Yamaha Engine Behavior

[djtech2k]

I just got back from my first trip with my boat. I saw some things that concern me so I want to run it by you guys to see what could be causing it or if it’s at all normal.

When I first started out in the boat, I noticed that the left engine was running a few hundred RPM less than the right. After a little while, I watched and could feel the left engine sporadically start to increase rpm suddenly and I could feel the boost. It would increase, go down, and go back up again. This was at higher speed. That was within the first 20mins. As the day went on, I would watch the rpm and the left was consistently 200-500 rpm less that the right. I also noticed that if I gave it throttle you speed up quickly, the left engine rpm would shoot up to 8000+ very fast and then it would fall down to the normal level and increase normally. That happened multiple times, but only when I tried to speed up quickly and only on the left engine. The best way to describe it was like in a manual trans car when the clutch starts slipping and the rpm shoots up and then falls back.

I had 5 adults on the boat. Early on, the top speed seemed to be low 30’s. As the day went on, the top speed was about 35’ish. The engines did not seem to idle poorly or sound bad at all. It was just the rpm thing that I noticed

Any idea what it could be or if I should be very worried? I have read some mention of some throttle stuff but not sure if this could be related.

Thanks in advance.

 

[djetok]

I just got back from my first trip with my boat. I saw some things that concern me so I want to run it by you guys to see what could be causing it or if it’s at all normal.

When I first started out in the boat, I noticed that the left engine was running a few hundred RPM less than the right.
After a little while, I watched and could feel the left engine sporadically start to increase rpm suddenly and I could feel the boost. It would increase, go down, and go back up again. This was at higher speed. That was within the first 20mins. As the day went on, I would watch the rpm and the left was consistently 200-500 rpm less that the right. I also noticed that if I gave it throttle you speed up quickly, the left engine rpm would shoot up to 8000+ very fast and then it would fall down to the normal level and increase normally. That happened multiple times, but only when I tried to speed up quickly and only on the left engine. The best way to describe it was like in a manual trans car when the clutch starts slipping and the rpm shoots up and then falls back.

I had 5 adults on the boat. Early on, the top speed seemed to be low 30’s. As the day went on, the top speed was about 35’ish. The engines did not seem to idle poorly or sound bad at all. It was just the rpm thing that I noticed

Any idea what it could be or if I should be very worried? I have read some mention of some throttle stuff but not sure if this could be related.

Thanks in advance.

The 200 rpms is normal. The other issue sounds like possible cavitation . Look for missing silicone or abnormal sealing around your intake.

 

[djtech2k]

The 200 rpms is normal. The other issue sounds like possible cavitation . Look for missing silicone or abnormal sealing around your intake.

Does cavitation make the boat shake, like a bent prop would on an outboard?

Sorry, I am brand new at this and never had a jet boat before so just trying to figure out where to look and what to look for. I will search for a video on what you are describing so I know what I am looking for.

 

[agannole93]

Does cavitation make the boat shake, like a bent prop would on an outboard?

Sorry, I am brand new at this and never had a jet boat before so just trying to figure out where to look and what to look for. I will search for a video on what you are describing so I know what I am looking for.

Cavitation if bad enough makes the boat shake. I've sucked up a few random things on our lake and it's definitely noticeable. What you have doesn't sound like cavitation to me, unless you're getting an audible sound and severe vibration. Others I'm sure can chime in.

 

[djetok]

Does cavitation make the boat shake, like a bent prop would on an outboard?

Sorry, I am brand new at this and never had a jet boat before so just trying to figure out where to look and what to look for. I will search for a video on what you are describing so I know what I am looking for.

I'm sure it could , your sucking air


Cavitation is a lot like a slipping transmission. Not that anything is slipping , it just feels like it.

 

[djtech2k]

Ok so how can I check this to see if it is cavitation or not?

For example, by intake do you mean water intake or air intake?

 

[VitaminSea]

Ok so how can I check this to see if it is cavitation or not?

For example, by intake do you mean water intake or air intake?

Cavitation is the result of air being introduced into the jet pump. If you boat is on a trailer, start by climbing under it and checking out the intakes. There have been instances, where the silicone might be gapping, or not smooth. This would effect the flow of water to the pump.

While under the boat, check the backside of the props to ensure there is nothing wedged in there.
Check your clean out plugs, maybe one was not properly seated, effecting the water flow.
WIth the engines off, put the throttle in FWD, and then look at the pumps from the back of the boat, look to see if there is debris there.

Use of a flashlight, and maybe post some pics and others can chime in. I’m sure others will chime in, as I’m far from an expert here, but this is a good starting point IMO.

 

[djtech2k]

I just went to look at the boat and noticed a few things. When I went to check the clean out ports, I noticed that the left engine hole is full of water. I was able to get the plug out and it drained, but it was not super easy to remove. The right hole seems to be stuck. I unlatched it but I cannot get it out. There is only a few inches of water in that clean out port. After trying to get it out for a while, it will not come out nor will it latch now. I put some ice in the hole hoping it will shrink enough to get it out, but no luck so far.

So is it normal for there to be water in there? I know I have read that the plugs can get stuck. The left eventually came out but I cannot get the right one out. What does this mean and what should I do about it? Would any of this clean out port stuff have any impact on the behavior I was seeing with the engines?

When you mention the silicon, where is that located? In the left engine, I can see down through the cleanout port down through the grates. I feel the impeller and I do not feel any tears/dents. When I crawled under the boat, I see the grates but not sure what to look for.

Here are some pics of anything that I thought was interesting or might be relevant.

 

[VitaminSea]

this is on my Ski, and for whatever reason the photo rotated when I uploaded it. See where the white is, where the intake tunnel meets the hull? This is where you want to look on the boat.



you are looking for missing silicon.

