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Popping from exhaust when starting warm


Noah H

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Just recently I’ve been getting a loud pop when I start my engine when it’s warm, it sounds like it is coming from exhaust. Could this be indication of a burnt valve or just in need of a valve adjustment? Ive adjusted my valves about 6,000 miles ago and haven’t had this problem since. Sometimes it happens when I crank the engine, other times it happens shortly after the engine is started, it also does sound like there is one or two valves that have a distinct tick which is quite audible but only is heard at cruising speed. I adjusted valves when running per spec. Motor runs well otherwise. 
 

thanks for any input. 
 

nh 

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9 hours ago, Noah H said:

Just recently I’ve been getting a loud pop when I start my engine when it’s warm, it sounds like it is coming from exhaust. Could this be indication of a burnt valve or just in need of a valve adjustment? Ive adjusted my valves about 6,000 miles ago and haven’t had this problem since. Sometimes it happens when I crank the engine, other times it happens shortly after the engine is started, it also does sound like there is one or two valves that have a distinct tick which is quite audible but only is heard at cruising speed. I adjusted valves when running per spec. Motor runs well otherwise. 
 

thanks for any input. 
 

 

Check your timing and look for a crack in your exhaust manifold. Remember any loud backfires?

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Popping in the exhaust usually means it didn't burn in the combustion chamber.  Had that happen to me years ago, one of my plug wires popped off the plug and that fuel air charge was getting ignited in the exhaust system.  Inside the car, 64 300, it sounded like putt putt, outside it'd shoot a tongue of flame and sounded like a gun shot.  My muffler on that side looked like a balloon someone seriously over inflated. 

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30 minutes ago, Sniper said:

Popping in the exhaust usually means it didn't burn in the combustion chamber.  Had that happen to me years ago, one of my plug wires popped off the plug and that fuel air charge was getting ignited in the exhaust system.  Inside the car, 64 300, it sounded like putt putt, outside it'd shoot a tongue of flame and sounded like a gun shot.  My muffler on that side looked like a balloon someone seriously over inflated. 


thanks sniper I’ll check the exhaust system. But the clacking sound when running I’m going to adjust the valves. I’ve ready so many hot clearances. 
 

.010 and .010 

.011 and .009

.010 and .013 

.08 and .011 

 

Read from the archives it states .010 and .010 for both valves. Should I be particular in choosing? It’s a 230 CU IN. Everyone’s different specs is kinda throwing a loop at me. 
 

thanks 

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Thanks, I’ve adjusted specs on valves and my engine does not clack anymore. More of a soft ticking after I set my valves running. I also connected a timing light to check my timing and it is correct too. Dwell angle/points as well. I did check my exhaust sniper and there is no bulging of any kind and does not sound like there is a definite exhaust leak. 
 

Still a popping sound. Sometimes when cranking other times you wait for a few seconds and it pops once started. 

 

Just did a 40 mile trip with no trouble, I’ve tried the dollar bill to the exhaust pipe and it doesn’t suck it in. Just would like to pin point the popping/backfire noise. Weird it’s only on startup. 

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8 hours ago, Sniper said:

 

Wiggle the plug wire around while you are checking it as that will help find an intermittent issue. 

Plugs changed and wires tested even across the board. Still backfires. This time after it’s started and ran for a few seconds. Could it be a float level that’s adjusted too high?

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34 minutes ago, Sniper said:

Depends, what did the old plugs look like?  Were they black and sooty?  Can you post a pick of them?  Did you inspect the cap and rotor?

Plugs had a slight Grey-ish brown. One (5) had a little more white than grey-brown. 

