Proper 1996 Merc 5.7L Idle RPM

[DYLBOSS]

I'm trying to figure out if my tach is correct. I bought an hour/RPM meter and have installed it. I have one wire wrapped around plug 3 a bunch of times and a ground going to the engine chassis. Tried 2 different spark wires too with the same results. So far things seem to be way out of wack. The meter tells me to configure it with 1 of 3 settings:

1. Spark plug fires twice per revolution
2. Once per revolution
3. Every other revolution

Not a mechanic by any stretch, and just trying to get to the bottom of this. Does anyone know what setting this should be on for my motor? So far, option 1 has produced the most reasonable results with 1300 RPM at idle. The others are 2k+.

When I looked online at the Merc engine specs it said that it should be in the 650 range. Is that right? Sounds very low.

I should add that I tried this in my driveway with muffs on, if that makes any difference.

Thanks.

 

[Bondo]

Re: Proper 1996 Merc 5.7L Idle RPM

When I looked online at the Merc engine specs it said that it should be in the 650 range. Is that right?

Ayuh,.... 650/ 700 rpms, in gear, in the water....
On the muffs, maybe 800rpms...

 

[DYLBOSS]

Re: Proper 1996 Merc 5.7L Idle RPM

Thanks. Ok, interesting then. Something's not right. My real tach is saying 800/900, but the new mechanic's tach is saying 1300. I don't get it.

 

[Oldskool]

Re: Proper 1996 Merc 5.7L Idle RPM

The proper setting should be #3. A four cycle engine, which you have, fires the spark plug every other revolution.

 

[dirtyoldman]

Re: Proper 1996 Merc 5.7L Idle RPM

The proper setting should be #3. A four cycle engine, which you have, fires the spark plug every other revolution.

Correct. Once every other revolution. I'd recheck the wiring. You'd know if you were idling at over 2k rpm.

 

[DYLBOSS]

Re: Proper 1996 Merc 5.7L Idle RPM

Correct. Once every other revolution. I'd recheck the wiring. You'd know if you were idling at over 2k rpm.

Ok, thanks guys. I'll double check the settings tomorrow.

 

[DYLBOSS]

Re: Proper 1996 Merc 5.7L Idle RPM

Ok, something's not right. Had the drive in gear at idle and the tach said around 750RPM. Looked at the other tach and it said 4000RPM. Don't know what is going on here. I have the tach set to every other revolution and I have the wire wrapped around the 3rd plug wire. Should it be wrapped around another wire?

 

[dirtyoldman]

Re: Proper 1996 Merc 5.7L Idle RPM

Ok, something's not right. Had the drive in gear at idle and the tach said around 750RPM. Looked at the other tach and it said 4000RPM. Don't know what is going on here. I have the tach set to every other revolution and I have the wire wrapped around the 3rd plug wire. Should it be wrapped around another wire?

It shouldn't matter which plug wire it's wrapped around. 750 RPM is too high, and your tach may be bad on the one that reads 4000rpm.

 

[180shabah]

Re: Proper 1996 Merc 5.7L Idle RPM

750 RPM is too high,

He is not in gear, in the water, so we don't know his real idle RPM.

 

[John_S]

Re: Proper 1996 Merc 5.7L Idle RPM

The inductive tach might be picking up signal from adjacent wires. Try wrapping the wire around the plug wire where it is the furthest away from the others.

I believe you have T5 and has to be in base mode at 650 rpms in neutral. I will recheck this evening. I just use a shop type tach that attaches to the coil lead.

 

[John_S]

Re: Proper 1996 Merc 5.7L Idle RPM

While I can't access the SM, I did find this in a '94 service bulletin: http://www.boatfix.com/merc/Bullet/94/94_02.pdf


Timing Engine or Adjusting Carburetor:
Because the Thunderbolt V Ignition Module has Idle Speed Spark Control, it has to be locked in ?Base Timing Mode? whenever you:
-Set or Check The Timing
-Adjust Engine Idle RPM
-Adjust Idle Mixture

BEFORE turning ignition key on or trying to start the engine, connect a jumper wire from the Timing Lead (PUR/WHT wire, see wiring diagram) to a good ground. This locks the Module into ?Base Timing Mode?. Now any of those three adjustments can be made.

IMPORTANT: The jumper wire must be connected to the Timing Lead and ground BEFORE turning ignition key on, otherwise the Module will not go into the ?Base Timing Mode?.

Adjustments are made the same way they were done on Thunderbolt IV engines.
The Idle Speed Spark Control in the Module is only active within a certain RPM range. Because of this, engine idle RPM has to be set exactly to the specified RPM shown and in NEUTRAL GEAR. The boat can be either in the water or on a trailer.

IMPORTANT: Use an accurate shop tachometer when setting engine RPM because it has to be at an exact RPM in forward gear.

Timing at Idle RPM: 10? BTDC
Idle RPM in Neutral Gear: 650

IMPORTANT: Be sure to disconnect the jumper wire between ground and timing lead or ignition will remain in ?Base Timing Mode?.

 

[DYLBOSS]

Re: Proper 1996 Merc 5.7L Idle RPM

He is not in gear, in the water, so we don't know his real idle RPM.

This is in the water and in gear.

 

[DYLBOSS]

Re: Proper 1996 Merc 5.7L Idle RPM

While I can't access the SM, I did find this in a '94 service bulletin: http://www.boatfix.com/merc/Bullet/94/94_02.pdf


Timing Engine or Adjusting Carburetor:
Because the Thunderbolt V Ignition Module has Idle Speed Spark Control, it has to be locked in ?Base Timing Mode? whenever you:
-Set or Check The Timing
-Adjust Engine Idle RPM
-Adjust Idle Mixture

BEFORE turning ignition key on or trying to start the engine, connect a jumper wire from the Timing Lead (PUR/WHT wire, see wiring diagram) to a good ground. This locks the Module into ?Base Timing Mode?. Now any of those three adjustments can be made.

IMPORTANT: The jumper wire must be connected to the Timing Lead and ground BEFORE turning ignition key on, otherwise the Module will not go into the ?Base Timing Mode?.

Adjustments are made the same way they were done on Thunderbolt IV engines.
The Idle Speed Spark Control in the Module is only active within a certain RPM range. Because of this, engine idle RPM has to be set exactly to the specified RPM shown and in NEUTRAL GEAR. The boat can be either in the water or on a trailer.

IMPORTANT: Use an accurate shop tachometer when setting engine RPM because it has to be at an exact RPM in forward gear.

Timing at Idle RPM: 10? BTDC
Idle RPM in Neutral Gear: 650

IMPORTANT: Be sure to disconnect the jumper wire between ground and timing lead or ignition will remain in ?Base Timing Mode?.

Thanks a lot for this information. This sounds a little over my head. I think I may just have the shop do this. I guess there's no real way to determine if the tach that I bought is accurate or not.

 

[180shabah]

Re: Proper 1996 Merc 5.7L Idle RPM

This is in the water and in gear.

Must be gettin'my threads mixed up, could have swore I saw on trailer, on muffs.

OOPS!!!

 

[DYLBOSS]

Re: Proper 1996 Merc 5.7L Idle RPM

Must be gettin'my threads mixed up, could have swore I saw on trailer, on muffs.

OOPS!!!

No, you are correct. However, I did try this both in and out of the water. Same results.