Idle Issue Mercruiser 3.0

I’ve done a bit of reading on idle adjustment, but haven’t found any info on my particular problem. My rpm at neutral idle is lower than at engaged “trolling” idle. It goes from about 700rpm when engaged to 550-600 rpm when in neutral. It bogs down in neutral and eventually stalls. It even bogs down when engaged but at least doesn’t stall. It also runs rough in neutral, some engine movement.

It should be higher rpm in neutral right (750 rpm)? I was going to adjust the idle but don’t understand how to make the idle higher in neutral than when engaged, which is what I’ve heard is supposed to happen.

I am assuming you have a 2bbl Mercarb - here is the final idle speed adjustment procedure.

First check / adjust timing.

Preliminary Idle Speed and Mixture
IMPORTANT: The following adjustments will provide
a sufficient idle speed and mixture for starting
engine. Final adjustments must be made with
engine running.
1. Back out idle speed screw until it no longer contacts
idle cam. Turn idle speed screw in until it just
contacts idle cam, then turn screw in an additional
two turns.


Final Idle Speed and Mixture
IMPORTANT: Boat must be in the water and engine
at normal operating temperature to accurately
check and adjust idle speed and mixture.
Carburetor should be set so that engine idles
smoothly within range given under “Specifications,”
with boat in the water, engine at normal operating
temperature and drive unit in forward gear. To adjust
idle speed and mixture, proceed as follows:
IMPORTANT: DO NOT attempt to compensate for
other engine problems (incorrect ignition timing,
faulty ignition components, low compression,
vacuum leaks, etc.) with carburetor adjustments.
This will only cover the problem, which must be
corrected if engine is to achieve maximum fuel
economy and performance.
1. Connect an accurate shop tachometer to engine.
IMPORTANT: DO NOT turn idle mixture needle
tightly into seat, as damage to needle and/or seat
may result.
2. If new or rebuilt carburetor has been installed,
turn idle mixture needle in (clockwise) until it lightly
contacts seat, then back out needle 1-1/4
turns. This will provide a sufficient setting to allow
starting engine.
3. Start engine and run at 1500 RPM until engine
reaches normal operating temperature.
! WARNING
DO NOT leave the helm unattended while performing
idle speed and mixture adjustments, following.
BE CAREFUL NOT TO ACCIDENTALLY
ACCELERATE ENGINE WHILE PERFORMING
ADJUSTMENTS.
4. With boat in open water, place remote control in
forward gear, idle position.
5. Disconnect throttle cable barrel from anchor
stud. Be sure not to lose spacer on anchor stud.
6. Adjust idle speed adjustment screw to obtain
550-600 RPM, in neutral gear.
7. With engine running at specified RPM, adjust idle
mixture needle as follows:
a. Turn idle mixture needle in (clockwise) until
the engine speed begins to drop due to lean
mixture.
b. Turn idle mixture screw out (counterclockwise)
until the speed begins to drop due to
rich mixture.
c. Turn screw in to a point between these two
extremes to obtain maximum engine smoothness
and RPM.
d. Readjust idle speed adjustment screw until
engine idles at 650-700 RPM in forward gear.

Thanks. Any comment on if it’s normal for the neutral idle to be lower than the engaged in gear idle? Shouldn’t it be the opposite?

That's not normal - should idle 50-100rpm higher in neutral than in gear. In gear idle should be @650rpm. Follow the process to set the idle - I bet the idle screw is holding the idle cam open too far in neutral or the throttle cable is out of adjustment and allowing the throttle plate to remain open slightly in neutral. It just takes a small amount to affect the idle.

So I haven’t messed with the idle screw yet because rpms have fluctuated up and down. Randomly at two different times it went to 900rpm engaged/trolling and would jump to 1000rpm at neutral. Every other time it’s at 700rpm engaged and 500-600rpm (eventually stalls) in neutral. I added stabilized to the fuel and added 5 gallons of 93. It was at 1/3 of a tank before that. It’s almost out of gas now but no improvement other than those two instances I noted above. The issue is I don’t know if it’s too rich or too lean, since it is usually bogging down but at times was too high. Also the temp so far is between 125 and 140... usually stays at 125 but I haven’t pushed it much since I’m trying to work out a few bugs. Any ideas? Should I just mess with the idle mix regardless? I don’t see how that would fix fluctuating idle issues but I’m brand new to this.

