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Author Topic:   Top Speed Declines 14-MPH

robbdrell posted 08-01-2005 11:29 PM ET (US)
I have a [18-foot Boston Whaler boat of unspecified model] with a 150-HP [Yamaha]. It has been running superbly as of late. Yesterday I noticed the boat vibrating especially at low speeds. Now I can only go 28-MPH where previously could do about 42-MPH. The boat normally shoots out onto a plane whereas now it has no guts. I looked at the prop and it appears ok but I'm no expert here. Any advice? Thanks
jimh posted 08-02-2005 08:36 AM ET (US)
Has the engine speed at Wide Open Throttle (WOT) changed, too?
bsmotril posted 08-02-2005 09:29 AM ET (US)
There is a good possibility you have lost a cylinder. On a V6, that is good for about a 2500 rpm drop in top end. When a single cylinder goes out, the first thing to do is pull the plugs and find which one looks different. That is the likely bad cylinder. If the plug tip looks damaged, or has metal deposits, do a compression test on the cylinder. If that is not a problem, I'd start with ignition in this order of replace/swap: Spark Plug, Wire, Coil. If that does not help, then looked for a clogged carb jet or injector that feeds that cylinder. BillS
lastcast posted 08-02-2005 09:22 PM ET (US)
I had an issue something like your own. Wide open throttle only put out 3800 rpms. Seemed to be missing. Especially when the engine was hot after some trolling. Power pack was the cause the mechanic said. I checked the condition of the plugs and could not tell the difference.
jimh posted 08-02-2005 09:27 PM ET (US)
There is nothing indicating the engine speed has declined. Just the boat speed. More information is needed here before a reasonable diagnosis can be made.
robbdrell posted 08-03-2005 12:39 PM ET (US)
what kind of information do we need to make a dignosis?
Buckda posted 08-03-2005 01:40 PM ET (US)
Rob -
JimH was trying to find out if your RPMs decreased as well as your speed, or if your RPMs stayed the same and your boat lost top end speed? Knowing that will give us a direction to go in.
Dave
jimh posted 08-03-2005 01:50 PM ET (US)
Read the first response. It contains only this question, still unanswered:
"Has the engine speed at Wide Open Throttle (WOT) changed, too?"
Plotman posted 08-03-2005 04:35 PM ET (US)
Spun hub?
Jerry Townsend posted 08-03-2005 05:01 PM ET (US)
As Jimh states - there is more information necessary - starting with "Has the engine speed at Wide Open Throttle (WOT) changed, too? So far, we only know the WOT speed has decreased and there is vibration.
Now, if the WOT engine rpm decreased - that suggests one or more cylinders are not firing. This can be checked with the engine running by either by 1) shorting a plug wire or 2) removing a plug wire from the plug - and noting if the engine speed further decreases - in which case, that is not the faulty cylinder. If, when shorting a plug wire or removing it from the plug, the engine rpm does not reduce - you have found a bad cylinder. You can also use an ignition analyzer or a spark plug wire ignition tester. Then check the plug wire and the plug - and replace as necessary.
The noticeable vibration at slow rpms suggests one or more cylinders are not firing.
If all cylinders are firing - the vibration is coming via some other rotating device - such as the prop. --- Jerry/Idaho
robbdrell posted 08-07-2005 06:14 AM ET (US)
thanks alot for the info. i'll have to get the boat back in the water to look at the RPM's @ WOT. in the mean time i'm going to look at the plugs then pull the prop. i'll keep you posted, Thanks
robbdrell posted 08-07-2005 06:19 AM ET (US)
con't.
unfortunately since i just bought the boat, i'm not sure that the RPMs are at WOT. any idea what it might be for a yama 150?
jimh posted 08-08-2005 05:36 AM ET (US)
Please include the engine speed and the boat speed at several different throttle settings. If you have the data for both before and after the onset of the problem it would be helpful in making some sort of diagnosis.

