Just out of curiosity, when you took it in did you just tell them to service the carbs, or did you describe the issue and ask them what they thought?

 

[EricJRW]

Re: Engine hesitation??

Just curious, so you can be at a constant throttle setting, just humming along, and the jar will occur? Or are you saying you tend to get the jar as you increase (or decrease) throttle?

Since ventilation is not likely, could it be cavitation? I understand the hydrodynamics of cavitation, but am not sure under which actual conditions (boat, motor, prop, load, etc.) it would occur.

 

[7lazy77]

Re: Engine hesitation??

Thanks for your responses fellers!

Incoop....When I took the boat in to have it winterized (october 2011), I had them go thru the carbs & compression test....everything checked out good according to my local boat shop. I have discussed the problem & they first told me that I needed a prop with less pitch, so I purchased a 11p Solas aluminum 3-blade prop from a 13p metal which did nothing. They then told me that it could be a spun hub. Haven't did the "mark" test yet, but with a new prop not sure if it would be the problem. I will do the mark test next time out to confirm though. The local boat shop seems to be stumped & out of other suggestions.

EricJRW....The "jar" happens while I am increasing the throttle & occasionally happens while I am at a constant set speed at higher rpm's. I am starting to believe it is a "ventilation" problem from my research of previous posts witht he symptoms I am having. It was my understanding that ventilation occurs when air pockets caused by turbulence develop around the prop & cause it to momentarily lose it's bite, which feels like the prop is spinning out?? If this is correct, then it would make sense in my case when the jar happens, rpm's jump & momentum of the boat slows down which I then back off the throttle a bit & slowly increase again. Sometimes it does it & sometimes it don't....nothing constant.

 

[lncoop]

Re: Engine hesitation??

Okay, I'm glad to hear compression checked out, and your definition of ventilation is spot on. It happened to me last Saturday when the vice admiral and I were the only ones aboard and I had the motor trimmed out as though the admiral and our friends were also along for the ride. However, I'm still not clear on what exactly is happening with the RPMs. If it's just your prop blowing out you should be able to attain the proper RPMs at a certain trim angle. Is that the case? Sorry, but I can't remember if you ever said.

 

[rogerwa]

Re: Engine hesitation??

As I said in a previous post, you need to eliminiate a slipping prop hub as well.

 

[rogerwa]

Re: Engine hesitation??

Sorry disregard last post. Didn't see the comments about chceking it..

 

[EricJRW]

Re: Engine hesitation??

I went through something similar, and it was a spun prop. What made it confusing is that I even used a prop that had it's hub replaced, but it spun out within 30-minutes of use. I later learned the job was done wrong, but I moved away from the prop with a rubber hub and got one that used splines (there are a few options, mine is in the sig).

I don't know other boats, but is yours a triple? The reason I ask is that in diagnosing my problem (which seems so obvious now), making sure I had clean water going into the prop was mentioned. Basically needed to make sure nothing in front of the motor can cause the turbulence resulting in ventilation (dirty 3rd pontoon would be a source of turbulence, but I was told to be sure even my two logs were clean - which they were).

If the A/V plate is at the "proper" depth, then this is very mysterious. One other thing I did, while underway, was look under the deck, from the front of the boat, to see what the water looked like. It's a bit scary to stick one's head off the front of the boat, but I made sure I had calm water, plenty of space, and no other boats around (oh, and obviously I was not the only person on board, and a person I trusted was at the helm.).

So that's why I'm curious if cavitation could ever be an issue with your setup. But since your on another prop with a different pitch, it seems unlikely.

Good luck... I'm looking forward to hearing you found the problem!

 

[lncoop]

Re: Engine hesitation??

Yeah, it's a mystery. Spun prop is always a possibility of course, but it seems unlikely since he's swinging a brand new one.