Backfiring

[Becky]

I have  a 1930 model A that keeps backfiring Help!! 

[Spinneyhill]

Backfiring is through the carburetor, after firing is through the exhaust. Which is it? Many people don't know there is a difference.

[Becky]

Through the exhaust 

[Guest]

Usually means a too rich mixture and/or a burnt valve.  A compression test and a leakdown test will tell you a lot.

[Becky]

Thank you!! Where can I get a compression gage and what should the compression read?

[PFitz]

Could also be caused by misfiring in the ignition system. When a plug (or plugs) is not firing, that unburned charge of air and fuel hits a hot spot in the exhaust system and lights off.

 

Check how the spark plugs look and see if any are more oily or sooty than the others. Might be a bad or shorted plug.

 

 A timing light will also show a misfire is it's being caused by other parts of the ignition system. Clip the timning light lead on to each spark plug wire in turn and look at the light, or if it's too bright shine it at a light surface like the palm of your hand. The flashes should be at even intervals like a drum beat. If it's skipping some flashes that's most  likely the one with a problem somewhere between the distrib cap and plug.  If it's more than just one wire it could involve more of the distributor and coil.

 

Paul

Edited July 24, 2019 by PFitz (see edit history)

[Becky]

Thank you! I will check these things out! 

[Becky]

Paul do you know where I can buy a adapter for a compression gage to test my 1930 model A?

[PFitz]

1 hour ago, Becky said:

Paul do you know where I can buy a adapter for a compression gage to test my 1930 model A?

Sorry Becky, I don't know where to get one because I was too cheap to look.

 

I made my adapter years ago out of an old spark plug. I broke off and knocked out the porcelain insulator. Then cut a pipe thread inside the spark plug base to take a male air line quick disconnect that would fit my compression gauge quick disconnect hose fitting. Then I took off the sparkplug metal gasket and used a rubber o-ring so I didn't have to get it more than finger tight to seal.

 

Maybe one of the other guys knows where you can buy an adapter ????

 

Paul

[Becky]

No problem!! Thank you that sounds like a good idea!! 

[Becky]

Paul do you know what number the compression should read 

[MCHinson]

From personal experience, I would suggest you change the condensor. A failing condensor is a common Model A Source of this type of problem.

[PFitz]

11 hours ago, Becky said:

Paul do you know what number the compression should read 

Becky,

 

Not knowing your engine's history, the compression number could be anywhere from around 60 lbs on up, depending on what pistons and head your Model A has. But the total is not what your looking for. Your looking for differences from cylinder to cylinder. You want to see which are the weakest cylinders to find out if the muffler explosions are being caused by an exhaust valve problem, - either burnt, or sticking open.

 

Differences in compression, cylinder-to-cylinder of 5% or less are considered normal. More than that needs to be checked further, such as put a couple of tea spoons of motor oil in the spark plug hole and check it's compression again to see if the number comes up, and by how much.  If it does, then you can use the compression fitting to run compressed air into that cylinder while it's on TDC (Top Dead Center) and listen at the tail pipe (exhaust valve leak), carburetor (intake valve leak) or a crank case opening (rings)  for the hiss of air leakage noise.

 

That's checking for possible mechanical causes. The timing light will show if the problem is coming from the ignition system.

 

Paul

 

 

[Becky]

Ok thank you!!

[Becky]

Ok Thank you Paul for all your good information!!! 😊

[Becky]

Backfiring now not starting at all replaced the condenser and points and plugs and rotor and cap adjusted the float in the carburetor set the timing help!! 

[Becky]

30 minutes ago, Becky said:

Backfiring now not starting at all replaced the condenser and points and plugs and rotor and cap adjusted the float in the carburetor set the timing help!! 

Very frustrating gotta go to a fundraiser tomorrow with this car 1930 model A ford coupe 40 hp stock engine 

Edited July 27, 2019 by Becky (see edit history)

[MCHinson]

The key to working on a car is to do one thing at a time. Where did you get the condensor? Just because a condensor is new does not mean it is good. There are certainly a lot of cheap foreign condensors being sold these days. Sometimes they are worthless. NAPA typically sells good ones. 

 

I would switch back to the original condensor and see if it then fires. If so, despite the fact that you may need a condensor to prevent backfiring, the new one you installed could be defective. 

 

Do you have a copy of the Model A Ford Mechanic's Handbook?  If so, review the trouble shooting chart on page 4-5. If not, I will let you know that the number one item listed for Backfiring is a Defective Condensor.

