Not making RPMs

R

ratteree

New Member
Aug 5, 2012
22
CA
Boat Info
Boatless
Engines
boatless
last weekend I went out on my 320 Sundancer with twin 350 mercruisers. I had 5 guys including myself on the boat. I had to have 2 guys get up on the bow just to stay on plane. with the guys on the bow I was able to maintain 21 knots and approx 3900 RPM on the port and the starboard engine was maybe at 410o RPM both at full throttle. If I matched RPMs I would quickly slip off plain and go down to about 14 knots and maybe 3200 RPM. I had the bottom scraped yesterday hoping that the diver would find a bunch of crap on the underwater gear but he said it wasn't really bad at all. some on the tabs, exhaust and minimal on the props. This is just popping out of nowhere and all of the other systems (as far as I can tell) are fine. There is no way the boat would be lagging with 5 guys on it so I know something is up. I will be loading the boat with 8 people tomorrow and I am dreading asking them to get up on the box to get some speed not only because it is not safe.

Any idea what the heck is going on?
 
I just got the boat last August so my honest answer is I have not and further... I don't know. Plugs and fuel filters I can manage. Wires? Not sure what you are referring to here. The wires connected to the plugs?

How do these impact RPM... serious question. I am mechanically curious.
 
I had the same problem you may not be reaching rpms do to some cylinders not firing I had two bad plugs and wires could not get over 3900 changed wires and plugs and all is good. Sounds like you need a tune up. And I am referring to plug wires.
 
perfect. heading to west marine now. keeping my fingers crossed
 
I had a similar sounding problem, the engine wouldnt rev past 4200rpm, the proper max revs should be 4600-5000rpm. So I replaced the distributor cap and the spark plug wires and instantly I was back to the proper max revs again.
 
julians said:
I had a similar sounding problem, the engine wouldn't rev past 4200rpm, the proper max revs should be 4600-5000rpm. So I replaced the distributor cap and the spark plug wires and instantly I was back to the proper max revs again.
Click to expand...

I threw in 5 new plugs on the engine that was really lagging and one in the other engine. I was only able to buy the 6 plugs yesterday afternoon. I will replace the other 10 this week. I will also have a look at the distributor caps. I am heading out on the boat today with a group of about 8 people. this will be as good a test as any. I'll keep you all posted.

By the way... could these plugs be any more difficult to access?! I have a new appreciation for these mechanics!
 
Hope that solves you problem if you still have trouble cap and wires would be next
 
You guys are all lucky. I have the same symptoms but after plugs and wires had the compression checked. Not good and some worse than others. There is no oil consumption so I'm hopeful no complete re-build. Hopefully it won't be too bad when it gets opened up. Anyone have any thoughts on this before I open the checkbook real wide?
 
When tested did he test it under load conditions just wondering if it fails under load conditions
 
I would think with weak compression you won't get a clean combustion but you should get pretty close to wot but if they are real bad then you loose power. How bad are they.
 
I would definatly have him give you number like if 1 thru 7 were like 160 and 8 was 140 I don't think that would be so drastic. If you have any below 100 you have big troubles or over 15% of each other here is some one else's questions with same issues.

The first thing you should do is hook a a small outboard portable tank (like a 3 gallon) filled with fresh new fuel to the engines fuel pump and try it. This will rule out any issues with the fuel supply and fuel quality. If the engine performs better then you know the problem is either in the tank, lines, or fuel quality. If there is no change then you can rule this out.

Next is checking engine compression and fuel pressure. They both take time to check so it's up to you which you would like to check first. For compression, your results need to be 100psi or better on all cylinders, and all cylinders within 10 to 15% of eachother. If you get any cylinder below 100psi, then the engine has mechancial problems.

For fuel pressure, here is how you check it.
Checking Fuel Pressure and Fuel Supply Vacuum
1. Connect a fuel pressure gauge to the shrader valve on the fuel rail.
2. Cycle key switch 2-3 times (OFF to RUN position) at 3 second intervals to reach
maximum fuel pressure.
3. Verify that the pressure is within specification.
4. If fuel pressure exceeds 44 psi (303 kPa), replace the fuel pressure regulator.
5. If fuel pressure is less than 40 psi (276 kPa), but greater than 13 psi (90 kPa):
a. Relieve the fuel pressure in the fuel rail. Refer to section 2A for fuel pressure relief
procedure.
b. Use a tee fitting and connect a vacuum gauge to the fuel inlet side of the Cool
Fuel Module. Do not remove the fuel inlet fitting adapter.
c. Cycle the key switch 2-3 times (OFF to RUN position) at 3 second intervals to
reach maximum pressure.
d. Verify that the vacuum from the fuel source is within specification. If the vacuum
exceeds 2 in. Hg (7 kPa) , excessive fuel restriction exists. Correct the fuel
restriction before proceeding.
e. With the vessel secured to the dock and the engine running in neutral, restrict the
fuel supply with fuel shut-off tool and verify that the Cool Fuel Module has the
ability to cause an inlet vacuum reading of 11 in. Hg (37 kPa) or greater. If the
vacuum reading is less than 11 in. Hg (37 kPa) with the fuel supply restricted, but
fuel pressure is within specification, replace the low-pressure pump.
6. If fuel pressure is 13 psi (90 kPa) or less and inlet vacuum is less than 15 in. Hg (51
kPa), replace high-pressure pump and fuel pressure regulator.

Start with that and let me know how you make out. Post back with any questions
Good luck!


These were on a 2007 8.1 just a few things to ask your mech
 
Wow. That's a lot of great detail on diagnosing. It may be a little above my pay grade to do however. It does not pass my "do no harm-do it yourself" limitations LOL.

More important, although I did not remember the numbers, the compression was checked by two different mechanics and both said that they were unacceptably different and one or more below minimum. Both said next step was to open it up and see. I just have not had the $ to do so.
 
WOW, where did you come from? Regardless, welcome aboard and your detailed advice is very timely for me. I just put on newly rebuilt carbs this spring after a winter layup and my STB engine idles like a champ, but will not rev over 1,100 RPMs. I have sprayed carb cleaner around the base of the carb without any increase in revs, I have removed the gas cap to eliminate any vent restriction. I have also changed the coil, rotor, cap and plugs (no wires or distributor sensor module as yet) and still no improvement, so I am thinking fuel issues are at play here. Could it be problems with the new carb even though it idles fine. There is no hi speed adjustment on this Rochester carb. I will remove the supply from the tank to eliminate those possibilities as you suggested. Now, that would leave the pump, inline filter at the carb or the carb itself?? Am I on the right track here? ~Ken
 
So to give an update from my original post. I managed to scrounge up 6 new plugs from the local shop and I put 5 of them in the lagging engine. FIXED!!!! She got right up on plane and cranked along. I ordered, and received, the remaining 10 plugs which I will put in this coming week just to do a little PM and make myself feel better. I wish all of my issues were solved this easily. Thanks for you help!
 
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