Mercruiser 3.0 Idle adjustment

[rmcel123]

I have a 2001 Bayliner Capri 1950 that has a Mercruiser 3.0 and uses a Merc carb #3310-807504A1. I bought the boat last year (1st time boater) and ran it for the year to see if I would enjoy boating. The boat ran fine for the most part but it was hard starting it at times, both cold and hot. I decided to rebuild the carb this year.

I got the kit from Mikes Carb shop online and proceeded to tear it down. First thing I noticed was the choke was wide open and would not move at all. Answer to hard starts when cold! I actually had to remove the butterfly plate on the choke and heat the shaft ends and force the shaft out. It was really stuck in place. I took every part down and cleaned them well and then proceeded to rebuild. I have 2 excellent resources at work that assisted when I had questions and I am very confident that the carb went back together properly with all adjustments made correctly. I cleaned the shafts and greased them a little to make sure they wouldn't freeze again.

The boat started right up when done, however it was idling high(1100rpm). It ran great for the test run. I was so pleased that it fired right up every time, even with a dead alternator! That's another story but a new one is now on the boat. I tried to adjust the idle using the idle screw and idle screw on the throttle but couldn't get it below 900 without it really running rough. I read through some more info and realized that I should disconnect the throttle while doing this. I have the idle down to 750-800 and it runs pretty well at that, however it is supposed to be 650-700.

I checked for vacuum leaks, bad wires, etc using a spray bottle with soapy water. Wires are all good, no changes in idle when wet. I retightened all carb bolts and sprayed and again, no changes in idle. I can get the motor to 650 but it runs like crap and usually dies.

Does the 3.0 really idle well at 650rpm? Should I just leave it at 750-800 and go with that? Any ideas what to check that may improve idle at lower rate? All filters and fluids are new. New plugs, rebuilt carb, gas is good. New alternator, good battery.

Thanks for any advice

Dave

 

[antsmands]

Re: Mercruiser 3.0 Idle adjustment

Hello Dave;

Check your idle speed adjustment which is used to set the initial idle speed. Back out on the screw until it no longer contacts the cam (boat not running), turn screw back in until it just touches the cam and then two additional full turns, then go about adjusting your idle mixture screw setting by seating it and then backing it out 1 1/4 turns. Start the boats on the water and make the final adjustments to the mixture to smooth the boat out. This should be done in idle with the throttle cable disconnected. If at this point the idle is still above 700 back out on the idle speed screw to get it down.

Tony D.

 

[rmcel123]

Re: Mercruiser 3.0 Idle adjustment

Thanks Tony. I have tried the setup you described for this carb a few times. Can't get that idle down to 700 and the motor is very unstable at that rpm. Unfortunately I have to do this using muffs but I do let the engine warm up before adjusting. Does anyone have a 650-700 idle on a 3.0 Merc that is quiet and stable? It sounds like a Harley when it is that low! Also, adding 2 screws to the idle screw makes it idle way higher and I can't adjust the idle jet down enough to get a steady idle at low rpm. Maybe my carb linkage is not correct? Anyone have a pic of their carb linkage I can compare too?

Thanks

 

[fishrdan]

Re: Mercruiser 3.0 Idle adjustment

I have the idle down to 750-800 and it runs pretty well at that, however it is supposed to be 650-700.

650-700RPM,,, "in gear" while on the water with a warmed up engine.

Leave it at 800RPM, then take it out to the lake, then do the final adjustments. You'll have to play with the idle mixture (base of carb) and idle speed (throttle arm) adjustments to get it to purr, back and forth until it's idling well.

My 140HP (old 3.0) idles a bit lower than 800RPM out of gear, but drops to around 700 in gear on the water...

 

[fat fanny]

Re: Mercruiser 3.0 Idle adjustment

Are u using a shop tach? or going of the boats tach? Have you checked the timming? Do you have the required fuel pressure? Have done a complete tune-up? (plug,wires,cap-n-rotor,oil change and fuel filters)

 

[airshot]

Re: Mercruiser 3.0 Idle adjustment

On my 3.0 I found the best way to adjust idle and mixture was to have boat in water with son-in-law driving at dead idle speed.
Made all my adjustments in this manner, idle speed is higher on muffs but right at the 650-700 while underway in the water. Also the idle is smoother while underway than when sitting still or on the trailer. Underway is where you need it to run correctly, on the trailer doesn't mean much. Hope this helps.
Airshot

 

[rmcel123]

Re: Mercruiser 3.0 Idle adjustment

Thanks for the advice. I guess that I will have to put the boat in the water and adjust it there to get a real reading. I don't want to take the carb apart again if I can help it so I was looking for something else! I guess the water is the something else.

The boat has new everything except cap and wires, although I did spray those during my testing and there was no change in idle so they should still be good. I will take the cap off and check for moisture and corrosion. They are probably due for a change soon anyways.