A few things to troubleshoot:

Do you know the history of the boat? Does it run ok at speed?

Temp: Sounds like you have the wrong thermostat or maybe no thermostat - engine should run at around 175deg and have a 160deg thermostat. An engine running too cold is almost as bad as too hot. It could cause problems such as the choke not functioning properly.

Carburetor: If it runs ok at speed, just not idling - Get can of carb cleaner, take the flame arrestor off and give it a good cleaning. Spray it into the vent hole on the top - this will get it into the blowl and clean the float and needle.

Fuel Tank: Possible you have a fuel tank problem - ethanol fuel and age deteriorates fuel lines - plumb in a portable tank to supply fuel / test. If it runs fine on a portable tank - Check the vent line, check the fuel feed line, check the anti-siphon valve. I myself had strange fuel delivery problems on my 185 - traced it to a deteriorated fuel line that was sucking air and depositing rubber debris into the carburetor and clogging up the fuel filters. Had to replace the fuel lines and clean/rebuild the carburetor. Later I had a high RPM surging problem I traced to a faulty anti-siphon valve. On a 20yr old boat all of the fuel lines would be suspect.

Fuel filter: Clean? What about the carburetor inlet filter?
Distributor cap/rotor in good shape?
Spark plugs / wires in good shape?
Correct firing order?
Timing set correctly?

In the end, it might be time to clean/rebuild the carburetor. Easy to do on a Mercarb, there are kits available for around $50. But I would verify all the things I listed above are in good order -- any one of those can cause problems and none are hard to check and correct if necessary.

ok it looks like the choke flap is stuck open. I can’t get it to rotate. I removed the thermostat (?) and see a screw that connects to the rod the flap is on... is that how I adjust it? I watched a video and it seems it should adjust at this point but it’s still stuck. This seems like the culprit of at least some of my problems (temp and idle?)

Supposedly the previous owner rebuilt the engine last year.

Are you sure this is a Mercarb? I have been assuming it was, but on a Mercarb the choke is electric and you adjust it by loosening the screws on the thermostat (round black thing on STB side of carb) and turn it lean or rich. The setting for a 3.0 is 1 tick mark to the left of center. Make sure you are getting 12v to the choke. But, the choke flap being stuck open will not affect how it runs and idles once it is warmed up.

Yes that’s what I have... so that plastic part spun but did not affect the choke flap. I eventually removed the screws entirely trying to figure out how to move the flap. It’s just stuck upright and I’m hesitant to force it to close.

Google Mikes Carburetor - he has a lot of videos about adjusting and rebuild carburetors, including the Mercarb. I bought my rebuild kit from him, watched his video and rebuilt mine myself. It sounds like something is off with your choke pulloff lever. This might be causing it to be hard to start cold, but I doubt it is your surging / stalling problem. Did you check the things I mentioned above?

I haven’t had time to check all of those but will. I just noticed the choke was stuck open and decided to investigate. I think the throttle arm was installed wrong? When I readjusted the choke it required repositioning the throttle arm. See how it was misaligned in the photo below after I followed the procedure to set the choke.

Sounds like your choke and maybe the linkages are all gunked up. Needs a good cleaning and adjustment.

I had a 3.0 for a long time and it was the best running motor i ever owned. Mine used a 140 deg. thermostat, per the shop manual. Maybe they changed...I dunno. But before you go further better find out what your specific engine requires.

Temp changes may indicate plugged passages somewhere, or more likely your impeller is shot as well..

cjnap said:

I haven’t had time to check all of those but will. I just noticed the choke was stuck open and decided to investigate. I think the throttle arm was installed wrong? When I readjusted the choke it required repositioning the throttle arm. See how it was misaligned in the photo below after I followed the procedure to set the choke.

Click to expand...

Throttle cable looks a little out of adjustment - it should push the throttle plate all the way shut when in neutral, but that looks a little too far.

So the throttle lever on the carb has a broken piece (the metal piece that you bend to adjust the choke engagement). It looks like the previous owner adjusted something to keep the choke open permanently which is why I thought it was stuck. I’m looking into replacing that throttle lever and will test the electric choke element. Hopefully don’t need a new carb, but the rebuild kits don’t seem to include these external components.

2.5 and 3.0’s were mercs best engines. I’m sure you’ll figure it out. Start w the simple things.

I’m the type that if my engine sneezes I’m buying a new carb. Rarely serves me well.