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jalexquijano

Deceleration popping at exhaust with new MSA Premium exhaust

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Just bought a new MSA Premium exhaust for my 1972 240z. Took it to a muffler shop to remove the former stock system and had it installed including the manifold to downpipe gasket and the 2 pipe to pipe gaskets. The kit brings a turbo muffler. The problem is that now whenever i speed up and reléase the car pedal (decelerate) the car pops at the tailpipe. It did not had this deceleration popping in the past with the former system. I was wondering if the gaskets provided by MSA are not of good quality or else?

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Nothing changed, the old system was just quieter and you couldn't hear it.

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Generally, this is cause by an exhaust leak further up stream. Is your emissions junk intact, removed, or correct blocked off?

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Either you have an exhaust leak, or the system is more restrictive and now your mixture is too rich.  

To find an exhaust leak, have a friend hold a bath towel over the tailpipe.  That will amplify the leak - and you'll hear it far more easily.  You'll be able to find it while your friend holds the towel, attempting to plug the pipe.  

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Thanks for your great recommendations. I did tried to lean the mixture and the popping at the tailpipe was still there when decelerating and more pronounced when going down hill. I was thinking that the 2.5" 3 bolt gaskets supplied by MSA were not of good quality and maybe i should purchase another set manufactured by Vibrant and sold by Amazon.

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I don't think its an exhaust leak. I've had cars with exhaust leak before and did not pop as u mentioned. Have u done a tune up?

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Maybe a vacuum leak.

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I'm with Mike on this one. His car doesn't have to have an exhaust leak to pop on decel. His new exhaust is less restrictive than whatever he had before. It's a sports car! Enjoy it!

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Racerx, exhaust leak will most definitely cause popping on loaded decel.

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Keep in mind it doesn't need to literally be from exhaust pipe (the leak). When I removed the air pump from my A14 B210, there is an exhaust port in the head, found near the tstat housing. I left this unplugged accidently. I had no ticking or audible exg leak. However it cause a wicked popping from tail pipe under heavy vacuum conditions.

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Exhaust will normally pop like this, you just can't hear it due to the muffler. Mike is probably correct.

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Just making some popcorn with the more free flowing exhaust I'd bet. 

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My car has no anti backfire valve, i had it removed when purchased. I have plugged all the balance tube and also removed all the emission crap! I even lowered the both gas mixture knobs  to 1.5 turns anticlockwise (leaned the mixture) to determine if the car was rich and the popping was still present on deceleration. Replacing all the spark plugs with Brand new NGK BP6ES plugs did not help. Deceleration popping still there.

 

Bottomline, the only way to muffle this popping is to go back to the original exhaust? This MSA PREMIUM EXHAUST SYSTEM comes with a Turbo Muffler while the other one was a simple Dynamax muffler.

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Backfires in Exhaust 
    
Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:
 

1)  When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system. 2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat. 3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there. 4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires. 5) Completely stock does not do this. The exhaust must be both free-flowing and have an open exit for the popping to occur.

Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust backfiring. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system backfiring.

A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction. 

 

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit.

Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition:

If exhaust system popping  is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.

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I'd heard it can be caused by a lean condition but never understood until reading that

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 Richen up your idle mixture and turn the idle speed up. Try 900 or 1000 rpm.

 

 

Some later Datsuns have a solenoid that holds the throttle open slightly during deceleration. There was a switch at 10 mph in the speedometer that turns it off for idle.

 

Even newer ones have a BCDD that it like a small carb withing the carb that prevents what you describe. Provides more fuel/air during high intake vacuum and turns off when it lowers.

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Unless its a normal sound from the new exhaust, its caused by fuel burning in the exhaust.  How can richening the mixture make it better???  Add more fuel to eliminate an issue with fuel burning in the exhaust...