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OK, this maybe a long shot.  Back in the late 60s I had a 1950 Plymouth 4 door.  One day I started the car and and heard a "pop' form under the hood.  i didn't seem to hurt the running of it so I went about my normal day.  A day or two later I was working under the hood doing something and a young neighborhood kid climbed up on the bumper and was asking me what the different things on the engine were.  "What's that?' I looked at what he was pointing at and said, "That's the air cleaner." Then it was something else, then something else, and then something else.  Again he asked, "What's that?" I looked and he was pointing to the distributor.  I told him it was the distributor.  As I looked though I noticed the distributor cap was not fastened  to the the distributor but was dancing around on top of the distributor. Neither of the bale clamps was holding the distributor cap to the distributor.  I was amazed the car would run in this condition but it did. I shut the engine down, locked the distributor back on and went on my merry way.  A few days later, again I heard the "pop" from under the hood when I started the car.  Got out and checked the distributor and sure enough the two bale clamps had released the distributor cap.  I examined the distributor cap and noticed the recesses where the bale clamps clipped had some of the bakelite chipped away.  Went to NAPA to get a new distributor cap and the guy behind the counter told me the new cap was a new design that had a hole through the side of the cap to relieve the pressure from crankcase gasses collecting under the cap and "popping off" when the points arc.  As I said, a long shot but when you mentioned the popping was coming from the drivers side rather than the exhaust side, thought I would add my 2 cents 

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Did my last post kill this discussion?  I hope not.  Although I had this condition in the late 60s, I think we now have an opportunity to run this to ground and find a solution for it.  The remedy may be as simple as having a distributor cap with a "bleed hole" in the side of it.  That's easy, and I think that was the solution for me in the 60s.  On the other hand we may want to examine what is causing the fumes to gather in the distributor in the first place.  Is there an oil seal/gasket in the distributor that is worn/missing  and needs to be replaced?  Is it something with the oil in the crankcase?  Is it  contaminated with gasoline making the fumes more volatile  and able to "pop off" from a spark .  Point being, although the hole in the distributor may prevent the "popping" it is not solving the problem of what is causing the problem in the first place.   

Noah H:  have you pursued this problem any farther?  Did you replace your distributor cap with one that has a hole in it?  Did your old distributor cap have a hole in it?  Have you changed the crankcase oil lately?   Can you still get the "popping" to happen with a warm start?  Best regards to all.

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2 hours ago, squirebill said:

Did my last post kill this discussion?  

I suspect the discussion died down because everyone interested made their suggestions and have no more to say about it.

 

Sometimes threads just die natural deaths.

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I was taught years ago that you can test valves by holding a dollar bill over the end of your tail pipe.

A engine is a air pump, it should blow the dollar bill straight out in the air.

Now if the bill flutters back and forth and gets sucked back to the tail pipe, you have something going on with the valves. Could be burnt valves, could be timing etc...

Just a quick simple test you can try to get a clue where to look next.  Really handy if you are out inspecting a used car to buy & condition of engine.

 

Another possibility to go along with @squirebill Years ago when I was 22 years old and had a 1961 Dodge truck that we generally just used to haul trash to the dump once a month, no local garbage service.

Wife was driving the /six 4spd down the freeway and it had a huge backfire. She really thought blew a tire and pulled over. All the tires were good and was running fine, she just drove off again.

So the next Saturday I went out and looked at it. There was a explosion in the crank case and the dipstick was 1" shorter and curled up. I had to stand on fender to pull it out.

 

That truck had a leaky carb, the oil had gas in it, the exhaust manifold leaked. We still drove it for two years after that, then went to little brother & he sold it still running a year after that. Just a lot of maybe here on condition of the motor.

 

What does the dollar bill test show ... run a compression test. how is the carb, you have any exhaust leaks at the manifold where a bad valve can suck in cold air?

Does the vacuum advance work properly. Lots of variables here to go wrong. Will need some serious testing & back & forth banter to fix it over the internet.

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I can add my .02 its probably pretty long and too wordy.
 

In summary I’d:

 

-compression test first and foremost. Wet and dry test

-clean & gap plugs

-clean cap & rotor

-hook up dwell meter & set points 

-how old are points & condenser? Plug wires? Consider replacement 

-inspect all internal distributor wires and fasteners. 

- hook up vacuum gauge at intake. Note needle movement. Set A/F mixture screw and ignition timing by vacuum. Confirm timing with timing light at front pulley. Monitor spark advance with timing light while revving engine. 
-Check tappet/valve lash

-Set idle speed

- check fuel pump pressure

-set float


If these steps won’t find your problem consider trading it in for a 1 year newer model. Lol. ?

