84 Johnson 40 HP Weak Top End/No Slow Idle

[foreman05]

Hey all just picked up my first boat a 1985 16 foot Mirrocraft lake fisherman II 76" beam I think with an 84 Johnson 40 HP tiller all in amazing shape. Took it out on the lake today to get a feel for it with my family of 5. Im guessing we weigh in at 850, we had no gear today.

The motor starts great, pull choke hit the button and we're running. Is it normal to not be able to fully twist it to slow and just have it idle slow? It only idles in neutral just above the shift position, this made trailering a bit more exciting than necessary lol. It will idle in or out of gear above the shift position no problem.

Also the top end seems like it was really struggling, almost like driving 30 mph in overdrive. I was expecting a wound out 2 stroke but it sounded like an overloaded 4 stroke.

I grew up with my uncles 16 foot deep v duck boats with old 25-30 hp tillers. even loaded down with all the duck gear and us two at 250 each would move about twice as fast as I got today.

I will GPS our speed Sunday, im guessing 8-10 mph maybe.

Anyway love the boat and excited to get my boys on the water.

Cheers,

 

[racerone]

You start with a compression test------Followed by a s[ark check.-----Sounds like it is running on one cylinder.

 

[foreman05]

The guy I bought it from said he just checked the compression at 140. I think it runs to smooth to not be running on both cylinders but I could be wrong.

 

[racerone]

Spark check.

 

[oldboat1]

btw, 50:1 gas mix ratio. Decals to the contrary, if any, do not apply. Fresh gas, TCW3 two cycle oil. Borrow, rent or buy a compression tester and test it yourself. Get an adjustable open air spark tester, and test magneto strength. 1/2 inch crisp spark should be easy. Both top and low end performance indicates the motor needs some attention.

 

[racerone]

The 2 cylinder 40 / 50 HP models never had stickers with 100:1 mix on them.-----That was a short lived concept on the 30 HP and smaller motors around 1986 to 1988 models.

 

[foreman05]

I will test compression and find an open air spark tester. He said the gas was fresh, I will dump it an mix fresh ethanol free gas @ 50:1. I just bought evinrude XD30 oil so that should be good.

Thank you all for the recommendations, anything mechanical has never really bothered me but outboards and carbs are not my cup of tea.

 

[Crosbyman]

feed it some Evinrude engine cleaner and some 2+4D fuel conditionner

can't hurt

 

[Huebillionaire]

The guy I bought it from said he just checked the compression at 140. I think it runs to smooth to not be running on both cylinders but I could be wrong.

I thought the same thing once time. It's very deceiving.

 

[foreman05]

Rented a compression tester, got just over 130 on the top about 135 on the bottom.

Bought a spark tester, seems to light it up pretty good. Also provides a damn good zap if you hold the coil side of the wire lol.

Can someone confirm the correct plugs? In it now are QL78C. I read somewhere QL78YC is what it needs then found a few Johnson charts calling for QL77JC4. I'd like to replace them and keep a few spares onboard.

I'd also like to buy a service manual but didn't see any at the boat shop, any ideas?

Thanks again!

 

[racerone]

Checked flywheel key ?

 

[foreman05]

Checked flywheel key ?

I did not check it, I assume its in there but don't want to take off the flywheel to check. I would know right away if that was the issue wouldn't I?

 

[foreman05]

I also have yet to find a fuel filter. Any ideas?

 

[racerone]

If key is sheared the timing will be off.----Up to you to do the checks on your motor.

 

[foreman05]

Well went to the lake again. Boat cruises at 7 mph according to my phone gps.
Fresh gas and new plugs were the only changes. I took the cowling off and used the throttle myself. Anything beyond the wot stop just makes it bog down. It did idle slightly better I think but still won’t let me go slow enough in gear. Guess I’m going to try and become an outboard mechanic.

 

[racerone]

Something is wrong.-----Some trouble shooting has to be done.--What fuel pump is on this motor, VRO or something else ?--Motor needs 4 things to run.----Correct amount of fuel.----Crankcase compression,----Spark at the right time.-----Compression in cylinders.-----So the work / initiative is up to you.-----Or run to a shop and pay for help.----Sorry to be blunt here.

 

[foreman05]

Racerone, don’t get me wrong here I have initiative and would prefer to fix on my own however after looking through the whole internet plenty of people seem to have the same problem but never post back what they did to fix it. I love forums but find that common among those who ask for help.
I picked up some earmuffs so I can run it at home. Also can’t believe how hard it is to find a service manual.

 

[racerone]

Sorry----You say it is in amazing shape ?----So are you judging this motor by a paint job or the way it performs ?----A service manual for 1980 to 1988 is all you need.----Checked the flywheel key ??----Do not run this motor until you find the issue.---A plugged carburetor could result in starving a cylinder of oil !------Ruptured fuel pump diaphragm is also a possibility.?

 

[tphoyt]

In my eyes being that this is new to you and have no history on it and it’s not running right first order of business would be to rebuild the carbs and fuel pump replace and fuel lines that seam dry and or cracked replace plugs and inspect plug plug wires and get a new water pump installed. Make sure the gas tank is clean and free of any old gas and contaminants. Then fire it up and see where your at. If it’s still not right move on to ignition. If it’s good change the oil in the lower unit and enjoy.
amozon should have you repair manual if you can’t find one here on IBoats.

 

[foreman05]

Yeah I would say its in amazing shape on the outside. Unfortunately for me we did not get a full test drive just a quick startup and run with muffs on. Being it popped right off I had no reason to think the top end wasn't there.

I just downloaded a manual so I am combing through that now.

I will check plugs after our outing, that should give me an idea if each cylinder is getting fuel. I will go through and replace what I can get parts for, I heard they aren't making Johnson parts any longer?!?

Would it make sense that in neutral it revs like crazy but in forward its a turd?