carb question

[Darrendude]

I have a 76' 120hp merc. The carb has 2 adjusting screws at the base. I think I remember reading somewhere on this thread that most people have these screws set at about 2 turns out . On my mine, the front screw has to be screwed in almost all the way or it runs very rough. I have remove both of these screws, inspected and cleaned but it is still the same. What should these screws be set to and what would cause my senerio?<br /><br />Thanks

 

[LuckyPenny]

Re: carb question

Sounds like your float needs serious attention. Check the float drop before you start tearing up the carb.

 

[rodbolt]

Re: carb question

the two turns out is a reference.<br /> can you adjust it so it idles well and accelerates smothly ?

 

[Darrendude]

Re: carb question

Yes I can rod , just thought something could be wrong since I need to screw the front screw in almost all the way. What exactly do these screws adjust? Air/fuel mixture?<br /><br />Thank you

 

[Don S]

Re: carb question

The screws should run about the same amount out, you may have some clogged air or fuel passages on the one side. A carb rebuild may be in order. This includes dissassembly, and soaking the carb in carb cleaner and blowing out all the passages, then resetting everything, not just spraying the carb with some carb cleaner. No such thing as a rebuild in a can.<br /><br />One thing I have done in the past that may clean out any minor clogs, dirt and a little water is to disconnect the throttle linkage, rev the engine to 4 or 5000 rpm, then quickly cover the carb top with your hand and choke off all the air while keeping the throttle all the way open, pull your hand off before it gets to slow and dies (It would be seriously flooded if it did die). Do this a couple of times and see if the idle screws will adjust more evenly.

 

[Darrendude]

Re: carb question

Thanks Don , I got a carb rebuild kit (a bag with a hundred gaskets,seal,and assorted other things lol . The instructions dont look real detailed but I have a service manual aswell. The screws are definatly not in the same position. Front screw is about a 1/4 turn out and the rear is about 1 1/2 turn out. Runs alright , alittle rough at low rpms , but from reading here, that seems almost the norm for these engines.<br /><br /><br />Thanks

 

[jimmythekid]

Re: carb question

maybe someone adjusted it that way to compensate for some other problem, like a vacuum leak maybe? I am not positive, but I belive the screws control air metering curcuits. If its screwed in all the way, its cutting down on the flow of air, its probly sucking in air from somewhere else below the carb. The leak would be on the same side as the barrel with screw thats turned in more<br />If you going to rebuild the carb, just make sure you get the carb cleaner that comes in a liquid form in a one gallon paint can, the kind with a parts tray in it. You can let the aluminum parts soak submerged in it. Just make sure to wear gloves and glasses, nasty stuff. The stuff in the spray cans never worked good for me on the internals, good for cleaning the throttle plates and external stuff, but not strong enough for the internals. Just make sure to use it on only the metal parts. Any rubber or plastic it will ruin.<br />Also what DonS said is true, if you slam the choke shut (or your hand) at wide open throttle, the vacuum created can sometimes "suck out" small obstrutions, you just have to make sure the throttle is open wide when you do it and the RPMS high enough so it can suck for a few seconds.

 

[KRS]

Re: carb question

Not enough suction to pull off a finger??

 

[Don S]

Re: carb question

Not enough suction to pull off a finger??

Not even close...........

 

[Buttanic]

Re: carb question

The screws control idle mixture fuel not air. The best way to ajust the idle mixture is with a vacuum gauge connected to the intake manifold. The screws are ajusted one at a time until you get the highest vacuum reading and then the idle speed is ajusted. If it did have a vacuum leak below the carb on one side the screw on that side would have to be screwed out not in to compensate for the extra air from the leak. It does sound like the carb may need a rebuild.

 

[Darrendude]

Re: carb question

Buttanic, if it needs to be screwed in more, what could that be compensating for?

 

[trog100]

Re: carb question

i think the mixture screw is what could be described as a "user controled" air leak.. it compensates for slight variations in the amount of gas that passes thru the slow running jet in fixed jet carbs.. usually the more u screw it in the more it blocks of a small air-bleed passage which alters the depression on the jet.. therefore making the idling mixture slightly richer or weaker if u go the other way and screw it out..<br /><br />it should be roughly where the book says it should be.. but thats only a rough initial set up starting point with everything unworn and as it is when the engine comes from the factory.. it its a mile out either way it suggests either a small airleak elsewhere.. incorrect fuel level in the float chamber.. partially blocked slow running jets.. wear around the butterfly spindle letting in air that shouldnt be getting in.. in fact all sorts of possibilities cos the idling mixture is quite critical for smooth idling..<br /><br />most of the error possibilities tend to weaken the mixture off compared to as new.. this would require the screw to be screwed in more than it should be to compensate.. things like too high a fuel level in float chamber or a choke flap that wasnt fully opening would have the opposite effect and need the needle to be screwed out more..<br /><br />if u try that bunging your hand over the top of the carb trick.. use the palm of your hand and be prepared to be slightly "un-nerved" by the power of the "suck".. he he <br /><br />trog100

