2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

[Mdavis]

Hi everyone and thanks in advance for your assistance....here is my ongoing issue:

• This happens every time I take the boat out fishing
• Boat will run WOT at 6,000 rpm's then after about 3 minutes will drop to 5,000 rpm's and stay there for the rest of the day
• Boat motor sounds awesome and no alarms when running at the 6,000 or 5,000 rpm's
• It's not the gas as I have gone through 3 - 30 gallon tanks this year
• After is sits for a couple of days and go back on water runs WOT at 6,000 for about three minutes than back down to 5,000
• This motor is on a Bass Buggy Pontoon

Has anyone else had this issue?

 

[Faztbullet]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

Does it lose speed when it drops rpm?

 

[Mdavis]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

Does it lose speed when it drops rpm?

Yes, when at 6,000 rpm's I go about 20 mph and at the 5,000 rpm's I get a top speed of about 17 mph.

 

[Faztbullet]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

You have a carb or injector problem. Luckily its a 4 stroke and lack of fuel will only cause a power loss, if a 2 stroke it would have shucked a piston by now..

 

[Mdavis]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

You have a carb or injector problem. Luckily its a 4 stroke and lack of fuel will only cause a power loss, if a 2 stroke it would have shucked a piston by now..

Is there anyway to clean injectors or is that somthing a shop will have to do? I'm pretty mechanical, but have never cleaned injectors before.

I was just about ready to put some seafoam in it for the year....should I hold off on that?

Thanks for the help!!

 

[Faztbullet]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

I send my customers off to have cleaned and balanced..
Brucatosvs.com

 

[Mdavis]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

I send my customers off to have cleaned and balanced..
Brucatosvs.com

Great and thank you very much for your help! Will send them off to be cleaned.

 

[CharlieB]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

Before pulling the injectors out I suggest a couple of simple tests.

Fuel pressure at speed, see if there is any difference before and after the motor slows.

Spark test, check spark to see if the maximum spark gap/arc changes between first starting then test again once the motor slows. If the ignition will arc a 1/2 gap cold, then only a 3/8 gap hot may indicate a problem with an ignition component once at operating temp.

 

[Mdavis]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

Before pulling the injectors out I suggest a couple of simple tests.

Fuel pressure at speed, see if there is any difference before and after the motor slows.

Spark test, check spark to see if the maximum spark gap/arc changes between first starting then test again once the motor slows. If the ignition will arc a 1/2 gap cold, then only a 3/8 gap hot may indicate a problem with an ignition component once at operating temp.

Interesting.....Any documentation out there on how to actually do the above two test?? I just need some directions/docs and I'm sure i could handle this.

This may be why..(when the motor warms up) the drop in rpm's.

Thanks for the input.

Come to think of it, when i use my trolling motor for most of the day and then restart the motor it will run at 6,000 rpm's again then drop down to 5,000. So maybe it cooled down enough then when it gets to operation tempature it faulters again.

I love troublshooting.

 

[Mdavis]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

Also, I should state that when it starts dropping you can hear a diffrent picth/sound to the motor. Does not sound bad, but it does sound different. Sometimes it bounces back and forth between 6,000 and 5,000 before going to 5,000 and staying there. Each time it bounces you can hear a pitch change to the motor. Like somting is 'Kicking in" then "kicking out". But, motor does not make any wierd noises and sounds great at both 5k and 6k rpms.

 

[Mdavis]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

anyone??

 

[CharlieB]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

An adjustable gap spark tester can be found at an auto parts store within a few phone calls.

Set the gap at 7/16 inch and test spark on each cyl one at a time. Increase the gap slilghtly and retest until you find the maximum gap that the ignition will still spark reliably.

As soon as the motor is warmed and begins to slow, once again test spark on each cyl at that maximum gap, if any do not now spark reliably begin reducing the gap until you once again get a reliable spark.

A hot blue arc is expected, any weakening will change to yellow or you will have to reduce the gap measurably.

Repost your findings.

Fuel injected motors have a shrader valve on the high pressure fuel line to the injectors, it has a screw on cap and under it looks a lot like the air pressure tire valve. Fuel pressure should not vary more than 2 or 3 lbs while the engine is running, no matter what the RPM, the pressure should remain steady.

I seriously expect no problem with the fuel pressure, but do expect you to find a difference in the spark between cold and once up to running temps.

Prove it, then we will proceed to figure out which component is the cause.

 

[Mdavis]

Re: 2008 4-Stroke 50 HP Merc WOT Down 1,000 rpms After 3 Minutes of Running

Thanks for your explanation and will get one tonight.