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Exhaust bang bang bang

T
twlldeen@gmail.com
Tue, Sep 14, 2021 4:18 PM

Exhaust backfires quite a bit if I put the trans in low going down a steep hill ..haven't had a carburetted vehicle in years and years so wondering if
this is normal?

Recently drove the coach across Vancouver island, Port Renfrew - Mesachie Lake mountain road and then on to several steep unpaved roads down
Riverbottom road in the Cowichan Valley. Coach ran perfectly, but backfiring was a little disconcerting.

Exhaust system is very new, stainless and extremely heavy duty BTW.

Larry

Larry - Victoria BC -

1977 Palm Beach 40,000 miles, PO said everything working but forgot the word NOT. New Atwood fridge, water heater & furnace. New SS exhaust system,
6000w Onan, Iota Converter, R134A A/C, New fuel lines & heat exchange hoses

Exhaust backfires quite a bit if I put the trans in low going down a steep hill ..haven't had a carburetted vehicle in years and years so wondering if this is normal? Recently drove the coach across Vancouver island, Port Renfrew - Mesachie Lake mountain road and then on to several steep unpaved roads down Riverbottom road in the Cowichan Valley. Coach ran perfectly, but backfiring was a little disconcerting. Exhaust system is very new, stainless and extremely heavy duty BTW. Larry -- Larry - Victoria BC - 1977 Palm Beach 40,000 miles, PO said everything working but forgot the word NOT. New Atwood fridge, water heater & furnace. New SS exhaust system, 6000w Onan, Iota Converter, R134A A/C, New fuel lines & heat exchange hoses
JR
John R. Lebetski
Tue, Sep 14, 2021 8:27 PM

Mine will “burble” during long engine braking, but I have my exact  GM copy exhaust system installed so it is barely noticeable.
I suspect your idle mixture may be adding extra fuel causing this to be more pronounced.  If you have 3” stainless this may exaggerate it. Correct
mixture is balanced best idle just at the lean drop edge.  “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean drop.

John Lebetski
Woodstock, IL
77 Eleganza II

Mine will “burble” during long engine braking, but I have my exact GM copy exhaust system installed so it is barely noticeable. I suspect your idle mixture may be adding extra fuel causing this to be more pronounced. If you have 3” stainless this may exaggerate it. Correct mixture is balanced best idle just at the lean drop edge. “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean drop. -- John Lebetski Woodstock, IL 77 Eleganza II
KV
Keith V
Tue, Sep 14, 2021 9:38 PM

Mine has never done that with or without a carb with headers and 3" exhaust with a single rumbly flowmaster 80 in the back
Popping would seem to indicate unburned fuel in the exhaust. How is that happening?


From: John R.Lebetski gransport7087@gmail.com
Sent: Tuesday, September 14, 2021 3:27 PM
To: gmclist@list.gmcnet.org gmclist@list.gmcnet.org
Subject: [GMCnet] Re: Exhaust bang bang bang

Mine will “burble” during long engine braking, but I have my exact  GM copy exhaust system installed so it is barely noticeable.
I suspect your idle mixture may be adding extra fuel causing this to be more pronounced.  If you have 3” stainless this may exaggerate it. Correct
mixture is balanced best idle just at the lean drop edge.  “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean drop.

John Lebetski
Woodstock, IL
77 Eleganza II


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Mine has never done that with or without a carb with headers and 3" exhaust with a single rumbly flowmaster 80 in the back Popping would seem to indicate unburned fuel in the exhaust. How is that happening? ________________________________ From: John R.Lebetski <gransport7087@gmail.com> Sent: Tuesday, September 14, 2021 3:27 PM To: gmclist@list.gmcnet.org <gmclist@list.gmcnet.org> Subject: [GMCnet] Re: Exhaust bang bang bang Mine will “burble” during long engine braking, but I have my exact GM copy exhaust system installed so it is barely noticeable. I suspect your idle mixture may be adding extra fuel causing this to be more pronounced. If you have 3” stainless this may exaggerate it. Correct mixture is balanced best idle just at the lean drop edge. “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean drop. -- John Lebetski Woodstock, IL 77 Eleganza II _______________________________________________ GMCnet mailing list Unsubscribe or Change List Options:
BV
Bill Van Vlack
Tue, Sep 14, 2021 10:57 PM

With a couple EFI systems I have, you can turn on or off 'Deceleration Enleanment' and allow/disallow that burble. I used to turn off/on the ignition
Dad's Dodge while coasting. BANG! I don't suppose that helped the muffler much.

