What tach do I need ?

mckeemm

Petty Officer 3rd Class
Joined
May 17, 2006
Messages
98
Hi I have a 77 merc 700 3 cyl.

I'm working through some typical problems.

i.e.

Not charging - Installed new rectifier now charges high (16.8 V) New battery, clean connections.

Inconsistent Idle - Checked and cleaned fuel system from tank to carbs (2), pulled carbs and installed new needle and seats set float level to book spec. Adjusted idle air bleeds to 1 1/2 turns out. Does not idle hot or cold, engine stalls when approaching idle. I need to keep the warm up lever about half travel, which makes for a hard shift.

Peculiar Starting - To cold start I must move the warm up lever to max and hold down the choke button, starts right away and I can move the lever back to half travel. If I move the lever closer to idle the engine stalls. If I attempt to start the engine with the warm up lever any where but wide open the engine fires but will not run. Fuel supply seems fine.

One thing I noticed was that it is possible to overpower the float valve if I really squeeze the primer bulb hard.

Could the floats be water logged (fuel logged)?

I’m looking for info on what type, part number and source of tach can be used to check my idle and WOT rpm.

Any help much appreciated.

Mark
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
28,277
Re: What tach do I need ?

Mark, For best operation, you should have a permanent tach mounted on the dashboard. If you poke around the disconut marine places, you can get a teleflex tach for $75 or so.

PS, either your idle speed is too low, your carbs are not adjusted correctly, or you have some other carb issue. Overpowering the float is normal for these motors.
 

Caddy_Kid

Petty Officer 2nd Class
Joined
Nov 18, 2005
Messages
113
Re: What tach do I need ?

I have Faria gauges on my boat, and they are very affordable, have a lifetime waranty, and are of great quality. I bought mine here: http://www.boatersland.com/faria.html. You can download the instruction manual from the faria site and see what pole setting you would need to use.

As far as idle, my 100 HP merc starts in much the same manner. Needs full choke and some idle throttle. Not a big deal as they are a bit cold blooded. However, it idles great at 1000 RPM (which is higher than it should, but as low as the old reeds will let it). Sounds like your idle mixture screws arent set quite correctly yet. 1-1/2 turns out is a good starting place. I tuned mine while it was idling in a bucket in the driveway. Fire it up, let it warm up a bit, then throttle it back until it is as low as you can get it to go. Then go back to the carbs and turn the screws a half turn or so at a time on each carb until it picks up a bit in RPM. Lower the idle throttle and repete. Thats what I did with mine anyway after I took the 3 carbs it has apart. You have probably already done this though.

You'll want it to idle around 800 or so out of gear and around 500 or so in gear. Mine is old enough and has low enough compression that 1000 RPM's is as low as she will go and still stay idling without coughing and eventually loading up and dying.
 

mckeemm

Petty Officer 3rd Class
Joined
May 17, 2006
Messages
98
Re: What tach do I need ?

Thanks guys,

I'll try that idle set-up proceedure in a barrel again. That's why I need a tach, it sounds like the engine is running real slow as I retard the throttle but will quit with the idle stop screw stlill a 1/4" off the original stop setting.

There appears to be a significant deadband in the throttle control. I may be asking the thing to run slower than is possible. without a tach it's hard to tell.

From other posts I have read I believe I need a 12 pole tach to run with my stator. I missed a Teleflex #58477P on ebay yesterday. Can anyone confirm that this tach will work for me?
 

mckeemm

Petty Officer 3rd Class
Joined
May 17, 2006
Messages
98
Re: What tach do I need ?

I did the idle mixture adjustment again and I have the engine idleing on the idle stop. The engine shifts fine and takes throttle without hesitation. I did this on the muffs so it's time for a water test. I may have to tweek things a bit with the engine under load but at least I know the proceedure. The idle air bleeds are very sensitive on these carbs.

Mark
 
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