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jalexquijano

Deceleration Popping At Tailpipe With New Msa Premium Exhaust System

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Just replaced my former stock 2" exhaust system with Dynamax muffler with an MSA Premium exhaust for stock manifold system:

Motorsport! MSA Premium Exhaust System, 70-72, 74 240Z-260Z (For Stock Manifold) - The Z Store! Nissan-Datsun 240Z 260Z 280Z 280ZX 300ZX ( Z31 Z32 ) 350Z 370Z Parts

anyway after installing it i am noticing a loud deceleration pop at the exhaust which i didnt had with the former exhaust system. so i tightened the tailpipe rear and front clamps as there is no gasket in that área. the only 3 gaskets that come with this exhaust system are the manifold to downpipe gasket and the two pipe to pipe 2.5 inches gasket. I even lowered the mixture to 1.5 turns clockwise to lean the mixture and there is still popping at the exhaust.  Any clues? My BTDC timing is set to 16 BTDC and My camshaft is a Schneider 274F.

 

Just to confirm i had no backfire with the former Steel stock exhaust. What could be causing this?

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Might sound like a silly question but the exhaust system you've changed to (in particular muffler/exhaust pipe) - is it smaller than previous system you were running.

 

I used to have an issue where it was a 2 and 1/2" exhaust that ran into a muffler with a 1 and 1/4 exhaust pipe outlet. Used to cause a similar issue, once I changed it to a 2 1/2 all the way through = no problems.

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You are going the wrong way, Too lean causes the popping and you probably leaned the mixtures out by fitting a freer flowing exhaust. Then you made it even leaner with the mixture screws.

Edited by d3c0y

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Might sound like a silly question but the exhaust system you've changed to (in particular muffler/exhaust pipe) - is it smaller than previous system you were running.

 

I used to have an issue where it was a 2 and 1/2" exhaust that ran into a muffler with a 1 and 1/4 exhaust pipe outlet. Used to cause a similar issue, once I changed it to a 2 1/2 all the way through = no problems.

the former one was smaller. This one has a 2.5 diameter bolted from the manifold to the tail pipe. The MSA exhaust comes with a Xcelerator turbo muffler:

 

http://www.apexhaust.com/product/automotive-light-truck/muffler/xlerator-big-max-turbo-mufflers/

 

My 1972 240z has the following specs:

 

Hitachi distributor with Pertronix 1761 ignitor module and ignition coil.

 

NGK BP5ES Spark plugs gapped at .031

 

Timing adjusted to 16 BTDC.

 

Schneider 274F camshaft with new rocker arms, retainer springs and lashpads.

 

Round Top 3 screws SU CARBURETORS Remanufactured by Ztherapy.

 

I removed all the emission devices and plugged the balance tube. the only thing plugge is the the pcv valve and the hose that goes to the crankshaft.

 

Please advise how to solve this deceleration popping when downshifting and releasing the gas pedal. If i place the shifter in neutral there is no popping.

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You are going the wrong way, Too lean causes the popping and you probably leaned the mixtures out by fitting a freer flowing exhaust. Then you made it even leaner with the mixture screws.

spark plug 3 may2017

spark plug 4 May 2016

 
Spark plug 3 and spark plug 4 at 2.75 turns clockwise. I am still lean? My car suffer from a fuel percolation problem and if i richen them more they get carbón fouled at idle.
Edited by jalexquijano

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Spark plug 3 and spark plug 4 at 2.75 turns clockwise. I am still lean? My car suffer from a fuel percolation problem and if i richen them more they get carbón fouled at idle.

Those plug pics look pretty lean to me, maybe new needles are required. Winding the seat down will richen the mixture throughout the range but mostly at idle as most needles do not have a linear taper.

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Those plug pics look pretty lean to me, maybe new needles are required. Winding the seat down will richen the mixture throughout the range but mostly at idle as most needles do not have a linear taper.

 

Needles? Both carbs were fully refurbished my ztherapy in 2015 including new needles. I dont think the needles are worn.

