Newbie looking for advice

[lucabrazi]

Just bought my first boat- small one with a 70hp motor. Motor was compression tested and runs strong. Based on the size of this little boat with a 70hp motor, I think it should FLY - but it doesn't: Why?

Two symptoms I'm hoping you'all can help me diagnose. First, it takes FOREVER to plane. If anyone else is in the boat they have to move forward in the boat to get it on plane. Second issue, once it does plane, the motor runs stronger when I give it only about 90% throttle. When I give it full throttle the engine loses RPM's slightly.

Attached a picture if that helps. Any help for this first-timer would be much appreciated!

 

[82rude]

Re: Newbie looking for advice

what rpm,s are you running at wide open?two things come to mind .you might have a spun hub or youre over propped.for example you have a 19p prop but should be running a 17 or even maybe a 15 .first you need to know the rpms at wide open throttle (wot).look on the prop it should say something like 14 x 17p for example.

 

[82rude]

Re: Newbie looking for advice

nice looking rig it should get up and go with a healthy 70 on it .arnt you a little small to be driving a boat?:lol:

 

[roscoe]

Re: Newbie looking for advice

Nice looking little boat.

The little guy in the boat thinks it IS flying, and its still on the trailer.

Ok, you have a long shaft motor, 20" long, see arrow in photo.





But your transom is only 15" tall.
Creates a bit of a mismatch.

To try to fix that, they added a jack plate to the transom, to raise the motor up. see photo.

It is hard to tell for sure, but the motor still looks too low to me.

This will cause planing and handling issues.

The motor should be lowered (tilted) all the way down, as close to vertical as possible.
This will give you the quickest planing.


Now, with the boat on the trailer, lower the motor down.
The large flat horizontal plate, just above the propeller, (anti-ventilation plate), should line up with the bottom of the hull, or be an inch above the bottom of the hull. Take a photo from the side, with the motor lowered down. Post it here so we can get a proper look at it.

See the lower red line in the diagram below.

The 3/4 throttle vs full throttle issue, sounds like a throttle cable / carb adjustment. It should not be done unless you have a repair manual in your hand, as you can easily adjust the fuel mixture too lean, or throw the timing off, if you adjust the wrong thing.

One thing you can check: With the motor off, push the throttle handle forward half way. Now remove the motor top cowl. and look at the carb butterfly, have someone continue pushing throttle forward as you watch the butterfly. The butterfly should move to straight horizontal at full throttle. If it is not going horizontal, or goes past horizontal, you have a problem.

Your motor may also only be firing on 2 or the 3 cylinders. A spark check should be performed.
And a compression check too while you are at it.

That looks like an awful lot of motor for that boat, be careful.

 

[Frank Acampora]

Re: Newbie looking for advice



Well, you have a small boat and a big engine. Contrary to what you might think, adding more horsepower to a small boat does not necessarily make it faster. You need to account for the extra weight of the larger engine--this is where some of your problem in planing out is. Here is a small hull now with a 125. ORIGINALLY I equipped it with an 85 and it was a DOG. Wouldn't get more than 38 MPH top speed. It took seasons of experimentation to get it up over 40 with the 85. A much lighter 60 that I had would run it to 45 though. So, engine weight and engine type does make a difference.

Your boat looks like a round bottom hard chine hull. Usually they are pretty good with handling horsepower but again, you have a BIG chunk of aluminum back there. You can try a cheap set of Smart Tabs and see if they help planning any.

NOW: In addition to what was said above, if you lose power at over 3/4 throttle, it could be fuel starvation. You need to check that all hoses and filters are not partially clogged. You need to check the tank strainers and fuel line quick-connects to be certain that these are not partially clogged. AND--Checking the carbs to see that the floats are not hanging up and the inlet needles are not sticking partially closed.

 

[lucabrazi]

Re: Newbie looking for advice



Thanks for the tips and advice everyone. Here's two more pics that give a better profile look at my setup. The anti-ventilation plate is right AT or slightly above the bottom of the boat- so it doesn't seem that is the issue. But if anyone sees something different let me know. Given that the boat jumps up on plane when I shift weight to the front of the boat makes me think that this IS about the heavy engine in back. I'll check spark and fuel delivery and see how that improves performance.

 

[JimS123]

Re: Newbie looking for advice

You got more than 1 problem...

The motor looks too low to me (but I'm only going by the pics I see). Need to view with a yardstick.
With that much HP and no plane, undoubtedly the prop pitch is wrong.
The cavitation (antivent) plate is damaged - don't know how that effects planing. (The skeg is also broken off)
Sure sounds like a fuel issue as well.
The motor is too big for the boat. I would be concerned about safety.

 

[lucabrazi]

Re: Newbie looking for advice

JimS123- Admittedly I am new to all of this: Can you explain what makes you concerned about safety? Again, my first boat so thanks for educating me. I'll look up what a "skeg" is and why mine is broken off!

 

[roscoe]

Re: Newbie looking for advice

Motor is definitely too low. Probably 2-3 inches too low. see photo

I would run an ad and try to trade that motor for a short shaft motor, 40 hp would be perfect.





Seriously, think about trading for a smaller short shaft motor.
It is a safety issue in that it is too much power, too much weight, poor handling.
That boat was not designed to run a motor like that.

It will solve several problems, including the probability that you are over powered, could be a legal issue depending where you are located.

 

[bonz_d]

Re: Newbie looking for advice

Could be mistaken but by the 1st photo in post 6 it looks to me as that motor is already attached to a jackplate.

Too much weight in the stern and not enough lift by the hull.

Want to see small boat, big motor, go fast?

mirage tunnel gopro video 101MPH - YouTube

 

[lucabrazi]

Re: Newbie looking for advice

Roscoe- Thanks for your detailed help.

Looks like I need to swap the motor or the boat. I don't need anything fancy- something to pull the kids in a tube and get a fishing line wet. As you can see this boat is pretty small. Given that I think I have a solid running motor- is upgrading to a slightly bigger boat that will be a better fit for this motor a bad idea?

Thanks

 

[bonz_d]

Re: Newbie looking for advice

Bigger boat is never a bad idea. Just watch what you buy. Don't want to pick up someone else's headache either.

 

[Solittle]

Re: Newbie looking for advice

Roscoe- Thanks for your detailed help.

Looks like I need to swap the motor or the boat. I don't need anything fancy- something to pull the kids in a tube and get a fishing line wet. As you can see this boat is pretty small. Given that I think I have a solid running motor- is upgrading to a slightly bigger boat that will be a better fit for this motor a bad idea?

Thanks

I wouldn't swap anything until you have what you got running properly. You have good advice here so far - follow it. My best guess is either you are not running on three or throttle cable issue. Motor sits to low and you may not have the right prop but neither of those fit the major symptoms you describe. That rig should jump out of the water and scream at WOT.

 

[lucabrazi]

Re: Newbie looking for advice

Thanks to everyone. I am familiar with how to check for spark and cylinder function on auto engines- anything different I should be aware of on an marine outboard? Put the motor in a trashcan and fire it up?

Thanks, The Newbie