1968 johnson 6 hp slow

[letitroll707]

My 1968 johnson 6 hp is running slow average speed around 5 mph. Rebuilt carburetor new cables coils points condensors spark plugs rebuilt fuel pump good compression dont know what im missing to get this motor running faster like it should. Please help.

 

[boobie]

Do a cylinder drop test on it to see if it's running on both cylinders.

 

[letitroll707]

the prop is in good condition , all the orifices on the carb have been cleaned and blown out with air, and it's on a 14 ft aluminum boat. Both cylinders had good compression how do I do a compression drop test?

 

[loiner1965]

Basically with engine running , pull off 1 spark plug lead with insulated pliers etc and listen to the engine.
If it stops then it was running on that cylinder alone....if it misfires and sounds lumpy then it was running on two cylinders.
Try it on both plug leads to be sure

 

[steelespike]

Disconnect one plug wire at a time and see if it will run.
Or loosen the wires at the plug and with it running pull one wire at a time with insulated pliers.
Link n sync. Be sure that as the throttle advances it opens the carb at the correct point.

 

[letitroll707]

Is there a chance the fuel pump is not pumping enough gas for it to hit high speed but enough to run low speed?

 

[Chinewalker]

Try the drop test before messing with fuel pump.

If you do suspect the pump, easiest way to confirm is to squeeze the primer bulb while running and see if it picks up - effectively making you the fuel pump.

 

[racerone]

Fuel pump function is easy to test.---Just operate the primer / manual fuel pump on the hose at full throttle.-----Any history on the motor ?---Any chance that mice have built a home in the exhaust housing and chohing the motor.-----Does spark jump at least a 1/4 gap or more ?

 

[letitroll707]

Did the drop test pull wire off top plug it dies pull off second plug still runs what do I do?

 

[Tim Frank]

Is this a new (to you) motor?
If so, depending on the load in the boat and trim, you may be getting all you can from that combination.
It's only a 6 hp. on a 14 footer.

If this motor has previously performed much better for you, check compression.

 

[letitroll707]

Its an old motor that we were given dont know any of the history on it we found this motor should get up to 15 to 20 mph but we can only get 5

 

[racerone]

It is only running on the top cylinder.----Check for spark as suggested earlier and check reed valves for the bottom cylinder.

 

[Tim Frank]

Its an old motor that we were given dont know any of the history on it we found this motor should get up to 15 to 20 mph but we can only get 5

Where did you "find' that.

Next step is check compression.

 

[nwcove]

Its an old motor that we were given dont know any of the history on it we found this motor should get up to 15 to 20 mph but we can only get 5

15-20mph with a 6hp? ..... not gunna happen unless you are heading down a river thats running at 10mph

 

[Tim Frank]

How many people are in the boat at 5 mph?

 

[letitroll707]

I was told by my brother that it should be that fast I always thought smaller hp like a 6 would just troll. There are 2 of us in the boat when we take it and we use it in the bay.

 

[Tim Frank]

I was told by my brother that it should be that fast.

Maybe you should get him to fix it.

If you are getting 5 MPH with 2 adults in a 14' boat, that's not bad.
If you want ~ 20 MPH, you'll need to upgrade your HP to at least 15.

You should still make sure whether you have a problem or not.
Try the drop test at a high idle.... sometimes if you do the test with a motor that DOES have both cylinders firing OK, if the speed is a tad too slow, it can cut out on one, but not the other.
Test the spark with a proper spark tester.

 

[Sandor27]

Does the motor sound like it is hitting full rpms at WOT? What pitch is your prop? Can it possibly have the sailboat 5" pitch prop on it? That would get you about 5mph. For comparison, I had a 1236 Jon boat and a 6hp that would get me and a buddy up to 8mph (not on plane) and me alone if balanced right about 12mph (just on plane) but no more.

 

[Salt Water Junkie]

I'm the brother of guy that started this forum , was wondering if you know if the 2 blade prop is the "sailboat" prop ?

 

[racerone]

A 6 hp is not going to move a 14' boat at 20 mph , simple as that !