embarrasing carb/idle adjust question

[superwooter]

i feel stupid even asking this, but would someone mind posting a pic or good description of where the idle adjustment screw is located? i have a 1978 70hp merc thunderbolt. i had the carbs cleaned by a boat mech buddy and i'm thinking he set the idle too high at 3 full turns out he said. my neutral idle speed is nearly 1100 rpm which i understand can be very bad for the gears in the lower unit. i'm taking the boat out this weekend and would really like to tinker with it.
any help is greatly appreciated,

wooter

 

[Chris1956]

Re: embarrasing carb/idle adjust question

The idle speed is controlled by a set screw called the idle stop screw. It will be located on one of the throttle levers(likely the inside lever) on the port side of the motor. Move the throttle to neutral from full throttle and observe the movement of the levers. The one that is connected to the carbs, will stop moving when the idle stop screw hits the block.

You may want to get a service manual and a timing light and perform a link and synch. It sounds like your mechanic is not up to it, and it is quite important that the carbs and ign timing be synched.

 

[Texasmark]

Re: embarrasing carb/idle adjust question

I had a 3 cyl 70 but that's been a while ago and I forgot a lot. Trying to remember.

The (each) carb has/should have, a carb low speed adjust which is a brass (usually knerled) screw with a very obvious Stainless Steel spring mounted on it that sticks out some 3/8" from the carbs alum housings. It may be in the front, or on the side; I think OMC has it in the front and I know Merc has it on the Port side. The high speed adjustment is fixed and not accessible.

When he says 3 turns out I think this is what he was referring to. To check, once you find the screw, observe where it currently is (using the slot position as a reference) and very slowly and delicately screw it cw UNTIL YOU FEEL RESISTANCE.....do not tighten as you can easily cut a groove in the needle and totally screw it up as an adjustable element counting turns or portions of turns as you turn. If you turn 3 complete revolutions you have your critter he mentioned. It usually isn't out that far, but I would look at linkage (below paragraphs) before I would mess with this.....so, for the time being at least, return it to the position where you found it.

Other things can affect idle, in particular the linkage. Easiest way to check that is to look at the engine while moving the remote throttle lever back and forth, like from idle to half speed position. The remote cable (end that's inside the engine compartment) has a movable end on it that is connected to your throttle lever that you are moving on the front end and on the engine end, to linkage which ultimately winds up controlling the spark position (under the flywheel) and the carb's butterflies positions.

As you move it back and forth, notice the linkage movement and look for a set screw or two (I think they used white Nylon with a similar SS spring for this on these engines) that contacts the linkage, in particular contact that occurs when you bring the remote back to the idle position....I THINK these are on the starboard side on those engines.

Tweaking that screw will control your idle speed, obviously CCW to reduce rpm's.

HTH

Mark

 

[superwooter]

Re: embarrasing carb/idle adjust question

thanks mark. i believe you are right about which screw is set at 3 out. when he had the carbs off the engine, i watched him turn them in to see where they were originally set. they were at 1 1/2 turns out. after cleaning and re-installing them, i didn't see him make the adjustment out to 3 turns because i was operating the throttle control. he reminded me how they were at 1 1/2 to begin with but that he turned them out a half at a time until he got to 3 turns out. i am 99.9% sure that he didn't make any adjustments at the linkage. i also don't think the engine had completely warmed up before he started making adjustments to that screw.

with the above in mind, would you recommend that i slowly turn the idle screw on each carb back in, say 1/4 turn at a time AFTER IT HAS TOTALLY WARMED UP until the engine starts to stumble and then turn them back out a half turn (got that idea from this thread under FAQs... http://forums.iboats.com/showthread.php?t=168855 )? this is what i had planned to do, but i didn't know for sure where the screw was located on each carb. the only time i've seen it was while the carbs were off the engine and i just couldn't remember after they were re-installed.

thanks again,

wooter

 

[superwooter]

Re: embarrasing carb/idle adjust question

i googled mercury carburetor pics and found several diagrams but none of them were labeled. they just had each part numbered but no reference as to what the different numbers where. below is a diagram of a mercury 700. is the screw you are talking about number 7? if so, i know what i need to do. if not, please direct me!
thanks,
wooter

 

[emckelvy]

Re: embarrasing carb/idle adjust question

#7 is Most Definitely the idle mixture adjusting screw. CW for leaner mixture; CCW for richer.

Try starting at 1-3/4 to 2 turns from fully lean and see what that does for you.

HTH...........ed

 

[Texasmark]

Re: embarrasing carb/idle adjust question

i googled mercury carburetor pics and found several diagrams but none of them were labeled. they just had each part numbered but no reference as to what the different numbers where. below is a diagram of a mercury 700. is the screw you are talking about number 7? if so, i know what i need to do. if not, please direct me!
thanks,
wooter

Yes, on #7. Regardless of where on the carb it's located, there is only one screw on the carb that looks as I said and looks like #7 with it's SS spring.

All the adjustments I ever made on carbs was to screw in carefully till closed (cw) due to the screw having right hand threads which is the thread all screws have unless there is a special application, like some wheel lug nuts on cars and trucks and weedeater nuts. Then back out (ccw) to 1.5 turns for starters. Then tweak till the engine is happy. Closing off the fuel supply means no fuel (just air) and that is as lean as you can get.

If the engine is happy at your current setting, I'd just leave it alone and tweak the linkage idle setting. You aren't going to hurt anything by feeding too much fuel into the carb like you would on a 4 cycle since oil accompanies the fuel and you aren't going to wash the oil off the cylinder walls like would/could happen on a 4 cycle.

Mark

 

[Chris1956]

Re: embarrasing carb/idle adjust question

Idle RPM is not set by adjusting carb mixture screws. Although you can influence idle speed by adjusting mixture, this is not the proper way.

Idle RPM is set after synchronizing idle timing with carb throttle plate and adjusting idle mixture to allow smooth acceleration and resonable idle quality. Idle speed is set by throttle stop screw on carb linkage arm.

 

[superwooter]

Re: embarrasing carb/idle adjust question

thanks for the info, guys. i'll post my results next week assuming i do actually take the boat camping this weekend. the weather has turn much colder than previously predicted. i think the high on saturday is about 45 and the low the night before is 30. i haven't decided for sure, but i may opt out of taking the boat because of the cooler weather. but i will keep all this in mind for when i do get to put the boat in the water again.