1979 Mercury 115 Inline 6 - Idle Speed too High

[wcsd106]

I recently picked up a boat with a 1979 Mercury 115 Inline 6 Cylinder engine. Serial Number: 5314846

The previous owner had allowed it to sit with fuel in the carburetors. The fuel had spoiled in both the carburetors and the fuel pump, so those components were cleaned and rebuilt. I installed new fuel lines throughout the system, and finally had a chance to fire it up today.

I connected the flushing muffs to the engine and fired it up. It took a few attempts, but once it started, it would fire up fairly easily.

The idle speed, however, is entirely too high. ~1800 RPM or so on the muffs.

• All 3 carburetors have an idle mix adjustment. I have set all 3 to 1 turn open as was called-for in the OEM manual.
• I have checked my primary pickup timing and it is set at about 6 degrees ATDC.. Within the 5-7 degree range specified in the OEM manual.
• I have checked the shutters on all 3 carburetors to ensure they are completely closed when the throttle is in neutral.
• I have attempted to locate a leak between the carburetor and the intake by spraying carburetor cleaner where the carb and intake meet while the engine is running; No change in RPM was observed

Any ideas, folks?

 

[jimmbo]

Is the idle speed screw on the throttle linkage arm contacting its stop? if it is, it just might be set too high. if the controls are attached an misadjusted cable might be preventing the linkage from moving enough

 

[wcsd106]

The idle stop screw does contact the block when the throttle is in neutral as it should. The primary pickup stop on the throttle lever also contacts its place on the block when at neutral.

Now for the interesting part... With the engine off, for obvious reasons, if I push the controls to WOT, the full throttle stop screw makes contact at its stopping point, but the maximum spark advance screw does not. I can, however, manually push the spark advance to allow the screw to make contact.

I know this is difficult to visualize without pictures, but I've run out of daylight today. I'll get some pics posted tomorrow.

Thanks.

 

[GA_Boater]

Now for the interesting part... With the engine off, for obvious reasons, if I push the controls to WOT, the full throttle stop screw makes contact at its stopping point, but the maximum spark advance screw does not. I can, however, manually push the spark advance to allow the screw to make contact.

I think you need to grease the two places on the distributor. If the grease doesnt work, you may need to take it apart and clean out the old hardened grease. Been there, done that.

 

[wcsd106]

Thank you, but this model of 115 is the distributor-less model. I have attached a picture of the throttle arm assembly.. Would it be wise to disassemble this and grease it as well?

 

[GA_Boater]

I just went by the serial number you gave, 5314846.

 

[wcsd106]

I have ran into that a few times already with this engine. The serial is correct, but depending on what site you go to, you'll get a different diagram. I think 79 was a changeover year on these.

 

[GA_Boater]

No biggie, wcsd. All these things make it harder on us trying to keep the old ones running.

I don't know much about your style of 115, just the earlier ones.

One thing you can try is to disconnect the throttle linkage to the carbs and see if you can get the idle RPMs lower by rotating the top carb linkage more toward closed. On these the throttle plates need to be completely closed for the right idle and on some models it isn't easy to get a good view of the plates.

 

[wcsd106]

Thanks. I'll give that a shot tomorrow. I also have a buddy who offered to let me bring it into his shop so we can put another set of eyes on it. I'm definitely taking him up on that offer.

 

[Chris1956]

Gee, back off the idle stop screw and adjust the throttle cable to push the throttle closed. That should help reduce idle RPM. Make sure the idle pickup timing and max spark advance screws are set properly. Make sure the carbs open all the way, before the throttle stop is engaged.

It sounds like at least the idle stop and throttle stop screws are set incorrectly. Idle on the flusher device should be about 1000 RPM.

 

[wcsd106]

Made a little progress today. I took the boat to a buddy's shop so that someone else could look for things I was missing. We got the idle down to about 1300 on the muffs, but it wanted to cut out when we tried to push it lower. The plugs had some light deposits on them, and I don't k ow when they were changed last, so he's suggested j put a new set of plugs and we'll start back at it.

We did have it idling at 1000 rpm for a short time, but it was idling rough.

 

[merc850]

1000 is still too high aim for 7-800 rpm.

 

[wcsd106]

Shouldn't 1000rpm on the muffs drop to 850 or so once I put the lower unit in the water?

 

[Chris1956]

Idle speed on the flusher will drop when the motor is placed in a lake. 1000 RPM is OK for initial setting on the flusher.

If she won't run smoothly below 1300 RPM, you probably have dirty or misadjusted carbs. 1200 or so is when the motor shifts to the main jets. Under that RPM, the idle jets and circuits control the fuel mixture.

Carbs initial setting is 1-1/2 turns open. The motor should run on that mixture at low RPM. Normally you would need to open them up a bit to acccelerate. That setting must be done in the water.

 

[wcsd106]

Just an update even though it will make me feel like an idiot. I got the idle pulled down by ear until it sounded good and the tach was still reading 1800 RPM.. Then it jumped to 3400 RPM. The engine sounded the same.

Took the tach out, and it was half filled with water.. Replaced it with a new Faria tach and it was idling around 850 RPM. Took it out to the lake yesterday and aside from running a little rich, which I can adjust out, it runs like a scalded dog.