Engine stalls at idle once warm

[dleedvm]

Hi everyone,
They got my Sabre in the water almost a month early this year. The ole A4 started right up and it runs fine at idle initially, and fine at cruise, but when I idle back coming into the marina, it runs rough and stalls eventually if I don't intervene. It will restart and run at idle as long as it is in neutral. Sometimes before it stalls entirely it will make a very disconcerting clanging sound, which I hope is just misfiring, since it runs smooth otherwise. I've done nothing special to the engine this year other than upgrade the water pump and I disconnected and reconnected the throttle cable thinking I was going to replace it (I didn't). I still have the mechanical fuel pump, if that makes any difference. Oil pressure at idle is about 15-20 psi. Thanks
David

[ILikeRust]

I would start with the fundamentals. Based on your description, my first thought is carburetion. Is it possible the idle screw got turned a little bit? Or perhaps there's just a little crud in the carb. That's where I would start - open up the carb and make sure it's clean; make sure the idle screw is set right.

Can you give a little more description of the "clanging" sound? Is it a rattling, like the valve lifters tapping? Or....?

[Dave Neptune]

Info?

David, how long since a tune? Is there a possibility you bumped the distributor when working on the water pump? If you did not bump the distributor it is probably just an idle adjustment or a possible carb becoming gumbed up.
Have you pulled the plugs to see how they look?
The noise could just be the gears clanking when she dies and the prop spins into the gear instead of being driven by the engine gears.

Dave Neptune

[roadnsky]

"it runs fine at idle initially, and fine at cruise, but when I idle back coming into the marina, it runs rough and stalls..."

David-
As the boys are suggesting, it sounds like a carb issue.
Here's an old post from Don that I've always kept handy that may help with your symptoms...

"The only thing I can think of that might explain your shut downs (presumably at or near idle) is that the idle mixture might be on the lean side and doesn't pick up the load as you come back past 1100 RPM or so. If turning the idle mixture screw in a half turn or so doesn't cure the problem, I'd check the two idle ports along the side of the top of the carburetor throat to see if one of them is clogged (especially the lower one).

At approximately 1100 RPM, the main discharge nozzle pretty much drops out and the idle system is supplying most of the fuel to the engine, and if the lower of the two idle ports is clogged, the mixture will become very lean during the transition to idle. If the engine can get to idle, and the upper port is open, it will able to carry the load, but only at around 800 RPM.

This situation is sort of like a flat spot during acceleration, only in reverse. The lower of the two idle ports is referred to by some (including the writer) as the "off idle" port, because this port has to kick in as you move the throttle forward to get you "off of idle" and on to the main discharge nozzle. If the lower port is clogged, you'll feel a flat spot until you get to the main discharge nozzle - if you can even get to the main discharge nozzle.

As you decelerate, you're depending on the lower idle port once again to kick in to carry you smoothly as you transition between the main discharge nozzle and the upper idle port."

[roadnsky]

And a little more...

More quoted info from Don's past wisdom...


"The following problems within the idle system (listed somewhat in decreasing order of likelihood) can cause uneven or unreliable idling:

IMPROPER IDLE SET SCREW ADJUSTMENT: Your idle RPM may be set – not by the idle set screw – but by the throttle cable itself. In other words, the idle set screw is backed off so far that the lower limit of the throttle cable is reached before the idle set screw. The problem with this scenario is that throttle cables do not have an accurate end point, and they don’t always stay where you put them. If you pull the throttle cable to its minimum position and walk away, it’s likely to spring back up or down a bit, moving the throttle control arm with it.

To check the idle stop, place the throttle lever in the cockpit at idle, then go down to the carburetor (with a flashlight and mirror if necessary) and note whether or not the end of the idle set screw is actually touching the idle stop coming out from the housing of the carburetor. If it is not, reposition your throttle cable so that the idle set screw does touch the stop.

This adjustment will probably result in too slow an idle, and the next time you start the engine, it will probably shut down as you slowly move the throttle to idle. So the next (and final step) is to readjust the idle set screw to the idle RPM that you want. In this new setting, the idle RPM will actually be set by the idle stop and not by the random positioning of the end of the cable.

CARBURETOR ISSUES AFFECTING IDLE:

1) Dirt in the idle jet.

2) A small speck of dirt in or near the upper of two tiny outlet ports of the idle system. This is the port (about the size of a fly speck) that you see just off the edge of the throttle valve when looking down from the mounting flange of the carburetor.

3) A faulty seal around the idle passage way between the upper and lower halves of the carburetor housing. This passageway is in the very center of the carburetor housing and is about the size of a drinking straw. If the gasket between the upper and lower housing is not completely sealed around this passageway, air can be drawn in and break the suction necessary to draw fuel up to the upper idle port.

4) A leaky float valve which provides such a rich mixture that it floods out the engine at idle RPM.

5) There is an air bleed hole in a small brass plug that is pressed into the lower face of the upper half of the carburetor that sometimes becomes blocked. This plug is located between the idle fuel jet and the venturi tube in the main throat of the carburetor and can be easily seen when the upper half of the carburetor is separated from the lower half. Sometimes a small speck of dirt lies on top of this small plug and interferes with the air being drawn in through the bleed hole to mix with the idle fuel.

