My WOT is 5600. She's good at 5000. Then went wide open for a second and it started breaking up, almost like a miss. Sparks plugs are fairly new. The rest of the rpm range was fine, no other issues. I almost never go to WOT unless it's to check things out. Ideas? Thanks.
Engine Breaking Up at High RPM
- Thread starter hotajax
- Start date
This can be a tough one over a forum. Nothing against what you wrote, but there's very limited info to go on here. Fuel and spark (air is almost never an issue) are the things to look at. Fuel quality being one probability, fuel supply being another.
Because it's easy and cheap, I would at least pull the plugs and check them, taking note to their order. Clean and reinstall. Plug wires are another possibility - inspect those (although you can't always tell from the outside - but make sure they're making a good contact. Check your main filter (boat mounted), the engine-mounted filter, the VST filter and even the pressure regulator filter (just because you're already in there).
If you have a vac gauge on your main filter (good idea to install one, if not), check that. If you have a pressure gauge, you can eliminate a lot of guessing and legwork by hooking it up the Schraeder valve and going for a run while you monitor it during the issue.
When was the last time you had your injectors cleaned?
Because it's easy and cheap, I would at least pull the plugs and check them, taking note to their order. Clean and reinstall. Plug wires are another possibility - inspect those (although you can't always tell from the outside - but make sure they're making a good contact. Check your main filter (boat mounted), the engine-mounted filter, the VST filter and even the pressure regulator filter (just because you're already in there).
If you have a vac gauge on your main filter (good idea to install one, if not), check that. If you have a pressure gauge, you can eliminate a lot of guessing and legwork by hooking it up the Schraeder valve and going for a run while you monitor it during the issue.
When was the last time you had your injectors cleaned?
hotajax said:
More common causes is VST pump screen. 95 out of 100 times that kind of issue is restricted fuel flow. When the motor bogs, check the primer bulbs to see if is is collapsed, If it is, the problem is a blockage or restriction somewhere between the primer and the tank. A plugged fuel vent can cause the same thing. In that case the primer bulb will collapse or get soft. If the vent is plugged, try running with the fuel cap off and see if the problem goes away
A leaky fuel canister ( the clear one on the motor can cause the problem, If you have the problem and the cowling is off, an air leak usually is indicated by the filter canister being full of gas. It is normally half to 2/3 full. There is a filter element in that clear canister that can get clogged. It is not all that easy to determine if it is dirty and restricting fuel.
On some SXs and I think yours is one, there is also an inline fuel filter (metal) if plugged, same thing , fuel starvation. Your motor may have fine screens on the output of the VST pump and on each injector. You need a fuel pressure gauge to diagnose an injector problem ( actually, it doesn't exactly tell you if an injector is bad but it will tell you that the VST and all plumbing before it are working)
Of course there are other things like spark that can be acting up but the low pressure fuel delivery components are more often the cause.
On that motor a clogged VST screen will often cause missing and bogging when the boat is bouncing around.
One test that you can do with care is to securely tie up the boat and rev the motor in gear to see if it starts to bog down at high revs and under load. Just be careful
Good luck
Dennis and Seasick: Wasn't there a write up somewhere of how to clean the VST? Or was it a link to another website? I did the low pressure pumps about five or six years ago, but they can go bad that quick? Thanks for the tips,these tips are not terribly expensive to take a look at.
The LP pumps "could" go bad that fast - they're just a thin, rubber diaphragm. If you have a service manual, there's a vacuum/pressure test that can be done. The VST filter is washable. When you remove the VST tank... check the tank for cleanliness, clean it and the filter hat. If you start with the pressure test on the fuel rail, though, that will eliminate (or show there's a problem) everything on the fuel side except for the injectors. It's an easy test to help narrow things down.
I'm not home to look at my book, but isn't the max rpm on an OX66 5500? Does the engine have a rev limiter like 4 strokes?
5,500 is correct. I don't have my book in front of me right now, either, but "most" engines I'm familiar with typically have a rev limiter somewhere in the 150 RPM range above MAX. But, I have no idea if that's correct for this engine. I would assume so since it's got a compuker, but you know what they say about "assuming"... :mrgreen: However, 100 RPM over is no big deal - especially just as a "check if all things A-OK, kinda thing". I'm sure Google could help with this question.
It seems to me that Hotajax knows something is no normal especially by the symptom of running at revs for a second or so and then bogging down.
A service manual will help and there are posts on the web about cleaning or replacement but there are a few hints that will help.
Basically, you have to remove the 8 or 9 or so screws on the VST top cover but you will have to remove the VST pump housing to get to those bolts. On some motors you have to remove other parts too. It is east to lose screws and washers or spacers. Pit rags in the bottom of the cowling so that is a washer drops it doesn't slide down the exhaust housing or get lost under other castings. Drain the VST tank to avoid spills later. There is a drain plug at the bottom of the tank.
