Mercarb idle speed screw and adjustment

Kubaat3lover

Chief Petty Officer
Joined
Jun 6, 2019
Messages
443
Hello folks,
manual says "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."
I will do that on the bench, do I have to "open" the choke or do it on the closed?
There is nothing in the manual but one of our colleagues from the forum did this and he opens the choke..

Thanks!
 

Kubaat3lover

Chief Petty Officer
Joined
Jun 6, 2019
Messages
443
Ay ay sir!
buut as far as I remember when I When I adjust the screw to where I need it as far as idle, it won’t allow the choke plate to move
 
Last edited:

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,954
Ayuh,..... When yer fine tunin' the idle, ya do it on a warm motor, choke off,.....
 

vroom ZOOM

Chief Petty Officer
Joined
Aug 15, 2017
Messages
414
Ay ay sir!
buut as far as I remember when I When I adjust the screw to where I need it as far as idle, it won’t allow the choke plate to move

YES! it should not move. The choke cam is not really a fast idle cam, it is not eccentric, i.e it has no camminess to it, it is perfectly round. It will not change the throttle plate position no matter what the choke position is. What it IS for, is when you have a warm motor, the idle stop screw holds the choke open, so the warm motor does not flood. Therefore, this is partly the reason why you pump the throttle when starting a cold motor, it releases the choke and lets it close.

Turn idle speed screw in until it just contacts idle cam, then turn screw in an additional two turns."
I will do that on the bench, do I have to "open" the choke or do it on the closed?
from what I just said, this means that it does not matter, choke open or closed. Don't adjust that screw on the bench, tweak it with the motor running in the boat for target idle rpms, then adjust the cable to give you a little "preload" so the screw is always putting a little bit of pressure on the choke cam, and so the engine always returns to idle fully. Two turns should give you a good starting point, but if it does not start, give it half a turn more and try again. Adjust the screw till you get 650-750 rpm idle, in gear. If your motor idles too fast, but when you unscrew the idle stop screw it stalls, play a little with the idle mixture screw at the base of the carb. What I like to do is with the engine running, turn it one way until the engine begins to stumble. Then turn the other way until it stumbles (generally about half a turn). Halfway between these is the happy point. unscrew the idle stop screw a little again, and repeat the mixture adjustment again, until you get down to your target idle rpms.
 
Last edited:

Kubaat3lover

Chief Petty Officer
Joined
Jun 6, 2019
Messages
443
wow What a great explanation, Thanks!
As you said the cam is rounded that’s the reason it confuses me how it works .
Thanks it will be my first adjustment.
I have also vacuum gauge but there is no place to connect it to the carb
 

Kubaat3lover

Chief Petty Officer
Joined
Jun 6, 2019
Messages
443
Achris, I cannot find your post where You wrote how to do mixture adjustment step by step in really plain language....Can You provide with help? Im going to the boat for final adjustments after rebuild
 
Last edited:

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Achris, I cannot find your post where You wrote how to do mixture adjustment step by step in really plain language....
I don't think I've ever written a post like that.

1. Warm up the engine.
2. Check timing is correct.
3. Adjust idle SPEED to spec.
4. Moving idle MIXTURE screw/s 1/4 turn at a time, turn screws CW (leaner) until engine slows slight. Turn screws CCW (richer) until engine speed picks up, then slows again.
5. Set mixture screws between these points. If in doubt, err on the side of richer rather than leaner.
6. Adjust idle speed to spec. (It may have changed in the process of adjusting the mixture)

Chris.
 

Kubaat3lover

Chief Petty Officer
Joined
Jun 6, 2019
Messages
443
I think I should put the engine into the Base Timing Mode also and I dont have timing lamp but ive checked timing 2 years ago and it was just fine.
Can I ommit this step and do the mixture adjustment ?
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Sorry, yes, base timing mode. And if your confident with the timing by all means omit checking the timing.
 

Kubaat3lover

Chief Petty Officer
Joined
Jun 6, 2019
Messages
443
Ok I’ve go timing lamp but I cannot localize timing tab/reference... it’s 2005 4.3 Tks
Forgive my ignorance...04583EACF-F150-4FF4-80EF-3A62F365F00A.jpeg
 

Kubaat3lover

Chief Petty Officer
Joined
Jun 6, 2019
Messages
443
all right, I’ve been at boat and did adjustments.
base timing mode set, idle rpm set to 580 rpms@ idle forward . And now, when turning mixture screw clockwise, leaning 3/4turns and the engine begin to drops. That’s correct but when turning CCW there is almost NO reaction the engine doesn’t drop. I did 10-12 turns and nothing. Why? So now Im not sure if the Mixture is set properly. When the engine drops on the lean side I did 3-4 turns CCW and I hope to be on the richer side as Achris said but how to confirm the setting? The engine runs perfect when hot , etc starts nice
Take care thanks for any input
 

Scott06

Admiral
Joined
Apr 20, 2014
Messages
6,428
all right, I’ve been at boat and did adjustments.
base timing mode set, idle rpm set to 580 rpms@ idle forward . And now, when turning mixture screw clockwise, leaning 3/4turns and the engine begin to drops. That’s correct but when turning CCW there is almost NO reaction the engine doesn’t drop. I did 10-12 turns and nothing. Why? So now Im not sure if the Mixture is set properly. When the engine drops on the lean side I did 3-4 turns CCW and I hope to be on the richer side as Achris said but how to confirm the setting? The engine runs perfect when hot , etc starts nice
Take care thanks for any input
Maybe tje passages are a little plugged? I would go back to lean stumble spot and go out 3/4 or one complete turn, 1.25 turns at most not 3-4 turns. Let it idle 15 minutes and pull a spark plug or 2 and read them . 3-4 turns out you will eventually foul the plugs with black soot. If it idles ok leave it alone normally 1.25 - 2 turns out works ok
 

Kubaat3lover

Chief Petty Officer
Joined
Jun 6, 2019
Messages
443
Passages are for sure clean and drained.
ok I’ll go back, try to lean the engine, the. 1-2 turns and check the plug. Thank you.
is there any procedure maybe? Looking for the peak in RPMs?
 

Harritwo

Chief Petty Officer
Joined
Oct 4, 2011
Messages
586
With engine off. Run idle mixture screw in until it lightly seats. Back out 2 complete turns. Now you have a starting point for the mixture. Start engine and follow Step 4 of AChris post.
 
Top