Backfire: Carb? Timing? Coil?

Hi all. I'm new here and would like some advice. In my young and foolish days (I'm still foolish, just not so young anymore), I wouldn't hesitate to tear into my '48 CJ2A and love every minute of doing so. Original engine, 12-volt, oil bath removed and replaced with a Kaier unit. But now... Here's what the patient is presenting: Start-up has always been immediate, even on cold days (I live in Utah) and when not started up for weeks or months. A few weks ago, I had to pump the pedal with full choke and let the starter cycle for 10-15 seconds before she would fire up. Lately, after starting and slowly driving, she has been coughing. Note that this was before Covid-19! The choke doesn't seem to be operating as before (length of time choke is kept on before closing butterfly). She runs really well once the engine warms up (after two or three miles) but the last time she was backfiring violently until I reached the 2 -3 miles. I drove her home, parked and that is where she sits right now. Waddya think? Carb kit? New carb? In my foggy way of thinking if the timing is off, she would put up a fuss regardless of engine temp. Coil? Thanks in advance for any advice from the Forum! VR - mj

I am thinking to start with, a good old fashioned tune up. When the fuel air mix is cold and the choke is on, it's hard to light the fire. A strong ignition is a big help. Points, plugs ,rotor, condenser, wires and a cap.

Thanks for the input, bmorgil. I repaced all you mentioned last Summer and have put only about 500 miles on since then but I will check points gap, condenser, and make sure all wires are in place and not broken, etc. I'll also check the carb butterfly and fuel pump and line before I do a carb kit (or maybe buy a new carb). Thanks again - I value your input! mj

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Originally Posted by Bohonkie

Thanks for the input, bmorgil. I repaced all you mentioned last Summer and have put only about 500 miles on since then but I will check points gap, condenser, and make sure all wires are in place and not broken, etc. I'll also check the carb butterfly and fuel pump and line before I do a carb kit (or maybe buy a new carb). Thanks again - I value your input! mj

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I'd put carb issues way down on the list of things - I'd lean heavily on electrical. Wires and the cap are suspect, but I'd not discount a condenser and burned points. All condensers are bad, it's just a matter of them deciding to show how bad they are... .

The really old guy we went to for help told us to change the condenser. "But, it's a new distributor." He says; "Do you want it to run or do you want to argue?"

So, when we decided to change the condenser, look at what we found....

Let us know what you find.

The fact "she runs really well when she warms up" is the big thing that makes me think electrical. Weak spark makes it difficult to ignite the rich fuel air mixture of a cold choked engine. This is usually where weak ignition shows up. I did not know you tuned it up recently. Let me ask this, is there a resistor in the 12 volt conversion between the coil positive and the ignition switch? If not is it a 12 volt non ballasted coil? If the coil runs on 12 volts all the time, it will burn the points. This is just something to be sure of. The vehicle usually starts on 12 volts and runs on 9 volts via ballasting.

If may be time to adjust the valves as well, just to cover all bases. I'm on board with the electric side rather then being fuel.

Haha, yes gm you are correct! We should increase the branches on the Fault Tree. As the motor warms and the block expands valve clearance increases. As the engine wears valve clearance decreases. Therefore as the engine warms, do to a tight valve lash from a normally wearing valve train, the lash will improve. Tight lash causes compression loss and produces the symptoms you mention. Also a restricted carb main circuit would act similarly. It would however show signs of lean under load.

I would start off by reading the spark plugs to get a general health of the engine. The plugs would give you a sense if it is running lean, too fat, or fouling. If the plugs look like the cylinder is cold/wet then look at your ignition system. If the plugs look OK, then look to the valve train to see if any intake valves are too tight. I would adjust both intake and exhaust for good measure since you are already into things that deep.

https://ngksparkplugs.com/en/resources/read-spark-plug

Thanks, all. I adjusted the valves about 600 miles ago and she doesn't act like she was before I did that (I've done it a feew times over the years when she starts losing power and acting like she's running on only 2 cylinders). I'll see if I have a set of points and condensor hanging around and check all the electrics. There is no resistor between the coil and ignition. Thanks again... please stand by... mj

MJ, it is possible the coil does not need a ballast resistor. If so it will say so on the coil. If it needs one and you don't have one, it could burn the points in a shorter period of time.