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Author	Topic: Top Speed Declines 14-MPH
robbdrell	posted 08-01-2005 11:29 PM ET (US) I have a [18-foot Boston Whaler boat of unspecified model] with a 150-HP [Yamaha]. It has been running superbly as of late. Yesterday I noticed the boat vibrating especially at low speeds. Now I can only go 28-MPH where previously could do about 42-MPH. The boat normally shoots out onto a plane whereas now it has no guts. I looked at the prop and it appears ok but I'm no expert here. Any advice? Thanks
jimh	posted 08-02-2005 08:36 AM ET (US) Has the engine speed at Wide Open Throttle (WOT) changed, too?
bsmotril	posted 08-02-2005 09:29 AM ET (US) There is a good possibility you have lost a cylinder. On a V6, that is good for about a 2500 rpm drop in top end. When a single cylinder goes out, the first thing to do is pull the plugs and find which one looks different. That is the likely bad cylinder. If the plug tip looks damaged, or has metal deposits, do a compression test on the cylinder. If that is not a problem, I'd start with ignition in this order of replace/swap: Spark Plug, Wire, Coil. If that does not help, then looked for a clogged carb jet or injector that feeds that cylinder. BillS
lastcast	posted 08-02-2005 09:22 PM ET (US) I had an issue something like your own. Wide open throttle only put out 3800 rpms. Seemed to be missing. Especially when the engine was hot after some trolling. Power pack was the cause the mechanic said. I checked the condition of the plugs and could not tell the difference.
jimh	posted 08-02-2005 09:27 PM ET (US) There is nothing indicating the engine speed has declined. Just the boat speed. More information is needed here before a reasonable diagnosis can be made.
robbdrell	posted 08-03-2005 12:39 PM ET (US) what kind of information do we need to make a dignosis?
Buckda	posted 08-03-2005 01:40 PM ET (US) Rob - JimH was trying to find out if your RPMs decreased as well as your speed, or if your RPMs stayed the same and your boat lost top end speed? Knowing that will give us a direction to go in. Dave
jimh	posted 08-03-2005 01:50 PM ET (US) Read the first response. It contains only this question, still unanswered: "Has the engine speed at Wide Open Throttle (WOT) changed, too?"
Plotman	posted 08-03-2005 04:35 PM ET (US) Spun hub?
Jerry Townsend	posted 08-03-2005 05:01 PM ET (US) As Jimh states - there is more information necessary - starting with "Has the engine speed at Wide Open Throttle (WOT) changed, too? So far, we only know the WOT speed has decreased and there is vibration. Now, if the WOT engine rpm decreased - that suggests one or more cylinders are not firing. This can be checked with the engine running by either by 1) shorting a plug wire or 2) removing a plug wire from the plug - and noting if the engine speed further decreases - in which case, that is not the faulty cylinder. If, when shorting a plug wire or removing it from the plug, the engine rpm does not reduce - you have found a bad cylinder. You can also use an ignition analyzer or a spark plug wire ignition tester. Then check the plug wire and the plug - and replace as necessary. The noticeable vibration at slow rpms suggests one or more cylinders are not firing. If all cylinders are firing - the vibration is coming via some other rotating device - such as the prop. --- Jerry/Idaho
robbdrell	posted 08-07-2005 06:14 AM ET (US) thanks alot for the info. i'll have to get the boat back in the water to look at the RPM's @ WOT. in the mean time i'm going to look at the plugs then pull the prop. i'll keep you posted, Thanks
robbdrell	posted 08-07-2005 06:19 AM ET (US) con't. unfortunately since i just bought the boat, i'm not sure that the RPMs are at WOT. any idea what it might be for a yama 150?
jimh	posted 08-08-2005 05:36 AM ET (US) Please include the engine speed and the boat speed at several different throttle settings. If you have the data for both before and after the onset of the problem it would be helpful in making some sort of diagnosis.