40hp johnson only goes 8mph.

[heapsofjeff]

I just bought a boat and it will only go 8mph. I have read others with similar size boats and engines get 24-30 mph. I cant seem to figure out what is wrong? It is a 1971 arrow glass fiber glass 15' boat. It has a 1971 Johnson 40 hp engine (40EL71B model) with a 10.5x12 prop. The engine seems to run good and smooth. I tried pulling the plug wire off 1 cylinder and it was obvious rough and slower. Tried the other and same, so seems both cylinders are working. I replaced the plugs with j4c plugs after reading the j12c that was in it was wrong, but no change. The boat does not have a tach so I am unsure if there is cheap way to rig one up to diagnose. Thanks for any things to try.

 

[jbuote]

Still new myself, but if you're running smooth, and you get throttle response as you expect, sounds right, feels right, just not going anywhere....
​Check for a spun prop hub...

That's my beginner guess, but others will probably have other thoughts, and would be more helpful..

For what it's worth..

 

[boobie]

What's the compression on it ??

 

[jimmbo]

Is the timing advancing fully and is the carb opening fully?

 

[F_R]

So, the normal answer would be that it is running on one cylinder. But in your original post you told us that you pulled the plug wires and proved that it is running on both. That makes the diagnosis a bit interesting. Some valid suggestions have already been made, so forgive my repeating them.

Does it rev crazy high but not push the boat fast? That would be a spun prop hub as already suggested.

But doesn't rev high? AND running on both cylinders? An inspection of the throttle linkage is in order. The spark has to go to full advance and the carburetor has to open fully. And they must be properly synchronized, but that is another story.

It could be a sick engine. Check the compression. But if that is the problem, I don't think you would get it to run at all.

Could be a rag or something sucked into the intake.

It could be running way too rich. Does it kind of just blubber along? Could be somebody has been "fixing" the carburetor. Could be somebody has installed the wrong high speed orifice plug, thinking more gas means more speed (wrong). Could be the choke is closed. Could be a ruptured fuel pump diaphragm.

If all else fails, go back and double check the one-cylinder theory----the most likely anyway.

 

[Vic.S]

I just bought a boat and it will only go 8mph. I have read others with similar size boats and engines get 24-30 mph. I cant seem to figure out what is wrong? It is a 1971 arrow glass fiber glass 15' boat. It has a 1971 Johnson 40 hp engine (40EL71B model) with a 10.5x12 prop. The engine seems to run good and smooth. I tried pulling the plug wire off 1 cylinder and it was obvious rough and slower. Tried the other and same, so seems both cylinders are working. I replaced the plugs with j4c plugs after reading the j12c that was in it was wrong, but no change. The boat does not have a tach so I am unsure if there is cheap way to rig one up to diagnose. Thanks for any things to try.

Consider a Tiny Tach https://www.tinytach.com/

 

[heapsofjeff]

I ordered a tiny tach, so will know the rpm when that comes in. I will have to look up and see how the timing is adjusted on this engine and check that. It uses a sheer pin on the prop so I don't think it can be a spun hub. The carb has a grill over the intake so I would think it would be hard for something to get sucked in. I am having to pull start it currently due to a bad starter, so I didn't have any luck checking compression. Any tips for doing it with pull start? I guess I will have to wait for the tach to have more info.

 

[racerone]

Facts for you.----Even with the drive pin the rubber drive hub ---INSIDE-- the prop could be slipping.----And you can get a good compression test with the recoil starter.---And does spark jump an actual gap of 5/16" or so with a spark tester.----Best to not spend money on this motor untill you gather some facts on it.

 

[clemsonfor]

I too am wondering if your making full RPMS or does it sound weak? You could also be sucking air from an air leak and not getting all the fuel it needs.

 

[F_R]

No Title

Rubber hub in prop

 

[82rude]

Check your carb settings.I had a 1970 33 evinrude that first time out ran really slow, all i did was adjust the carb a little and she took off like a scalded cat.Its been a few years but if i remember correctly the carb was originally set at factory recommendations then i had to turn the high speed in 1/2 turn maybe?Easy to do if you have tiller drive but if you have remote get help from a friend.

 

[heapsofjeff]

Wow I had no idea the prop had the rubber inside it like that, although I do have 2 props and tried them both with no change. It was at that point I realized they were both the same pitch. Just one was in better condition and the other had a few nics in it.

 

[racerone]

Likely there is no high speed needle on the motor in question.

 

[heapsofjeff]

The service manual shows it does have a high speed needle, I will see if it really does tomorrow and make sure

 

[jimmbo]

I see the manual saying so too http://www.boatinfo.no/lib/evinrude/...rude.html#/108

I checked 1971 brochures. They say adjustable jets(evinrude), or adjustable High and Low speed jets(Johnson)

The 71 40hp was based on the Gale powerhead(like a 1957 35hp, but with 43.9 displacement), not the E/J design used last in 1970. That might explain the adjustable High speed needle

 

[F_R]

Well I'll be darned, I just checked the book and it does show an adjustable high speed jet. That could be the problem if it is set way too rich. It would just sort of blubber along. Normal opening would somewhere in the neighborhood of 5/8 to 3/4 turn open. Adjust from there, at full throttle, on the lake.

 

[heapsofjeff]

Went to pick up boat from storage lot today and realized the wood inside the fiberglass on the rear of the boat where the motor mounts is rotten. It is allowing the motor to lean quite a bit. It wasnt rotten where the motor mounts but at the ends where it connects to rest of boat. Ended up spending all morning taking that apart and got all the rotten wood removed so can replace once it dries up inside there. Probably will be able to repair next weekend and then see if the motor does any better with it not leaning outward.