MANIFOLD LEAK: You could have a vacuum leak in the manifold gasket or carburetor flange gasket. Some folks have been successful in diagnosing a vacuum leak in these areas by spraying some starting fluid around the manifold gasket and carburetor flange gasket while the engine is at idle. If the engine RPM changes at all while spraying the starting fluid, the gasket is almost certainly leaking.”

Hope all of this points you in the right direction.
Please report back and let us know what you find...

[dleedvm]

Engine stalls at idle

Thanks for the input, everyone! I have to admit, I have a bit of a phobia regarding carburetors that goes back to my childhood and experience with Testors airplane engines! Any change in a set screw with those things rendered the engine useless in my hands. It is entirely possible that I jiggled the distributor while resting on top of it for what seemed like hours on end removing the old water pump. I will look at that. I have the electronic ignition kit that I need to install too, so maybe I can kill two birds with one stone there. I also have to admit that I don't think I emptied the separator bowl on the mechanical pump during winterization. The fuel level is also quite low (they launched boats before anyone opened their pumps this year) and I didn't treat it with anything this year. I did notice that when it is idling rough, opening the choke kills it entirely. I don't think I messed with the idle setting but anything is possible. The boat has always idled somewhat fast. In gear it will do about 2 kts at idle, which is actually a little too fast in the marina, so I typically take it out of gear as I'm coming in and drift most of the way until it is time to maneuver. Thanks
David

[sastanley]

dleedvm,

I used to mess up those silly little super-simple motors too. I had a tiny little 2-stroke motor with a reed valve, a glow plug and it ran on JP-5 jet fuel! - Mine ran more on the bench lashed to a vice than it ever did on an airplane...

As for the carb, just follow the MMI book. It has all the 'stock' settings, and then using the accompanying docs you can tweak it to run perfectly for your situation (like what Jerry just posted & other PDFs available in the FAQ section.)

Don Moyer's rhetoric in his printed word, in my opinion, is a perfect balance between successfully dragging a newbie thru kicking and screaming for the first time, as well as being excellent review for the experienced mechanic with many useful tidbits along the way they may have forgotten since the last read.

[roadnsky]

...I did notice that when it is idling rough, opening the choke kills it entirely.

David-
Hmmm, after suggesting everything I posted, that symptom suggests to me more a clogged jet or float valve than a lean mixture.
Soon Dave Neptune will chime in again with his input.
(He's our resident carb guru)
As he asked earlier, a look at your plugs will tell us something.
In the meantime, I've attached a doc regarding clearing carb clogs.

I have the electronic ignition kit that I need to install too, so maybe I can kill two birds with one stone there.

I might suggest here, that you wait until you've fixed this issue first so you know you've solved it as well as to not possibly compound it?
(YMMV)

[Carl-T705]

If you don't want to "tinker with anything" get a can of " sea foam" at the auto parts store , pour it in the fuel tank and go out motoring for an hour or more. This stuff has never failed to show an improvement in anything I've poured it in, from the weed wacker to a semi truck. Let us know which route you take.PS If you do the sea foam carry an extra fuel filter it may loosen up old crud in the tank as well.

[ILikeRust]

The boat has always idled somewhat fast. In gear it will do about 2 kts at idle, which is actually a little too fast in the marina, so I typically take it out of gear as I'm coming in and drift most of the way until it is time to maneuver.

The idle speed is a dirt simple adjustment - mine was idling a bit too high also, so I turned it down a hair. I had the same issue - in gear, even in idle, was still a bit faster than I would like.

Do you have the Moyer Service and Overhaul Manual? If not, you really should get yourself one - it's an extremely worthwhile investment if you're going to own that boat for a while. I have found it invaluable.

Don't fret about adjusting the idle speed - it's just a simple screw that adjusts where the throttle stops moving. If you go too low, you just give it a teeny tweak with a screwdriver.

Same thing with the idle port adjustment screw - turn it al the way in, then turn it back out about 1.5 turns. Then just give it very tiny tweaks to find the sweet spot.

[Sony2000]

Possibly diconnect the throttle cable while you are setting up, or adjusting the carb when the engine is running. Just one less complication, to get a SOLID idle.

[Dave Neptune]

Checks

David, you said that adding the choke just killed it faster and that indicates to me that you are to rich, not plugged up.
Again did you look at the plugs? If you do and they are sooty try opening the idle screw 1/2 turn out to lean it a bit.
I'd give that a try first before tearing things apart.
Also get a 1/2" open end wrench and check to see if the dist is snug as you may have moved it if it was a bit loose.

Dave Neptune

[Mo]

Dave and David, you made me think of something. If David winterized the engine with oil in the spark plug holes he should pull the plugs and clean them up, check the gap, and each plug for spark. My tiny little mind is thinking this could be fouled plugs.

[hanleyclifford]

Into the Unknown

That's what I used to feel when adusting carburetors until I installed the air/fuel ratio gauge. For about $100 you can know exactly how you are burning fuel and take the guesswork out for good. www.cyberdynegauges.net

[dleedvm]

Wow! Lots of great ideas. I haven't checked the plugs yet, so will do that this weekend. I will replace them or clean and gap if necessary. I will also check the distributor. The source of the clanging Dave suggested makes perfect sense, because it seems to happen only when the boat is still moving pretty swiftly (phew!). I will definitely drain the carburetor and will try sea foam. My boat currently has just an inline polishing filter and I have a new one with hoses ready to go (the current hoses aren't USCG approved). I have no idea how old the current filter is. Thanks everyone!
David