3300 Top RPM on 1998 380 Sundancer. Why?

R

rgj

New Member
Nov 29, 2018
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Boat Info
330 1997 sundancer and 338 1999 sundancer
Engines
454
Calling all gearheads!

On my 1998 380 Sundancer. Owned boat for about 5 months. Engines will not drive boat faster than 19.5mph and RPM on boat engines does not go higher than 3300. This is on both engines. Bottom was just repainted and no issue with growth on props or bottom. High vibration at low and high RPM's. I am changing the spark plugs but I think the plugs were changed before I bought the boat. No overheating at any time. Fluids are all normal. Does anybody have any advice on a prioritized checklist to diagnose and fix this problem? Thanks in advance!
 
How long ago was the bottom done. Where do you boat. When was the last time props were tuned. Did the boat perform better when you bought it
 
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First thing I would check would be motor mounts and alignment. This not likely the cause but very easy to do and can cause these problems. Then the props next. Sounds like you could be way over propped. Also, would wonder about the wires for plugs. Gut tells me you have prop issues. Without more information it is really just a guess, not knowing the service history of the boat. The performance is so out of norm for this boat, it makes one wonder what kind of maintenance if any it has had before you purchased it. What did the survey show as issues
 
rgj said:
Calling all gearheads!

On my 1998 380 Sundancer. Owned boat for about 5 months. Engines will not drive boat faster than 19.5mph and RPM on boat engines does not go higher than 3300. This is on both engines. Bottom was just repainted and no issue with growth on props or bottom. High vibration at low and high RPM's. I am changing the spark plugs but I think the plugs were changed before I bought the boat. No overheating at any time. Fluids are all normal. Does anybody have any advice on a prioritized checklist to diagnose and fix this problem? Thanks in advance!
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Has the boat been used regularly in the 5 months since you bought it? If the boat ran fine 5 months ago and then sat unused for 5 months in salt water I can guarantee you the bottom and running gear are all fouled and causing your problems. Hire a diver for $100 to clean the bottom and running gear.
 
i only used the boat at low speeds like under 8 knots so I never noticed the problem. Just tested boat at high speed since I was bringing it to a marina to get bottom painted. Boat was tested right after bottom paint so bottom is smooth as baby's butt.
 
Thanks guys for the responses. This is an amazing resource!
 
rgj said:
i only used the boat at low speeds like under 8 knots so I never noticed the problem. Just tested boat at high speed since I was bringing it to a marina to get bottom painted. Boat was tested right after bottom paint so bottom is smooth as baby's butt.
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But you haven't answered the most important question. How did the boat perform when you bot it? What changed?
 
When I first bought the boat, we drove it at around 17 knots with no problem for about 15 miles bringing it to its new dock. Seemed to work fine. I did not test the boat at full throttle. No survey done. (yes, i know)
 
rgj said:
When I first bought the boat, we drove it at around 17 knots with no problem for about 15 miles bringing it to its new dock. Seemed to work fine. I did not test the boat at full throttle. No survey done. (yes, i know)
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Ok since bottom was clean, I'd start with a compression test. If you have good compression you can then start messing with ignition, props, etc. good luck.
 
rgj said:
When I first bought the boat, we drove it at around 17 knots with no problem for about 15 miles bringing it to its new dock. Seemed to work fine. I did not test the boat at full throttle. No survey done. (yes, i know)
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change your profile to reflect your new boat, please.
I have a mile and a half thread about a port engine that is really starting to point to fuel restriction on high demand. Runs great at lower speeds/RPMs. Collapsing fuel line or weak high pressure pump is starting to look like the culprit. Search me, you'll find it.
Are you saying that both engines will make 3300 rpm, but no more, and with the same throttle position?
BTW, the 380 didn't show up until 1999. Are you sure of your model, or the year? Big differences during that transition period engine and interior wise. Hull, it was the same. Your Hull ID on the transom will tell the truth.
 
Thanks I will check it out. Yes 3300 rpm but no more full throttle. I am pretty sure its a 1999.
 
Hi guys, update on this. Plugs and wires changed. Drove the boat 30 miles and super smooth and no vibration at around 16 knots. No problems at all for about 2 hours. Top speed is now 20 knots which is an improvement. Starboard top rpm is 4000 up from 3300 but port still stuck at 3000. Port is actually performing worse than before tune up! How could this be?! Also, when I lowered speed to around 5 knots, the port engine would lose all power until I increased thrust. This is new for this engine. A buddy recommended Sea Foam to add to the gas to clean the fuel injectors. Any thoughts?
https://www.amazon.com/dp/B001DKQOM...&ref_=pd_luc_rh_crh_rh_sbs_sem_03_01_t_img_lh
 
Magic elixers rarely work, but it's cheaper than pulling the plenum and cleaning or replacing the injectors unless you are doing all the work yourself. If time is on your side, try easy cheap stuff first, but keep your eye on what's happening. With mine, it has been definitely more than one issue.
 
If you have the electric fuel solenoids try flipping them to bypass. Had a similar problem on my port engine. Couldnt get above 3300-3400 rpm. Turned solenoid to bypass and was able to reach my normal range.
 
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