Engine Dies at Idle

[ontsen10]

Hi,

I have an Atomic 4 Gas Inboard on a 1978 Catalina 27. It is still sea water cooled. Whenever I pull the throttle lever all the way back to idle, the engine dies. This just started randomly after running at idle just fine for over a year when I first got the boat.

I have done the following to try to fix it:

1. Cleaned the carb and air intake filter
2. Oil change
3. Replaced the primer hand pump (positioned vertically correctly)
4. Replaced the fuel filter
5. Replaced the spark plugs (I know this wouldn't help for the problem, but did it since I was working on the engine anyways)

The engine runs and sounds brilliant so long as the throttle lever is slightly forward giving the engine a bit of juice.

Any thoughts on how I can fix this or troubleshoot would be much appreciated!

[Vermonstah]

You have come to the right place. You will find great support here.

Second item, do you have the MMI manual? if not, get it - worth its weight in gold.

I assume you have tried to adjust the idle screw on your carb. How does engine respond when you adjust idle screw?

And do you know what carb you have? Slight variations between different models, though unclear whether relevant here, you still want to know exactly what carb you have.

With some more info on this, the experts here will help you get your engine dialed in. They have certainly helped me!

Brian

[Dave Neptune]

ontsen10, first welcome to the MMI Afourian Forum. Sounds like a couple of possibilities and a bit more info will help.

At what RPM's does it die and where does it stay running?
It could be the idle speed screw that holds the carb butterfly in it's idle position or the idle stop. You just may need to speed the idle up a bit.

The carb's all have 2 idle "circuits" that feed the engine at idle and "low speeds". They are positioned in the carb where the butterfly valve sits at idle. What you will see is two little openings one above the throttle plate (butterfly) and one below, IE stage one and stage two. If these and there passages are not clear you will have idle problems.
If the "choke" helps it idle I would look to the idle ports and get a good air hose on them.

Dave Neptune

[BunnyPlanet169]

Another Welcome!

What RPM are we talking about, running vs dying?
Have you adjusted the idle screw? Starting full in, and then back it out 1 1/2 turns?
Have you checked your throttle and choke linkages for sloppiness?

EDIT: See John, above. "primer pump"? If this is a squeeze bulb, please consider removing it. It's not designed for inboard use.

[JOHN COOKSON]

Originally Posted by ontsen10

Hi,

1. Cleaned the carb and air intake filter
3. Replaced the primer hand pump (positioned vertically correctly)
!

Sometimes the only way to get the idle ports Dave N. mentioned cleared is to run a fine wire through them.
What is the "primer hand pump"? I don't have one on my 1980 model built in 1979 A4 equiped Cat 27.
Is there a PVC valve on your engine?
Another welcome aboard the forum.


TRUE GRIT

[ontsen10]

Thank you for all of the welcomes. I'm so glad I found you guys.

It sounds like my best move is two things:

1. Tweak the idle screw to speed up idle, but where is this idle screw? There seem to be several screws around the carb that I can tweak and I would hate to mess with the wrong one. Is there a picture or diagram that someone could point me to?
2. Pull the carb off and air hose or fine metal thread the holes. However, since the choke is operating as it should, is this indication that the carb is probably good to go?

Regarding the other questions and info:

1. I will be sure to get the MMI manual, glad you mentioned it.
2. I'm not sure what carb I have, but it definitely was replaced in the last few years prior to me getting the boat. It is still bright and shiny.
3. Regarding the choke, it operates as it should. I pull the choke out when firing up and if I don't push it back down within 20 seconds or so, the engine dies.
4. Unfortunately there is no RPM gauge on it. Just water temp, alternator power, and oil pressure.
5. The throttle and exhaust linkages look good.
6. I'm not sure what a PVC valve is, but I don't think so.
7. The primer bulb "hand pump" must have been installed quite recently, probably a few years ago. It sits in between the fuel tank and the fuel filter.

[ontsen10]

Jeff -

Just saw your note about the primer squeeze bulb. I will definitely remove it if you recommend so. Do you why it would have been installed in the first place?

