# Need Major Help With J12 Build, will not plane 25hp Mercury EFI 4Stroke



## Gbeach407 (May 12, 2017)

Hello everyone I'm hoping some of you might have some thoughts and suggestions as to help me with my recent Carolina Skiff J12 build. I attached a few pictures below. I purchased the hull about a year ago and just recently bought a 2006 Mercury EFI 4 Stroke long shaft tiller off CL. With that said I mounted the seat and the engine myself. The issue I am having is I cannot get this boat to plane or get past about 10mph. The first time I took it out I was by myself and didn't have much time to test the boat so I launched at a local lake ramp and immediately was dangerously porpoising. I adjusted the manual trim pins all the way down but still was having the same issue. I realized that I didn't have enough weight in the front so packed it up for the evening and went home. Needless to say I just tested the boat for the second time yesterday and had similar results. The boat was not porpoising but the engine couldn't get the boat on plane even at full throttle.I had myself (185lbs), my girlfriend (125lbs), my dog (40lbs), gear (40lbs), and one 56lb sand bags in the very front storage. I figured with this 25hp Mercury that this skiff would be flying. Even loaded down yesterday, we where only able to go about 10mph and never get on full plane despite being at full throttle. I'm convinced there is something wrong. Either weight distribution, tilt/trim, or possibly engine not functioning at max RPM despite indicating max throttle on the tiller handle. Please let me know your thoughts... I will post a video later if possible.


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## Guest (Jul 15, 2018)

Hey Bud, that motor is very noticeably around 4-5 inches too low! Raise it up untill the cavitation plate is flush with the bottom of the hull using a straight edge and it’ll run!


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## yobata (Jul 14, 2015)

If the hull isn't water logged my first step would be to get a tachometer


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## Guest (Jul 15, 2018)

Your gonna need a jack plate from the looks of it. You can find manual plates on CL used for cheap if ya shop


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## Guest (Jul 15, 2018)

And like Yobata said, after you get the motor mounted at the correct height... get a tach!


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## nautilott (Oct 29, 2017)

It's a little hard to tell from the pics angles, but it looks like the motor's position is driving the transom up which would drive the bow down. Might try raising the trim angle up a couple of holes.


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## Vertigo (Jun 3, 2012)

Looks like you have a clamp on 20" shaft motor on a skiff designed for a 15" shaft. You need to find a way to raise the motor about 5". You're probably going to need a transom extender or a jack plate. Search for used on Craigslist. These things don't wear out.


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## Gbeach407 (May 12, 2017)

Hey guys I really appreciate all the feedback so quickly! Here is a little update and more specs on the boat. First off I'm pretty positive this particular J12 has a 20" transom. Also the engine is bolted on the transom. I posted some more pictures below. In addition I'm pretty confident the hull is not water logged. As far as your suggestions for a jack plate, I immediately was thinking along those lines once this issue started happening. I've also concluded I need to move the bench seat placement up about a foot or so. My only question is can 3" - 5" of motor height really bog down a 25hp 4stoke enough like this to not make it plane or hit maximum speed?

*Update: ... just took it out with another knowledgeable adult on board this afternoon on a calm lake. Still having same issues that have been reported. We tested all the manual trim position pin heights and determined the lowest setting (closest to the boat) was the best and allowed us to get on a semi-plane going roughly 10-13mph. With that said with this engine configuration, this boat should be hauling it. We seem to think the engine is running at lower rpm than indicated when on full throttle on the tiller handle, it even sounds that way when listening to it. (Unless this 4 stroke is really quite) Still we ran around for about an hour and half no major issues with the engine just refused to get us up to top speed despite being on full throttle. We decided to call it quits and took it home. After taking the cowling off we noticed some significant slack in the throttle cable connection points, we discovered that when putting the tiller handle on full throttle there was still about a half an inch of play to give. We decided to adjust the connection points and tighten the screw so the slack is now gone. Unfortunately, we did this at my house and have yet to test and see if this fixes anything. (In the event this fix is a failure or effects the engines idle/running, it can easily be adjusted back.) 


