No, the first season I owned the boat, it had the original Champion plugs and it ran in the normal 4400-4800 RPM range. The boat was previously owned by another gentleman who also kept the boat in freshwater year-round.
Incorrect spark plug gap cause loss of RPMs at WOT?
- Thread starter impy03
- Start date
No, the first season I owned the boat, it had the original Champion plugs and it ran in the normal 4400-4800 RPM range. The boat was previously owned by another gentleman who also kept the boat in freshwater year-round.
370Dancer
Well-Known Member
- Oct 2, 2006
- 2,489
- Boat Info
- 1998 370 Sundancer
- Engines
- 380hp MAG MPI Gen VI with V drives
I heard you say both engines are not making RPMs. Move away from the engines and to things that are common to both engines, like, say the bottom growth.
Good suggestion. Unfortunately, I'll have to wait another couple of months before I can jump in the water with some snorkel gear to check it out as water temps are only in the 50s and I don't have a trailer for the big gal.
Update: I cleaned the flame arrestors (both extremely dirty) and replaced my props with some new aluminum OEM spec 3 blade 14x19 and ran the boat this past weekend. No change in RPMs at WOT, still idles and runs well but only achieves 3600-3800 revs for both engines. One thing I noticed is that my electronic choke is wide open at a cold start for both carbs, so I am assuming the chokes are both shot and need to be replaced or adjusted.
At this point, I'm almost leaning towards biting the bullet and buying (2) reman carbs (with a 6-month warranty) with the electronic choke to throw on both engines to see if that does indeed resolve my issues with hard start and low RPMs at WOT.
At this point, I'm almost leaning towards biting the bullet and buying (2) reman carbs (with a 6-month warranty) with the electronic choke to throw on both engines to see if that does indeed resolve my issues with hard start and low RPMs at WOT.
scoflaw
Well-Known Member
- Aug 10, 2011
- 6,395
- Boat Info
- 1999 Powerquest legend 260 sx 502 mag
2005 Baja 242 islander 496
- Engines
- 502 mpi Bravo 1
496 mag B1
Not the problem with the WOT.
The carbs or the issue with the electronic choke?
scoflaw
Well-Known Member
- Aug 10, 2011
- 6,395
- Boat Info
- 1999 Powerquest legend 260 sx 502 mag
2005 Baja 242 islander 496
- Engines
- 502 mpi Bravo 1
496 mag B1
if your chokes are stuck open that won't effect anything except cold start. At wot all your fuel is flowing thru the main jets and doubtful they would be plugged up
10-4
After some further research, I've seen where faulty ignition sensors can start causing strange problems so I think I'm going to go ahead and replace those as well since they are relatively cheap. Hard for me to believe they would both be going bad at the same time though.
Espos4
Well-Known Member
Read post #22 again.
If the water is too cold, Hire a diver and have the bottom checked and cleaned if necessary.
You mentioned a compression test in post #10 but no results. You say you’re not a mechanic so I would recommend performing a proper compression test and posting the results here.
Last edited:
BillK2632
Well-Known Member
I think you need to check and eliminate two basic things:
Timing - you have changed plugs wires, but did you check / set the timing and make sure it is advancing? Do you have Thunderbolt IV or V ignition? If so, make sure it did not get left in base timing mode. Still think it is unlikely timing is off on both motors, but it could be and is easy to check/set.
Fuel - The one common thing these motors have is the fuel supply and that will definitely cause problems at higher RPMs - could be something as simple as a faulty anti-siphon valve or a loose fuel line connection sucking air. On a boat this old, all of the fuel lines would be suspect and should have been replaced by now. Plump up a fuel line and run it on a portable tank.
I would do both of these before you start replacing more parts.
Timing - you have changed plugs wires, but did you check / set the timing and make sure it is advancing? Do you have Thunderbolt IV or V ignition? If so, make sure it did not get left in base timing mode. Still think it is unlikely timing is off on both motors, but it could be and is easy to check/set.
Fuel - The one common thing these motors have is the fuel supply and that will definitely cause problems at higher RPMs - could be something as simple as a faulty anti-siphon valve or a loose fuel line connection sucking air. On a boat this old, all of the fuel lines would be suspect and should have been replaced by now. Plump up a fuel line and run it on a portable tank.
I would do both of these before you start replacing more parts.
joeyleggz
Well-Known Member
- Jan 15, 2013
- 878
- Boat Info
- current boat: 1997 sea ray 330 express cruiser
- Engines
- twin 454's efi
How about something as simple as checking are you getting wide open throttle on your linkage ? why don’t you put the hammer down all the way and go check the carburetor linkage and see if there is any more travel in it?
I have the TB IV ignition, so I will still need to check the timing and also verify it is not set in base timing mode.
As far as fuel goes, I disconnected all the fuel lines added teflon tape to all threaded connections and re-torqed. I then drained and replaced the fuel/water separator filters and verified that I had no water in my fuel. I then pulled the anti-siphon valves and to my surprise, they do not have the ball valves/spring assemblies which I am assuming a previous owner deleted. I made sure both pickups were not clogged as well. I would like to replace my fuel lines as they do show age, but they are proprietary and use male/female threaded connections from the pick-up tube to the filters and then to the engine itself. I'll need to do some research on the best way to change these out or if I can pick up matching replacements.
