Idle Air Screw - No Effect.

StuartT

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Have been having idling problems on my 2000 90 hp ELPTO 2 stroke. Coughing, spitting. dieing, just continually rough. Above 2000rpm hpwever, it runs like a champ.

When I adjust the 3 idle mixtures, the top carb screw seems to have no effect, even when fully screwed in. The carb is mounted tightly, and the cover plate above the mixture screw is tightly secured with a good gasket. It would seem logical that this is the cause of the rough idle, but before I go tearing into it, I would appreciate any thoughts as to why the idle screw is ineffective?
 
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CharlieB

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Re: Idle Air Screw - No Effect.

Idle mixture screws on those carbs starting point is 1 and 1/2 turns OUT from 'lightly seated'.

Idle mixture should be set while idling IN THE WATER in forward gear, adjust to best idle speed using only the mixture screws then open each an additional 1/8 turn.

Rapidly accelerate the craft to WOT. Any bog, cough, or hesitation then open the idle mixture screws and additional 1/8 turn, repeat acceleration test.

If this proceedure fails to cure your idle problem then test compression, test spark, if both are well within acceptable range, clean carbs completely, installing new carb kits, then readjust from the beginning.
 

StuartT

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Re: Idle Air Screw - No Effect.

Thanks for the suggestions. I had been setting the mixture screws in gear as well as on trailer, but the top carb has acted unresponsive in both cases. This afternoon, I went ahead and removed the air box in preparation to install new carb kits and noticed something I had missed before. This air box surface opposite the #1 carb (top) was wet with oil residue, the surface opposite #2 was just slightly wet, and the bottom surface opposite #3 was pretty much dry. The differences make me suspicious, especially when the #1 carb does not respond to idle adjustment. Does this sound like defective reeds in the top cylinder? Excessive preload? There is also a check valve in there somewhere (never actually seen it and do not know it's purpose). Could this be a contributor?

I did a compression check. #1 = 120, #2 = 117, #3 = 118. All a little low, but close in comparison. I am not sure how to test spark, but the motor is totally smooth above 2000 rpms so I am guessing spark is okay. Just for the record, WOT is about 6000 (under propped) and top speed is 45 mph on a 16' fiberglass hull.
 

achris

More fish than mountain goat
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Re: Idle Air Screw - No Effect.

One of the tests I do on engines I'm tuning/servicing is a 'cylinder drop' test... I have a pair of plastic pliers, used in the electrical industry for pulling HRC fuses. While the engine is running I use the pliers to pull one of the plug leads off... 2 things should be noticed... 1. as the lead comes off the plug cap you should be able to hear the spark jumping under the lead, thus confirming you have spark on the plug, and then 2. You will have the engine drop revs... you should get the same revs drop on each cylinder.... That indicates that each cylinder is contributing the same to the running of the engine...

Just one thing to note on these engine... They are recognized as the worst idlers out there... My brother has one, I installed it from new for him... It idles like a pig.. above 1500 rpm and it's great... just idles like crap...

If you're worried about the carb spitting, then while the engine's running put the back of your hand close to the carb throat.. you shouldn't feel any spitting back on your hand... if you do, check reeds... (given that compressions are ok).. If it's the top cylinder, I'd look at the top crankshaft seal too...

Chris..........
 

StuartT

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Re: Idle Air Screw - No Effect.

Thanks Chris. Because I have already removed the carbs, it's to late to run the engine to check for blowback on my hand, but I will say that the amount of oil on the air box cover was small. Obviously damp with oil in front of the #1 carb throat, but not excessively dripping. I do intend to pull the reed block and inspect.

You are obviously in the buz, can you speculate on the cause of the #1 carb air screw having no effect wherever it is adjusted.
 

achris

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Re: Idle Air Screw - No Effect.

...can you speculate on the cause of the #1 carb air screw having no effect wherever it is adjusted.

Would have been nice to have done the cylinder drop test.. I suspect the carb was not running at idle... Hence no effect with the mixture screw... Maybe a bit of dirt in the fuel idle circuit... Make sure you give it a good clean out, and blow all the passages with clean, dry compressed air.

I've just have a thought... When were the fuel pump diaphragms last changed? You know they are supposed to be done annually... If the pump is weak at low speeds, the top carb may not be getting enough fuel... Worth looking at....

Chris...........
 

StuartT

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Re: Idle Air Screw - No Effect.

Good thought Chris. This 8 yo engine is new to me so I have no history. I will do the fuel pump along with the carbs just to get them current.

I just pulled the #1 reed block and it looks (to me) perfect. All reeds are clean, laying dead flat with no chips and no corroded or damaged areas. They all lifted easily with light compressed air blown from the backside.

