1978 Johson 115 wont idle

[Rmoore45]

I just bought a 1978 Invader with a 115 Johnson on the back. I can get it to start and run but it wont idle, it wont go below 2000 rpm. I'm thinking its a timing problem so I posted pictures of the throttle and advancing linkages. Also, I cant seem to find a high idle on the controls for the boat. There is no button or little lever that moves the throttle into the start position.

Anyone know how to make this motor idle?

Thanks, Ranger

 

[mark1961]

Re: 1978 Johson 115 wont idle

It probably wont idle because of restricted/blocked (dirty) low speed circuits in the carbs.....try squirting some premix down the carb intakes when you are backing it down and see if the situation improves.....remove, clean and rebuild the carbs if it does.

Post up a pic of your control unit so we can see what ya got......

 

[metriccrescentwrench]

Re: 1978 Johson 115 wont idle

....it wont idle, it wont go below 2000 rpm.

Do you mean that it won't stay running at idle, or that when it's running it won't go below 2000 rpm? If that's the case try backing off this screw just a LITTLE BIT:

Note that you might need to adjust the trunion on the throttle cable a bit to keep tension so that the throttle will go to the stop after you back off that screw. Also, when you start messing around with these adjustments it's best to do a complete "link-n-sync" as called out in the factory service manual as all the adjustments affect everything else. However, it won't hurt to back off on the idle stop screw just a bit and see what happens.

Also note that idle speed is supposed to be measured with the boat in the water and underway with the engine idling in gear. In that condition idle speed should be between about 650 and 850 rpms.

 

[Rmoore45]

Re: 1978 Johson 115 wont idle

The motor will not stay running when I back off the throttle, it spits and sputters then dies. I pulled off the carbs today and cleaned them out, still the same problem. I didn't take out the little press in plugs in the carbs which might be the problem. I checked the timing with the reeves method and its dead on 24 degrees when cranking will yield 28 degrees when running. To get around the going in gear problem I just disconnected the shifter cable.

I guess I'll order some carb kits and do it right. I suspect flooding might also be a problem.

Thanks, Ranger

 

[metriccrescentwrench]

Re: 1978 Johson 115 wont idle

The motor will not stay running when I back off the throttle, it spits and sputters then dies....
I guess I'll order some carb kits and do it right. I suspect flooding might also be a problem.

OK, guess I wasted a post then. It looks like the idle stop screw is turned pretty far in, your idle problem might explain why someone did that.

It sounds like you're on the right track with getting the carb kits. Also, have you checked compression yet?

 

[emdsapmgr]

Re: 1978 Johson 115 wont idle

The idle on that engine is not set by adjusting the carbs. The idle is set by adjusting the idle timing. The screw that adjusts the idle timing is the one that Wrench posted. Agree that it looks like it is screwed in pretty far, probably resulting in a high idle. If the engine occasionally coughs while idling- that may be a "lean sneeze." A lean sneeze occurs when the dedicated idle passages in the upper carb body have a fuel restriction. Those passages are pretty small and fuel can dry up in them over many years. An aerosol carb cleaner like a Gumout product will clean them out. Usually you don't need to remove the core plugs inside the carb during cleaning.

 

[Rmoore45]

Re: 1978 Johson 115 wont idle

Ok did a compression test and the results are good. 125-130 on all cylinders. Couldn't get an exact measurement because the compression tester I have is a piece of crap, however it was shooting past 120 up to about 135 on all cylinders so I'll average that to 125-130.

I can get the motor to run about 1800-1900 rpm consistently but when I back off on the throttle it just dies. Even if I go and do it by hand on the motor, it still dies. I did notice one thing while playing with the timing, whenever it drops below a certain rpm (around 1500) it immediately looses about 200-300 rpm and the motor starts shaking. I'm betting its missing on at least 1 cylinder. My theory is that once it gets below that rpm the coil quits working resulting in a miss.

One way to find out, spark plug tester. My question is how far should the spark jump in open air on these motors? I have heard 7/16"

 

[metriccrescentwrench]

Re: 1978 Johson 115 wont idle

Yes, about 7/16" is what everyone says.

In my opinion you're more likely to find that the problem lies in the carbs. There's just no good reason for a plug to quit firing right on cue as you drop the throttle below a certain point. HOWEVER, I've been wrong before. If you do the spark test the usual way you'll be testing at cranking speed (maybe 300 - 400 rpm ?) so if you see a good spark that will essentially eliminate your theory. But in any case that will be good information.

 

[emdsapmgr]

Re: 1978 Johson 115 wont idle

When that engine idles, normally the carb butterflys are closed. Any air going to the engine at idle goes through a small hole(s) in the middle of the butterfly plate on each carb throat. Since these plates are closed, the carb draws fuel from the small dedicated idle passages (only) in the upper carb bodys. If these passages get restricted (dried fuel over time) they won't flow any fuel and the engine won't idle correctly. As soon as you advance the throttle just a little (and those butteflys open slightly) the carb will draw fuel from other jets. Usually the engine will run somewhat normally once off idle. Only way to fix this type of idle problem is to disassemble the carbs completely and clean them, paying close attention to the fuel flow out of the idle passages.

 

[Rmoore45]

Re: 1978 Johson 115 wont idle

Since these plates are closed, the carb draws fuel from the small dedicated idle passages (only) in the upper carb bodys. If these passages get restricted (dried fuel over time) they won't flow any fuel and the engine won't idle correctly.

Are you saying that only the upper carb has idle passages and the bottom one doesn't? I think I'm misunderstanding but just checking. Both of the carbs on my motor have idle passages, which is why asked.

I ordered some carb kits, the float needle looks like it was a little worn and there was a small amount of gunk in the carbs. I took a small wire (the kind used for cleaning torch tips) and cleaned out the idle passages. They seemed fine but I decided to take out the plugs in the carb, the passages were clean but I want to be thorough. The floats were in good condition but I'm going to put them in a pool of gasoline for a while to make sure that they don't sink over time.

Another problem was that I think I had the wrong spark plugs, I was running NGK BR7HS but now I have Champion L77JC4 like it recomends. I still havent checked the spark on the coils but I bought a ignition tester so I will probably do that later today.

Just waiting on carb kits now there is always something else to do on the boat though. Like trying to the get trim n tilt working.

Thanks, Ranger

 

[Rmoore45]

Re: 1978 Johson 115 wont idle

Is it sad that it took me this long to realize that Johnson is spelled wrong? Lol

Anyways I rebuilt the carbs, bolted them on, connected all the linkages, and started it up. After it warms up for about 30 seconds @1500, I can drop it down to around 1000 rpm and let it idle. It didn't idle all that great and spit and sputtered every once in a while and occasionally died so I checked the ignition coils with my spark tester. 3 out of 4 coils jumped a 7/16" spark, but #3 cylinder didn't. I swapped coils and determined that the coil is bad and its not the trigger for that coil.

Now to order a coil for it.

 

[Rmoore45]

Re: 1978 Johson 115 wont idle

A+ for Iboats store, ordered the coil tuesday and it was here friday.

Installed it in less than five minutes, checked the spark with my tester and it jumped a 7/16" gap no problem. Hooked everything back up and fired it up and man does that motor run good now! I let if warm up for a bit then brought it down to idle, I actually had to idle it down a little. Motor runs smooth and fires right back up after you kill it.