 

[f_inscreenname]

Re: carb question

I don't think it would apply here but wouldn't low manifold pressure cause this also? I have had a couple hot rod motors with big cams over the years. All had the set screws in real far. For that matter on a Camaro I had a while back that I had to use a vacuum reservoir for the power brakes it was so low. The bleed screws on the carb were also in deep. I just took it as trade off for the big cam.

 

[Buttanic]

Re: carb question

On a big cam engine the problem is because of the low vacuum. The throttle plates are open more than on a normal cam to get the idle speed up and the carburator is no longer operating on the idle circuit. The solution in that case is to drill a 1/8 hole in the trottle plates to get extra air and be able to close them so the carb operates on the idle circuit and the idle mixture screws become useable. Idle air bleed is controled by a fixed size orfice and is not ajustable. The idle air bleed mixes air with gas before it reaches the mixture screws. The mixture screws control this mixture of air and gas as it is sucked into the venturi and mixes with the air coming in through the carb barrels . This is to help atomize the gas as it enters the carburator venturi. Think of air bleed as a soda straw with a small hole in it just before your mouth. As you suck up the soda a small amount of air would mix with it. Anything that could cause low manifold vacuum could be the problem such as leaks, burnt valve seats, worn cam lobes, broken valve springs. Normal vacuum should be 15 to 20 inches and steady at idle, anything under 15 inches or a vacuum that is not steady indicates a problem other than the carburator itself.

 

[Darrendude]

Re: carb question

Thanks Buttanic<br /><br />According to my dash vacuum gauge, it has about 18-19 in. of steady vacuum at idle.

 

[Darrendude]

Re: carb question

I rebuilt the carb today and I still have the same thing . Front screw needs to be in nearly all the way. I cleaned and blew out everything. It seems to run good but I noticed a small hole in the exhaust manifold about an inch from the head. No water comes out just hot air. I knew I had an exhaust leak (tick tick) but not a hole. It is on the bottom side (natually lol) I am trying some JB weld on it to see if that stops it. JB is good up to 600 degrees. Has anyone tried this?

 

[Buttanic]

Re: carb question

The front idle mixture screw could have been damaged from overtighting it sometime in the past. Also the front air bleed orfice could still be plugged. If you could borrow another carburator to try at least you might be able to eliminate the carb as the problem. This is a shot in the dark but which cylinder has the exhaust leak. Possible there could be an internal leak in the manifold from the exhaust side to the intake side.

 

[Bondo]

Re: carb question

JB is good up to 600 degrees. Has anyone tried this?

And Your Exhaust Gases run 1200*.................Nope,..... I've never tried it...........

 

[Don S]

Re: carb question

It seems to run good

I have had more customers say this than I care to think about. And when I here it a red flag goes up.<br />I have found these "Good" running engines with 0 compression on 1 or 2 cylinder, or spark plug wires crossed, timing so far out it's amazing it ran at all, vacuum leaks so bad it won't idle below 1000 rpm, spark plugs with the porcelen blown out of the center and hanging by the plug wire. Spark plug wires melted to the exhaust pipe, and all the time the customer is saying "It runs GREAT"<br />Now you have a hole in the exhaust manifold, and since the exhaust manifold and intake are all the same, what shape do you think the intake is in, or anything else on the engine. What else has a hole in it.<br />Nobody here has the ability to hear or see your engine, and your last few posts are beginning to sound like you have more problems than you are letting everyone know about. Who knows, maybe the carb is just flat worn out or the passages are corroded from water. If that's the case you can rebuild the carb all day long and it won't fix it.

 

[jimmythekid]

Re: carb question

is the intake manifold and exhaust manifold on the same side of the head? if they are the likelyhood of the gasket being bad is good. Have you checked for a vacuum leak? Thats where I would start. Is the "tick tick tick" noise coming from the same end of the engine as the turned in screw? I would bet money you have a vacuum leak, probably from the intake gasket. No JB Weld wont work, you need a new manifold.<br /><br />PS if you did not soak (submerged)the metal parts of the carb in cleaner at least for half a day you wasted your money/time on rebuilding the carb. IMHO<br /><br />DonS the hardest part in our respective line of work is getting the customer to tell us the whole story <br /><br />too bad its not EFI it would have been fixed by now. Sorry guys I couldn't resist