RPM Differences

[estoltz]

2006 340 with 8.1s and V-Drives. I purchased it in September of 2015. At the time survey showed Max throttle RPMs at port 4720 and starboard 4900. The surveyor added it to the list of discrepancies to be addressed before closing. We took the boat out a few times last season and found that it ran OK but that the starboard engine required slightly more throttle to keep RPMs matched. I also noticed a little vibration so I decided to tackle new plugs. I started by changing the plugs on the starboard engine outboard side and found 2 items. First the plugs looked brand new. As we had only put about 4 hours on the boat last season since closing on it I assume that the previous owner had the plugs changed to address the RPM issues. Port engine plugs have also been changed. Second thing I found was that the third outboard plug wire (counting from the front of the boat) was not even plugged in and the plug had never fired. I plugged it in and when I took the boat out this weekend I'm not at port 4730 and Starboard 4500. I'm a bit confused and I'm now looking for advice on how to start off troubleshooting this further. Should both engines be at the same RPM at full throttle and about where should that RPM be or is this difference not that big of a spread and I shouldn't worry about it?


Thanks everyone

 

[estoltz]

Scott,

Your ears must have been burning. I was just reading Jimmy Buoy's post on which you guided him on a similar issue. As luck would have it I have a free haul out (part of a we owe from the sale) for cleaning. I will ask them to pull the props and scan them for me while they have it out.


Any other thoughts or items to check while I am waiting for them to haul it?

 

[juggernaut1]

I happen to run my 38 flat out on the weekend in open water. At the end of the run one motor ran just over 4700 and the other just under 4700 based on the smartcraft - about 40 or so rpm difference from memory. It hit 29.5 knots according to the smartcraft which accords with the GPS readings I was seeing. Bottom paint was last done back in November and I haven't cleaned it since. Boattest ran this boat at 30.2 knots and around 4750 rpm from memory. My boat is fairly loaded as we stay on it over weekends and have an inflatable on the bow - so speed and rpm seems about right in the circumstances. The only thing is I don't know how accurate the smartcraft readings are and whether they can be relied upon.

Notwithstanding the above, are your wires fairly new? I had an rpm issue on one motor that developed during a trip back in November and discovered one of the wires was arcing across to the exh manifold. Changes all wires and now all good.

 

[estoltz]

So wires seem fairly new. I'm going to pull the maintenance records from the marina since I am the second owner and the first had all work done by them. And as for fuel filters yes they were done in November.

 

[mainstream]

Hello
I bought my 370 back in 11 as a left over with 46 hours on it but it had sat for almost 3 years and barley ran after putting about 100 hours on it I had a similar issue where my port would only spin at about 4000 at WOT and the specks for 8.1 are between 4400 and 4600 depending on load after having my mechanic in the ER while under way with his scan tool he noticed that at lower rpm all was fine at higher rpm was where the issues started where on engine started missing causing the rpm drop we changed plugs and wires and all has been fine since I now get to WOT and am within 100rpm depending on loading so wires and plugs can cause your issues I would start there I have them do a shake down cruise every spring to make sure all is good. Good luck

 

[rondds]

Replacing plugs on these motors is tricky. Very hard to get the plug wire back on properly, and hard to tell if it took a proper seat.

I had the same issue when I changed plugs the first time. Boat was NOT running right so I pulled over, opened the hatch and heard a ticking. One wire was dangling and arcing to the manifold. Amazing how the computer can compensate for this.

As mainstream pointed out, for inboards, 4400-4600 RPM at WOT. As Scott pointed out, you need a prop examination.