Also sounds like you need to purchase some “manhole cover” rebuild kits. It’s an easy process once you have the plugs removed. Search for stuck clean out plugs on the forum, lots of post.

Generally the kits are $40-50 each, but I have noticed the price has crept up some. You will need two, one for each plug

Yamaha OEM Part - MANHOLE COVER REPAIR SET
Part Number F0R-67609-09-00

Not related, but you should also look to replace your Anodes. The one pictured is certainly pitted and has dissolved what appears to be a considerable amount.

 

[VitaminSea]

Also, how is your throttle engagement? Smooth? I recently had to replace my throttle cables and it made a big difference with the throttle holding RPMs and being able to better manipulate RPMs.

 

[djtech2k]

this is on my Ski, and for whatever reason the photo rotated when I uploaded it. See where the white is, where the intake tunnel meets the hull? This is where you want to look on the boat.

View attachment 131976

you are looking for missing silicon.

Also sounds like you need to purchase some “manhole cover” rebuild kits. It’s an easy process once you have the plugs removed. Search for stuck clean out plugs on the forum, lots of post.

Generally the kits are $40-50 each, but I have noticed the price has crept up some. You will need two, one for each plug

Yamaha OEM Part - MANHOLE COVER REPAIR SET
Part Number F0R-67609-09-00

Not related, but you should also look to replace your Anodes. The one pictured is certainly pitted and has dissolved what appears to be a considerable amount.

Thanks.

So I assume that in order to see what you see in that picture, its under the boat and maybe behind the metal grates? I don't think I got that far under my boat. So the white in that picture looks like it is in a small crack or area near the impeller?

When I look in the cleanout ports, I can see that it is not super clean or whatever. There is a bit of film or what have you in that port and/or on the plug that I was able to get out. I was thinking of cleaning that up and but and maybe taking some steel wool over it to scratch off any extra surface. The right ones that is stuck did come up a little bit but still not out. The left one is out and I can put it back in/out, but I do need to tug on it. I have not cleaned it yet.

I did already order the yamalube silicon spray since I saw that in some videos where people spray that on the plugs when they remove the plugs for when they store the boat.

I will check out the rebuild kits. I was hoping I wouldn't need them but its not a huge deal if I do. I am far more concerned about the RPM jumping around on the left engine.

I think I know what you mean about the anodes. I am guessing its that very gnarly looking piece of metal.

 

[djtech2k]

Also, how is your throttle engagement? Smooth? I recently had to replace my throttle cables and it made a big difference with the throttle holding RPMs and being able to better manipulate RPMs.

Well, the throttle was not hard to move but I did notice that it was kind of sensitive. I mean that when moving forward for example, I would move it and keep moving it with no/little change to the engines, and then all of a sudden the engines would pickup which sometimes made it jerky. It did not seem like it was slipping or anything but that the throttle would go from nothing to a jump, so maybe more of a sensitivity. It seemed to move a lot with no change but suddenly pickup in a certain spot. Maybe its normal on a jet boat. Its all new to me.

 

[djtech2k]

Ok, so I just crawled under the boat to get a better look at the left engine, as you mentioned. Here are the pictures. There was a tiny piece of grass/twig in there and its in the picture. No idea if this could be the issue or not. I did not see any kind of silicon or anything like what your picture showed.

 

[VitaminSea]

Good find on the twig. I would be concerned with the spots on the tunnel, as circled below, as well as the end of the black plate where it meets the hull. Looks like its missing some silicone, and not smooth. Ideally, you want the tunnel to be smooth fiberglass.



The “white” stuff in the picture of my ski is some adhesive from the factory, not chips.

As for lubrication of the plugs, any silicone spray could be a good start for that stuck plug. The Yamalube stuff is great, but pricey and I would use that to spray the engine down, and the pumps. Once you rebuild your clean out plugs, you will be amazed at how easy they are to remove and reinsert.

 

[djtech2k]

Ok so where that black plate meets the hull, it should have silicon? What kind of silicon?

The tunnel that leads to the impeller was definitely not smooth. It has whatever that stuff is that’s kind of rough. I assume it’s from stuff in the water. So it should be smooth? Is it just a matter of cleaning it? If so, any suggestions?

So do you think what you see in the pics could cause what I described?

 

[VitaminSea]

Not an expert here, so hopefully others will chime in. Use a marine grade silicone. I would try a cleaner wax if you have some lying around, if that doesn’t work you might need an acidic hull wash and something to scrap the growth off the hull (plastic putt knife or plastic razor blade).

you could certainly head back out before you addressed this and see what impact the twig had.

 

[djtech2k]

Sorry for the newbie question.

For the silicone, does it come in a tube I guess? And I assume you just fill in around that black plate? Anywhere else?

Any suggestions for the type of any of this stuff? I have none of it so I will be buying whatever I need.

 

[BrentEB]

Do you know when the spark plugs were last changed? Old plugs can make the engines run a bit flakey.

On my boat the engine speed is pretty constant from "neutral" up to about where the throttles are straight up and down. What's happening I believe is that the outlet "buckets" are being raised to full open, then the engine speed begins to increase.

 

[djtech2k]

I do not know for sure but I think it was last year. The last owner is a local guy that I know and he had the boat for 2 years. He put about 30 hours on it last year and had not used it this year. He did not change the plugs this season so I assume it was done last year.

 

[djetok]

Sorry for the newbie question.

For the silicone, does it come in a tube I guess? And I assume you just fill in around that black plate? Anywhere else?

Any suggestions for the type of any of this stuff? I have none of it so I will be buying whatever I need.

Yes you get it in a tube. 3m 4200 is what I would use. You can get it at home depot or lowes.