how do properly adjust idle mixture on 4 cyl mercruiser?

My dad's mid '70's vintage mercruiser with the 4 cyl GM engine has a bad "dead spot" sputter as you accelerate off idle. If push the throttle slow, it gets past it OK, but if push it quick, the bobble is pretty bad, it may or may not backfire, and occassionallly stalls.<br /><br />Some history... he was having problems with the rough idle that he thought was ignition related, so he bought new plugs, wires, distributor cap, rotor, points & condenser. He had put all this stuff in and I arrive to look over his shoulder just as he's putting it back together. He's careful to put the wires on the distributor just like they came off, but I comment that it can't be right... who's ever heard of a 1-2-3-4 firing order?? He's adamant that's how they were, I check manual, it says 1-3-2-4... look on block it also says 1-3-2-4 so that's how we put it together. Fired it up on the muffs, adjusted timing to 8 deg's as per spec, and adjusted idle mixture screws ~ 1 3/4 turns out to get us close until final tune on the water. Seemed to be nice smooth idle. On water, it runs good except for above mentioned dead spot. Adjusted idle mixture screws, which seemed to have only a minimal effect on the deadspot...deadspot might be marginally worse when the idle is smoothest. (?) I thought maybe choke was partially engaged, so I turned cap on choke about 1/8 turn to "lean"... no effect.<br /><br />I didn't say that once past this spot, the engine runs better than I ever remember it running. I've got to check it against my boat, but I'm sure he's gained 5+ mph. Amazing how firing the plugs in the right order smooths things out!! If it was wrong for quite a while, it's amazing to me that it ran as well as it did.<br /><br />Seems like a mixture problem to me... air leak, or choke issue or ???? What do you think?<br /><br />Thanks for your input...<br /><br />Brent

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

maybe a couple more specific Q's would help...<br /><br />1) what's the proper way to adjust the idle mixture screws? The manual was vague on that. Said screwing them should cause rough idle at some point, then screw them out until idle slows, proper setting is somewhere midway between these 2 settings. Also it has 2 idle mixture screws on the base of the carb, one at the back, one at the front. Manual says adjust one then the other. So, these 2 do the same thing? 1 for each barrel?<br /><br />2) thinkin' about it now, maybe I should've tried turning the choke cap to "rich" a bit... if it has an air leak maybe it's too lean and some choke should help. At least that would tell me that it's too lean and not too rich. Make sense? Do hesitation & a dead spot like that usually come from something being too lean rather than too rich?

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

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Re: how do properly adjust idle mixture on 4 cyl mercruiser?

see if a change of topic title and a bump gets any interest....

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

ok, i'll take interest, but i aint no carb expert by any means. i would even say that carbs are still mysteryious to me. anyways, i think the idle mix screw are only for idle. they'll have no effect on the stumble that yer expericeing. the idle mix screws only effect idle. when the throttle plates are closed. the screws allow fuel to enter there at the tip of the screw. as the throttle plates start to open even a little bit, the, i think it's called the main metering starts to take effect and the mix screws do nothing as the throttle plates are opening and the fuel starts to go through the main metering circut of the carb. maybe accelerator pump? float level, gooy, pluged passages in the carb, but not mix screws. i aslo feel that if ya had a vacum leak, that your idle mix screws would do nothing as in the same effect as opening the trottle plates. now hopefully someone who really knows carbs will come by and tell me i'm full of it and set ya straight. <br />sounds about right for the way yer adj. the mix screws to me. the happy medium. but like i said, i aint no carb expert.

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

Hey BF,<br /><br />Sounds like a stick choke to me. I would recommend searching the word SEAFOAM on this forum and see how it's used. This is a great carb cleaner/fogging oil. After athorough cleaning, I would then check the choke settings. It could be the idle mixture screws, but it is best to make sure it is clesan first and the choke is working properly. If none of this works for you, it could be time for a rebuild. <br /><br />First and foremost, get the manual or you will be up the creek without a paddle (or engine)<br /><br />good luck<br />e

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

thanks for inputs folks..<br /><br />Eggo... yes, I'm still suspicious of choke too, when manual choke on my old truck is set poorly, it has a similar bobble on the low end... I did the seafoam thing to it last season... didn't fix crumby idle, but if wires were crossed, that's no surprise! It could be that disassembling the carb and giving it a good going over/cleaning would be a good idea. We have a good mercruiser manual, but the carb setting section is vague... Probably because I'm supposed to know that already <br /><br />Ziggy... I hear what you're saying. Maybe a clogged carb passage (?) Is there a separate fuel circuit that looks after the mid range throttle position? There sure doesn't seem to be a problem with fuel flow once it gets past that bobble. I'm more familiar with 2 stroke carbs than automotive style ones.<br /><br />Thanks again for the input!<br /><br />Brent

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

if push it quick, the bobble is pretty bad, it may or may not backfire, and occassionallly stalls.<br />

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Sounds like my old Invader(1980). Merc 120(GM 2.5)with a Rochester 2 jet. Bout every two years I'd have to put an accelerator cup(pump)in the carb, problem solved. I found out later that was a common problem with the later ones. Go ahead and give the carb a rebuild.<br /><br />The mixture screws are just for idle like ziggy says. Start with both screws about 2 turns out and adust both equal amounts to achive best idle. If one is 2 and 1/4 turns out the other one should be too. In is lean out is rich.<br /><br />If the choke comes off(open)when the engine warms you don't need to fuss with it. Its job is done once the engine is warms and wouln't effect your problem.

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

thanks... I'll try and scare up a carb rebuild kit before I head back to his place tonight... Is the accelerator pump business part of a standard kit, or is that something I'd have to get separately?<br /><br />I don't have the manual with me now, hopefully there's a section on carb rebuilding in there!<br /><br />thanks<br /><br />Brent

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

Is the accelerator pump business part of a standard kit

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Yes, the kit should have everything you need. there was a time you could get the pump separately but I haven't been able to lately.

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

My 1985 3.0 has your exact problem. I haven't tried to mess with it yet, I just accelerate SLOWLY for now. I'll be interested to see what you find out. My accel pump seems to work fine, as it dumps a ton of fuel in when you "hit it". A buddy of mine recommended checking the vacuum advance. (It has brand new plugs/wires/cap/rotor).

Re: how do properly adjust idle mixture on 4 cyl mercruiser?

I am pretty sure your problem is the accelerator pump. To adjust the idle mixture use a vacuum gauge attached to the intake manifold. There is a port on the manifold where some models had a vacuum gauge attached as part of the instrument panel. If not then there is a 1/8 pipe plug in the manifold. Turn the idle mixture screws in and out until the vacuum is the highest.