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I'm not suggesting the needles are worn, I'm suggesting they are not the right needle for your engine.

The needle is like the fuel jets on a Weber type carb. The taper on the needle regulates the amount of fuel that passes through the carby for a given piston/damper position.

I think your needles may be too lean for your engine setup.

Edited by Cozza

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Why NGK BP6ES instead of BP5ES? My engine has a Schneider 274F Camshaft which is hotter than stock! I need to compensate and avoid the plugs from getting carbón fouled at idle:

 

Because the ones you have now look like glow plugs but if all your doing is idling around then, yes, keep hot plugs

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FWIW i have a 310 duration cam with near .600" lift and i run BP6ES plugs, with a stock gap and 50mm carbs...

 

Funny how the questions have been answered and we are all told we're wrong. You're right there is nothing wrong with it, carry on!

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I need your advice. Should i swap to ngk bp6es and gap at .031 or at .035? I have a petronix 1761 ignitor module installed at the distributor and not points.

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The popping is due to camshaft duration and overlap, you now have an exhaust system larger, that has less back pressure, thus allowing unburnt fuel to pas through the slightly open exhaust valves.

With back pressure, the unburnt fuel was more kept in the cylinders.

Little bit richer, on mixtures, back timing to 14 or 12 btdc, then check plug colours and run a range or two different, to decrease the plug fouling.

 

BTW, we visited your country last year, then a run through the Canal. Also Costa Rica, Nicaragua, great beaches, best barbeques, and amazing wildlife! Just regular tourists!

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Dat2Kman,

 

Okay. I found a broken stud which i will replace this weekend including a new manifold to downpipe gasket. Hope it solves the deceleration backfire (popping when releasing the gas pedal) at the tailpipe. I am also enclosing a picture of the new exhaust. Cant stand the popping and not interested on going back to the Steel rail. How much horsepower could i have gained with this new exhaust? I will try and richen the mixture a Little bit more and lower the btdc from 16 to 14 btdc.  

manifold To downpipe broken stud

New exhaust full

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Funny how the questions have been answered and we are all told we're wrong. You're right there is nothing wrong with it, carry on!

I was thinking the exact same thing........

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I have not stated anyone is wrong here! I am not an expert nor a mechanical engineer! Just trying to understand the deceleration popping at the tailpipe when going down hill or realeasing the gas pedal at 2nd or 3rd gear. I cannot hear it when the car is in neutral and i let the gas pedal go.  

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The popping is due to camshaft duration and overlap, you now have an exhaust system larger, that has less back pressure, thus allowing unburnt fuel to pas through the slightly open exhaust valves.

With back pressure, the unburnt fuel was more kept in the cylinders.

Little bit richer, on mixtures, back timing to 14 or 12 btdc, then check plug colours and run a range or two different, to decrease the plug fouling.

 

BTW, we visited your country last year, then a run through the Canal. Also Costa Rica, Nicaragua, great beaches, best barbeques, and amazing wildlife! Just regular tourists!

Next time you come to Panama i will get you a VIP Tour at the New Locks and let you test my 240z!

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You don't get it in neutral because there isnt the load on the engine and the airspeed into the intake, so it's not as lean. - Now you can't say it hasn't been explained between all the posts here.

 

Just run the standard gap on the plugs, like just install them out of the box they will be fine.

If you buy a thing called a colour tune (glass spark plug) you will be able to see if it's running rich or lean too. Can you take it to a dyno and get an AFR probe put on it.

 

I'd bet this is all related to your car stopping when it's idling in traffic. 

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i keeped bp6e with pertronix on stanard gap, done around 10000km on that setup. plug as stated looks lean but the popping sound is something you will have to live with. You need to check your O2 readings really to get the most out of it.

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so it does not do any effect if i gap them at .031 instead of .035?

That is such a small difference you will never notice, You actually want the biggest gap you can possibly run, I would run a .5 gap.

Its only when talking high hp turbo cars that you run into issues with bigger spark gap.

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