1975 Johnson 115 Fuel / Throttle Problem

[QuackAddict84]

Hello - I'm working on a 1975 Johnson 115 hp. I've worked on many outboards, but never one this big. It sat for a few years, and I managed to get it started, but it didn't run well at all. I figured carbs were the culprit, so I rebuilt them. Now it seems to start with some effort. The problem is that when you start to open the throttle under load, it quickly dies out. The strange thing is that if you skillfully milk the choke for a while as you get onto the throttle, it will suddenly start running great, plane out, and get up to speed with full power. However, when you slow back down, it has exactly the same problem. I drove it a mile or so at WOT with no problems, until I stopped. It idled down fine, but as soon as I pushed on the throttle again, it instantly died out. Again, milking the choke as I start to get on the throttle, it finds a sweet spot and runs great again. I have rebuilt lots of smaller marine carburetors, and never had a problem. It would surprise me if I did something wrong, but I'm willing to take the carbs apart again based on any advice I can get. It kinda acts like a car engine would if it had a bad accelerator pump, but I couldn't find anything that resembled an accelerator pump on the outboard carbs. It also doesn't appear to have any lean/rich adjustment needles anywhere. Thanks a lot for any input, I really appreciate it.

 

[ezeke]

Re: 1975 Johnson 115 Fuel / Throttle Problem

The V4 engines in this series have fixed jets and no adjustments for lean/rich, plus they use choke plates in the carburetors to enrich the fuel/air mix for a cold start.

Two things are absolutely necessary for a cold start: 250 RPM minimum cranking speed and properly working chokes. The ignition system cannot work without a fully-charged, powerful battery. The chokes have to close 100%, no exception, and stay closed while you crank the engine until it fires. The only way to verify that is to take the air silencer cover off of the carburetors so that you can watch while someone cranks the engine.

Once you have that set, you need to verify that the throttle plates are closed and perfectly synchronized when at idle and perfectly horizontal at WOT. You also have to be sure that the timing advance is working properly as you move the control arm or the warm-up lever.

 

[onesubdrvr]

Re: 1975 Johnson 115 Fuel / Throttle Problem

Just wondering,

When you rebuilt the carbs, did the rebuild kit come with the little clips that attach to the needle and the float?

I was having a similar problem with mine, milking the choke (well, on my 82 it's a bypass solenoid), would keep it running, but as soon as it would come off, it would die, so I rebuilt the carbs,.. at first I was a little discouraged when I pulled them off because they looked spotless, pulled all the orifices out, and the needle / seat / etc., and everything was perfect, rebuilt anyway, and as I was installing the new seats / etc., I noticed a little clip in the bag that goes around a collar on the needle, and the float, so that when the bowl is empty, the weight of the float pulls the needle down, and you're not relying on gravity and a couple / few pounds of fuel pressure.

Just a thought
Wayne

 

[emdsapmgr]

Re: 1975 Johnson 115 Fuel / Throttle Problem

Probably a carb issue. It is possible the engine is still running a little lean if it bogs. Make sure the idle in gear is between 650-750 rpm, set when the boat floating in the water. If the idle is too low, you could bog. Also, those early crossflow carbs never had mid jets, for midrange throttle. So, you had to have the idle jets clear and fully open. Possible one of the jets or one of the idle passages in the carb body may still be slightly restricted. Pull out the idle jets for visual inspection and blow some aerosol carb cleaner through the idle passages-check/compare the exit spray flow out of each passage with the same passage in the other carb...to make sure they flow similarly.

 

[ezeke]

Re: 1975 Johnson 115 Fuel / Throttle Problem

In case you are not familiar with the exit passages of the idle circuit, they are located in the floor of the carburetor throats, behind the throttle plates. They are similar to the ones in the attached thumbnail.

 

[QuackAddict84]

Re: 1975 Johnson 115 Fuel / Throttle Problem

Took the carbs back apart and cleaned the idle jets real well. It started right up on the trailer. I'll let you know how it runs in the water tomorrow. Thanks so much guys; I really appreciate it.