 

For not starting, the probable causes are 1. Battery Voltage Low; 2. Defective Coil; 3. Defective Condensor; 4. Open Ignition Switch or Cable. 

 

The preliminary test of the igniton system with the key off is:

 

1. Test for 6 volts at battery connection on starter.

2. Test for 6 volts at both terminal box wiring nuts.

3. Test for 6 volts at both coil terminals.

4. Place a piece of paper between the point contacts in teh distributor to keep points open. 

5. Turn ON ignition switch and test for 6 volts on the open point arm.

 

This test checks all wiring connections from the battery to the points. If any of the preliminary test failed, trace the fault to a disconnected or broken wire in the circuit. 

 

 

 

 

 

 

Edited July 27, 2019 by MCHinson (see edit history)

[TerryB]

No spark perhaps?  Check points to see if they are opening at the correct time.  

[DonMicheletti]

When did it start backfiring and did you do anything before it began?

[Becky]

5 hours ago, Becky said:

Very frustrating gotta go to a fundraiser tomorrow with this car 1930 model A ford coupe 40 hp stock engine 

 

3 hours ago, MCHinson said:

The key to working on a car is to do one thing at a time. Where did you get the condensor? Just because a condensor is new does not mean it is good. There are certainly a lot of cheap foreign condensors being sold these days. Sometimes they are worthless. NAPA typically sells good ones. 

 

I would switch back to the original condensor and see if it then fires. If so, despite the fact that you may need a condensor to prevent backfiring, the new one you installed could be defective. 

 

Do you have a copy of the Model A Ford Mechanic's Handbook?  If so, review the trouble shooting chart on page 4-5. If not, I will let you know that the number one item listed for Backfiring is a Defective Condensor.

 

For not starting, the probable causes are 1. Battery Voltage Low; 2. Defective Coil; 3. Defective Condensor; 4. Open Ignition Switch or Cable. 

 

The preliminary test of the igniton system with the key off is:

 

1. Test for 6 volts at battery connection on starter.

2. Test for 6 volts at both terminal box wiring nuts.

3. Test for 6 volts at both coil terminals.

4. Place a piece of paper between the point contacts in teh distributor to keep points open. 

5. Turn ON ignition switch and test for 6 volts on the open point arm.

 

This test checks all wiring connections from the battery to the points. If any of the preliminary test failed, trace the fault to a disconnected or broken wire in the circuit. 

 

Ok I will check all of the above thank you!! 

 

 

 

 

 

[Becky]

I will check all of the above thank you!! 

[Becky]

3 hours ago, TerryB said:

No spark perhaps?  Check points to see if they are opening at the correct time.  

I will check the points thank you!! 

[Becky]

I will check the points thank you!! 

[Becky]

3 hours ago, MCHinson said:

The key to working on a car is to do one thing at a time. Where did you get the condensor? Just because a condensor is new does not mean it is good. There are certainly a lot of cheap foreign condensors being sold these days. Sometimes they are worthless. NAPA typically sells good ones. 

 

I would switch back to the original condensor and see if it then fires. If so, despite the fact that you may need a condensor to prevent backfiring, the new one you installed could be defective. 

 

Do you have a copy of the Model A Ford Mechanic's Handbook?  If so, review the trouble shooting chart on page 4-5. If not, I will let you know that the number one item listed for Backfiring is a Defective Condensor.

 

For not starting, the probable causes are 1. Battery Voltage Low; 2. Defective Coil; 3. Defective Condensor; 4. Open Ignition Switch or Cable. 

 

The preliminary test of the igniton system with the key off is:

 

1. Test for 6 volts at battery connection on starter.

2. Test for 6 volts at both terminal box wiring nuts.

3. Test for 6 volts at both coil terminals.

4. Place a piece of paper between the point contacts in teh distributor to keep points open. 

5. Turn ON ignition switch and test for 6 volts on the open point arm.

 

This test checks all wiring connections from the battery to the points. If any of the preliminary test failed, trace the fault to a disconnected or broken wire in the circuit. 

 

 

 

 

 

 

 

[Becky]

Just now, Becky said:

 

I think the coil might be bad? 

[Becky]

1 hour ago, Becky said:

 

How do you test the coil to see if it is bad

[MCHinson]

Here is a previous discussion that should help you diagnose your problem.