Idle screw and backfire
 

I know there are numerous threads on backfiring and have read most of them.
I was getting some frequent pops when I really tried to put the pedal to the floor.
I notice a lot of people commenting that this type of problem could be timing or the float in the carb. However, I tried some common easy steps first and this is where my question comes in....
I opened up the gav more than it was, around a turn and 1/2.
I also quit leaving the spark all the advanced and put it 3/4 down.
The thing I am not sure about is that I adjusted the idle screw on the zenith.
I screwed it in all the way then opened it about half a turn. This seemed to really help the idle on the engine. After driving it for about 30 mins I had only one backfire but it seemed to have more get up and go.
Should I just run it lie this or is this not the solution? Should I go ahead and open up the carb and adjust the float?
Thanks

Re: Idle screw and backfire
 

Backfiring on acceleration is probably due to electrical issues; on deceleration is fuel. When the gas pedal is down, especially at low speed, lots of air/fuel mix is being taken in which raises the pressure in the combustion chamber, which makes it harder for the spark to jump the plug gap, which raises the resistance and ultimate discharge voltage, which enables the spark to find an alternate, unproductive path to ground, which makes the fuel mix fail to ignite, which makes the unburned fuel go out in the exhaust where it can be set off as a backfire. The problem could be anywhere in the ignition system, including hard-to-see cracks or tracks over the surface of the plastic distributor parts. Check everything closely. Don't screw around with the carb unless you've got more compelling issues than you describe; there's a lot more ways to make things worse than there are to make them better, IMHO.

The GAV should be about 1/4 turn open after warmup; one turn or more is way too much for normal good running. The idle adjust should be set midway between open-too-far and open-too-little, both "too" conditions being judged by the engine starting to stumble and run rough. If you've got air leaks around throttle shaft (common) or around gaskets, this adjustment, which bleeds air into the idle circuit, will need to be closed down more than otherwise to get the right balance. The opposite is true for the GAV; it must be opened more to provide extra fuel to balance the air leaking in. With a good tight system, you might get away with closing GAV completely. Spark advance is more controversial: anywhere from half-way to all the way down is used by different folks, depending.

If car is backfiring while running down a long grade with throttle closed, especially in 2nd gear, opening up the GAV as much as one or two full turns will help. Under those conditions--and similarly when shifting gears--the only fuel supply is the idle circuit but the high-revving engine needs more. Insufficient fuel to support proper combustion causes unburned gas to accumulate in exhaust, leading to backfiring.

Steve

Re: Idle screw and backfire
 

Based on what Steve wrote, it sounds like you may have an air leak somewhere in the intake system, be it worn throttle shaft, base gasket, manifold gasket.

Re: Idle screw and backfire
 

Make sure you have the right idle screw. Ive got a couple of dozen old ones...you can see a difference in the dia. and length and # of threads on them.
Wick

Re: Idle screw and backfire
 

You say that you haven't checked the timing. You tried to do some easy steps first.

Timing the Model A Ford is actually very easy. You should make sure that is correct since it is such an easy check before you start changing wiring, replacing gaskets, rebuilding carbs, etc.

Print out the following website, take it out to your garage and follow these steps.
http://www.abarnyard.com/workshop/timing.htm

Below is my attempt to simplify the timing procedure.

Pull the timing pin out and put pointy end into timing gear cover.

Use hand crank to turn motor over (while using other hand to push in on timing pin) until the timing pin drops into the engine further.

Then remove the top cap on the distributor. With the spark lever inside the car all the way up, grab the rotor (round object that turns inside the distributor) and make sure it is sitting as far clockways as it can sit. If it looks like the picture below, the car is right in the sweet spot.

http://www.abarnyard.com/workshop/images/distrib-2.jpg

If it doesn't look like the photo, remove the rotor, loosen the screw and turn the cam below and re tighten. Then check the rotor again, (making sure it is turned clockwise to remove slack)

Re: Idle screw and backfire
 

Got the timing dead on. Readjusted the idle screw.
But it is still doing it. I can drive it fine without
Any pops but it is when I really push the accelerator down
It pops and doesn't have the power I think it should have.
Guess replacing gaskets is my next step.

