# popping noise from engine



## Tom738 (Jun 1, 2010)

shirbon said:


> i have a 71 Nova with small block 400, when sitting at idle there is a pop, pop-pop coming out the exhaust tail pipe, more so at a faster idle than slow idle though it still makes the sound. wondering what is the most likely cause, missfire - back fire etc etc ?, and what would be the cause of these or whatever the problem may be ? Thanks.


I would check engine timing, distributor, spark plugs, etc...

But I have near zero experience, so take that with a grain of sand.


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## Windows on Wash (Aug 30, 2011)

What kind of cam is in it as well?

Tuning is usually the issue. Unburned fuel in the exhaust stream creating a little backfire in it.


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## Brainbucket (Mar 30, 2015)

Bad cam maybe. Small blocks back then were bad about that. I use to build the crap out of 400sb. Do you have a exhaust leak? Air entering hot exhaust can do that but bad cam, lobe rounded off, came to mind as soon as I read that. But now as I think about it, crossed ignition wires? It just happened or after repairs were made... More info needed.


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## 47_47 (Sep 11, 2007)

Brainbucket said:


> But now as I think about it, crossed ignition wires? It just happened or after repairs were made... More info needed.


Firing order should be 1,8,4,3,6,5,7,2. If wires were just changed, I think 5 & 7 wires may be crossed.


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## Rough Rooster (Feb 7, 2015)

Two suspects have been mentioned.
Here is another;
Check the underside of distributor cap and look for carbon tracks between terminals next to coil screws. Screws too long or screwed down too tight and the ignition pulse will arc from terminal to the coil screw leaving a carbon track you can see. If this is showing, then replace cap and coil being careful not to tighten screws too tight.
Another possibility is ignition burn-through the rotor. It will be visible, especially on the white plastic rotors.
Happy hunting!

RR


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## shirbon (Oct 2, 2014)

its a new cap and rotor and the wires are in correct order. After I get my brakes bleed I will take for a ride and see if it pops while driving


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## Bondo (Dec 8, 2007)

Ayuh,.... A compression test might well give ya some hints on what's up with it,....

Many of the things mentioned above can be diagnosed with a compression test, 'n leak-down test,....


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## Brainbucket (Mar 30, 2015)

Compression test some times won't revile a bad cam if there is any lobe left. Engine isn't spinning fast enough to see bad cam unless the valve doesn't open at all. Leak down test won't revile a bad lobe. It will revile a burnt, bent valve, bad rings, or head gasket. Only removing the valve cover and watching the rockers move is the best way. What kind of exhaust is on the car? Stock or performance? What kind of ignition is on this car. Stock with points or has it been upgraded to HEI or performance ignititon? If it is poping, is it missing at the same time? More info....:yes:


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## 47_47 (Sep 11, 2007)

Agree, more info needed. What else did you do? Did you change the plugs and possibly bend one (close) of the ground electrodes? 
I would start with a cylinder balance test to try and pinpoint what cylinder(s) are effected. Hook up your tach and with a pair of insulated spark plug wire pullers, pull one wire at a time and note the rpm drop. 1,4,6 and 7 should be about the same and 2,3,5 and 8 about the same. You will may notice these drops different if your carb isn't balanced.


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## Rough Rooster (Feb 7, 2015)

Reread original post and saw it is a '71.

A very common problem on this year model is the throttle shaft having worn bushings and leaking air around it causing a popping putt sound in exhaust. Check the throttle shaft for tightness and for a simple test coat the outside area around shaft with grease and check to see if problem is helped.

RR


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## 47_47 (Sep 11, 2007)

Many good posts about common problems from a '71 car. Just wonder how many parts are still OE on a 44 year old car? This thread did take me back though and it took me a while to draw a good secondary parade.


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## Brainbucket (Mar 30, 2015)

Rough Rooster said:


> Reread original post and saw it is a '71.
> 
> A very common problem on this year model is the throttle shaft having worn bushings and leaking air around it causing a popping putt sound in exhaust. Check the throttle shaft for tightness and for a simple test coat the outside area around shaft with grease and check to see if problem is helped.
> 
> RR


Yes. Very common. That would cause a vacuum leak if the throttle shaft bushing was worn. Just grab it and see if it moves up and down and side to side. It shouldn't. I like the grease check. Not as flamable as carb cleaner. :thumbup:


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