Trying to learn as much as possible about why the engine system got modified to its current condition before optimizing it...

[BunnyPlanet169]

I had one (once), but I've removed it for safety.

Any inboard component on a gasoline fuel system should be UL listed / USCG approved for inboard gasoline use. Technically, that entails a fire test (among other things), where the component cannot fail/leak after 2 1/2 minutes of flame exposure.

Squeeze bulbs are only intended for outboard use, where any fuel leak is open air, and usually visible and smellable.

I think it's worth going over the fuel system of any relatively 'new to you' boat to become familiar and comfortable with the component choices made.

Idle screw, shown here from MMI panorama: http://www.moyermarine.com/360/r2.html

[JOHN COOKSON]

The first thing you need to try is to adjust the idle mixture. As BP said turn the needle in until it seats lightly then back it out 1 1/2 turns for an initial setting. After this inital setting you can "play" with the idle mixture\idle speed till you get a setting for both you like. I don't have a picture of the idle mixture adjusting screw. Maybe someone in the forum can help out. (LONE RANGER to the rescue?)
Once again as Dave N. mentioned if closing the choke helps the idle is too lean some way some how.
Comments;
The carb may be bright and shiny on the outside but what does it look like inside? (Sort of reminds you of certain people doesn't it?)
Your choke sounds normal to me.
The linkage "looks good" but is it adjusted correctly? (Sort of reminds you of certain people doesn't it?)
The primer bulb is most likely to prime the filter after a change. Didn't pick up on it the first time around.

TRUE GRIT

Edit: BP came up with a picture while I was typing.

[ontsen10]

Awesome. Thanks!

I'll adjust the idle screw later today and post a reply back on how that works.

[JOHN COOKSON]

10
The needle adjustment controls the amount of air in the idle circuit not gas. The more you turn the needle in the richer the idle mixture. You may know this already?

TRUE GRIT

[Dave Neptune]

ontsen10, there are two idle screws. There is the idle speed screw that is on the linkage side of the carb and it controls where the throttle lever stops. The other is the mixture adjustment on the top of the carb with a spring under it. It is for adjusting the "air in the idle mix of fuel from the idle jet" so tighter or in is less air for a richer mix and out is leaner.

If you have a PCV valve it will be in place of the slash tube. The slash tube comes out of the block in front of the carb and points to the flame arrestor on the front of the carb. If the slash tube is missing and the tube is hooked to the manifold with a plastic device inline you have a PCV valve.

Jerry should be posting some of his wondroius pics with lil arrows to identify things.

Dave Neptune

[ontsen10]

Dave Neptune

Any chance you can show me a picture of where this other idle screw is located?

"There is the idle speed screw that is on the linkage side of the carb and it controls where the throttle lever stops."

I went down to the boat and tuned the idle air screw and found the optimal spot for it, however, the engine is still dying at idle so I'd like to tune the idle speed screw as a next step.

[BunnyPlanet169]

It's on the picture below, at about 9:00 to the throttle arm pivot. Your arm may look different, but there should be a small screw there....

[Marian Claire]

Ontsen 10.
As others have mentioned it is important to know what your RPM #s are. If the idle speed adjustment is letting the RPM drop below 700+- it may be to low and adjusting will help. But if you adjust the idle speed and you have to have 1000+- RPM to keep running then the issue is probable in the carb.
I just do not want you to fix the symptom and leave the problem. IIRC it is recommended that you should not shift in and out of gear above 1000 RPM.

Welcome to the forum.
Dan S/V Marian Claire

[ontsen10]

I was able to get the idle RPM just under 800 and everything seems to be running well! I also tightened up the nut holding the idle arm on the inside compartment, which could have caused it to get loose in the 1st place.

Thanks again for all of your help...I'm really getting to know these Atomic 4's!

[Marian Claire]

Good deal. Thanks for the follow up.
Dan S/V Marian Claire

[Dave Neptune]

ontsen10, nice and thanx for getting back with a "success".

Dave Neptune