I still want to hear what you all think and value your comments so please keep them coming. Take a look at the pictures below. If you all think its still an issue with motor height, something more serious with the engine, possibly a new prop. I really just want to get this thing dialed in!

Thanks,
GBeach


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## Guest (Jul 15, 2018)

It is very clear in the pics that your motor is way too low! If you’ll hold a straight edge on the bottom of the hull, the cavitation plate should be level or slightly above the hull! If you don’t raise the motor to the correct height, you are just chasing your tail!


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## Guest (Jul 15, 2018)

Also, keep trying to run it that way and you’ll be rehubbing a prop!


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## predacious (Sep 1, 2016)

carolina skiffs have problems planning.it's a common thing with them...
balance is important - a 4 stroke merc,like that 25 you have isn't a light motor - looks like it's electric start - that means it's in the 200lb range.not sure where your battery is mounted,or the fuel - I see a bench/storage seat - it's close to the transom.with the weight of the motor and the person operating the boat - and again,depending on where the other mentioned things are,that's a considerable amount of weight in the aft.this causes,the boat to not want to plane .balance is important on a Carolina skiff,as well as any small boat.if you've the battery,and the fuel,in that storage seat - I suggest you move it further forward...I don't see any pics of the entire boat and how it's set up - so im not sure what's going on...

leaving those battery cables hanging and swinging around isn't the best approach...

believe it or not,carolina skiff likes the antiventialtion plates just about even with the bottom of the hull.

smart tabs are also highly recommended by Carolina skiff on their j series skiffs

25hp,is a lot of hp on a j12,lot of weight too - after you put the smart tabs on,the hull will plane easily ...


side note:

it's not recommended by Carolina skiff to drill holes and mount things to the interior deck.besides the deck being cored with foam,that means it won't hold fasteners - improper sealing techniques will cause problems with the hull getting waterlogged.it's best to laminate blocks - not wood blocks ! to the deck,when mounting equipment...


last side note:

slipping hubs - if the hub is bad,the boat will act as if it's in neutral - the load to push the boat overcomes what the hub can handle,and the prop blades stop rotating.i doubt your problem is a spun hub....


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## Guest (Jul 15, 2018)

predacious said:


> carolina skiffs have problems planning.it's a common thing with them...
> balance is important - a 4 stroke merc,like that 25 you have isn't a light motor - looks like it's electric start - that means it's in the 200lb range.not sure where your battery is mounted,or the fuel - I see a bench/storage seat - it's close to the transom.with the weight of the motor and the person operating the boat - and again,depending on where the other mentioned things are,that's a considerable amount of weight in the aft.this causes,the boat to not want to plane .balance is important on a Carolina skiff,as well as any small boat.if you've the battery,and the fuel,in that storage seat - I suggest you move it further forward...I don't see any pics of the entire boat and how it's set up - so im not sure what's going on...
> 
> leaving those battery cables hanging and swinging around isn't the best approach...
> ...


Motor is way too low “clearly visible in pics”
Will eventually spin a hub if continues to try to run wot with motor mounted at this height!
Engine height is most likely also causing the porpoising OP mentioned in first post. 
All JMHO!


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## MooreMiller (Sep 10, 2016)

Not the best method but has worked for me. I’ve beat the tar out of it and the mount is still very strong. Cheap too. 

I get 24-30 mph with a 30hp merc/hatsu/Nissan, depending on prop and load. I have a j14.

I really should fill in those bolt holes.


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## Gbeach407 (May 12, 2017)

Okay cool thanks for all the info fellas. After looking at the picture below where I highlighted the red area .. I think its clear I need to get the motor up a few inches. I'm going to buy a jack plate and probably go ahead and get the smart tabs as well just to be safe. As far as the engine I believe it weighs 175lbs. I'm going to try to avoid rigging the fuel up front but if it's a necessity for weight distribution then I will do it. Also going to remount the bench seat further forward to balance the weight out better, right now its way too far back. Planning on getting a lot of these things ordered,installed and tested soon so I'll keep everyone posted on the results!

Thanks


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## DuckNut (Apr 3, 2009)

Having your lower unit where it is won't affect you as much as described.

