81 Johnson No Low End

[Doug 70 Johnson 1981]

1981 Johnson 70 hp, Model #70ELCIM

I picked this motor up this spring to replace a 1975 we've repaired a few times...same 14' boat glass boat.

The carbs were cleaned (twice to be sure). It starts easy, idles smooth, has great mid-range power (two tubes), and is very smooth on the top end, but it goes to nothing out of the hole. Compression is good (cold and hot), coils are fine (swapped in another set we had to be sure), timing has been reset, link and sync many times to be sure, etc. As per an old thread, we disconnected the rectifier to be sure it was not a factor.

From a cold start, it comes out of the hole a little less than expected, but not too bad...and then out of the hole power disappears, sometimes immediately, to the point it will only just get past idle. lt doesn't stall, just bogs down, and does not seem to be missing. Sometimes it breaks free, and then goes great mid to top end.

One thing we cannot resolve is that the hood makes a huge difference. Without the hood, it's does not loose all of it's power. I cannot find an impingement or short, the air hole is clear, but there was some loose foam. I thought maybe a small piece of foam made it into a carb, but can't imagine it would clear after the hood is removed.

We're stumped and could use some help here. Perhaps the hood is holding heat close to the power pack and it is failing at transition, but, I'm guessing at this point.

 

[Rudy Brown]

Re: 81 Johnson No Low End

Someone once said that if a motor runs better with the hood off, theres a good chance theres an exhaust leak. The motor is injesting exhaust gases thru the carbs.

 

[Walker]

Re: 81 Johnson No Low End

Pull the hood off while the motor is running. If the rear lock shocks you you've found at least part of the problem. Those motors are notorious for damaging and shorting from the rear latch to the lower sparkplug wire.

 

[Doug 70 Johnson 1981]

Re: 81 Johnson No Low End

Thanks to both of you.

I checked and no short. Symptoms also persisted after hood removal.

I did find an exhaust leak, and not where I had looked before. I noticed small droplets of water spurting out what appears to be a vent hole in the adpater plate below the powerhead. That resolves the matter of the hood, as I understand all idle gases are going out the upper inner exhaust housing, and would therefore be accumlating under the hood during idle.

Is there reason to believe replacing the gasket above the inner exhaust housing adapter will improve my low end power under all conditions...with and without the hood on?

 

[tashasdaddy]

Re: 81 Johnson No Low End

let me try to grasp this. if you throttle up slowly its does great. but if you gun it , it basicly looses it. is this correct. if so have you thought about the fuel pump.

 

[Doug 70 Johnson 1981]

Re: 81 Johnson No Low End

Thanks for your interest in my problem.

Starting cold, it will come up on plane, but not as well as the old 1975 70 hp, broken piston and all. Once running a few minutes, there are times when it will not even idle at full speed in neutral. However, if I can get it through that, it runs like a dream. Ex. fI circle the tubes a few times, seems to clear itself, and then pulls them just fine, up and down through throttle range, as long as I don't go too low. Once I dump a kid in the lake, I have to go through it again, and some times I simply cannot get them up and troll them back to the dock.

The exhaust leak I found makes a lot of sense to me and the parts person I just ordered the gasket from. What I'm not sure of, is if replacing this gasket will add horsepower when the hood is not on. I certainly hope so.

 

[tashasdaddy]

Re: 81 Johnson No Low End

do the gasket. i think you need to check compression again. unplug big plug the controls, put shifter in forward full throttle, all plugs out. insert guage, jump soleniod 4 revolutions each. post findings please.

 

[Doug 70 Johnson 1981]

Re: 81 Johnson No Low End

She runs fine now...

I figure it was a longstanding exhaust leak, as the inner exhaust housing was plugged up pretty good, and all of the vent holes along the the side of the inner exhaust housing were completely clogged. Fumes were leaking under the hood because one of the rear bolts coming down through the adapter plate had no head, and the bolt next to it which comes up from the inner exhaust housing had fallen out and into the exhaust housing, allowing air to blow straight up through it. It also appears exhaust may have been pushing out through the base gasket below the manifold...which having changed them a couple of times on the old motor, I did not want to do again.

As for the compression test, when I get the compression tester back here again, I'll check it out. The bottom cylinder was about 10 lbs lower than the top two during previous checks, but we knew that wasn't enough for that little power.

Thanks from me, and the kids, for the support!!!