Varying RPM 19S

[BLR]

I have a 2008 19S 4.3 Mercruiser. The RPM's are varying by 200 RPM's back and forth. Have 210 hours on the boat. Anyone have any ideas why this would be happening? Boat runs well otherwise. Thank you for any ideas.

 

[Espos4]

I have a 2008 19S 4.3 Mercruiser. The RPM's are varying by 200 RPM's back and forth. Have 210 hours on the boat. Anyone have any ideas why this would be happening? Boat runs well otherwise. Thank you for any ideas.

Varying at idle, at cruising speed?
Can you provide more details?

 

[BLR]

Varying at idle, at cruising speed?
Can you provide more details?

Once I get on plane and cruising this is when I notice the varying RPM's. The boat will be cruising at a certain RPM and then increase by 200 RPM's and then return. I will set the RPM's at 3800 RPM and it starts the variations. It's feels as if the throttle is being manually controlled.

 

[Chip S]

Some times corrosion inside the tachometer can cause this. On the back of your tachometer is a switch labelled 4, 6, 8. Mark where it is and then turn it back and forth several times. This will knock any internal corrosion off the contacts. After turning it back and forth several times set it back to where it was. A 4.3 is a six cylinder so it should be "6", but make sure you know where it is set before you start moving it. This trick got my tachometer to be rock solid. Before it would wander.

 

[BLR]

Thank you Chip S for your idea I do understand. But I can actually feel the drop and pick up of the engine. I will add I do feel there is too much play in my throttle cable. Thinking that might be the issue.

 

[Scott06]

Whats the maintenance history of the boat? Rule out the play in throttle cable but incant see that varying rpm. I would take a look at the contents of your fuel filter and look at ignition components like cap rotor and plugs. See if the timing is moving around at speed

 

[Chip S]

Whats the maintenance history of the boat? Rule out the play in throttle cable but incant see that varying rpm. I would take a look at the contents of your fuel filter and look at ignition components like cap rotor and plugs. See if the timing is moving around at speed

+1 If you can hear/feel the engine rpms changing and you haven't changed the fuel filter yet, do that first. It's easy, relatively inexpensive, and has a good chance of fixing your problem.

 

[BillK2632]

Also check the anti-siphon valve and fuel line connections to ensure you do not have a fuel restriction.

 

[BLR]

Thank you everyone for your help, I appreciate it very much!

 

[BLR]

A big thanks to Chip S for his excellent advice! The problem was in the fuel filter. I found the fuel filter frozen onto the mounting plate and could not remove. Removed the assembly from the motor and placed in a vise and was able to remove. I found the date on the filter to be July 2007, this would have been the original filter. The filter was full of water. Installed everything back in the boat and headed to the lake. Happy to say the boat ran without any RPM surges! I bought the boat used and did not know the the maintenance history. I was told the dealer did all the work on the boat. From now on I will be replacing the fuel filter at every oil change which is once a year.
Thank you everyone and especially Chip S for the advise! I appreciate having this forum to ask the group questions.