1986 Johnson 70 hp low RPMs

[GreenAndPoor]

This engine was rebuilt and only has 30 hours on it but I can only get it up to 4000 RPM and 10mph at full-throttle. My understanding is that it should be hitting 6500 and at least 15mph. Do I need to adjust the idle? Is this as easy as turning a screw? The choke lever on the control seems really tight and it starts on the first try every time. Any advice would be appreciated - first time boat owner here.

 

[flyingscott]

Only 6000 rpm for that motor. Do a compression test spark test and make sure the throttle linkages are moving correctly.

 

[fireman57]

.oops, didn't read far enough. Yeah, he's right

 

[Chinewalker]

Okay, back up a bit. 4000 RPM and only 10mph? 6000 RPM and only 15mph? What are you running this motor on?

 

[GreenAndPoor]

Okay, back up a bit. 4000 RPM and only 10mph? 6000 RPM and only 15mph? What are you running this motor on?

It's a 17.5 ft Fiberglass Beachcraft... I DID have 4 passengers on board...

 

[flyingscott]

It's a 17.5 ft Fiberglass Beachcraft... I DID have 4 passengers on board...

What do you mean by low RPM? I would seriously suspect your tach or a spun hub in the prop. Try a different prop because if your tach is right then you possibly have a 20" shaft on a boat that needs a 25" or a spun hub.

 

[GreenAndPoor]

What do you mean by low RPM? I would seriously suspect your tach or a spun hub in the prop. Try a different prop because if your tach is right then you possibly have a 20" shaft on a boat that needs a 25" or a spun hub.

Well it's my understanding that at full-throttle I should be getting around 6000 RMP. My tach says that I'm getting only around 4020 and I can tell that my speed readout is correct because there's no way I'm getting more than 10MPH at full. I read some other similar threads that said I should try cleaning out the high-speed jets. What do you think? Is it stupid to try and do a carb cleanout on my own with nothing but YouTube to guide me?

 

[GreenAndPoor]

What do you mean by low RPM? I would seriously suspect your tach or a spun hub in the prop. Try a different prop because if your tach is right then you possibly have a 20" shaft on a boat that needs a 25" or a spun hub.

Oh and PO said that they did a full rebuild a year-and-a-half ago which is why there are only 30 hours on it. Is it dumb to assume that a full rebuild should have addressed these issues?

 

[flyingscott]

You need to do a compression and spark test you also need to make sure all the linkages are moving correctly.

 

[GreenAndPoor]

You need to do a compression and spark test you also need to make sure all the linkages are moving correctly.

Ok, will do. Thanks for your help!

 

[jakedaawg]

Did this motor and prop ever run good on the current vessel? It may be as simple as putting the correct prop on.

 

[jakedaawg]

another thought...if this is a 30 some year old boat it could be waterlogged up in the floatation. Not easily fixable if thats the case.

Not all old boats are good.

 

[flyingscott]

My concern is the speed and rpm don't make sense. I would still try a different prop

 

[GreenAndPoor]

Did this motor and prop ever run good on the current vessel? It may be as simple as putting the correct prop on.

The PO told me that he was getting up to 6000 RPM and 30MPH last season on this prop.

 

[racerone]

I see wild guessing and speculation here.---I do not see the results of a compression test and strong spark test.

 

[GreenAndPoor]

Update: Sparks and compression are good. Got up to 25 mph with just me and my brother in the boat. The tach was jumping all over the place so I think that it's the problem - I was getting incorrect RPMs. BUT...I'm now worried about waterlogged floatation. When my boat is in the parkade I notice an always wet tiny trickle under the bilge drain on the ground ... but there's no water in the bilge. I don't see a drain hole from the hull to the lower bilge either so I can't see how water could get in or out. Any thoughts?

 

[ob]

Can't offer up anything with respect to the boat leakage but an erratic tach reading may point toward an issue with the engine voltage regulator/rec. Bare in mind that depending upon the weight of boat and occupancy weight , the engine needs to be propped accordingly. Not uncommon for boaters to own two props of different pitch depending upon their intended use of their boat and or number of persons aboard.

 

[Chinewalker]

Take it to a truck weigh station. Poke around online to see if you can get weights for what the boat, motor and trailer SHOULD be, then compare to what they are in actuality... If you're way over (taking into account fuel & gear in boat, too) then you're likely waterlogged. Poking around under the deck might tell the tale, too..