1984 Johnson Wont Plane

[mrkrsll]

I have a 1984 15hp Johnson that I cant get to plane. I have it on a 12' fiberglass boat which weighs about 130 lbs. Im 215 and about 125 lbs in gear at the stern and the max speed i can get is 12 mph. The motor runs great. Dosent stall, no hesitation. Any ideas? Is there too much weight for this motor?

 

[ondarvr]

Move some stuff forward and see if it helps, but there can be many other reasons.

 

[mrkrsll]

Move some stuff forward and see if it helps, but there can be many other reasons.

Correction i ment 125lbs at the bow

 

[interalian]

Did it ever plane or is it new-to-you? Compression test, spark test would be my suggestion after you try weight re-distribution.

 

[mrkrsll]

Did it ever plane or is it new-to-you? Compression test, spark test would be my suggestion after you try weight re-distribution.

It's a new motor to me.

Thanks! I'll be checking both tomorrow.

 

[JDusza]

Also, bring your 125 pounds to mid-ship.
And, yes. Especially new to you, should be go over the basics; compression, spark and fuel.
Compression should be minimum 90 psi with both cylinders within 10%. This is a great little and proven engine.
Look at spark with an air gap spark tester. Look for spark to jump 3/8 to 7/16" gap. Could be firing on only one cylinder.
Look for ignition linkage to advance trigger at throttle up.
Be sure no air leaks in fuel line up to carburetor.
Be sure both low and high speed circuits of carburetor are clean and that float valve works properly.
Lots to look at to be sure, but, then you'll know....

 

[Tim Frank]

You say it is a new motor to you....what about the boat?
If you already had a 12' boat that only weighs 130 pounds, it sounds like it would need a short shaft motor.
Any chance you bought the motor without verifying proper shaft length, and have a long shaft 15 HP?
That would certainly hamper planing.

 

[bspeth]

+1 on the shaft length.Also my buddy had a 12 fiberglass fishing boat and the hull design was just so that it just wouldn't let it plane no matter what we did with weight distribution with people and gear,but get some weight up front after verifying you have the right shaft length. The cav plate should be even with the bottom of the transom.I actually think a 6-9.5 would be better for your set up.

 

[mrkrsll]

Also, bring your 125 pounds to mid-ship.
And, yes. Especially new to you, should be go over the basics; compression, spark and fuel.
Compression should be minimum 90 psi with both cylinders within 10%. This is a great little and proven engine.
Look at spark with an air gap spark tester. Look for spark to jump 3/8 to 7/16" gap. Could be firing on only one cylinder.
Look for ignition linkage to advance trigger at throttle up.
Be sure no air leaks in fuel line up to carburetor.
Be sure both low and high speed circuits of carburetor are clean and that float valve works properly.
Lots to look at to be sure, but, then you'll know....

Thanks for the info!

 

[mrkrsll]

No Title

You say it is a new motor to you....what about the boat?
If you already had a 12' boat that only weighs 130 pounds, it sounds like it would need a short shaft motor.
Any chance you bought the motor without verifying proper shaft length, and have a long shaft 15 HP?
That would certainly hamper planing.

The boat and motor are both new to me. The serial number on the motor is J15RCRM which I think its a short shaft...

+1 on the shaft length.Also my buddy had a 12 fiberglass fishing boat and the hull design was just so that it just wouldn't let it plane no matter what we did with weight distribution with people and gear,but get some weight up front after verifying you have the right shaft length. The cav plate should be even with the bottom of the transom.I actually think a 6-9.5 would be better for your set up.

The cav plate is even with the bottom of the transom, ive posted a pic.

 

[mrkrsll]

Got bigger problems. Took the boat out to test the motor again today and the transom cracked under load and water came rushing in. Thank god I was close to shore....