 

Edited by keithb7
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On 9/14/2021 at 4:23 PM, keithb7 said:

I can add my .02 its probably pretty long and too wordy.
 

In summary I’d:

 

-compression test first and foremost. Wet and dry test

-clean & gap plugs

-clean cap & rotor

-hook up dwell meter & set points 

-how old are points & condenser? Plug wires? Consider replacement 

-inspect all internal distributor wires and fasteners. 

- hook up vacuum gauge at intake. Note needle movement. Set A/F mixture screw and ignition timing by vacuum. Confirm timing with timing light at front pulley. Monitor spark advance with timing light while revving engine. 
-Check tappet/valve lash

-Set idle speed

- check fuel pump pressure

-set float


If these steps won’t find your problem consider trading it in for a 1 year newer model. Lol. ?

 

 

Dwell is set at 38 degrees, I have not tried a compression tester, but motor was rebuilt about 7000 miles ago, but that might not prove anything. Inside the distributor looks brand new. I have confirmed that the advance is working via the timing light. Just pop! And away she goes. Vacuum is reading about 17 1/2. Idle for my car is about 500 with the fluid drive. The points/condenser are about 3 years old, but I have been daily driving the car. I have set my valves hot and running 8 intake and 10 exhaust. 

 

I will consider that :) thanks 

 

 

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On 9/14/2021 at 4:05 PM, Los_Control said:

I was taught years ago that you can test valves by holding a dollar bill over the end of your tail pipe.

A engine is a air pump, it should blow the dollar bill straight out in the air.

Now if the bill flutters back and forth and gets sucked back to the tail pipe, you have something going on with the valves. Could be burnt valves, could be timing etc...

Just a quick simple test you can try to get a clue where to look next.  Really handy if you are out inspecting a used car to buy & condition of engine.

 

Another possibility to go along with @squirebill Years ago when I was 22 years old and had a 1961 Dodge truck that we generally just used to haul trash to the dump once a month, no local garbage service.

Wife was driving the /six 4spd down the freeway and it had a huge backfire. She really thought blew a tire and pulled over. All the tires were good and was running fine, she just drove off again.

So the next Saturday I went out and looked at it. There was a explosion in the crank case and the dipstick was 1" shorter and curled up. I had to stand on fender to pull it out.

 

That truck had a leaky carb, the oil had gas in it, the exhaust manifold leaked. We still drove it for two years after that, then went to little brother & he sold it still running a year after that. Just a lot of maybe here on condition of the motor.

 

What does the dollar bill test show ... run a compression test. how is the carb, you have any exhaust leaks at the manifold where a bad valve can suck in cold air?

Does the vacuum advance work properly. Lots of variables here to go wrong. Will need some serious testing & back & forth banter to fix it over the internet.

I have tried the dollar bill test and it has never sucked it in. It pushes it straight out. But does wave around.  The exhaust Is about a year old, but the gasket between the manifold and the pipe looks pretty chewed up, I will try tightening it up more. 

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On 9/14/2021 at 9:49 AM, squirebill said:

Did my last post kill this discussion?  I hope not.  Although I had this condition in the late 60s, I think we now have an opportunity to run this to ground and find a solution for it.  The remedy may be as simple as having a distributor cap with a "bleed hole" in the side of it.  That's easy, and I think that was the solution for me in the 60s.  On the other hand we may want to examine what is causing the fumes to gather in the distributor in the first place.  Is there an oil seal/gasket in the distributor that is worn/missing  and needs to be replaced?  Is it something with the oil in the crankcase?  Is it  contaminated with gasoline making the fumes more volatile  and able to "pop off" from a spark .  Point being, although the hole in the distributor may prevent the "popping" it is not solving the problem of what is causing the problem in the first place.   

Noah H:  have you pursued this problem any farther?  Did you replace your distributor cap with one that has a hole in it?  Did your old distributor cap have a hole in it?  Have you changed the crankcase oil lately?   Can you still get the "popping" to happen with a warm start?  Best regards to all.

Thank you for your reply, I have not checked my distributor for a hole. Oil does leak from the block between the distributor mount. I have just changed the oil with vr-1. My dispstick does not smell like gas and is not milky. But it still pops after a warm start. 

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