boat wandering at slow speeds/lack of plane with 4.3 Merc

[bashr52]

I just upgraded form a 15' trihull with an O/B to a 21' center console with a 4.3 I/O. At low speeds the boat wants to wander, you can't hold it in a straight line. I'm constantly correcting left or right to keep the nose from drifting one way or the other. This is my first experience with a bigger boat and an I/O, is that normal? Once its got some power and the speed is up, it seems to be fine.

Also, with just me on board I cannot get it to plane at full throttle. I'm assuming there is to much weight toward the back with the I/O back there and empty storage up front. I can only get about 4000 RPM out of it, but with some weight up front and on plane the RPM will rise to 4300 or so. I had a 17 pitch prop on the boat when I got it, but during the test drive with 3 guys in the back it wouldn't plane, so the marina dropped it to a 15 pitch to try and get some RPM up. It did do a little better if I went to the front to get the nose down after that. It is the 2 barrel version of the 4.3, I think it may just be on the small side for a boat this size?

 

[oldjeep]

Bow wander is normal, stop steering it back and forth. Small adjustments and then wait a few seconds.

Sounds like you have too much prop if you can only hit 4300 RPM at WOT

 

[jkust]

Yep, bow wander is standard. Could be any number of issues from extra weight from water infiltrating the wood and foam, low compression, less than clean hull...lots of things. What is the make, model and year of the boat?

 

[Alumarine]

You might have a tired engine or one that needs a tune up. Or a lot of water in the boat.
A 15 pitch prop should do better than that.

 

[Assassin3F]

Target RPM for most boats is between 4200 and 4600 RPM. Before doing anything you need to find out what your wide open throttle range is supposed to be for that engine to be sure you have a problem.

A couple of items; Seems basic but make sure your trim is all the way down. Some times the gauge is showing its way down and it isn't, take it to the stops then back off a slight amount. Make sure the prop isn't damaged...if your trailing the boat to the lake then its obvious but if it is docked in the water you gotta make sure its right.

Get a copy of the propeller guide from your drive's manufacturer (Brochure). If its an older boat there is a sticky here in the adults only section that can get you access to a number of out of publication manuals and brochures. The issue could be the diameter of the prop or the dia/pitch/material combination. Most mechanics will just replace like for like when it comes to dia...but a significant chunk of owners will try to play with propeller diameter to improve a hole shot or some other reason that makes perfect sense at the time. It is an easy thing to check if you have the guide/brochure and can save you time and money in the long run if the problem is the prop dia. Just cause it fits does not make it correct for the engine.

Other item if you are trying to diagnose water in the boat etc. is weigh the trailer with the boat on it then weigh the trailer empty you can figure the boat weight at that point most boats will have a dry weight in their manual/brochure for you to compare it to. It'll give you at least some idea if that is the problem.

The above is the easy stuff now you start getting into more complicated territory. Check your Tach gauge and confirm it is reading correctly...I usually don't trust dash board gauges for something like that simply cause I feel like they are not specific enough for my tastes. They are fine to flag a potential problem but for diagnosing the boat I want to know.

Assuming it is correct you start going over the engine. I would probably start by removing the flame arrestor with the boat off and pushing the throttle handle at the operators location all the way to the forward stop. Look into the carb and make sure the lower plate is wide open. If it isn't you need to adjust the throttle cable to make sure it is cracked wide open. (Note you will need to open the choke plate with your finger to inspect the throttle plate and I am assuming you have a mechanical throttle of some sort.)

At that point you probably need to get into some deeper stuff. That would depend on the engine. As a for example I had a hard starting engine but general maintenance had been neglected. I replaced the impeller for the crankshaft mounted water pump and found an impeller which was too large for the pump housing and mis-installed. Swapped it for an OEM impeller from Volvo and since the switch it starts first time every time RPM range also jumped about 300 RPM. If the general maintenance condition is good you should plan on doing a full tune up and possibly compression testing the engine at that point.

All of the above is basically hoping for a simple fix before worrying about a big one.

