how to read RPMs on my engine

[robinsbd]

Hi, I am adjusting my carb and i am trying to read RPM. I tried using a difital timing light but it fluctuates too much.

Is there a better way? I have a digital meter and I tried hooking it to the ignition coil negative with red wire and negative on battery with black wire. I dont get any reading at all with the meter.

Right now, I'm just going by the dash tach, but I dont know if that is accurate.

Also, a related question: The dash tach says I'm idling between 750 and 800. Is this OK for shifting my drive?

 

[TyeeMan]

You can wing it using the in dash tach but to set the ripms correctly you need a shop tach. That said, the boat also needs to be in the water and to properly set idle rpm the boat needs to be secured to something and in forward gear.

Depending on year of boat/engine, idle speed is probably 650-700 rpm, , in gear.

I would say 800 r's in neutral is a bit high. <750 would get you more into the window

 

[alldodge]

Manual shows 650-700 rpm for idle.

Your helm Tach may be correct or a bit off, don't know without an accurate one to judge. Some are very accurate some are not. I would say if you slow the idle down and it appears to slow then it may be off. If you can slow the idle down and it doesn't then it may just be close enough

 

[Scott Danforth]

Digital multi meter, on DC and read frequency in HZ Then multiply by 60

 

[Kloctower]

A digital aftermarket will do wonders for you. Check out eBay

 

[Fun Times]

Hi, I am adjusting my carb and i am trying to read RPM. I tried using a digital timing light but it fluctuates too much.

Assuming the engine in question is the 1997 5.7L mentioned in your signature line, Are you grounding out the Purple/White wire to go into Base Timing Mode for whenever you, Set or Check The Timing, Adjust Engine Idle RPM, Adjust Idle Mixture...If not the timing will fluctuate a lot. Grounding the wire should help stabilize the timing marks.

Is there a better way? I have a digital meter and I tried hooking it to the ignition coil negative with red wire and negative on battery with black wire. I dont get any reading at all with the meter.

What meter model do you have? Is the meter an independent meter of the digital timing light you own or does the light have an tachometer display?

Right now, I'm just going by the dash tach, but I don't know if that is accurate.

Also, a related question: The dash tach says I'm idling between 750 and 800. Is this OK for shifting my drive?

800 RPM would be entering high middle ground for a still somewhat safe margin RPM that you could still shift into any gears as for keeping things within the safety zone...900+ RPM wouldn't be the best idea.

When you are able to a get a good working digital RPM meter connected and you start to find the right idle RPM range with the engine warm, you should be seeing a small fluctuation in RPM of 643 - 648 RPM....If you're above say 670 RPM, then if you listen real close to the engine idle RPM, you should be able to actually hear the engine RPM drop down into the 643 - 648 range as you slowly lower the idle RPM when backing off of the idle screw. Once you find that RPM range mentioned, now try revving the RPM up to 1300 RPM and be sure the RPM goes back down into the 648 range within one or two seconds. After revving, The RPM will usually drop to the 700 range before dropping/stabilizing around 648 when all is adjusted correctly...If you don't seem to notice the idle drop after a second or two or three, etc., then while it's not a big deal/concern, it just tends to indicate the idle screw may be backed out/off a bit to much but as long as you can hold the 643 -648 range after revving the engine, your still okay. Moral of the story is there is a fine point of adjustment once you get to know the system.

 

[robinsbd]

Assuming the engine in question is the 1997 5.7L mentioned in your signature line, Are you grounding out the Purple/White wire to go into Base Timing Mode for whenever you, Set or Check The Timing, Adjust Engine Idle RPM, Adjust Idle Mixture...If not the timing will fluctuate a lot. Grounding the wire should help stabilize the timing marks.

Yes, I grounded the purple/white wire, which locks the ignition module into base timing mode. Interesting thing here is that I can set the RPM in base timing mode using the dash tach, but when I take it out of base timing mode, the RPMs increase a little.

What meter model do you have? Is the meter an independent meter of the digital timing light you own or does the light have an tachometer display?

Bosch FIX 7677. This is a separate device from the timing light.

 

[Scott06]

When you take it out of base mode the Tbolt V will advance the ignition to "maintain a calibrated idle speed" per merc

You may have seen this as you seem knowledgeable on the system. http://www.boatfix.com/merc/servmanl/16/16b4r2.pdf

Pages 4B-15 -18 describe it.

Maybe your dash tach is reading high and where your setting the idle screw is below what the ignition module is trying to get to ? I'd set it and not worry about it too much. Unless it's clunking harder than an alpha normally does it will be fine.

fWIW I have an old sun pro clamp on tach dwell with multiple analog scales and it didn't like my Tbolt ignition either. Ended up getting this

http://www.tooldiscounter.com/ItemD...&kw=SHETA100&gclid=CMHoge7tnNACFQpYDQodPlYI-A

Works pretty awesome as a tach for any cylinder and cycle combo. It wireless for the tach so I was able to verify my indash tach up to 4500 rpm and get WOT rpms on my old outboard. The analyzer also does ignition voltage output to the plug so I could take baseline readings while it's running well.