4hp johnson slower than expected

[Beakster]

Hi, I have a 1972 4hp johnson with the weedless drive.

I've put it on an 8foot inflatableand I'm only getting 5kts. The boat only weights 25kgs and I'm of average build.

Is that to be expected? I think my main question is if I'm able to find a standard rather than weedless drive will it make a difference to the speed?

 

[cyclops222]

What GPS data speed are you expecting. On a 4 hp cylinder head rated, worn out motor?

 

[Lpgc]

As a kid I had an 8ft inflatable (with wooden transom) and 3.5 Suzuki, dad made up a stiff wooden floor for it, it would plane with us both in it.

 

[Beakster]

What GPS data speed are you expecting. On a 4 hp cylinder head rated, worn out motor?

I just borrowed a 90s johnson 4hp standard drive from a guy in the marina and measured 12kts. Able to get the dinghy up on plane and significantly faster than mine.

So now the question is, is it faster because of the standard vs weedless drive, or is there a problem with my motor. If it's a problem with the motor I can likely fix it. But if it's caused by the different drive/prop I'm as well buying a different motor.

 

[racerone]

Test / inspect / diagnose this motor.-----Likely has issues.

 

[cyclops222]

Old motors were rated by cylinder head power. Which excluded all gear train losses. So your motor is not really the prop hp.

 

[Beakster]

Old motors were rated by cylinder head power. Which excluded all gear train losses. So your motor is not really the prop hp.

I doubt that the 90s 4hp johnson is significantly more powerful that the 70s 4hp johnson. They are both 2 strokes and about the same weight. Certainly not such a significant power difference.

Test / inspect / diagnose this motor.-----Likely has issues.

I think the consensus is that a 4hp johnson should be faster than 5kts in such a small light boat. If you are sure that this isn't a prop issue (e.g. the smaller prop of the weedless is causing the speed limitation), then I will start tonight with a compression test.

 

[racerone]

The weedless drive is at an angle.-----So some of the power pushes UP and not FORWARD.

 

[Beakster]

The weedless drive is at an angle.-----So some of the power pushes UP and not FORWARD.

Enough to stop my little boat getting up on plane? I tried all the trim settings and nothing helped. When I put my friends standard drive on and have it throttle the front end shot up in the air.

 

[airshot]

Need to do a thorough test on your older motor, she might be tired. It has been my experience with the weedless drive that it does not push as well, but only 1 or2 mph, not double the speed ! Same with weedless props on electric trolling motors..

 

[Beakster]

Need to do a thorough test on your older motor, she might be tired. It has been my experience with the weedless drive that it does not push as well, but only 1 or2 mph, not double the speed ! Same with weedless props on electric trolling motors..

I measured the compression today at 75psi and 83psi. Not great.

I then saw someone selling an 1990 Evinrude 3hp model: e3bresr. I did a compression test on it and it was 120psi on each cylinder. It's not running but it's been sat for a decade so suspect a carb rebuild is needed.

As mentioned above they changed the HP rating from flywheel to prop, so as I understand it, this 1990 3HP is actually the same engine as the 1972 4HP.

I'll get it running this weekend and see how fast it goes.

 

[flyingscott]

compression might be ok. Sounds like you are running on 1 cylinder. Check you points and coils

 

[DeepCMark58A]

compression might be ok. Sounds like you are running on 1 cylinder. Check you points and coils

Correct even on 1 lung they will start right up and idle.