How "steady" is your idle RPM?

[TomG]

After a flawless summer of running my A-4, I am beginning to notice some specific peculiarities of my little machine. I have a MMI rebuilt late-model engine with EI, electric fuel pump, direct drive turning an Indigo prop, with the bigger 55A alternator. The engine starts immediately, and pushes my Tartan 30 at 2,200 rpm at 6.7 knots without quarrel. Since improving the exhaust system over the winter, the engine has never "just quit" or been slow to respond to throttle or run hot or anything that was wasn't 100% normal. Not to hex myself, but the engine has performed perfectly.

The one thing that I am wondering about is the engine's proclivity to "hunt" for idle. After the engine warms up, idle RPM in neutral vary from around 750 to 1,000 RPM regardless of throttle position. In gear, the idle holds rock steady at 800 RPM. The engine doesn't surge and the RPM change is only noticeable on the tach in neutral. It seems like the RPM holds steady and then will change only after racing the engine (off the idle jet) and then back to idle.

My question is, is this normal for the A-4? The engine is operating so smoothly I hate to try to tweak a non-problem if this is typical for the A-4.

A little cold front pushed through the Chesapeake Bay last night so I'm out sailing today. Hope everyone else gets some sailing in today!

[romantic comedy]

The fist thought that comes to mind is an air leak. A small air leak might cause the variation.

[Loki9]

I don't think it's normal, at least, my A4 doesn't do that.

[thatch]

Two Things

Tom,
Two possibilities come to mind, first if you have a PCV valve, I would temporarilly plug it and see how she idles. Since that valve is basically a spring loaded vaccum leak, contamination will sometimes result in an erratic idle. Secondly, with my A4, I will sometimes notice that if I am not in the neutral "sweet spot" she may sort of "cycle" in and out of gear.
Tom

[edwardc]

Tom,

Mine doesnt "hunt" like that, and I have a PCV valve.

I've always thought that a hunting ide was the result of fine dust-sized patricles going thru the idle jet. Might be time to drain the float bowl and check the gas that comes out.

Do you have a polishing filter?

[Sony2000]

After putting a project engine back together repaired it wouldn't give me a solid idle that motors do. But all that changed after I set the "dwell angle" of the points.

[thatch]

Which Tom?

Edward,
Not sure which Tom you were addressing but I will answer anyway. The reason that I mentioned the PCV valve was not to open up that "can of worms" again, but merely to eliminate that as a possibility for the idle glitch. As I'm sure you know a "sticky" PCV valve can indeed cause those symptoms. Another cause for the "hunt" could be in the distributor where, with the advance mechanism hanging up, the engine would be getting varrying advance signals that will also change the idle speed. I have delt with higher performance engines that required a "recurve" of the distributor to achieve a solid predictable idle.
Thatch

[Dave Neptune]

Go with~~

Go with T Hatch on this one. For an idle to vary like that it takes something to vary and most fixed things won't cause a variance. I will point out that a we do not use a return spring for the idle stop like a automotive application, how you pull the throttle back and how hard can cause a change in idle speed but will not cause it to hunt. Could be a slight varitation in idle fuel preassure which would be no biggie.
After plugging the PCV if so equiped the idle may not be as smooth because it will be a tiny bit richer, so if the idle becomes steady then just replace the valve and be done wirth it. If not the idle may be set a tiny bit rich which in some cases can cause a bit of "hunting" however lean usually is steady but rough.

Dave NEptune

[ndutton]

See if you get the same symptom with the alternator belt removed.

[67c&ccorv]

Neil...are you thinking the alternator regulator charging control?

[ndutton]

Neil...are you thinking the alternator regulator charging control?

You know it.

At the RPM he's talking it might be right on the threshold of charging/no charging. RPM goes to 1000 and the alternator kicks in applying load to the engine which drags the RPM down to 800. At the lower RPM the alternator drops out unloading the engine and we start the cycle again. In gear the added alternator load is not as profound so therefore is not apparent.

I'm not saying this is happening for sure, that's why I asked about a no belt test. I'd like to eliminate the possibility before tearing into a good performing carburetor.

[hanleyclifford]

During the "cycle" do you notice any voltage variation?

[TomG]

Thanks for the advice and opinions, everyone.

While the vacuum leak idea certainly has merit (and easy to check by plugging the PCV line), I'm really leaning towards the alternator. I have the 55A alternator that goes into a Blue Sea 120A ACR then to two group 24 AGM batteries. When I start, the voltage at my engine gauges shows a little under 13v. Once I get the motor running, about 10-20 seconds after start, I'm guessing the ACR decides to "do its thing" and starts charging the battery(ies). The load on the engine is quite noticeable as the voltage goes to around 14 or so to put the trons back into the bank. I think perhaps I've set the idle with the alternator drawing a max charge and when it drops off, I'm getting the extra rpm. I think I'll try both approaches: take the PCV out of the loop first and try it then run without the alternator and see what she does.

Thanks to everyone for the help!

[hanleyclifford]

Tom, I thought that alternator had it's own internal regulator.

[ndutton]

Awaiting Tom's test data, the mere thought that a PCV system might be the cause of this is giving me a . . . .

Well, let's say the wife is looking at me like it's her birthday or sumthin'