Odd idle issue

christian

christian

Senior Member
Car runs very well at idle, and moderate to high speed. But ive been suffering from some annoying hesitation as I give the car some gas from idle, not under acceleration, just when i crack open the throttle plate as im transitioning from idle, it momentary but annoying.

I find that if I open my idle screw out to 2 and a half turns from seat, all this hesitation is gone, yet im left with a terrible idle, so Ive been running a 1 turn from seat for a good idle and just dealing with this issue as of late. GAV has no effect on the hesitation.

carb has been thoughly cleaned with flow tested jets installed, nice tight throttle shaft, new gaskets etc.

I doubt this is an electrical problem as the car runs great otherwise. does it whether hot or cold. also, not a fuel sloshing issue, or a float inertial issue (when coming to a stop) , as I experience this with the car when sitting also.

i experimented with a higher idle, and that got rid of it, but when I put it in gear i cant avoid grinding so i dont want to keep it like that.

im kinda dumbfounded, any ideas?
 
For what it's worth, I talked to my Dad about this issue, he knows nothing of model A's, but knows a good bit about carbureted engines, he was thinking vacuum leak. Since I need the airscrew turned in a tad more than usual for a good idle, yet under throttle there isn't enough vacuum to support the fuel being sucked in until the throttle plate is open a bit, hence why I need the airscrew way open to eliminate the off idle stumble.

It's plausible I guess, but that wouldn't explain why it runs fine in every other power band except off idle. I would think with a vacuum leak I would have to run the GAV way open to drive well yet I can push my fordor to 65mph with the GAV virtually closed and runs very smooth....

I dunno......

The other day I took some vacuum readings, fluctuated wildly between 16 and 23. Then fed MMO into the intake via the wiper hook up. It smoothed out the idle a lot, I also added a bit too much MMO to the fuel, I attributed the off idle stumble to the double almost triple dose of MMO, but I've since diluted the fuel by burning a few gallons and adding more

I guess my next step is to take more vacuum readings. If there is a vacuum leak it isn't at the intake or manifold that's for sure, must be in the valve train if there is one.
 
Drive the car for about an hour, got it good and hot. Then checked vacuum, got similar results from last time, at idle, very quick even fluctuation between 16 and 23. Compression on warm engine with throttle wide open showed from 1 to 4, 97,92,85,90.
 
question with the base idle set very low like the model A should be at operating temp and the timing advanced some approx 1/4 down, the gav open 1/4 turn, how far open is your air/fuel mixture screw from the closed position to obtain the smoothest idle ..
 
Try increasing the size of the hole under the choke plate just a tad. I find the new fake gas changes things especially with the zenith carburetors. I also increase the jet sizes a little. I have a flow testing setup to monitor what I am doing. You are running a Zenith, right?
 
I agree with your dad--vacuum leak. But, I don't know why opening the GAV doesn't help at least a bit.

At open throttle, the amount of air getting in due to the leak becomes insignificant compared to the main flow. Remember, the expression "stepping on the gas" really means "stepping on the air."


Steve
 
For optimum idle and driving, the GAV likes to be about 1/8 when warm. Also best idle is obtained at about one turn out on the airscrew from seated.

For now I can live with this minor stumble, if I give it a good bit of gas off idle I don't have the stumble. But what bothers me are these vacuum readings. If I had a leak somewhere wouldn't the vac reading be low and steady ? The readings I'm getting fluctuate from low to high, which tells me that if there is a leak it must be upstream from the intake, since at least some of the motor is pulling full vacuum ( the high reading when it fluctuates) and some parts of the motor don't pull enough vacuum ( the low end when the needle fluctuates)

I'm thinking I should be looking into the valve train.... Any thoughts. I'm not familiar with these style valves, what should I look for if I wanted to inspect them. Also as far as my compression readings, they aren't very good I know, but could be worse. If I had valve issues, wouldn't my compression numbers be worse? Or do compression tests not always show valve problems ?
 
Here's my 2 cents: Most off idle stumble issues are timing issues if the carb has no enrichment mechanism.

Lean mixtures burn slower than rich mixtures.

Model A carb has no enrichment mechanism, hence will burn lean transitioning off of idle. Lean mixture will desire advanced timing to produce power.

Advance your timing, reset idle mixture and speed and try again.
 
I appreciate all of your responses to the effect of setting timing, and adjusting the carburetor and such. All of these thing have been done. I can set the air bleed screw out some for a rough idle and no stumble, or i can set it in a bit for a richer mixture and smooth idle and stumble while transitioning from idle, but none of this changes the 800 pound gorilla that my idle vacuum readings bounce wildly from 16 to 23. I dont want to sound like a jerk since im here asking for your help, but i need ideas that can pin point this vacuum issue. these readings tell me there is something wrong with the motor, but im new to this and need some guidance.
 
christian said:
idle vacuum readings bounce wildly from 16 to 23.
Click to expand...


Causes: ignition miss, sticking valve, compression issues (usually severe), weak spark (very similar to ignition miss).

Advance you timing a few degrees (move the adjust down halfway), reset idle speed, what does your vacuum do?

Check plug gaps, observe spark and make sure it is healthy.


Jumping vacuum readings are more related to ignition and mechanical issues (like valves).
 
stouchton said:
Causes: ignition miss, sticking valve, compression issues (usually severe), weak spark (very similar to ignition miss).

Advance you timing a few degrees (move the adjust down halfway), reset idle speed, what does your vacuum do?

Check plug gaps, observe spark and make sure it is healthy.


Jumping vacuum readings are more related to ignition and mechanical issues (like valves).
Click to expand...


great will try these tonight. I have a suspicion that my idle issue is just a symptom of a greater issue, which is shown by the vacuum readings. come to think of it, I had this same problem with my model B carb, just not as bad.
 
as far as valves go, if it were the seats, that would show up on a compression test, but if it were the guides, that would not show up on the compression test ?
 
Just contributed to the other thread where you stated you had a carbon suppressor cable - that might be your problem. They limit the current, thus creating a weaker spark. Jumps almost the same distance, but has a "weaker" spark (you can tell, it will be less brilliant).

Drops efficiency of lighting the mixture, causing issues primarily at idle.

Might explain your jumping vacuum, but you would hear the miss in the exhaust.
 
Compression tests do not tell the whole tale, for things are moving slowly.

A sticky valve, or valve spring issue might sneak past a compression test.
 
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