When I put this all back together, I will pay particular attention to syncing the carbs and will also do a cylinder drop test as a follow up. Appreciate your (and Charlie's) assistance.
 

hkeiner

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Re: Idle Air Screw - No Effect.

As another troubleshooting test, you might consider spraying some fuel mix (oil:gas 1:50) into the questionable carb while idling to see if it makes a difference. If it does, if further indicates that the carb is not feeding enough fuel at idle and you can continue with troubleshooting the carb. If it does not affect idle, you may then consider more exotic non-carb related causes.
 

Ed R

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Re: Idle Air Screw - No Effect.

hey, top cyinder sounds like its not fireing! did you put your hand over carb to see if motor picks up RPM ? do you have spark ?
 

Ed R

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Re: Idle Air Screw - No Effect.

reading all posts, you went too deep too fast! you have a idle resriction problem !
 

StuartT

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Re: Idle Air Screw - No Effect.

reading all posts, you went too deep too fast! you have a idle resriction problem !

A little history. I bought this motor a month ago. It ran rough at idle, but at 1800 up, it was smooth as glass. Hit WOT with no hesitation. We wanted to take a weeks vacation with the new motor on our boat, so I paid an authorized Merc dealer for an hour of labor to check timing, idle, oil delivery, etc., etc. It still ran poorly at low rpms and idle during the week at the lake, and when I got home Saturday, I was focused on the #1 cylinder idle adjustment being ineffective. (I gave up on the dealer for now, was a waste of $130.)

Today, I rechecked the compression, then decided to look at the reeds on #1 because of the blowback residue I found on the inside of the air box, which is how I got to this point.

And while I reported the reeds looking good above, after looking again and noting that they do appear to all be fully flush, some of them click slightly when I tap them with my finger. Humm,??? I will probably replace them all now that I am in this far.

I do wish I had had some of the excellent information above before I tore into it. Ready, Shoot, Aim!
 

achris

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Re: Idle Air Screw - No Effect.

Slow down there Stuart.... The reeds have a tolerance on them... They can be up to 20 thou open and are still considered ok... That's from the Merc service manual... Check it with a feeler gauge...

Chris........
 

Ed R

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Re: Idle Air Screw - No Effect.

Its back basics, you have to understand carbs, idle, off idle, and high speed. we are just trying to mix air an fuel. put motor back together, I will walk you though it . eddie.
 

achris

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Re: Idle Air Screw - No Effect.

Before we all go for a walk, let's use the opportunity to get some basic maintenance done. If the engine was still all together I would not disagree with you, but since it's already in bits... He's got the carb off, so strip it and clean it, while the reed plate is off, check the reeds. And let's get the fuel pump 100% so we've not chasing our tails....

Also he has high speed.. that's working...

Chris.........
 

StuartT

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Re: Idle Air Screw - No Effect.

I forgot about the tolerances. I had read that before, but this is all a quick and steep learning curve for me. Lots of gas auto and marine/construction diesel experience, never a two stroke marine.

I just checked the #1 reed block sitting on my shop bench. The other two are still on the motor as I have yet to pull them. I can get a .015 feeler under one of the reed pedals but a .010 would not pass under any other. Are we saying okay to this block then?
 

achris

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Re: Idle Air Screw - No Effect.

...I can get a .015 feeler under one of the reed pedals but a .010 would not pass under any other. Are we saying okay to this block then?

Yep....:cool:
 

StuartT

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Re: Idle Air Screw - No Effect.

Just ordered all the gaskets and kits, will arrive next week. I will post again here as I put this all back together. Thanks for all your assistance to date.
 

Ed R

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Re: Idle Air Screw - No Effect.

Hey, guys, slow down. air to fuel, think.
 

StuartT

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Re: Idle Air Screw - No Effect.

Eddie, be more specific. "Thinking" isn't getting me anywhere (but that is to be expected). Maybe the others get it, but you need to talk baby talk to me.

I will mention that while I await parts, I did pull the #2 and 3 reed blocks and found two reeds exceeding .020, so replacing the entire set is certainly worthwhile. I did think about flipping them, but for $21 apiece, not worth taking the chance. The cylinder walls looked clean and unscored which made sense as compression was pretty much identical on all three cylinders. The carbs, it turns out, were spotless inside and the floats set correctly according to the service manual. New kits will provide insurance, and I'll have the opportunity to blow out all the passages.
 

achris

More fish than mountain goat
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Re: Idle Air Screw - No Effect.

Have you done the fuel pump yet?

If you still have trouble with #1, then you may have to remove the flywheel and top cap to replace the top crankshaft seal...

Chris.........
 
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