Idle Issue - Mercruiser 165 Inline 6

I have a '76 Mercruiser 165 L6 with a One drive.

It has a Rochester 2GC carb and I just rebuilt it myself. The problem is I can't seem to get it to idle high enough. Idle mixture screws are 1 1/4 turns from seated.

With the idle screw turned all the way in, I'm not even getting 500 rpms. When I removed the clip that keeps the screw from vibrating out, I was able to turn the screw in a tiny bit more and was then able to get the correct idle speed.

Is there something wrong with my carb that I am having to turn the screw ALL the way in to get a decent idle speed?

Re: Idle Issue - Mercruiser 165 Inline 6

your adjusting the wrong one.
your trying to attain your idle SPEED by turning your idle MIXTURE screw. they are 2 different screws.
the idle speed screws is on the side of the carb, where the linkage hooks up.

You are doing this on the water correct? setting idle SPEED while in foward gear. And with the throttle cable disconected.

And I assumed you set your timing first.... correct?

Re: Idle Issue - Mercruiser 165 Inline 6

Let me clarify some things:

Dwell is set to 33, timing to 6 degrees per the manual.

I do know the difference between the idle mixture and idle speed screws. I set the idle mixture screws at 1 1/4 turns from seated and have not adjusted them any further. I have only been adjusting the idle speed screw and with it turned all the way in, I couldn't get higher than 500 rpms. Adjustment was done on the water, in gear.

I would assume that the 1 1/4 turns on the idle mixture would be a good enough ball park to at least get a good idle speed. Do I need to mess with these first or is there another issue with the carb that I'm not aware of?

Re: Idle Issue - Mercruiser 165 Inline 6

have ya done a compression test? so you know ya've got something to work with..... what are the numbers? http://www.boatfix.com/merc/Bullet/97/97_25.pdf

i'm wondering what your output pressure is on your fuel pump..... should be 3-6 psi..... did you replace your fuel pump filter?

i still question how you rebuilt the carb. and think this could be of issue... i soaked mine in carb cleaner + washed and scrubbed everything in soapy water. 2 times as my carb was known to be filthy and gummy. + compressed air dried paying close attention to blowing out all the little passages....

i agree, you shouln't have to turn the idle speed screw all the way in. question for the carb. gurus though. don't the idle speed screw open the throttle plates? if so, don't this render the idle mix screws inop as they only adj. the mix at idle, which would be with the throttle plates closed? speaking of which. does your idle speed screw in fact, as your screwing it in. start to open the throttle plates?

just looked at my aristocraft 2gc. i see the idle speed screw on it is screwed mostly in too. as you know, i used the 2gc off my trisonic for my rebuild. pretty sure my speed screw in not all the way in after the rebuild. though in some......but not bottomed out as you descibe, nor like my unrebuilt one off the aristocraft which was all the way in too.......

Re: Idle Issue - Mercruiser 165 Inline 6

I am guessing you set the timing at 6 deg BTDC. But at what RPM did you set it? Also how did you set the idle mixture?

Re: Idle Issue - Mercruiser 165 Inline 6

Yes timing was set at 6 degrees BTDC. It was set by holding the throttle at 650 rpms and then adjusting timing.

I have not adjusted the idle mixture besides setting it to the initial setting. Maybe I need to mess with that and see if it affects the idle, since the idle speed screw is already turned all the way in?

I have not done a compression test, but it's on the agenda for one of these days. Will also test fuel pressure and let you know.

Re: Idle Issue - Mercruiser 165 Inline 6

I think you should adjust the mixture screw. Turn in till it starts to stall then out till it starts to stall then split the difference. Reset timing and idle.

Re: Idle Issue - Mercruiser 165 Inline 6

Windykid said:

I think you should adjust the mixture screw. Turn in till it starts to stall then out till it starts to stall then split the difference. Reset timing and idle.

Click to expand...

Ok, I'll probably try that tonight, but do I need to set the throttle to 600 - 650 rpms before I adjust the mixture screws or should I adjust them where it is currently idling (~450)?

Re: Idle Issue - Mercruiser 165 Inline 6

cnr2003 said:

Ok, I'll probably try that tonight, but do I need to set the throttle to 600 - 650 rpms before I adjust the mixture screws or should I adjust them where it is currently idling (~450)?

Click to expand...

It will be a little difficult at 450.

Re: Idle Issue - Mercruiser 165 Inline 6

you should set the mixture screw at the lowest possible rpm.
and the engine should idle with the IDLE SPEED screw all the way out.
If it does not then there is a problem.