Re: Idle screw and backfire
 

Have at it if you want but I'm betting it will be a lot more productive, and easier, to first check for electrical problems, for all the reasons I described earlier. I suspect that under load your high voltage is finding another path to ground. No spark at plug gap means both no power from that cylinder and backfiring (popping) when the unburned gas winds up in the exhaust system.

Try really easy things first: are your plug insulators and coil/distributor wire and distributor cap and body really clean and with no scratches or carbon tracks? Are the plug gaps clean and correct? Try watching for errant sparking at night: start the motor, raise the hood, suddenly race the engine with throttle control, watch for light show over the surface of the distributor. Are any of the spark plug wires really close to the distributor body?

Steve

Re: Idle screw and backfire
 

I agree with Steve. It sounds electrical. Do you have a good condensor in there, and have you checked your points?

Re: Idle screw and backfire
 

Wires? I'm running brass strips

Re: Idle screw and backfire
 

And plug gaps are correct and points are clean

Re: Idle screw and backfire
 

check the manifold gaskets or spray some WD40 on the gasket and check to see if the idle picks up.

Re: Idle screw and backfire
 

Lets not forget coil polairty, the wire between the upper and lower distributor plate and the connection to the popout cable in the distributor.

Re: Idle screw and backfire
 

Check for negative polarity on the coil secondary output using a dc voltmeter, connecting the positive of the voltmeter to ground and touch the negative lead of the voltmeter to a spark plug terminal while the engine is running - the meter should jump upscale. If not reverse the primary coil leads and repeat the test. The reason behind all of this is that it is easier for the spark to jump from a hot temperature surface to a cold surface - ie from the plug center electrode to the ground terminal of the spark plug. If you have the negative polarity on the secondary of the coil, then connect the coil to a tester. Does the missing happen only after the engine has been run say 20 minutes? If so, the coil may be breaking down - try another coil.

Re: Idle screw and backfire
 

You want him to connect a standard DC voltmeter to a sparkplug wire with the engine running????!?! :confused::eek:

Re: Idle screw and backfire
 

Not a problem using a DC voltmeter - have done it many times. The needle does not kick up very far.

Re: Idle screw and backfire
 

Some of the older automotive volt meters are built a bit more rugged and have a less sensitive movement, such as 1,000 ohms/volt. Many of the newer multimeters that most of us use have a more sensitive movement with 10,000 or 20,000 ohms/volt, and are more likely to be damaged by high voltage. Just to be safe, you could connect a resistor of 20,000 ohms to 1 megohms between the meter leads, and you can still see the upward movement of the needle when connected as Bill suggests.

Re: Idle screw and backfire
 

Tom,

Do you think my Simpson 260 could handle this?

Re: Idle screw and backfire
 

It probably could, but just to be safe I'd connect a resistor between the two leads. I used a 40,000 ohm resistor on my 20,000 ohms/volt DC meter and the needle moved enough to clearly see the spark polarity.

Without the resistor between the two leads the needle flicked rather violently up the scale. Always use the highest DC volts scale, of course.

Re: Idle screw and backfire
 

I have the same problem,checked timeing ,spark plug gap , reset point gap,went thru the carb.float set at 1 inch. new gasket ( carb to manifold) even changed the polarity on coil ,fuel lines all clear ,fuel flow is good to carb. drained gasoline from tank ,checked stand up fuel screen on fuel cut off switch, idles ok but on acceleration coughs and sputters ,pull out choke and it picks up speed runs with choke bout half way out and GAV open couple of turns,let off choke stars same ole thing ,thinking maybe the high speed jet or passage in Carb. is clogged up . Gonna Disasemble and soak it in Carb CLeaner tomorrow .I'll let you know results.

Re: Idle screw and backfire
 

Never underestimate the Pierce Simpson 260