Low RPMs at WOT

[tia34698]

Boat info: 23' wellcraft openbow, 5.7 mag mpi (300 hp) bravo one outdrive, 18 pitch prop.


Engine starts and runs. Sounds great cant hear any problems at idle. When boat is as full throttle only getting 2800-3000 rpms? Engine still sounds perfect at full throttle cant tell anyhting is wromg other than low rpm's and low speed??? Any suggestions on where to start.

Thanks,

Chris

 

[HT32BSX115]

Re: Low RPMs at WOT

Howdy,

Yes.

Right here: List of possible causes of low WOT (Wide Open Throttle) rpm.


Cheers,

Rick

btw, that's one of the items in the (infamous) "Adults Only Sticky" at the top of this forum.......

 

[tia34698]

Re: Low RPMs at WOT

Update:

I was checking to make sure that all sparkpug wires where connected, and one of the plugs broke in half....right in my hand? Could this be the problem?

Chris

 

[Bondo]

Re: Low RPMs at WOT

Engine still sounds perfect at full throttle cant tell anyhting is wromg other than I was checking to make sure that all sparkpug wires where connected, and one of the plugs broke in half....right in my hand?

Ayuh,....Hard to believe it's runnin' Perfectly on 7 cylinders....

When was the boat's last Tune Up,+ Service done,..??

 

[tia34698]

Re: Low RPMs at WOT

Sounds perfect....no weird noises? What would the symptoms be if it was running on 7 cylinders?

Chris

 

[HT32BSX115]

Re: Low RPMs at WOT

Update:

I was checking to make sure that all sparkpug wires where connected, and one of the plugs broke in half....right in my hand? Could this be the problem?

Chris

That could easily be the problem. It could cause an intermittent or continuous misfire which would lower WOT RPM a little. What was your WOT RPM before?

Sometimes they sound "alright" on 7 cyls

If you haven't replaced the dist cap, rotor, wires, plugs etc in a few years, now would be a pretty good time to do it.....

 

[tia34698]

Re: Low RPMs at WOT

Has been runnning at 2800-3000 WOT rpm since I bought it. Runs ok with just 4 people, but loaded it with 8 people and wouldn't plane....So that is when I started looking for problems. Boat is in my driveway now, headed to the store to get rotor, cap, plugs and wires. Will put those in, and update after this weekend.

*****If anyone has any other suggestions, please keep the comments coming!!!

Appreciate the help.

Chris

 

[HT32BSX115]

Re: Low RPMs at WOT

Well,

you have more than one problem going on there.


You may even have a defective TACH and it is in fact turning the correct RPM......

Otherwise,

2800-3000 is FAR TOO LOW for that engine and you need to get it fixed before you damage it further!

It should turn somewhere around 4800-5000 RPM (Look at your Mercruiser owners manual....It should indicate the WOT range...somewhere around 4400-5000 for a 5.7L MPI engine)

 

[tia34698]

Re: Low RPMs at WOT

Is there a way to test the TACH? Any suggestions if still does the same thing after tune-up?

Thanks,
CC

 

[45Auto]

Re: Low RPMs at WOT

How fast is it going at WOT (GPS speed if possible. Boat speedos are notoriously inaccurate)? If your speed is in the mid to high 40's, you're probably just below 5000 RPM with an 18" prop.

 

[HT32BSX115]

Re: Low RPMs at WOT

Is there a way to test the TACH? Any suggestions if still does the same thing after tune-up?

Thanks,
CC

Well there's ways to verify if it's lying or not...

Using GPS as suggested will provide accurate speed. You should be able to determine your drive ratio. It's usually on the side of a Bravo on the serial number placard. (Probably 1.5:1 etc)

Using GPS speed and about 12-15% "slip" you can use a Prop-Slip calculator to see if you're in the "ballpark"

http://www.go-fast.com/Prop_Slip_Calculator.htm


You should however, probably get a known tach to check your WOT RPM. They can be off. If it is indeed off, you could chasing a "problem" that does not exist......

 

[wanna-be]

Re: Low RPMs at WOT

agree w/above...try another tach thats tried and true. you won't hear anything unless its under load and then with exhaust and wind its very difficult to hear misfire, pinging etc. you may want to do a compression test...sb done once a year anyway... could be valves aren't closing properly. also inspect spark plugs. if they're too dry and white youre over heating (detonating) and you don't want to burn out a piston or tuplip valves, been there and done that.

 

[tia34698]

Re: Low RPMs at WOT

If I remember right last time speedo said 35 at 2800-3000 rpm at WOT

 

[HT32BSX115]

Re: Low RPMs at WOT

There's not much more to say.

You must know speed and RPM accurately to diagnose any performance problem.

While you're getting that info together, if you have not done a normal tune up to include filters and all the ignition items (cap, rotor, plugs, wires), you should do that before you proceed.

 

[tia34698]

Re: Low RPMs at WOT

What is the electronice part that is located under the rotor? It has some corrosion on it, does it need to be replaced?

 

[HT32BSX115]

Re: Low RPMs at WOT

Difficult to say without knowing what you have.

Mercury uses serial number to identify parts/model etc.

Since it's a Bravo 5.7MPI, newer (2000+?) I suspect that you have a Thunderbolt "x" electronic ignition system.

If there's corrosion in there it may be part of the problem.....or not.

 

[tia34698]

Re: Low RPMs at WOT

**Update**

Changed sprk plugs, rotor, cap, fuel filter/seperator, cleaned flame arrestor, uncloged pitot hole for speedometer... Will put in the lake tomorrow after work, and se how it runs.......


Reading through manuals and it keeps referencing "High Tension Lead". And not to remove it unless necessary? Can someone explain.

Thanks,
Chris

 

[tia34698]

Re: Low RPMs at WOT

One other question?

Could a loose/stretched belt, bad alternator or a weak ignition coil cause a loss of power at WOT? Not giving enoug juice to the plugs?

 

[tia34698]

Re: Low RPMs at WOT

Any comments on last 2 questions? Loose belt, coil, alternator?

Thanks,

Chris

 

[Bondo]

Re: Low RPMs at WOT

Ayuh,... Probably Not....