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38 special backfire troubles


sloperlad
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I have a 38 special that is back firing through the  carter carburettor. Have taken the carby off , had it serviced by a specialist and refitted. Replaced the condenser, checked plugs and leads and points. Distributor was rebuilt when engine was done with  about 500 miles now done. Vehicle sat for about 3 weeks at a detailers and  moved periodically. The car starts and idles well it justs splutters and back fires when gas peddle is applied  trying to go for a drive. Detailer let battery run flat and jump started it. since then runs like a 3 legged dog. Any suggestions?

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Detailer likely jump-started it with a hot 12-Volt charger at 55 Amps or more.

Possible damage, depending on how long he had it hooked up and cranked it. Consider fried points, condenser, etc.

 

Over the three week period he may also have left the ignition on with the car not running, and may have damaged the points in that way. Try filing smooth and straight and re-gapping in case they are not right.

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Is the choke working? It should be trying to close or partially close with the engine off and the throttle a little bit open if the engine is cold. If that is OK,

 

1) With engine shut off, but having just been run so you know the carb is full of gas,

 

2) Choke open, or propped open if engine is cold, so that you can see down there, and the throttle is closed against the stop screw for slow hot idle, engine is still off,

 

3) Look down carb throat and open throttle gradually. you should not be able to move the throttle in the open direction at all without seeing gas squirt out the accelerator pump nozzle.

 

If it passes that test, move on to the distributor.

 

1) Recheck the points gap, set if necessary. Make sure the return spring is present on the points.

 

2) Check the vacuum advance. It probably has a metal line you would need to disconnect, most conveniently at the carburetor. Either suck on it with a mityvac, or just suck on it. It can't leak any air, at all. Leave the distributor cap off so you can see it move, and see it move back when you let the vacuum off.

 

3) There are 2 little wires in the distributor, one grounds the breaker plate to the case, the other connects the points to the binding post. They are special wire made to take about a zillion bendings without breaking, because they are moving while you drive. Make sure they are both there, and are not broken. The one for the points is insulated, make sure any bare spots are not touching as the breaker plate moves with vacuum. The wire can be broken inside the insulation. Hint: it should not stretch. The ground wire is bare, and easier to see if broken. The popping would probably be in the exhaust rather than the carb, but you may as well inspect them while you are in there. It is common to find those wires screwed up.

 

4) Recheck ignition timing.

 

5) Recheck firing order. See which way the rotor turns, find number 1, and just follow it around the cap in the firing order, same direction as the rotor turns, checking as you go.

 

Still backfiring? You might have a bent pushrod or stuck valve or both. Is the valvetrain unexpectedly noisy? If the trouble isn't caused one by one of the things outlined above, and it isn't fuel delivery as Daves1940Buick56S suggested, you have a choice. You could go straight to basics and do a compression test and a leakdown test. OR you could pull the valve cover if it's easier, take the spark plugs out, and have a friend crank the starter while you watch the valve stems. They should all move about the same, and if one is different it should stick out like a sore thumb after you watch it come around a few times.

 

Edited by Bloo (see edit history)
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10 hours ago, Bloo said:

2) Check the vacuum advance. It probably has a metal line you would need to disconnect, most conveniently at the carburetor. Either suck on it with a mityvac, or just suck on it. It can't leak any air, at all. Leave the distributor cap off so you can see it move, and see it move back when you let the vacuum off.

 

3) There are 2 little wires in the distributor, one grounds the breaker plate to the case, the other connects the points to the binding post. They are special wire made to take about a zillion bendings without breaking, because they are moving while you drive. Make sure they are both there, and are not broken. The one for the points is insulated, make sure any bare spots are not touching as the breaker plate moves with vacuum. The wire can be broken inside the insulation. Hint: it should not stretch. The ground wire is bare, and easier to see if broken. The popping would probably be in the exhaust rather than the carb, but you may as well inspect them while you are in there. It is common to find those wires screwed up.

 

I've been hunting an intermittent exhaust backfire with my '38 Century.  The only time it seems to occur is after a period of constant 40 ~ 50 mph operation followed by letting-off the gas, as with coasting to an intersection to stop.  Was happening 50% of the time and I did find a terrible looking primary wire inside the distributor which I replaced with one I fabricated and swapped the condenser.  No change, so still thinking ignition, I ran the car on three separate occasions with the coil 'hot-wired' to the battery to eliminate the wiring harness, ignition switch and associated connections.  I couldn't make it backfire on the first two trips, so I was beginning to think I was on to something, but it did finally pop out the exhaust on the third run.  So, yesterday I pulled the distributor, removed the breaker plate and inspected, cleaned and lubricated the centrifugal advance mechanism.  It was dirty and a little sticky, but I think it was functioning.  I then used my hand vacuum pump to check the vacuum advance and found that it will not hold vacuum.  The arm moves if I keep pumping, but as soon as I stop pumping the spring returns it to the retarded position.  I'm hoping this is the culprit.  I ordered a new vacuum advance, new primary and ground wires and cap spring clips (mine are awfully weak) from Bob's.  This is probably a good week for the car to be down, as it looks cold and rainy until Friday this coming week...

Edited by EmTee
typo (see edit history)
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11 hours ago, Bloo said:

Either suck on it with a mityvac, or just suck on it. It can't leak any air,

 

Do the same thing with the fuel line going back to the tank. Cap off the tank filler and vent. It should be air tight. If not the fuel pump could be supply a mixture of air and fuel that will cause a lean condition.

 

Try to do your diagnostics in a methodical sequence. Addressing recent events can lead you to miss an underlying problem that has been there all the time. As in, step one is a compression test.

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3 hours ago, Ben Bruce aka First Born said:

Likely a rich condition?

 

Yes, or anything that causes a misfire, even lean. The gas that doesn't burn winds up in the exhaust pipe.

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For what it's worth, some afterfiring in the exhaust of a carbureted car is normal. When coming downhill on compression, the intake manifold vacuum is at its absolute highest. The throttle is closed. The idle jet is still under the throttle plate. Quite a bit of fuel/air mixture gets sucked through the idle jet, with almost no air to mix with. There is a difference though between normal popping and big explosions that threaten to blow the muffler apart. I know you guys know that, but I am throwing it out there anyway because It just occurred to me that we have had fuel injection as default (in the US) for about 30-35 years now. We have had catalytic converters quieting things down even longer. There might be people reading this thread who have never heard the "normal" noise.

 

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$10 says it's ignition related. Someone up above mentioned that if they had to jump-start the car, the might have fried the points and condenser. I'll bet that's the issue, but most likely the coil got fried. Or perhaps they left the ignition on and cooked it. But the stuttering and backfiring initially may seem like a fuel issue, it's almost always ignition. Replace the points, coil, and condenser and see what you get.

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Hi All checked. Had auto electrician out , all electrics re serviced found a loose wire in the distributor not earthing properly and also installed another new ignition coil. Low and be hold started and ran fine. I should have called in the experts at the start. An expensive way to learn . 

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