 

[wahlejim]

Your Wide Open Throttle should be 4600rpm, verified with a shop tach, not just your tach on the boat. First step I would take is to get your boat wieghed to eliminate water logging issues. Water trapped in your foam or your bilge would definitely cause issues. Other than that, the mercruiser forum is your next stop. They will tell you to go through this list first:

http://forums.iboats.com/forum/engi...6-list-of-possible-causes-of-low-wot?t=295070

Do this if the weight checks out. Be ready with test results if nothing in this list fixes the issue.

 

[bashr52]

The deepest section of my lake isn't large enough to get the boat on plane and let it stabilize to see where the RPM's fall once its up to speed. The rest of the lake is rather shallow, like 5-6 feet deep and the weed growth this year is so bad I won't run it much above idle to avoid wrapping weeds around the lower unit and plugging up the water intake. The top of the flame arrestor is labeled Max RPM range is something like 42-4400 RPM or something. The prop is a brand new 15x15.25. I have the 17 they took off as well to play with. I checked out the transom yesterday to rule that out, and the design is such is that it's solid fiberglass everywhere except for a section down the center where the outdrive is mounted. No way to check for foam intrusion unfortunately. I will be doing a full tune-up before I store it for the winter, as this being a new boat to me I need to validate the condition. The engine is a new jasper rebuild, installed 3-4 years ago. I have all the old maintenance records form the Marina where it was kept/serviced, I will look into them and see when the last time things were done.

 

[Alumarine]

I thought WOT for the 4.3's was 4400-4800.

 

[Philster]

Bow wander: If you leave it alone, it'll wander left/right and pretty much go straight, so that it's more of a wiggle and won't really go off course.

 

[Starcraft5834]

21 ft. 4.3 only 4K, difficulty getting on plane.. 4 blade 15 pitch is my recommendation.. a little wander normal.... just when you think you need to correct, dont touch the wheel,, you will probably straighten out or start going the other direction......

 

[JoLin]

I thought WOT for the 4.3's was 4400-4800.

It is. I think here's something going on with the OP's boat besides prop pitch. With a 15 pitch, he should be at or even above 4800 at WOT. Compression test, timing check and tune-up would be my starting point.

My .02

 

[jkust]

Op never responded to the question asking make and model of engine other than to say 4.3 and 2bbl carb. Is it a post 1995 or earlier? The wot range is different depending on which year it is. I am presuming it is and so he should be at the top of the wot range mentioned above of 4800rpms and with these low pitch props, he should be blasting past that number to the point where the Rev limiter should be kicking in.

 

[Starcraft5834]

Op never responded to the question asking make and model of engine other than to say 4.3 and 2bbl carb. Is it a post 1995 or earlier? The wot range is different depending on which year it is. I am presuming it is and so he should be at the top of the wot range mentioned above of 4800rpms and with these low pitch props, he should be blasting past that number to the point where the Rev limiter should be kicking in.

ya, unless he's water logged............21 footer with 4.3, pretty heavy boat...

 

[jkust]

ya, unless he's water logged............21 footer with 4.3, pretty heavy boat...

Yeah especially if it isn't even an MPI with the extra ponies.

 

[lineman09]

ya going from a tri hull to a v hull is a little weird to get use to at first .. I went from a glastron 156 to a 18ft v hull ....

 

[rs1lollie]

you never mentioned trimming the outdrive to the sweet spot when WOT, mine gains speed and rpms trimming up....

 

[bashr52]

No Title

The boat is a 1994. Engine was replaced with a Jasper rebuild due to a crack in the block, and I believe was replaced with a 4.3 of the same vintage. With just me on board tach hits 4k at WOT and with the outdrive trimmed full down, the bow comes straight up, but will not plane. It sure makes a ton of noise but doesn't go anywhere. With one person up front to hold the bow down, it will come up, hold for a second or so, then flatten out on plane and the speed and RPM's immediately start to come up. I can't say for sure what RPM it will finally peak out at on the tach as I'm running out of deep water at that point and need to throttle back.

When I water tested the boat before purchase there were 3 guys in the back, and it wouldn't plane with the 17 pitch, even if I moved up front. We switched it to a 15, found a bunch of water in the livewell that was pumped out, and it got up on plane by just moving one guy halfway to the front, and could be throttled back at that point and stay on plane as RPM's were climbing.

The previous owner always fished by himself, so the mechanic at the marina that was showing me the boat thought he could get away with the 17 pitch since he was lightly loaded.

Here's a picture: