'51 Plymouth deceleration grumble? VIDEO

[hep2jive]

Hello ev'ryone!

 

I have a 1951 Plymouth and when I decelerate it makes a weird grumble pop from the tailpipe. I have had the valves adjusted a few hundred miles ago. I think there must be an air leak someplace? the exhaust was pretty gumbly but I fixed that with opening the points up a little bit- they were a little close. but there is this weird sound after I rev the motor. I don't think it does it while I'm driving but I might be able to hear it... I have uploaded a video of the noise. Thanks for reading. The car runs fine and seems alright.

 

Here is the link to hear the noise:

 

 

 

Edited September 4 by hep2jive

[Plymouthy Adams]

is the end of your pipe a total square cut.....you may wish a bit of a bologna cut here...especially with the deflector in place....

[Ivan_B]

Isn't that just the regular deceleration misfire noise? If I recall correctly, it might be running too rich, and the un-burnt fuel is popping in the exhaust. Is your carb well adjusted, including the accelerator pump?

[greg g]

Is your choke opening fully? Is the throttle butterfly closing  completly?  What muffler is it running and where is the muffler located? What is timing at idle.

[hep2jive]

11 hours ago, Ivan_B said:

Isn't that just the regular deceleration misfire noise? If I recall correctly, it might be running too rich, and the un-burnt fuel is popping in the exhaust. Is your carb well adjusted, including the accelerator pump?

I'll lean out the carb and see if that works. I've never adjusted the accelerator pump. 

[hep2jive]

11 hours ago, greg g said:

Is your choke opening fully? Is the throttle butterfly closing  completly?  What muffler is it running and where is the muffler located? What is timing at idle.

The butterfly was removed. The muffler is a new muffler ( replaced old muffler with the same thing. The old muffler did the same thing) car runs idles smoothly just seems to have that popping when in neutral and I rev. I'll check timing but I doubt it's moved.

[greg g]

You said you would lean out the carb.  Turning the idle mixture screw only effects the air going through the idle circuit.  Does nothing when the throttle opens.  What carb do you have?  If it's a stock carter I would check the step up jet to assure it is returning to its closed position properly.  You might also consider using a vacuum gauge to see what it may tell you.  There is a good article about how to use and interpret vacuum gauge readings on the 2nd chance garage web site.

[lostviking]

This was said already, but that is very typical of either a exhaust leak near the manifold, or somewhere before the gases cool much, or too rich...or both. Check the plugs and see if it's running rich. Check the timing and make sure it's not retarded, and doesn't jump around.

 

What kind of muffler? Low restriction? Every time I've put a low restriction exhaust on anything, it would do that until I got the fuel adjusted to compensate for the lean condition I created.

[hep2jive]

12 hours ago, lostviking said:

This was said already, but that is very typical of either a exhaust leak near the manifold, or somewhere before the gases cool much, or too rich...or both. Check the plugs and see if it's running rich. Check the timing and make sure it's not retarded, and doesn't jump around.

 

What kind of muffler? Low restriction? Every time I've put a low restriction exhaust on anything, it would do that until I got the fuel adjusted to compensate for the lean condition I created.

It was a rather inexpensive muffler. Not sure the brand. Does it create a lean condition? What do you mean by, “it would do that until I got the fuel adjusted to compensate for the lean condition I created."

[Ivan_B]

If I am not mistaken, when you install a more free-flowing (and noisy) muffler, it leans out the engine due to increased exhaust flow, so you need to adjust the carb accordingly  

Also, make sure that your choke is fully open.

 

13 hours ago, greg g said:

Turning the idle mixture screw only effects the air going through the idle circuit.  Does nothing when the throttle opens. 

I was under impression that the idle circuit is always participating in the mixture flow (to a greater extent with the closed throttle, true).

Edited September 5 by Ivan_B

[hep2jive]

20 minutes ago, Ivan_B said:

If I am not mistaken, when you install a more free-flowing (and noisy) muffler, it leans out the engine due to increased exhaust flow, so you need to adjust the carb accordingly  

Also, make sure that your choke is fully open.

 

I was under impression that the idle circuit is always participating in the mixture flow (to a greater extent with the closed throttle, true).

So maybe it's a too lean? I'll play with the carb and see what she does

[Ivan_B]

Maybe. Did you look at the plugs, already? I just did a quick google search for "deceleration popping" to refresh my knowledge, and there appears to be a couple of potential causes for this.

[hep2jive]

Plugs were black lol. Leaning out carb. That might help. Thanks everyone. 

 

So when I get a cheap muffler it runs lean? 

[Ivan_B]

Not necessarily. When your new muffler offers less flow restriction, the engine might run somewhat leaner. An expensive straight pipe will do the same 😉

Edited September 6 by Ivan_B

[hep2jive]

When I lean the carb out the car shakes and wants more gas. Should I then turn up the idle screw? 🪛 

[Sam Buchanan]

13 minutes ago, hep2jive said:

When I lean the carb out the car shakes and wants more gas. Should I then turn up the idle screw? 🪛 

 

The quick and dirty way to set idle mixture is to first set idle speed for a slow, reliable idle. Then VERY slowly turn the mixture screw clockwise which makes the mixture more lean. When the idle speed begins to slow down (this can be a very subtle difference) make note of the position of the slot in the screw. Now slowly turn the screw counterclockwise to richen the mixture, once again listening for idle speed to come back up and them begin decaying. Note the position of the screw. Now turn the screw back clockwise until you achieve the smoothest idle, will probably be about midpoint between the two positions you noted. This adjustment needs to be done slowly while listening to very subtle changes in idle speed. Repeat the process until you are sure you have the mixture screw set for smoothest idle.

 

Using a vacuum gauge can be more accurate but a sensitive ear can often get mixture set close to where max vacuum occurs, at least close enough for satisfactory operation. You may need to reset idle speed after getting a good mixture setting. I find mixture needs to be adjusted in spring and fall when ambient temps change from summer/winter. Mixture is dependent on the density of air which changes with temperature. Your modern car does this automatically, we get to pamper our old cars with periodic attention to detail to keep them happy. 

Edited September 6 by Sam Buchanan

[hep2jive]

Thank you everyone. Somehow I got it right. Runs very quietly and super smooth. Just rolling along! Thank you! 

[Ivan_B]

Sam is correct, do you have a vacuum gauge? Adjusting the carb should be done together with the ignition and idle speed. Vacuum gauge and dwell/RPM meter combo are very useful to have. Get these off eBay, they are cheap. Not knowing what you are doing is not a very effective and reliable way to get things right 😅

[Sam Buchanan]

No telling how many motorcycles and VWs I tuned up without a vacuum gauge or dwell meter, probably helped me educate my ears. A few days ago I got out the vacuum gauge for the P15 after the recent discussion, turns out my "earcrometer" tuning was in agreement with the gauge. 

[hep2jive]

I'll did it by ear and feel. Took the car for a test drive felt great. I drive about 25k a year and drive all year here in the mid west. I think she'll be fine for now!

[greg g]

What gas mileage are you getting? Sooty plugs and poor mpg, point toward a possibly faulty step up valve.  

[hep2jive]

On 9/7/2024 at 10:58 AM, greg g said:

What gas mileage are you getting? Sooty plugs and poor mpg, point toward a possibly faulty step up valve.  

I get pretty good milage as of now. Running smoothly quietly and efficiently! Thanks the help everyone