Idle Air Screw - No Effect.

[Lone Duck]

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.

Pay special attention to the idle circuit in #1 carb. strip that carb completly and soak it. use a strand of automotive copper wire and check the passages and blow them out repeatedly . use a straw on wd4o or the like and look for fluid following the circuit . Study it and get to know where the passage starts and ends. Check the tip of your idle screw. Someone may have screwed it in to far and broke it off. May even have put a new one in and not cleared the passage. From what I have read on this thread I feel it is definitely #1 carb idle passage.

 

[StuartT]

Re: Idle Air Screw - No Effect.

Chris, the diaphram kit for the fuel pump is among the parts I have ordered. I will rebuild the pump as part of the re-assembly.

Lone Duck, I will be diligent in cleaning the carbs, especially #1. Thanks for the suggestions. I am also going to follow up on Chris' observation that the #1 carb linkage may have restricted the carb from getting all the way to idle. And, I am looking carefully at the throttle plate to make sure it is seating properly in the throat. There has to be a reason #1 is not having an effect on idle performance.

 

[dspence]

Re: Idle Air Screw - No Effect.

I have exactly the same symptoms with the same engine, in fact I started a thread today before I found this one -

http://forums.iboats.com/showthread.php?t=278080

Here's some more details on my symtoms that may well have the same cause as this thread:
>>
All idle screws on the carbs are set to 1 1/2 turns. I tried tweaking these screws. Fully closing the screw on the top carb makes absolutely no difference, opening it too far makes it run rougher. Over-opening the screws on cyl 2 or 3 makes it rev faster, closing too much bogs the engine. Interestingly, the engine runs absolutely no differently if just spark plug 1 is disconnected, or if just spark plug 2 is disconnected, but it stalls if just 3 is disconnected. Not sure if this is normal or not.
>>

My engine is very low hours (just 100hrs) and there is definitely nothing wrong with the idle circuit in the top carburetor (have stripped it twice.) Can't guarantee the fuel pump is perfect thought.

I'm following this thread with interest... perhaps this engine just idles like a dog, as someone above said

 

[StuartT]

Re: Idle Air Screw - No Effect.

Now there are two of us with the exact same problem. Welcome to the thread dspence.

The 4th post from Chris talks about a "cylinder drop" test, and it looks like dspence did it, with less than stellar results. Now I am really pumped to get back together next week and do this test as well to compare.

Anyone else have this engine that can share?

 

[longshanks]

Re: Idle Air Screw - No Effect.

Ok guys, I'll tread lightly here and hopefully not hijack this post. FWIW, I have a 1998 triple 60 merc that has many of the same issues. I can't make this sucker idle smoothly, but it's a great motor. It's attached to a jet pump, and the only thing that really bothers me is that it really hesitates when I open the throttle fully. After reading CharlieB's post near the top of this thread, I have the following questions

1) would further loosening all three idle mixture screws by 1/8 turns potentially cure this hesitation?

2) to what degree will changing the idle mixture screws influence fuel consumption of this motor?

thanks guys

 

[StuartT]

Re: Idle Air Screw - No Effect.

No problem with asking your question longshanks. Hopefully you will get an answer that works for you. I am on hold awaiting parts and since they just shipped from New York today and are coming cross country to the northwest, we have a few idle days anyway.

Stuart

 

[achris]

Re: Idle Air Screw - No Effect.

Ok guys, I'll tread lightly here and hopefully not hijack this post. FWIW, I have a 1998 triple 60 merc that has many of the same issues. I can't make this sucker idle smoothly, but it's a great motor. It's attached to a jet pump, and the only thing that really bothers me is that it really hesitates when I open the throttle fully. After reading CharlieB's post near the top of this thread, I have the following questions

1) would further loosening all three idle mixture screws by 1/8 turns potentially cure this hesitation?

If the hesitation is due to the mixture being too lean, yes...

2) to what degree will changing the idle mixture screws influence fuel consumption of this motor?

Virtually undetectable.... The mixture screws only control the fuel:air ratio when the engine is idling. Once you start to get to about 1200 rpm (under load) the idle circuit is playing no more part in the running of the engine...

I would be concerned about the idle quality. Those engine idle very nicely... Find out why it's not idling properly. It could be mixture screws, but check everything else too. Compression, spark (those engines really hate old plugs), then check the mixtures.

Chris.........

 

[StuartT]

Re: Idle Air Screw - No Effect.

Completed the refurb today and splashed for adjustment and a test run. Extreme rough running now gone, just an occasional spit, although I would not exactly call it a smooth idler. But it is considerably better than before, and above 1500, really runs beautifully. Dead smooth all the way to 5500 rpm. Great accelleration on a 13.25 x 23 SS prop.

What solved the problem? Not totally sure. I install new reed sets, rebuilt the carbs, the fuel pump, and painstakingly went through the carb syncronization. I did not check the timing as I do not have a timing light, but the shop I previously took it to indicated it was correct, although I do understand I could have influenced it with my adjustments. The carbs were very clean inside and the floats adjusted correctly, so I am not thinking they contributed. The fuel pump had the newer plastic diaphrams so I think it was rebuilt by someone else not all that long ago, but it is still a possibility. The reeds less likely, the syncronization a strong contender.

I should point out that we all missed one thing. The 75 and 90 hp preset idle specs on the carbs calls for #1 carb at 1/2 turn out, #2 and #3 at 1 1/2 turns out. Apparantly the #1 idle circuit must be run leaner and Merc calls this out in section 3B-1 of the service manual (at least the December 1997 copy I have).

Thanks for the help you guys offered. I learned a lot through all this and I'm glad I took it on.

Stuart