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Loss of RPM at WOT
I have a 2005 F150 Yamaha 4 stroke. It is currently Propped 14 1/4 18. This motor runs very smooth and initially has great power at 5300 RPMs. But last on a clear day with not much wind current or waves, it will run at 5300 at WOT for a couple of minutes at 40 mph (pushing a 97 21 ft Kenner center console) and then fall slowly to 4400 or 4300 at WOT. If I stop the boat with engine at idle for a bit i can repeat this process. I want the motor to run at about 5800 RPM WOT and may need a 17 pitch prop to get there in the future, however after some research I am beginning to think that the HP fuel pump is not able to keep up a high RPM anymore. Maybe this fuel pump is in the early stages of going out and I am losing fuel pressure at WOT until the motor loses power in a very smooth manner at lower fuel pressures? If this is the case, then the motor is running lean while decreasing RPM to a rate that the Fuel pump can keep up with. I want any thoughts and opinions before I spend the money on a new fuel pump. There is no other detectable fuel problem and pressure loss has not been verified with a gauge while running under load.
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need to test fuel pressures and vacuum.
if there are restrictions, or air leaks preventing enough fuel to get to the injectors it should be easy to find -
My F150 WOT normally spins 5,900. My boat spec's are about the same as yours but will turn at WOT to about 46 MPH (per GPS and 35 gallons of fuel on board)
About a year ago, RPMs dropped to about 4,300 (full throttle)
It would do "IT", about 80% of the time.
I had the original spark plugs in it, (about 200 hours) and was
told by the local shop they needed replacement.
Put in a new set, issue GONE. I cannot repeat it if I wanted to.
BTW, the OLD plugs looked brand new, gaps were still in spec...Scott
1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXRComment
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Awesome thanks. I will do this first thing. I really hope that does the trick.Comment
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How old are the plugs I wonder?
Not to change the subject as it has nothing to do with his issue....anybody want to guess the first question an unfamiliar mechanic would ask about this motor?Comment
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I have only owned this Motor for 2 weeks so I don't know the age of anything.Comment
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Suggest a water pump replacement unless you can find out exactly when it was replaced last....
But hang in there. I think your current issue will not be to tough to diagnose and repair. Fuel pressures and plugs are good places to start. Btw, you should be able to do better than 5300 WOT in that boat. Betting the fix will take care of that too. Kenner is a bay boat, right?Last edited by pstephens46; 11-23-2016, 09:41 PM.Comment
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Yes sir it is a bay boat. Normal load. It has great power at 5300. I can only imagine what It will have bear 6k. Thanks KS again.Originally posted by pstephens46 View PostComment
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A bit off the topic of your thread, but worth advising, is after buying a used engine to inspect all vital system components. Most boaters do not keep an accurate maintanance date list. If the previous owner did not and you want a reliable engine, go through each system thoroughly. Don't know when last the carbs were clean, fuel filter changed, water pump replaced, plug gaps checked, greased components greased, etc.?? Then best breakdown/remove components to determine what's good and what needs repair/replace.Originally posted by crvbal49 View Post
You may be one who maintains a clean engine, but don't expect the previous owner to have done the same.Jason
1998 S115TLRW + 1976 Aquasport 170Comment
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After that running issue is eventually fixed(the guys here will get you going), you'll possibly need to re-prop.
When they re-powered mine in 07, it was improperly propped and WOT would just barely hit 5K. It was flat out a dog and I don't think it'd lift a skier out of the water(TERRIBLE).
Went to the prop shop with my aluminum and bought a SS letting them know the RPM's I had and what I needed.
COMPLETELY DIFFERENT ENGINE.. I'd put money it's darn near as fast out of the hole as my old Evinrude V6 150, a tad hair slower on the top end- definitely heavier...
As the engine is "NEW" to you, drain the VST and catch what comes out in a clean can. (starboard side, thin, maybe 6" long, no w yellow, plastic hose with a drain screw at the base.). You can squeeze the primer to flush out as well..
Please post back what you findScott
1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXRComment
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I replaced the spark plugs last night and tested it this morning. No change. Starts out at 5400 at 42 mph on a smooth river, works its way down to 4900 after 1 minute and continues to drop from there but stabilizes at 4400 at 33mphOriginally posted by TownsendsFJR1300 View PostComment
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I will do that this evening and post again Thanks.Originally posted by TownsendsFJR1300 View PostComment
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what your describing is typically either a lack of fuel in the VST due to air leaks,weak lift pumps or a restriction .
OR the VST prefilter is clogging OR the VST filter is clogged.
myself I would install a water pump kit,T-stat,pressure valve grommet then replace the fuel filters and remove and clean the VST.
water pump takes about 1.5 hrs.
make sure you remove the slinger under the impeller plate and inspect the seal garter springs.
the VST takes 1.5-2 hrs.
the VST filter,if not damaged,can be cleaned.
the VST gasket,if not damaged or swollen can be reused.
if the maint history is unknown this would be the first things on the to do list.Comment
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I am new to outboard motors, I know what the VST is and where it is but don't know what it does exactly and why air leaks would cause less fuel. Would an air leak cause fuel to leak? there are no signs of fuel leaking... I will go through the filters and the VST and VST screens later today and post back. Thanks for all of the comments. any informative links on VST and its function would be helpful.Comment
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if there are leaks on the suction side of lift fuel pump it will suck air into the system .
if not enough fuel gets to the VST then there is not enough fuel to keep the rail pressure up to spec.
VST stands for vapor separator tank.
it trys to keep the high pressure pump from trying to pump air or fumes instead of liquid fuelComment
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