1967 100HP Johnson Runs Better with Door Open

[exodave]

This engine seems to run better with the front hatch on the motor cover open. Whenever it wants to die while warming up, all I have to do is open that hatch and it picks right up. Are we running too rich or something?

 

[Will Bark]

Re: 1967 100HP Johnson Runs Better with Door Open

Or you could have an exhaust leak coming up into the engine.

 

[exodave]

Re: 1967 100HP Johnson Runs Better with Door Open

Bingo! I'm sure that's what it is. The gasket around the lower unit junction is shot and I do notice exhaust creeping up into the powerhead area when the cover is off.

 

[F_R]

Re: 1967 100HP Johnson Runs Better with Door Open

There are a couple of reasons for this. Most people will be quick to point out that the big rubber seal between the exhaust housing and lower covers may be shot, which is a legitimate reason. However, what most people don't realize is that there is a foam seal on top of the water pump cover that may be shot or completely missing. Related to that is if it is a long shaft, there are two O-rings on the extension tube surrounding the drive shaft. Those O-rings may be missing, or even the whole extension tube. If any of these items are at fault, it will pump exhaust smoke up under the cowling, big time. Only a little bit of smoke will smother it. BTW, if you are attempting to run it in a barrel or something, you are fighting a losing battle. They just wont run in a barrel with the hood on. Get I on a boat on the lake before deciding if there is a problem.

EDIT: I just realized that you said it is a 100hp. What I said applies to the 75hp, but the 100 is similar with the exception of my description of the seals on the pump and extension tube. But they still gotta be there.

 

[exodave]

Re: 1967 100HP Johnson Runs Better with Door Open

thanks a ton for the detail. I am 100% sure that the part of the gasket (between the extension and the lower unit) the piece which goes straight across the middle and separates the exhaust pipe from the void that goes straight up into the cover is completely gone, so I think I'll start with replacing that and report back if it doesn't work.

 

[F_R]

Re: 1967 100HP Johnson Runs Better with Door Open

If you get into it that far, you can see the parts I mentioned.

 

[exodave]

Re: 1967 100HP Johnson Runs Better with Door Open

Finally realized I should update these old posts from last season (which was my first season with this motor)...

So, what I finally realized, after a small (but alarming) engine fire was that the fuel pump diaphragm had developed a quickly expanding hole, which was throwing fuel all over the engine compartment. Put in a new pump and she ran like a beast, better than ever, straight to the ramp to put it all away for the season. After all that, I now have the motor completely tore down and am doing a slow, thorough restoration over the winter. I have discovered a couple things:

1. There must have been a few smaller fires already which had destroyed the o-rings around the LU extension and the big thick rubber gasket at the bottom of the exhaust cover. These were all in decent shape when I first inspected at the beginning of last season.

2. It also burned up the bottom of the telltale tube, which complicated diagnosing overheating issues because suddenly we had nothing to indicated proper water pressure. Amusingly enough in hindsight, I think a leak in the thermostat was putting out the fires.

So now, it's time for some more elbow grease and a few more parts. Among other things, I'm manually removing all carbon buildup from the cylinders, all new gaskets, all new wiring, and at the very least, good visual inspections of anything I can get to. I do like this motor, and am looking forward to having it looking and running even better in a couple months.