Variable Idle ???

[IndyA]

Gentlemen: I've got a 29 coupe with rebuilt carb & motor (motor has 600 miles on rebuild) ; all manifold nuts are tight; head bolt torque checked; accelerator bracket oiled; no vacuum accessories (wiper, etc); advance rod works as it should and with all that ..... the idle speed changes at almost every stop (with motor warmed-up). Is this typical until motor gets well broken-in or are there other possibilities.... Thanks for all your help....

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Does the motor slow down or speed up?


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[IndyA]

Yes, generally the motor speeds up when coming to a stop .... about a third of the time, the motor slows down to point of engine shut off .... but of course, I adjust the accelerator lever so that doesn't happen... good question & thanks.

[1crosscut]

With the engine running spray starting fluid all around the manifold where it attaches to the engine block. If the engine speed changes while you are doing so you have an intake manifold leak.

[[email protected]]

The motor speeds up just before running out of gas such as when you close the fuel valve and leave your engine running. The motor will run about one half minutes or so until the gas in carb. is used up.

If float is set to shut off the gas flowing into the carb. at less than one inch and you come to a sudden stop, the motor will die or nearly die.


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[RawhideKid]

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Come on guys, what is the first rule of troubleshooting?

Get all the facts!!

What type of carburetor is it?

Where is the GAV set at?

How long ago has it been since it was cleaned and adjusted?

Who adjusted it?

Now, if it is a Zenith, the float level is way too low causing the very predictable problem of stalling at stops.

If it's not a Zenith, and it IS a Tilly, you can cause the same trouble with the same fault.

You guys are so funny poking around in the dark.

Someone else really needs to do a Search "stalling at stops"

[IndyA]

The GAV knob is turned fully clockwise.... it is a Zenith Carburetor. I tried the Starting Fluid test.... no change in idle speed. Appreciate all the help.....

[RawhideKid]

...

[RawhideKid]

If the GAV is closed you won't have any fuel going to the idle circuit, hot or cold! You need to open it at least ¼ turn, just to get it to idle. I would also like to know where your float level is set at. Just because you have opened the idle valve on the front of the carburetor doesn't mean you are getting fuel in at low RPMs.

Start the engine, open the GAV ¼ turn, set the idle screw in the front of the carb about 1½ turns out from full bottomed. Let the engine warm up a bit, just a couple minutes is enough.

NOW with spark lever UP fully, unscrew the idle speed adjustment screw, the one on the throttle butterfly with the spring on it, to ½ turn clockwise above engine stall. So, unscrew it til the engine stalls, after engine stalls, screw it back in ½ turn.

Restart engine, open adjustment screw on front of carburetor to smooth idle.This is the idle adjustment. Now adjust GAV to rough idle, note position, open from that position ¼ turn. This is the fuel adjustment, try not to lean it out (clockwise to stop) as this will give you better mileage, it will cause overheating.

[100IH]

In some instances, float level too high will also stall the motor. The slosh can flood the engine. Still, post #9 is a good path to follow. Don"t be afraid to try each setting of GAV, idle mixture and idle speed screw - a little more - a little less while listening and feeling the engine. Float level is more set in stone. get it right and leave it.

[RawhideKid]

Oh crap, totally got into the adjustments and forgot about the float level!

Remove the carb from the manifold, remove the center bolt which holds the carb together. Turn the top half upside down, you can even place it on a flat surface and get the float seam height. This should be right around 1.000 to 1.010. This can be accomplished by adding or subtracting the gasket/washer under the float needle valve.

I have done several and never had a seam deeper than that, so it should be close. The most important thing is to get the seam totally parallel to the top gasket flange. This means the front, and rear of the float seam parallel and side to side, all the same 1in.

I have found some floats that the top has been caved in and I have still been able to get it close to the 1in depth. This is your float level, 1inch. The extra .010 isn't going to make or break the float level, but with the float resting on the needle it must be flat/parallel to the gasket surface.

Once you have that 1 in flat, replace it on the bottom and screw it back together, reinstall on car, THEN adjust it as above.

[IndyA]

RawHideKid.... Many thanks for your sage advise.....both posts....I'll do the measurements this weekend..... IndyA

[Jacksonlll]

Next time it does it, get out and close the lever on the carb until the screw hits it's stop. If it moves and slows down, you throttle is not closing. Add a redundant closing spring on your throttle linkage.

[SeaSlugs]

Originally Posted by Jacksonlll

Next time it does it, get out and close the lever on the carb until the screw hits it's stop. If it moves and slows down, you throttle is not closing. Add a redundant closing spring on your throttle linkage.

Or fix it right and find out why its hanging up without adding more springs to wear out the throttle shaft faster... Dont forget to oil the throttle bracket! Also Ive seen things too tight and the car body (ie steering column) slightly flexes going over bumps so depending on your stopping incline angles it can change things.

With the hand throttle all the way up it should take 3-4 "clicks" of the lever down to get it to start to idle faster to create a "buffer" so to speak for the flexing body.

or carpet/floor board dragging on the accelerator pedal...

[Y-Blockhead]

Originally Posted by RawhideKid

If the GAV is closed you won't have any fuel going to the idle circuit, hot or cold! You need to open it at least ¼ turn, just to get it to idle.

It is my understanding that the GAV doesn't effect the idle circuit...

[RawhideKid]

Originally Posted by Y-Blockhead

It is my understanding that the GAV doesn't effect the idle circuit...

Oh yea, great, start your car and close the GAV and then tell me that.

[DJ S]

Originally Posted by RawhideKid

Oh yea, great, start your car and close the GAV and then tell me that.

The GAV doesn't effect the idle circuit. My car idles with the GAV closed and so should everyone elses. Sounds like you need to learn the basics. http://modelabasics.com/carb%20basics%202.htm

[Will N]

Originally Posted by Jacksonlll

Next time it does it, get out and close the lever on the carb until the screw hits it's stop. If it moves and slows down, you throttle is not closing. Add a redundant closing spring on your throttle linkage.

X2. I've had to gently bend the arm on the throttle linkage towards the carb in order to allow it to let the throttle to fully close.

[Y-Blockhead]

Originally Posted by DJ S

The GAV doesn't effect the idle circuit. My car idles with the GAV closed and so should everyone elses. Sounds like you need to learn the basics. http://modelabasics.com/carb%20basics%202.htm

Thank you DJ S

[Big hammer]

Originally Posted by Will N

X2. I've had to gently bend the arm on the throttle linkage towards the carb in order to allow it to let the throttle to fully close.

I've had too do this also, changing from a Tilly to a Zenith carb!