Me and a friend did check the linkage a few weeks back on both carbs, and neither had any play or additional travel in it.
On another note, I keep researching and going down this rabbit hole of potential issues and found some information regarding tachometers. Could it be possible that one or both of my tachs are failing causing the engines not to advance timing under load? The tachs work fine at idle but I have been noticing that sometimes they will "float" around when running and if I hit rough water or tap on them, they typically return to what I think is the correct RPM reading.
scoflaw
Well-Known Member
- Aug 10, 2011
- 6,395
- Boat Info
- 1999 Powerquest legend 260 sx 502 mag
2005 Baja 242 islander 496
- Engines
- 502 mpi Bravo 1
496 mag B1
There is no base timing lock on a TB4. Idle it down to about 650rpm and point your light at it.
Your tachs do not control the engine. However, a flaky tach can sometimes be fixed by exercising the cylinder selector switch on the back side.
Get rid of the teflon tape and use paste. Even if you used fuel-rated tape (or did you use the white stuff?), get rid of it. Pieces can break off.
If I may be blunt for a minute, it sounds like you're doing A LOT of internet searching trying to find a solution without much base knowledge on this subject (for example, the question about your tachs). The issue could be something with both engines, but based on the info so far, the most likely culprit is still a dirty bottom. Everything else you are doing - including all the time spent researching - might be all for not. Pay for a diver or a haul out or buy a wetsuit
And... replace the AS valves with properly operating ones. This is a huge safety issue.
Get rid of the teflon tape and use paste. Even if you used fuel-rated tape (or did you use the white stuff?), get rid of it. Pieces can break off.
If I may be blunt for a minute, it sounds like you're doing A LOT of internet searching trying to find a solution without much base knowledge on this subject (for example, the question about your tachs). The issue could be something with both engines, but based on the info so far, the most likely culprit is still a dirty bottom. Everything else you are doing - including all the time spent researching - might be all for not. Pay for a diver or a haul out or buy a wetsuit
And... replace the AS valves with properly operating ones. This is a huge safety issue.
BillK2632
Well-Known Member
I would be looking at that fuel line - I started having running issues on my boat (1999) and futz will a lot of different things like you. My issue was similar to yours except I had a surging issue at high RPM. I finally found two issues:
1. Anti-siphon valve was corroded and sticking - replaced that $10 item.
2. Fuel feed line from the tank, even though it was A1 rated for alcohol, it was deteriating from the inside and would collapse due to the suction at higher RPMs. When I took the old line off and split it long ways, the inside was just crumbling even though it looked fine on the outside. It was then that I realized what the little bits in my fuel filter that looked like pepper were. Also, make sure you clean out the small fuel inlet filter where the hard fuel line connects to the carburetor.
Replaced both and my problems were solved. I which I had taken the time to run the engine on a portable tank, it would have saved me several months of screwing around with all the stuff you are looking at.
Also, regarding teflon tape, be careful with that on fuel line fittings, it just takes the smallest piece of that to get into the inside of the line and cause a problem - I personally don't use it and when properly tightened I have never had a leaking issue.
All this plus what Dennis said - he posted while I was writing this. Do the cheap and easy things first. Cleaning bottom and running from a portable fuel can are cheap and easy!
1. Anti-siphon valve was corroded and sticking - replaced that $10 item.
2. Fuel feed line from the tank, even though it was A1 rated for alcohol, it was deteriating from the inside and would collapse due to the suction at higher RPMs. When I took the old line off and split it long ways, the inside was just crumbling even though it looked fine on the outside. It was then that I realized what the little bits in my fuel filter that looked like pepper were. Also, make sure you clean out the small fuel inlet filter where the hard fuel line connects to the carburetor.
Replaced both and my problems were solved. I which I had taken the time to run the engine on a portable tank, it would have saved me several months of screwing around with all the stuff you are looking at.
Also, regarding teflon tape, be careful with that on fuel line fittings, it just takes the smallest piece of that to get into the inside of the line and cause a problem - I personally don't use it and when properly tightened I have never had a leaking issue.
All this plus what Dennis said - he posted while I was writing this. Do the cheap and easy things first. Cleaning bottom and running from a portable fuel can are cheap and easy!
Just to conserve writing a book, I'll say I appreciate all the help to everyone and I'll just plan to stick with checking the hull, fuel lines, anti-syphon vales, and doing another compression check. I'll report back once all is tested. Thanks again!
BillK2632
Well-Known Member
I think that is a good plan at this point - if it doesn't fix it then at least you have ruled out some things.
While I know it is frustrating, all of these experiments do provide some value in establishing a baseline moving forward. Make sure you document all this so you know what was checked / replaced, and when.
Been a while since an update, but it has been determined that the harmonic balancer is failing on one engine, which I would assume would cause some timing issues. I have a new one on order and will update again once I get it installed and get the engine timed correctly. I also went ahead and replaced fuel lines and anti-siphon vales just to alleviate any potential issues with the fuel system. Fingers crossed this will resolve my RPM issues.
SCORPIO
Well-Known Member
Do you have zebra muscles in you area? If so, they are worse than barnacles. Grass, muscles etc will all cause issues. Start with a clean bottom and see what you get. The fact both motors are doing the same thing leads me to believe it's something common, like a fouled bottom.
