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29 Model A keeps backfiring thru carb.


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Need help please !   Hello everybody. Hope everyone is safe from all that is going on in the world. I need some help. I finally got my 29 sorted out (or so I thought).  I've got a newly rebuilt Zenith 1 carb on my car and every so often at 1/2 throttle or so, it backfires thru the carb. Not always but enough for concern. I feel like I've got the timing  dead on and I've followed the carb tuning that I found on another site. No change however. Any ideas or suggestions would be appreciated.

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I would add that the Nu-Rex timing light kit makes it to where you don't have to "feel like" you have the timing right because you will be able to see when it is actually firing.  I know a lot of people turn their noses up at it, but the simple truth is the amount of backlash is different in every Model A and you can't actually know what is going on with any amount of consistency without seeing it on a light:

 

https://www.brattons.com/ignition-timing-tools.html

 

That also makes it easy to use the light to check for things like the pulley mark dancing all over the scale due to point float or what not.

 

 

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Generally speaking a backfire (which technically is back through the carb) is caused by a lean mixture.  An afterfire (which is technically gas firing in the muffler or exhaust pipe after the engine) is caused by a too rich mixture.

 

Common usage has people WRONGLY calling any combustion outside of the cylinder a backfire.

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Lets hope not a nexhaust valve is on it's way out.

 

Hard to believe the Model A would run fairly well other then this pop and it be caused by the timing being off so much..

A scant to retarded overall (and not noticable to a novice as car appears to move along )it gets hot to overheat after some driving of just several hard miles.

When To advance timing overall  the starter begins kick back a bit(you notice that) when starting at full retard but car starts and when timing is advanced the worst effect is a rolling idle.

 

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Some years ago I drove a borrowed Model-A on tour. It had a backfire and the issue turned out to be that the plate in the distributor was "dished", resulting in marginal contact.

A bad condenser will also cause the issue you describe 

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6 hours ago, 46 woodie said:

The Model A Ford does not use plug wires, they use a brass connector, from the distributor to the plug, it's impossible to screw up.


 

You have obviously never experienced some of the repair shops I have seen. 

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