TOPIC: 1966 100 HP Johnson- Throttling and Accleration

I got my boat on the water yesterday and it started like a champ, idled well and even accelerated ok, at first.

The Problem- If into gear and went WOT or even when from a trolling speed to a much faster speed, the engine would rev, then drop way back and then slowly kick in. I could eventually get it up to 4500 rpms.

When I switch tanks from the old Johnson and connected the new brp tank, the first 5 minutes or so, the performance was even worse. I couldn't get about 20MPH and previously it was topping at 34mph.

There were also a few isolated times when I would go to slow down from WOT, that the motor would begin slowing down, then accelerate and then slow down again.

I have changed the coil to an auto coil and changed the spark plug wires, condensers, points.

I haven't done a fuel or carburetor rebuild. I've been told to leave the carburetor alone if possible because this style is very complex.

I have also not changed the throttle cable.

Any suggestions on a systematic way to troubleshoot without being near water?

Sincerely,
David

Sounds like a fuel delivery problem. Did you open the tank air valves(if equipped with them ).Next thing to check or just rebuild to eliminate as a problem is the fuel pump( easy job if you have the rebuildable type). Could be the fuel bulb bad on the fuel line,but I doubt it. One way to check the pump is to have someone squeeze the bulb while underway. If she starts running right , there's your sign. Of course you have to be on water. The carb hopefully is alright. Never did one of those and from what I've read don't want to.

Do you think it is possible that the diaphragm is filling up with fuel and then all of sudden expelling of it into the carbs is why there is a delay and then it runs normally for a bit until I slow down. Even when I slow down, the boat will sometimes have a bump in acceleration?

Do you have two tanks? Does it do it with both of them? Is it new gas? It definitely sounds like a fuel flow problem. I rebuilt both the fuel pump and the carb on that 80hp, not a real big deal. Pump is real easy.

Bob

Same problem with both tanks. In fact, when I switched to the new tank, the performance was worse for the first 5 minutes or so. Then it started acting like the first tank.

I am going to start rebuilding the fuel pump.

The carbs are a different story. Those look pretty darn complex. I would probably have to outsource that one.

Gas was fresh. I had stabil and sea foam in it. RPM's were about 4500. Manual says 4500 to 5500. Jim L looked at a video and we think maybe the engine needs to go up a peg because it looked like the bow was plowing.

We also discovered tonight when I took off the hood that one of the spark plugs ended up dripping oil. Hopefully, that means I just didn't tighten it enough. So, I have to explore that problem too.

Other than that, it had a nice pleasant sound and did have some nice pickup when it worked.

Good first day.

Ah brother in arms. These babies are a handful right??
Sounds like a fuel pump problem to me and it's a pretty easy rebuild with all the parts available and relatively cheap. Your '66 and my '67 are pretty much the same since I converted to the older ignition setup like you have. Difference is fuel pump. You have the older type with a glass bowl? You can easy convert to the newer type. Look at marineengine.com and check out the parts list for both years.

Funny you should say that about the new type of fuel pump. I bought the parts a while ago to rebuild the pump, but I didn't do it. Is there any benefit to updating to the 1967 fuel pump? I have to see if the bracket is different or if I could make it work.

I don't believe there would be any benefit switching over. The pump you have fit lots of older omc's. Had that design on my '58 35hp super seahorse and also on a '60 75hp starflite. When I rebuilt one of those it was because the rubber diaphragm was weak and soft and stretched out. Just wouldn't push the fuel fast and hard enough.

Just wanted to post an update in case anyone else has these issues.

I rebuilt the fuel pump today and reassembled it they way I took it apart. I have a manual and I had looked over it several times, so I was relatively confident. With a sense of accomplishment at rebuilding the fuel pump, I went into the house to chat with JimL and show him pictures of the diaphragm. It really didn't look that bad, so I wanted a second opinion. I was hoping the diaphragm would have a tear or something to explain the issue.

While I'm texting with him, I am looking at the manual and notice that the short spring and cap within the fuel pump had been installed on the wrong side of the diaphragm which means it would be rather difficult for a vacuum to be created.

So I disassembled the pump and reassembled it the correct way. I am hopeful that this will solve the problem. Now on to some electrical and steering issues.