My carb is an oddball version of the standard 2bbl Mercarb (3310-807504A1). It seems to be the one between the old and new versions. It has 3 adjustments on the choke arm while the old one has only one. The setup procedure for this carb says that it was best to make it a little leaner and set the choke to 2 tics leaner as well because the old settings were too rich. I may set those back to normal to see if the extra gas will make it idle smoother.

I will keep you posted

Dave

 

[rmcel123]

Re: Mercruiser 3.0 Idle adjustment

As promised, an update on my idle problems. I got the boat in the water last night and took it for a spin. Started right up with one pump of the gas and then back to neutral. Boat runs great now, no bogging issues at all. The throttle is very responsive, strong hole shots, no issues at all.

Except the dreaded idle! I have a Snap On timing light that has a tach feature on it. In gear at "idle" the rpms drop to 750-800 and the motor idles well.In neutral and out of gear the idle is about 900-1000 and it idles solidly, that is, no sputtering, shaking motor, etc. I played with the settings and in gear I could get it down to 600-650 but the motor ran poorly and stopped a few times. Also, wouldn't start easily with the jet/throttle set that low. I backed it off to the higher setting. I did back it off a little since the boat "ran on" when I turned it off. It now shuts off without any issues.

The only things left to do is replace the cap and wires and install a water/fuel filter. This may improve things but I have my doubts. I think that I am going to leave it idle a little high and live with it. The boat goes into gear easily without the nashing of gears, it has a solid clunk. I have checked/replaced just about everything that it could be so I don't know what else it could be.

Does anyone have a 3.0 that idles quietly at the correct rpm?

Thanks for all of the help.

Dave

Thanks for the advice. I guess that I will have to put the boat in the water and adjust it there to get a real reading. I don't want to take the carb apart again if I can help it so I was looking for something else! I guess the water is the something else.

The boat has new everything except cap and wires, although I did spray those during my testing and there was no change in idle so they should still be good. I will take the cap off and check for moisture and corrosion. They are probably due for a change soon anyways.

My carb is an oddball version of the standard 2bbl Mercarb (3310-807504A1). It seems to be the one between the old and new versions. It has 3 adjustments on the choke arm while the old one has only one. The setup procedure for this carb says that it was best to make it a little leaner and set the choke to 2 tics leaner as well because the old settings were too rich. I may set those back to normal to see if the extra gas will make it idle smoother.

I will keep you posted

Dave

 

[Don S]

Re: Mercruiser 3.0 Idle adjustment

I have adjusted many that idle at 700 to 750 in neutral and 650 to 700 in the water with no problem. You do have to have a good tuneup on them with proper timing and idle mixture for them to run properly.

 

[fat fanny]

Re: Mercruiser 3.0 Idle adjustment

Don is spot on it does require all fresh parts to start with and a little tweeking but it does payoff. I have had no issues with runability by following the service manual proceedures. Double check your timming after u change the cap,rotor and wires. Good luck!

 

[airshot]

Re: Mercruiser 3.0 Idle adjustment

Are you sure your tach on the timing light is correct. It did take some time and fine tuning to get my 3.0 where it is supposed to be and on ocassion it has stalled with the low idle, but only on ocassion and it does start right back up. Not sure what you mean by rough idle at 700 rpm, mine idles well at 700 but idles better at 750/800. Having the boat in the water creates enough drag to lower the idle speed a bit, while trying to do it on a trailer with a muff will give you false readings. You might try borrowing another tach and compare RPM to be sure yours is accurate. Follow Don's advice and check/replace everything in your tuneup.

Airshot

 

[boateecat]

Re: Mercruiser 3.0 Idle adjustment

I have a '99 Bayliner 1952 cuddy with a mercruiser 3.0. This boat is new to me last year and I am also a newbie boat owner. After experiencing stalling on acceleration and poor idle, etc. early this season, I rebuilt my mercarb in June with Mike's Carb kit #529 (same kit as yours). The kit came with 2 different needles; one was solid and the other had a spring. The instructions tell you to use the solid needle initially and if you have issues with the engine idle, replace it with the spring needle. The only problem with that is with the spring needle, you can lose power in a tight turn at speed. I used the solid needle, and I have some minor issues with rough idle and stalling at 600-650...but smooths out nicely somewhere between 650 & 700 rpm, in the water, in gear, at idle (trolling speed). I didn't bother to change to the spring needle, since I am OK with my idle just under 700 rpm.

I also completed Don's list as far as tune up and tweaking of the A/F is concerned. Like airshot said, make the A/F adjustments under the same conditions...on the water, in gear, at idle speed. Not only is it the best way...it is the ONLY way to do it.


Joe

 

[Pdevito]

Is there a special tool to read rpm. My auto tach reads between 680 and 720 and 800 on muffs. When doing cars in the old days it seemed to be a stady number. Also should the clamp be on the #1 plug wire?

 

[achris]

Old thread. Start a thread of your own.

Chris.......