The bolts ( 3 I believe) that hold the housing have washers on the back and front and may have spacers. It is easy not to see that there are washers and they will often fall down when the bolts are removed.
There is a rubber gasket between the VST body and the top cover. Although the Yamaha recommendation is to always replace it, you may be able to reuse it. It is expensive for an rubber gasket. More on that later.
When you pull the pump internals, you will see a plastic filter on the bottom of the pump. That is the screen that gets plugged. It twists off and if you look through the round hole facing some light, you should be able to make out your hand or fingers. It everything look opaque, the screen is clogged. New ones are not exactly clear though. You have to decide on wehther to clean the filter or replace it. If you are going through all the work AND the tank itself looks clean, I suggest you replace it especially if you don't know when it was last (if ever) changed.
I reason I mention a clean tank is that if the tank has gunk, dirt particles or a white residue you have other issues that are clogging the screen and not addressing those can result in clogging your new and somewhat expensive filter.
So assuming the screen only needs to be cleaned or replaced, do so and reassemble. Now comes the hard part. That rubber gasket likes to expand over time and getting it in the groove can be hard to impossible. Use some marine grease to hold it in place, if not it may shift and you wont know that until everything is together and there is a leak. Some folks claim that putting the gasket in the freezer for an hour or so shrinks it and makes it fit better. If all else fails, buy a new gasket.
Reattach the cover and remount the VST housing to the block making sure you put all the washers and spacers where they came from.
Finally, before firing up the motor, use the primer bulb to refill the VST tank. That motor does not like to run dry.
It isn't as complicated as it sounds. If I find a link to the pictures, I will post it.
Good luck
A service manual will help and there are posts on the web about cleaning or replacement but there are a few hints that will help.
Basically, you have to remove the 8 or 9 or so screws on the VST top cover but you will have to remove the VST pump housing to get to those bolts. On some motors you have to remove other parts too. It is east to lose screws and washers or spacers. Pit rags in the bottom of the cowling so that is a washer drops it doesn't slide down the exhaust housing or get lost under other castings. Drain the VST tank to avoid spills later. There is a drain plug at the bottom of the tank.
The bolts ( 3 I believe) that hold the housing have washers on the back and front and may have spacers. It is easy not to see that there are washers and they will often fall down when the bolts are removed.
There is a rubber gasket between the VST body and the top cover. Although the Yamaha recommendation is to always replace it, you may be able to reuse it. It is expensive for an rubber gasket. More on that later.
When you pull the pump internals, you will see a plastic filter on the bottom of the pump. That is the screen that gets plugged. It twists off and if you look through the round hole facing some light, you should be able to make out your hand or fingers. It everything look opaque, the screen is clogged. New ones are not exactly clear though. You have to decide on wehther to clean the filter or replace it. If you are going through all the work AND the tank itself looks clean, I suggest you replace it especially if you don't know when it was last (if ever
) changed.I reason I mention a clean tank is that if the tank has gunk, dirt particles or a white residue you have other issues that are clogging the screen and not addressing those can result in clogging your new and somewhat expensive filter.
So assuming the screen only needs to be cleaned or replaced, do so and reassemble. Now comes the hard part. That rubber gasket likes to expand over time and getting it in the groove can be hard to impossible. Use some marine grease to hold it in place, if not it may shift and you wont know that until everything is together and there is a leak. Some folks claim that putting the gasket in the freezer for an hour or so shrinks it and makes it fit better. If all else fails, buy a new gasket.
Reattach the cover and remount the VST housing to the block making sure you put all the washers and spacers where they came from.
Finally, before firing up the motor, use the primer bulb to refill the VST tank. That motor does not like to run dry.
It isn't as complicated as it sounds. If I find a link to the pictures, I will post it.
Good luck
Here is a Youtube for an F115
https://www.youtube.com/watch?v=gY2RFaahio4
Your VST pump is easier to get to I think. The last part of the video is the most applicable
https://www.youtube.com/watch?v=gY2RFaahio4
Your VST pump is easier to get to I think. The last part of the video is the most applicable
seasick said:It seems to me that Hotajax knows something is no normal especially by the symptom of running at revs for a second or so and then bogging down.
A service manual will help and there are posts on the web about cleaning or replacement but there are a few hints that will help.
Basically, you have to remove the 8 or 9 or so screws on the VST top cover but you will have to remove the VST pump housing to get to those bolts. On some motors you have to remove other parts too. It is east to lose screws and washers or spacers. Pit rags in the bottom of the cowling so that is a washer drops it doesn't slide down the exhaust housing or get lost under other castings. Drain the VST tank to avoid spills later. There is a drain plug at the bottom of the tank.
The bolts ( 3 I believe) that hold the housing have washers on the back and front and may have spacers. It is easy not to see that there are washers and they will often fall down when the bolts are removed.
There is a rubber gasket between the VST body and the top cover. Although the Yamaha recommendation is to always replace it, you may be able to reuse it. It is expensive for an rubber gasket. More on that later.
