Engine idle question

notjustair · Apr 7, 2014

I've noticed this with all of my carbed engines and always wondered why.

Why is it that when you are running down the road and push in the clutch the engine idles faster until you come to a stop? I'm not talking emissions stuff or EFI engines. I mean ones without any computer or electrical connections other than the coil. I was driving the grain truck back from town yesterday. Came to the dirt road and pushed in the clutch. I purposely pushed in the clutch really early to give it lots of time at idle to see if it would go all of the way down. Idle stayed above 1200 rpm until I got to about 10 mph and then idled down to 800 or so. It is a 1958 with a cable choke. There's no connection to speed. My restored old cars do it to. I originally thought maybe the wind through the rad was pushing the fan, but my old cars are air cooled. No fan up front.

Anyone got an ideas?

Wayne in MN · Apr 7, 2014

The carb may have a dashpot to prevent rapid deceleration when the foot pedal is quickly released. Look on the side of the carb for a silver dollar sized unit with a shaft sticking out that touches the throttle linkage.

Martin Reece · Apr 7, 2014

hi,
Vacuum ?

Billpa · Apr 7, 2014

It sounds to me like the motor mounts are bad , pushing clutch in pushes entire engine front , just a thought do you have a stiff clutch ?

Bob Bancroft · Apr 7, 2014

You may be getting just enough drag through the pilot bearing and the clutch to give you a little higher idle speed.

Steve@Advance · Apr 7, 2014

The clutch is what I'm thinking too. Some of the spring type pressure plates have weights that give a little more force when the engine is revved up. Maybe it doesn't fully release until the speed comes down. Still strange that it would make that much difference.

Wonder if putting it in neutral would make a difference?

robgIN · Apr 7, 2014

wayne has your answer , most carbureted cars and trucks have a dashpot to prevent stalling . Works kinda like the thing on your screen door to keep it from slamming shut.

k6zrx · Apr 7, 2014

None of the old cars I have do this. Idle drops right down to normal when the clutch is pressed.

I would say it must be drag in the pilot bushing.

Is it hard to get into gear (grinds) when going into gear from neutral, even with waiting a few moments to let the gears spin down after pressing the clutch? If so, the clutch might need adjustment, or something is dragging.

Motor mounts can cause adjustment to go bad as they get old, soft, and rotten. On both my DeSoto and Dodge, it was impossible to get them into reverse/first (column shift) after replacing the mounts. Over time, adjustments were made to compensate for the softening motor mounts.

New cars, yes I have seen this action. My wife's Audi did what you are seeing. Never seen it in a carburetor vehicle.

Josh

Bob Bancroft · Apr 7, 2014

Thinking more about it- this is just the "cart pushing the horse", as it were.

Just the other side of trying to shift with a revving engine- sometimes there's enough drag in the clutch assembly to clash gears- whereas the same machine will shift at idle.

Bob Harvey · Apr 7, 2014

I concur.

rustred · Apr 7, 2014

maybe you should say what vehicles and carb. if your throttle linkage is adjusted and not binding or if it has a cable having the right adjustment.
you have something not letting the throttle plates fully close. no such thing as engine idles faster when clutch is depressed. as soon as foot is off gas pedal, engine is at idle and in gear or out of gear has nothing to do with it.
you have other problems.
the holly carbs were kind of finicky .

Bob · Apr 7, 2014

"most carbureted cars and trucks have a dashpot to prevent stalling"

I'm gonna "call you out" on that.

Yes, SOME did, especially Fords, but they (IMHO) were FAR outnumbered by vehicles that did NOT have a throttle "dashpot".

ANOTHER possible cause for what the OP believes he is observing would be a failed brake booster creating a vacuum leak and high idle as long as the driver's foot depresses the brake pedal even a little.

Engine idle question

notjustair

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Wayne in MN

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Martin Reece

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Billpa

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Bob Bancroft

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Steve@Advance

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robgIN

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k6zrx

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Bob Bancroft

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Bob Harvey

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rustred

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Bob

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