Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

[cps22]

I have a 2005 2+2 115hp.

It runs great when running on all 4cyls, perfect actually.

At times, idling is the problem. Not all times, but about half the time it idles very rough, and smokes a lot. Other times it idles perfectly for long idles into large marinas.

I'm wondering that perhaps the fuel or spark isn't being stopped below the ~1900 RPM and thus causing the rough idle.

So the question, what exactly causes the motor to switch from 4 to 2 cyls?

Things I've tired:
Rebuild carbs, twice, once this year, once last year, no change, inside of carbs clean.
New spark plugs, adjusted the "sync" of carbs per manual, etc.
Substituted a new coil through each of the 4 coil positions, no channge.
Also tried different gas, no ethanol, hitest, etc.


What actually shuts off the 2 cyls at low RPM, is it centrifugal, vacuum, magic?

Thanks in advance,

CPS22

 

[CharlieB]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

Air flow. The lower two carbs do not have idle circuits and do not pass enough fuel to support combustion, only enough for lubrication. As the throttle is opened, air flows and fuel begins to get pulled through the main jet. Around 1500 the airflow is enough that the fuel amount is enough for combustion. The lower cyls have an accelerator pump to give a small shot if fuel as the throttle is opening to assist in accelerating the motor.

Synching the carbs, float levels, idle mixture adjustment of the top carbs, accel pump adjustment, timing, all must be perfectly set following the sequence laid out in the factory service manual.

Check this thread to see if your manual is still available

http://forums.iboats.com/showthread.php?t=309079&p=2072211#post2072211

or here

http://1manual.com/Mercury manuals/Service_Manual_Index.html

 

[dlplost]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

check the carbs to see if they are all in the correct positions...

 

[Faztbullet]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

There is also 2 injectors that squirt fuel into cylinders from the accel pump, they are just really a needle and seat with heavy spring.....

 

[cps22]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

Thanks for the suggestions Captain,
The carbs are in sync, float levels checked twice (once this year, once last year), and timing adjusted per Merc manual.

The weird thing is that the idle problem is intermittent. My wife and I are boating in TN on Norris Lake, put on 87 miles today. Been in and out of several marinas and sometimes the idle is good, other times its rough with noticable smoke. Note that the idle time is often 10+ min into these marinas.

When I adjust the idle needles, I start with 1 &1/2 turns out, and adjust very slowly in both directions. From the forums here I understand that the adjustments are subtle, but I don't seem to notice much change unless I turn more than 1/4 turn.

One more observation, if the motor is off for awhile, like when we've been in a marina getting gas, or snacks, it usually idles great when leaving. No smoke, no rough idle, smooth as can be.

However, the idle is bad after running awhile. Note that I'm not hot-rodding, nor backing off the throtle fast, just backing off slowly until I feel the 4cyl go to 2cyls then go all the way to idle. About half the time the idle is rough. Rough enough that if I don't put it in neutral and rev it up a bit, it will die. Again, the weird thing is that sometimes the idle is OK, no smoke.

We've put on over 700 miles in the last 16 days so I've had lots of time to observe the motor's operation.

Any other ideas considering that the idle problem is intermittent?


Dlplost, not sure what you mean if the carbs are in the same position. I assume you mean in sync, yes they are adjusted per merc manual specs. Note the problem is intermittent too.


Faztbullet, not sure what you mean by injectors with the spring.. are you speaking of the low speed adjustments on the upper carbs? If so, I've adjusted numerous times trying to get a smooth idle. When the motor is idleing smooth, the adjustments seem to make a difference, when the idle is in its intermittent "rough idle mode" the adjustments don't seem to make a difference.

 

[docster]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

the nature of this motor is intermittent rough idle. even when adjusted perfectly it will do that. just a poor design really. the only way i see to make it idle smooth all the time is to get rid of that 2+2 system. There is a way, im just not sure how yet. I know people who have had it done, a friend of mine has exact motor I do on his pontoon and he took to a shop near us that set it up so it will run on 4 cyl all the time. His idles incredible all the time. But the shop wont tell me what they did and im not giving them money to do it just yet. So its possible, just have to figure out how. it may call for routing your own fuel lines and putting on 4 identical carbs and bypassing the accelerator pump altogether. Thats what im thinking. I will probably try it this fall and see what happens...just need to find 4 carbs for cheap that fit this motors intake

 

[Faztbullet]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

Faztbullet, not sure what you mean by injectors with the spring

Follow the accelerator pump hose to under the switchbox and you will see two brass injectors that the line is attached to and re-routed back. You could also have a check valve/s in the recirculating system or behind the reed block that is defective causing fuel to puddle and causing the rough idle and smoke. When it sets,the fuel will weep down thru the crankcase allowing it to idle correctly and when you rev it it it clears it out. It could also be something really simple like a inlet needle sticking causing a carb to flood slightly....

There is a way, im just not sure how yet. I know people who have had it done, a friend of mine has exact motor I do on his pontoon and he took to a shop near us that set it up so it will run on 4 cyl all the time.

I have seen this done but never successfully as the engine idles TWICE as bad and shakes like a dog pooping a peach seed!!!!! That the reason it a 2+2 design because its a looper and they dont idle good at low rpm.

 

[cps22]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

Good information everybody, thanks.
When I get back from vacation I'll check the check valves. I assume that once removed, they can be checked with a vacuum pump while noting the vac level that they open.

Thanks,

CPS22

 

[LCSD147]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

I have the same problem with my '03 model. Fresh plugs and fuel filter annually helps. Also,
Fresh fuel with ethanol treatment. I've been wanting to trade it for a new four stroke.

 

[Texasmark]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

Ok you guys in the know. Just what was Merc thinking when they went to all this trouble to kill 2 cylinders when 4 cylinders was the main reason I liked the Mark 30, Mark 55, and later versions and having 4 cylinders slow speeds and especially idling and all were soooooo smooth. I really think it sucks and that is one reason I bought a 90 3 cyl.

For a guy who had one and got tired of all this, could you trash the bottom 2 carbs and replace with another pair of top carbs and get full 4 cyl 24/7?

Mark

 

[Faztbullet]

Re: Mercury 2+2 Question, what causes it to switch from 4 to 2 cyls?

Just what was Merc thinking when they went to all this trouble to kill 2 cylinders when 4 cylinders was the main reason I liked the Mark 30, Mark 55, and later versions and having 4 cylinders slow speeds and especially idling and all were soooooo smooth.

Because the Mark 30,45,55 and the small bore inline 4 cylinders were all crossflows.....when they went to the big bore looper it killed the idling, that why most loopers are a 3 cylinder or V-4/6.