What I do is disconnect the carb,
set the dwell,
set the base timing,
back out the idle speed screw,
screw in the idle mixture screw to a light stop then back it out 1 turn.
Start the motor. Don't bother looking at the tach at this point because it doesn't matter.
Engine should idle but be rough.
slowly turn the idle mixture screw in until the engine wants to die.
then slowly back it out until it smooths out, the rpms should be coming up at this point.
keep backing out the screw, (slowly, as it 1/8th a turn and wait 10 to 20 seconds in between each turn) the rpms will climb and climb, then when you hit the sweet spot the rpm's should drop slightly. That's where you want it.
Then, put the drive in foward gear and set the rpm's using the IDLE SPEED screw.
You may have to spin the speed screw in first to keep it from stalling, then backing it out to get the correct rpm.
Do not use the boats tach to set your rpms. get a decent shop tach.
Sears has an actron tach/dwell meter for 30 bucks that works great.
Then i reconnect the carb, making sure when the shifters in neutral the throttle plates are completely closed, and when the shifter is all the way foward the throttle plates are at 90 degrees to the venturi.
Double check your timing
check your timing advance
go for a ride

Re: Idle Issue - Mercruiser 165 Inline 6

Ok, I had the engine idling at around 500 rpm's and the idle mixture screws at 1 turn out from lightly seated. As I turned the mixture screws in, the rpm's began to rise and kept rising until the screws were nearly seated. What does that mean? Will it hurt anything if the mixture screws are turned in close to seated? With the screws about 1/4 turn from lightly seated, I was getting around 800 rpm's with the drive in gear (in the driveway) and was able to back the idle speed screw out slightly to get the proper idle speed.

This was all done using a shop gauge to verify the rpm's.

Re: Idle Issue - Mercruiser 165 Inline 6

it means your leaning the motor out. when you back them out the mixture will richen up.
lean mixture = motor rev's up
richer mixture = motor rev's down

Re: Idle Issue - Mercruiser 165 Inline 6

cnr2003-
The idle speed screw on my 2GC also has to be screwed 'almost' all the way in, however, I am able to achieve 750rpm's at idle in gear.

captJason said:

slowly turn the idle mixture screw in until the engine wants to die.
then slowly back it out until it smooths out, the rpms should be coming up at this point.
keep backing out the screw, (slowly, as it 1/8th a turn and wait 10 to 20 seconds in between each turn) the rpms will climb and climb, then when you hit the sweet spot the rpm's should drop slightly. That's where you want it.

Click to expand...

captJason-
Approx. how many turns out is the 'average' setting for the mixture screws on the 2GC's in your exprerience? I know that every carb is a little different, but I'm wondering what the ballpark figure is that we are talking about.

Also, I'm sending you a PM about a carb adjustment issue. Don't want to thread jack.

Re: Idle Issue - Mercruiser 165 Inline 6

If the engine increases revs as you turn the mixture screws almost all the way in it means you carb, or something that can let fuel in, is allowing too much fuel.... Is the choke open all the way? Float level? Check to make sure the float isn't flooded...

The initial setting of 1 1/4 is usually pretty good. It should have allowed you to get the right idle speed with the speed screw. Double check your timing also, make sure it hasn't drifted... and check the clear line fro the fuel pump to the top of the barrel on the carb hasn't got fuel in it....

Chris..............

Re: Idle Issue - Mercruiser 165 Inline 6

achris said:

If the engine increases revs as you turn the mixture screws almost all the way in it means you carb, or something that can let fuel in, is allowing too much fuel.... Is the choke open all the way? Float level? Check to make sure the float isn't flooded...

The initial setting of 1 1/4 is usually pretty good. It should have allowed you to get the right idle speed with the speed screw. Double check your timing also, make sure it hasn't drifted... and check the clear line fro the fuel pump to the top of the barrel on the carb hasn't got fuel in it....

Chris..............

Click to expand...

The choke is open all the way. How would I check the float for flooding?

I'm starting to think I don't have the right fuel pump for the application. I'm guessing there is too much fuel pressure and thats why I needed to turn the mixture screws so far in.

I'm taking it out today to see how it runs with the motor leaned out a bit. Then I suppose I'll be testing the fuel pressure.

Re: Idle Issue - Mercruiser 165 Inline 6

cnr2003 said:

, I was getting around 800 rpm's with the drive in gear (in the driveway)

Click to expand...

Driveway won't cut it. You need resistance to the prop turning.

Re: Idle Issue - Mercruiser 165 Inline 6

and backpressure on the exhaust