In one of the pictures showing the prop, it looks like the prop could be way too much pitch preventing the rpm's from getting where they need to be. 

You should have something near 9 1/4 X 10


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## Guest (Jul 17, 2018)

DN, I respectfully have to disagree with ya on this one. It will cause everything he described.


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## fishnpreacher (Jul 28, 2018)

DuckNut said:


> Having your lower unit where it is won't affect you as much as described.
> 
> In one of the pictures showing the prop, it looks like the prop could be way too much pitch preventing the rpm's from getting where they need to be.
> 
> You should have something near 9 1/4 X 10


That's the first thing I noticed. It looks like too much pitch for a 25hp.


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## Tiggerj1215merc4 (Jul 10, 2020)

Gbeach407 said:


> Hello everyone I'm hoping some of you might have some thoughts and suggestions as to help me with my recent Carolina Skiff J12 build. I attached a few pictures below. I purchased the hull about a year ago and just recently bought a 2006 Mercury EFI 4 Stroke long shaft tiller off CL. With that said I mounted the seat and the engine myself. The issue I am having is I cannot get this boat to plane or get past about 10mph. The first time I took it out I was by myself and didn't have much time to test the boat so I launched at a local lake ramp and immediately was dangerously porpoising. I adjusted the manual trim pins all the way down but still was having the same issue. I realized that I didn't have enough weight in the front so packed it up for the evening and went home. Needless to say I just tested the boat for the second time yesterday and had similar results. The boat was not porpoising but the engine couldn't get the boat on plane even at full throttle.I had myself (185lbs), my girlfriend (125lbs), my dog (40lbs), gear (40lbs), and one 56lb sand bags in the very front storage. I figured with this 25hp Mercury that this skiff would be flying. Even loaded down yesterday, we where only able to go about 10mph and never get on full plane despite being at full throttle. I'm convinced there is something wrong. Either weight distribution, tilt/trim, or possibly engine not functioning at max RPM despite indicating max throttle on the tiller handle. Please let me know your thoughts... I will post a video later if possible.


Try moving your bench up a foot or so. As far as you can move it and still use tiller arm comfortably. I have same boat with a 06 merc 15 4stroke n been having the same issue. By myself it flies with a light boat. Add my gf and a ice chest full of ice I'm like a barge. Maybe making 8-9 knots. If I make my gf walk to the front she lays down and might not run as fast but at least she flattens out


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## albuilt51 (Sep 30, 2016)

From experience:

1) *You have to be 100 percent sure your CS hull isn't waterlogged or any attempted performance upgrade will be minimally effective at best.* Weigh the bare hull and compare to new specs (230 lbs.). This can be done with one or two bathroom scales. A waterlogged hull can more than double the weight of the bare hull and perform as if it were made from concrete.* I always assume there is waterlogged foam under the deck of a used CS until proven otherwise.* Weighing a CS before purchasing is as important as doing a compression test on a used outboard motor. Repairs can far outweigh the value, most times even if picked up for free, as the only way to remove the waterlogged foam is to cut the deck out and physically remove it, (foam will NOT dry out, nor will it drain - regardless of how many holes you drill).

2) My J12 was a dog with a 9.9 Evinrude 2-stroke and just me aboard. Only way to get up on plane was to move as far forward as possible without letting go of the tiller. With two people on board - forget it. Such are the characteristics of most 12' boats. *Adding more HP adds more weight with little or no net gain.* It wasn't until I raised the motor enough that my anti cavitation plate was 1"-2" above the bottom of the hull AND added a SE Sport Hydrofoil, (Attwood or other brands will work as well), then performance was everything I could expect from such a short, wide, flat bottom boat. Up on plane in mere seconds, very controlled handling, remained on plane even at lower speeds. Having two people on board did not affect performance very much at all. A night and day difference!

Of course, making sure your prop hub hasn't spun, having the right pitch prop, and knowing your RPM's can fine tune your performance. Tachometers are available on Amazon for as little as $20 and installation takes only a few minutes.


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## Shallows (Mar 29, 2020)

This is a two year old, but... yeah gotta be low compression


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