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merc 3.0 idle adjustment

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vroomZOOM

Regular Contributor
Hello

you will probably think this is a dumb question but I can't seem to get the idle right on my 3.0 merc. I have replaced the coil cap and rotor. carb has been winterized with fuel stabilizer all the time. I can get the engine to idle at about 900 rpm not in gear fairly well, but it will not start without giving it a bit of throttle and warming up for a bit. also during shifting when the shift interrupter kicks in it may sometimes stall. I feel like taking out the fast idle cam and grinding it to give it a bit more "caminess", but I think there has to be another way to make this motor cooperate.
 
Don't be grinding anything. . Most likely needs a carb kit. But try this first.
The mixture screw for the carb. Is it visible or is there a lockout on it.? If you can close the mixture screw the engine should stall. If it doesn't stall 1 of 3 things may be at fault. The carb needs a rebuild or you have a vacuum leak. Or the ignition timing is off. Do you have access to a timing light ?
 
900 is way to high even in neutral.
Spec is 650-750 in gear.
Engine should if all is good idle in neutral around 800 max.

The lower/closer to 650 in gear the better for the shifting.

Stalling when coming out of gear if cable and shift linkage is all good, typically is shifing to slow. Quick and firm to neutral.

If you cant idle down to spec then as suggested, a carb rebuild.
If only one idle mixture screw its a Mercarb.
If two idle mixture screws its a Rochester
 
well, here is the way i tweaked it. it idles at about 750 in neutral quite nice now. haven't tested it in gear yet. yes it will stall if I close off the idle circuit screw. I adjusted it so it idles smoothly, without any missing. what my question was, should I have to give it a little throttle on a cold start? I am thinking that if I grind a little off of the idle cam it will push on the throttle more when cold but when the choke opens it will let it down to the ground out area and the engine will idle slower. im trying to make the engine start from cold without having to push the throttle open, should it be like that, or do they initially have to take some coaxing?
 
There is no Idle cam on a marine engine (no eccentric), unless your carb has been replaced with an automotive carb.


Do not modify anything. If it is not working correctly the way it was designed something needs service.
 
what my question was, should I have to give it a little throttle on a cold start?
Click to expand...

Ayuh,..... Hot, or cold,.... Carbed motors like alittle throttle when startin',.....
 
If you are asking what is proper starting method,

1 pump throttle a couple times
2 leave throttle at ~1-2 oclock
3 turn key and start engine.
4 set idle at ~ 1500 rpm and maintane for 1-2 minutes when engine is cold maybe longer.

4 cyl can take a bit to warm up.

If you are getting idle change/response from adjusting idle mixture the your carb may be in good shape.
750 in neutral base idle is good no need to adjust lower if engine runs good in neutral and in gear.
 
Chris nailed it in post #5 regarding a marine carburetor not incorporating a fast idle cam.
It may be possible that someone has install an incorrect carburetor on this engine.
 
RicardoMarine said:
Chris nailed it in post #5 regarding a marine carburetor not incorporating a fast idle cam.
It may be possible that someone has install an incorrect carburetor on this engine.
Click to expand...


I have to ask, what is the purpose of this post?
Anyone have a clue?
 
Yes, the idle mixture screw works quite well, it will kill the motor if I go too rich or too lean. I set it halfway between those two and it idles well. was the motor initially from the factory supposed to start cold without throttle? my previous boat did not have the option of advancing the throttle without putting it in gear. I hope you get my idea about the fast idle cam, I have a spare one so I will try it and tell you how it pans out.
 
All carbed I/O's require what i described on cold start up.

Marine carbs CANNOT have high idle cam on carb. Coast Guard regulation/law.

Do not put one on.

what you show in picture should be a smooth "cam" the main idle screw sits on. It is smooth for a reason and if not it should be smooth.
 
kghost said:
All carbed I/O's require what i described on cold start up.

Marine carbs CANNOT have high idle cam on carb. Coast Guard regulation/law.

Do not put one on.

what you show in picture should be a smooth "cam" the main idle screw sits on. It is smooth for a reason and if not it should be smooth.
Click to expand...


uh oh that kinda sucks. (im in canada tho, so maybe here its allowed)
 
still doesnt make sense. I cant see any law on the us coast guard that doesnt allow a fast idle warm up cam. on the mercarb exploded diagram it is called a fast idle cam. Why would they even put that thing there if it does not do anything? wierd. the fact that my previous boat did not have a fast idle on the controls (not possible to advance throttle by itself at all) means that these things should be able to start by themselves without giving it throttle.
 
Listen
Its your boat do as you please.

If you use your head and think about it you may realize why NO carbed marine engine has a high idle cam on the carb.

So think before you kill someone!

Your on your own!
 
kghost said:
Listen
Its your boat do as you please.

If you use your head and think about it you may realize why NO carbed marine engine has a high idle cam on the carb.

So think before you kill someone!

Your on your own!
Click to expand...
well it seems like I am an idiot, so what is the reason? I am still not getting this.:( its got what is called a high idle cam there for a reason???? or is the mercarb just a knockoff of the gm carb and they didnt even bother to get rid of it. really this is not making sense. and yes I would like to find out and read the actual us coast guard reg on that. I am not picking on anyone here, I just like it when everything is there for a purpose and I need to know why its there. I am trying to expand my knowledge on this stuff.
 
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If you are in gear heading towards a dock or a person/people in the water, and your choke malfunctions and the choke rod lifts and the high speed step of the cam is now your idle setting which could be 1000-1500 rpm or possibly higher if adjusted wrong and you cant slow down and you cant shift out of gear due to high rpms.....

YOU FIGURE OUT THE REST!!
 
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vroomZOOM said:
well it seems like I am an idiot, so what is the reason? I am still not getting this.:( its got what is called a high idle cam there for a reason???? or is the mercarb just a knockoff of the gm carb and they didnt even bother to get rid of it. really this is not making sense. and yes I would like to find out and read the actual us coast guard reg on that. I am not picking on anyone here, I just like it when everything is there for a purpose and I need to know why its there. I am trying to expand my knowledge on this stuff.
Click to expand...

If that carb has a stepped cam it is wrong.
Someone changed it and should not have.

I have worked on a few thousand carbed boats and I can garentee they all have smooth idle cams. NO STEPS.

Who cares if it is actually a regulation. If it is it is written for the manufacturers.

I understand you want to understand but if you dont take the words of some of us WHO KNOW. What good is the answer.

No one is trying to trick you.
It is smooth for safety, End of story!
 
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ie the radius is fixed for the arc that sees the idle screw....

All the factory service manuals I checked (3) refer to the part as the "Idle Cam"......

I checked the CFRs regarding the carburetor - no mention of fast idle cams....I believe its just a prudent design for the various examples cited previously...
 
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