When you pull the pump internals, you will see a plastic filter on the bottom of the pump. That is the screen that gets plugged. It twists off and if you look through the round hole facing some light, you should be able to make out your hand or fingers. It everything look opaque, the screen is clogged. New ones are not exactly clear though. You have to decide on wehther to clean the filter or replace it. If you are going through all the work AND the tank itself looks clean, I suggest you replace it especially if you don't know when it was last (if ever
I reason I mention a clean tank is that if the tank has gunk, dirt particles or a white residue you have other issues that are clogging the screen and not addressing those can result in clogging your new and somewhat expensive filter.
So assuming the screen only needs to be cleaned or replaced, do so and reassemble. Now comes the hard part. That rubber gasket likes to expand over time and getting it in the groove can be hard to impossible. Use some marine grease to hold it in place, if not it may shift and you wont know that until everything is together and there is a leak. Some folks claim that putting the gasket in the freezer for an hour or so shrinks it and makes it fit better. If all else fails, buy a new gasket.
Reattach the cover and remount the VST housing to the block making sure you put all the washers and spacers where they came from.
Finally, before firing up the motor, use the primer bulb to refill the VST tank. That motor does not like to run dry.
It isn't as complicated as it sounds. If I find a link to the pictures, I will post it.
Good luck
Seasick: Thanks very much for this info. Other question: How does a dapper gentleman such as yourself from NYC know so much about engines?
hotajax said:seasick said:It seems to me that Hotajax knows something is no normal especially by the symptom of running at revs for a second or so and then bogging down.
A service manual will help and there are posts on the web about cleaning or replacement but there are a few hints that will help.
Basically, you have to remove the 8 or 9 or so screws on the VST top cover but you will have to remove the VST pump housing to get to those bolts. On some motors you have to remove other parts too. It is east to lose screws and washers or spacers. Pit rags in the bottom of the cowling so that is a washer drops it doesn't slide down the exhaust housing or get lost under other castings. Drain the VST tank to avoid spills later. There is a drain plug at the bottom of the tank.
The bolts ( 3 I believe) that hold the housing have washers on the back and front and may have spacers. It is easy not to see that there are washers and they will often fall down when the bolts are removed.
There is a rubber gasket between the VST body and the top cover. Although the Yamaha recommendation is to always replace it, you may be able to reuse it. It is expensive for an rubber gasket. More on that later.
When you pull the pump internals, you will see a plastic filter on the bottom of the pump. That is the screen that gets plugged. It twists off and if you look through the round hole facing some light, you should be able to make out your hand or fingers. It everything look opaque, the screen is clogged. New ones are not exactly clear though. You have to decide on wehther to clean the filter or replace it. If you are going through all the work AND the tank itself looks clean, I suggest you replace it especially if you don't know when it was last (if ever
I reason I mention a clean tank is that if the tank has gunk, dirt particles or a white residue you have other issues that are clogging the screen and not addressing those can result in clogging your new and somewhat expensive filter.
So assuming the screen only needs to be cleaned or replaced, do so and reassemble. Now comes the hard part. That rubber gasket likes to expand over time and getting it in the groove can be hard to impossible. Use some marine grease to hold it in place, if not it may shift and you wont know that until everything is together and there is a leak. Some folks claim that putting the gasket in the freezer for an hour or so shrinks it and makes it fit better. If all else fails, buy a new gasket.
Reattach the cover and remount the VST housing to the block making sure you put all the washers and spacers where they came from.
Finally, before firing up the motor, use the primer bulb to refill the VST tank. That motor does not like to run dry.
It isn't as complicated as it sounds. If I find a link to the pictures, I will post it.
Good luck
Seasick: Thanks very much for this info. Other question: How does a dapper gentleman such as yourself from NYC know so much about engines?
Hotajax, I got a good laugh out of your comment!
I am fairly familiar with the OX66 Saltwater motors since I have one as do several of my club members. The real reason is that being from NY, I don't trust folks outright and that includes mechanics, especially 'certified' ones
Over the years, I have learned that some mechanics my friends have used are pretty knowledgeable and I hold special esteem for those who when asked will share their knowledge. Actually, I like to tinker, do a lot of research and try real hard to understand how those motors work (and that isn't easy since I have never been able to find a good reference manual). By training, I am an electrical engineer so I have a slight edge when working with wiring diagrams and electrical troubleshooting.There is also a ton of info on the internet. The trick is to determine what is based on fact and what is based on hearsay or rumor. I am also fortunate to be in a club with many seasoned boaters who do their own work. I often use their expertise for questions about things I don't have a lot if any direct experience with
As to your powers of observation, I may have a little more knowledge then the average boater on the Yami front but beware, I can often be a grumpy old codger!
Thanks again for the kind words.
Seasick
The beauty of owning an OX66, or any engine that has been around 18 or so years; is that any problem you may have has already been lived by someone else willing to share their experiences. Hence the added beauty of owners forums. So the problem was in the VST?