 

Popping in the exhaust goes away when you lean out the mixture.  The info above is backward, unless its a specific carb flaw that I'm not aware of.  If that's the case, the engine would flood at idle too.  High volume air draws fuel.  A closed throttle plate stops that volume.  Velocity past the fuel port decreases with volume.  CFM (volume) is required to draw fuel, not simply velocity.  If you had high velocity air at idle, you'd either flood or have a very, very high idle.  A typical carb flows 6-10 cfm at idle, a Weber 32/36 flows around 235 cfm wide open.  At decel (6-10 cfm) the fuel in the pipe is most likely fresh oxygen.  Add gasoline and you get popping.  

 

The notion that failed ignition is the problem only adds to the evidence that's its potentially too rich already.  

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My 1960 vw would blow 3 foot flames out the header, after I richened the crap out of the carb in an attempt to make more than 36 hp.

Great fun when terrorizing a small Wisconsin town in my youth.

 

 

When my Infinity stand-alone in my Toyota gets programed, it will not have an off throttle fuel cut so that I can do the same thing. :)

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You can mess with that on most EFI systems including Megasquirt and FiTech.   :devil:

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Unless its a normal sound from the new exhaust, its caused by fuel burning in the exhaust.  How can richening the mixture make it better???  Add more fuel to eliminate an issue with fuel burning in the exhaust...

 

Popping in the exhaust goes away when you lean out the mixture.  The info above is backward, unless its a specific carb flaw that I'm not aware of.  If that's the case, the engine would flood at idle too.  High volume air draws fuel.  A closed throttle plate stops that volume.  Velocity past the fuel port decreases with volume.  CFM (volume) is required to draw fuel, not simply velocity.  If you had high velocity air at idle, you'd either flood or have a very, very high idle.  A typical carb flows 6-10 cfm at idle, a Weber 32/36 flows around 235 cfm wide open.  At decel (6-10 cfm) the fuel in the pipe is most likely fresh oxygen.  Add gasoline and you get popping.  

 

The notion that failed ignition is the problem only adds to the evidence that's its potentially too rich already.  

 

He said he leaned out the mixture, I said richen it... back to where it should be for proper idle.

 

 

 

The BCDD (not used here) is a small carb within a carb that adds gas and air to remove the over rich deceleration. It's like decelerating with you foot lightly on the gas.

 

 

I don't see any problem here. The engine has always done this you just couldn't hear it with a muffler and resonator on it.

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I'd heard it can be caused by a lean condition but never understood until reading that

Yes, I went through that issue after installing the 48mm mikuni on my L20b, bigger jetting corrected the issue, if you hear the popping when you get on the gas hard followed by a loss of power, it's running too lean, you can do damage to the pistons, so get it right ;)

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Out of curiosity, went on line and typed msa exhaust popping and seems like its common.

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I hope its common as i have already leaned the mixture to 1.5 turns clockwise and retarded the timing to 10 btdc and still pops at the exhaust when decelerating. In view of this, the lean or richen the mixture or advancing or retarding the timing at the distributor with the MSA PREMIUM EXHAUST SYSTEM will not eliminate the deceleration popping as others have recommended. Maybe one of the gaskets provided by MSA is not sealing correctly.

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Or you're chasing your tail and it is what it is.

 

 

 


My car has no anti backfire valve, i had it removed when purchased. I have plugged all the balance tube and also removed all the emission crap! I even lowered the both gas mixture knobs  to 1.5 turns anticlockwise (leaned the mixture) to determine if the car was rich and the popping was still present on deceleration. Replacing all the spark plugs with Brand new NGK BP6ES plugs did not help. Deceleration popping still there.
 
Bottomline, the only way to muffle this popping is to go back to the original exhaust? This MSA PREMIUM EXHAUST SYSTEM comes with a Turbo Muffler while the other one was a simple Dynamax muffler.

 

The AB valve isn't a piece of 'pollution crap' so you've basically shot yourself in the foot removing it. Get another. It's job is to open during high vacuum conditions during deceleration and let air into the intake. This leans the mixture and lowers the vacuum level.

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