140hp johnson wont go fast enough to plane

keithy

Cadet
Joined
Mar 2, 2015
Messages
17
Hi, I am a new baot owner. My new to me boat is a 91 hydrasports with a 140hp johnson outboard. Took it out for its first test run and the engine idles a little rough but cranks right up. When running it sounds good but will no go faster than 8 mph. When i throttle up the rpms seem to go up but the boat will not go faster. I believe it feels like it is bogging down. The engine will never die out though. What could be the issue? Take it easy on me as this is my first boat and am only partially clued in.
 

vasy

Petty Officer 2nd Class
Joined
Dec 14, 2014
Messages
193
Do you know how many RPM does it rev up to?
Prop size?
 

keithy

Cadet
Joined
Mar 2, 2015
Messages
17
It runs fine until about 3000rpm and continues to up normally after that except no extra speed. I did not continually run the engine in the higher rpm for long because of the vibrating and sound of it.

I do not know the prop size, I can not find markings on it anywhere. it has quite a few minor dings on the blade ends though. I will look again today when I get home.
 
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Willyclay

Captain
Joined
Sep 8, 2006
Messages
3,239
Welcome to the iboats.com forum! The members here are great and can help you solve any problems with your boat, motor and trailer. IMHO it could be any of several issues such as a spun prop hub, clogged high-speed carburetor jets or running on less than all cylinders. Some basic trouble-shooting will be necessary but I will let one of the forum experts post in with advice on how to proceed. One thing you might post that may help other members trying to understand your problem is what RPM's are you seeing at full throttle? If no tach, that may also be an issue for those trying to help you. Good luck!
 

Bosunsmate

Admiral
Joined
Apr 7, 2012
Messages
6,129
To test if its a spun prop hub. Mark the outside metal casing and make it a straight line with paint to the inner rubber hub. If its a spun hub then when you are out on the water and giving it some power then the inner rubber bushing will move but the outer prop wont meaning the line wont line up anymore.

The rough idling needs to be tracked down, is its spluttering or backfiring?

You should do a compression test too
http://forums.iboats.com/forum/engi...ion-test-by-jb?t=313963&highlight=compression

A basic tester is around $10
 

keithy

Cadet
Joined
Mar 2, 2015
Messages
17
Not backfiring, it sounds more like random missing or possibly sputtering. I read that a spun hub would make the rpm race up quickly, this seems to go up at a normal running speed.

What I plan to do tonight is put some of the gas in a jar and check if it settles, grind the prop blade edges, replace the spark plugs, and I will add a compression test to the list, even though the dealer said its good.

I will also add that this boat looks well maintained, and I will wager that everything on it is most likely oem or oem replacement. I am mechanically inclined, but know nothing of outboard engines.
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Do a cylinder drop test on it to see if it's running on all the cylinders.
 

keithy

Cadet
Joined
Mar 2, 2015
Messages
17
Compression: 136 129 131 129. Looks good.
Plugs have slight glossy black between electrodes but not much.
Prop hub looked okay, but i did the kick test to check. Looks good.

I couldn't get a hose down to the bottom of the fuel tank, maybe I need a smaller hose? Instead I pumped it out the fuel line to the motor. After 20 minutes there is what looks to be fine dirt at the bottom of the jar.

I can do the drop cylinder test tomorrow. What else should I look for?

I'll get new plugs before I put it on the water Friday.
 

keithy

Cadet
Joined
Mar 2, 2015
Messages
17
I did something simple. Its dark out so while cranking after the compression test I looked for spark. 3 of the four ignition coils have spark under the boot. I'm going out on a limb to say the fourth one is bad.
 

V153

Lieutenant Commander
Joined
Apr 16, 2011
Messages
1,764
Sounds more like a spun prop to me. At 3000 rpm and climbing that boat oughta be doin a lil more than 8 mph. It oughta be at least tryin to get on plane. How trashed is the prop? Any idea of pitch or diameter? Can you post a pic of your set-up?
 

crazy charlie

Vice Admiral
Joined
May 22, 2003
Messages
5,352
It may be something as simple as the incorrect prop that is cavitating....loosing bite on the water..will sound kind of normal but a little bit louder.Are we also assuming that the boat bottom is clean of any barnacles and growth?? Charlie
 

V153

Lieutenant Commander
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Apr 16, 2011
Messages
1,764
Did you remember to unhook the trailer? Heh heh heh Sorry ...
 

keithy

Cadet
Joined
Mar 2, 2015
Messages
17
No Title

17.6ft deep v hull. Coil should come in today. The hull is very clean, excellent shape and still has gelcoat everywhere. How do I tell what the prop size is?
 

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keithy

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Mar 2, 2015
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This is a 91 hydrasports 1750 DC, does anyone know the fuel tank capacity and how long I can expect to run?


Found it to be 42 gallons
 

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Bosunsmate

Admiral
Joined
Apr 7, 2012
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17.6ft deep v hull. Coil should come in today. The hull is very clean, excellent shape and still has gelcoat everywhere. How do I tell what the prop size is?


Prop will/should have a number engraved on its side somewhere.
Post another photo horizontal from straight behind the anti cavitation plate of the motor and getting the boat in line, hard to tell in that photo but your motor may be sitting too high and cavitating as the boat lifts up
 

keithy

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Joined
Mar 2, 2015
Messages
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No Title

There is nothing I can find engraved on the prop. Is there a tool I can use to measure?

I replaced the coil, I used an inline spark tester afterwards on all plugs. All four now have spark. I put everything back together and tomorrow I will run it on the water.
 

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racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
36,030
If it is a factory prop there will be numbers on it !-----What colours / stripes are on each top coil wire ?-----That is the orange coil wire.
 
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