Wandering idle

[Matt Harwood]

I thought about putting this in the Buick section, but I thought it might be useful to others here. I've had more than one car where the idle is different almost every time I drive the car. I'm willing to think it was a quirk of one car, but I've had many of them. Sometimes the idle is nice, sometimes too high, sometimes too low, and I find myself constantly fiddling with it since it goes all over the scale.

 

Most recently, my '41 Buick has been tough to pin down. It will idle perfectly at any speed I choose, but it's different every time. I set it when it's warm, and when it's cold it's too low. I bump it up a little bit to get it smoother when it's cool and it idles at 900 RPM when it's hot. Manual suggests about 500 RPM would be right, and I can get it to idle there no problem, but when I first start it, it's like 250 RPM and pretty lumpy.

 

So one, why does the idle go up so much when it's hot?

And two, at what temperature should I set it? Manual says hot, but is there a too hot? Should I just set it hot (since that's where it will spend most of its time) and let the cold idle suck for the first five minutes? Should I dial in more choke so it stays on the high idle cam a little longer?

 

It's just weird.

 

 

[ply33]

I am far from an expert, but I've noticed on my old car that it takes a longer warm up period for the engine to idle nicely with the choke fully open if the manifold heat control is not functioning properly.

 

Just wondering if your heat riser is working properly. . . Could be the "hot spot" on the intake manifold isn't quite hot enough when your (I assume) automatic choke has fully opened.

[Rusty_OToole]

Most common reason is worn throttle shaft in the carburetor. This lets air leak in, increasing idle speed. Does no harm if the mixture is adjusted to suit. But, the throttle doesn't always come to rest in the same spot so the amount of air varies, changing the idle speed.

[carbking]

While I would agree to a degree with Rusty about the throttle shaft clearance; I would say the most common cause of a "hunting" idle would be improper use of a vacuum gauge to adjust the idle. Following closely in second would be choke/fast idle not properly adjusted. One other possibility, although probably not overly high on the frequency would be a fatigued throttle return spring.

 

Jon.

[edinmass]

Hunting idle on fuel injection is common. Not so much on a carburetor car. I would look at the vacuum brake booster and the system, as well as look closely at all vacuum lines. I have seen poor motor mounts suffer a vacuum leak when the motor flexes and the hose draws more air. I would also use a propane bottle to check for other leaks. 

[Rusty_OToole]

On rereading the original question it seems the idle speed is directly related to engine temperature, fast when it is warm and slow when it is cold.  If this is the only difference, it is more or less normal. That is why they put a choke on the carburetor and a device to raise the idle speed when the choke is on. For a 41 Buick this should be automatic, at least other expensive cars had automatic chokes by that time. I can only suggest inspecting the carburetors, choke and throttle mechanisms and make sure they are to factory specs adjusted correctly and working correctly according to the factory manual. It doesn't pay to go getting ideas of your own when it comes to carburetors.

[Matt Harwood]

Thanks for the feedback, guys. I think this confirms what I was thinking, that the choke may not be adjusted 100% properly. My Buick drops down to the lowest idle setting almost immediately after starting and that's just too low until it gets up to operating temperature. The throttle shafts are a good suggestion, too, but the carburetors are freshly rebuilt with new bushings so I'm hoping that's not the case. It looks like the idle adjustment screw is firmly hitting the stop every time. I'll set it when it's hot, then adjust the choke to stay open a little longer than it does. I do have it adjusted more or less to spec, but my car isn't 100% stock in terms of heat risers and choke stove pipes so that may require some extra tweaking. I think more choke might be the right idea.

 

Thanks for the extra information.

[Ken_P]

12 hours ago, carbking said:

While I would agree to a degree with Rusty about the throttle shaft clearance; I would say the most common cause of a "hunting" idle would be improper use of a vacuum gauge to adjust the idle. Following closely in second would be choke/fast idle not properly adjusted. One other possibility, although probably not overly high on the frequency would be a fatigued throttle return spring.

 

Jon.

Jon, what do you mean by improper use of a vacuum gauge to set the idle? Thanks!

[carbking]

Setting for the highest vacuum with a non-airbled carburetor.

 

Jon

[Oldtech]

Check that the arm mechanism on the outer end of the throttle  is fastened solid to the throttle shaft.  It is common for them to just be riveted and come loose.  Stromberg or Carter carb?