--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

With a couple EFI systems I have, you can turn on or off 'Deceleration Enleanment' and allow/disallow that burble. I used to turn off/on the ignition Dad's Dodge while coasting. BANG! I don't suppose that helped the muffler much. -- Bill Van Vlack '76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid November 2015.
T
twlldeen@gmail.com
Wed, Sep 15, 2021 12:11 AM

Keith V wrote on Tue, 14 September 2021 14:38

Popping would seem to indicate unburned fuel in the exhaust. How is that happening?

I don't know, that's why I asked. :)

Larry

Larry - Victoria BC -

1977 Palm Beach 40,000 miles, PO said everything working but forgot the word NOT. New Atwood fridge, water heater & furnace. New SS exhaust system,
6000w Onan, Iota Converter, R134A A/C, New fuel lines & heat exchange hoses

Keith V wrote on Tue, 14 September 2021 14:38 > Popping would seem to indicate unburned fuel in the exhaust. How is that happening? I don't know, that's why I asked. :) Larry -- Larry - Victoria BC - 1977 Palm Beach 40,000 miles, PO said everything working but forgot the word NOT. New Atwood fridge, water heater & furnace. New SS exhaust system, 6000w Onan, Iota Converter, R134A A/C, New fuel lines & heat exchange hoses
T
twlldeen@gmail.com
Wed, Sep 15, 2021 12:18 AM

JohnL455 wrote on Tue, 14 September 2021 13:27

Mine will “burble” during long engine braking, but I have my exact  GM copy exhaust system installed so it is barely noticeable.
I suspect your idle mixture may be adding extra fuel causing this to be more pronounced.  If you have 3” stainless this may exaggerate it.
Correct mixture is balanced best idle just at the lean drop edge.  “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean
drop.

I have no way to check or test emissions, should I adjust the carburetor? How do I do that?

BTW, I have no idea what this statement means  :d "Correct mixture is balanced best idle just at the lean drop edge."

Larry

Larry - Victoria BC -

1977 Palm Beach 40,000 miles, PO said everything working but forgot the word NOT. New Atwood fridge, water heater & furnace. New SS exhaust system,
6000w Onan, Iota Converter, R134A A/C, New fuel lines & heat exchange hoses

JohnL455 wrote on Tue, 14 September 2021 13:27 > Mine will “burble” during long engine braking, but I have my exact GM copy exhaust system installed so it is barely noticeable. > I suspect your idle mixture may be adding extra fuel causing this to be more pronounced. If you have 3” stainless this may exaggerate it. > Correct mixture is balanced best idle just at the lean drop edge. “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean > drop. I have no way to check or test emissions, should I adjust the carburetor? How do I do that? BTW, I have no idea what this statement means :d "Correct mixture is balanced best idle just at the lean drop edge." Larry -- Larry - Victoria BC - 1977 Palm Beach 40,000 miles, PO said everything working but forgot the word NOT. New Atwood fridge, water heater & furnace. New SS exhaust system, 6000w Onan, Iota Converter, R134A A/C, New fuel lines & heat exchange hoses
JH
James Hupy
Wed, Sep 15, 2021 12:21 AM

I did that same thing with my 1938 Oldsmobile.  Blew out the muffler, and
broke a couple of valve springs. That ended that engine, and allowed me to
swap in a 303 Olds V-8. And Headers, and dual exhaust.
You have something hot enough inside the exhaust system to ignite a
combustible mixture of gasoline and oxygen. If you get rid of either one of
the three things I just mentioned,  you will stop the banging. I would
concentrate upon the gasoline source if it were me. High engine rpm on
deceleration creates a very high vacuum with nearly closed throttle plates.
If they are open a bit too far, then air and fuel can enter the exhaust
system unburned, where it is ignited by hot exhaust components. Air can
also enter via worn throttle shaft bushings, even with throttle plates
closed. Time for a very close inspection of your carburetor. Just what I
would do.
Jim Hupy
Salem, Oregon

On Tue, Sep 14, 2021, 4:36 PM Bill Van Vlack bill.van.vlack@gmail.com
wrote:

With a couple EFI systems I have, you can turn on or off 'Deceleration
Enleanment' and allow/disallow that burble. I used to turn off/on the
ignition
Dad's Dodge while coasting. BANG! I don't suppose that helped the muffler
much.

--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath,
Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o
mid
November 2015.


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I did that same thing with my 1938 Oldsmobile. Blew out the muffler, and broke a couple of valve springs. That ended that engine, and allowed me to swap in a 303 Olds V-8. And Headers, and dual exhaust. You have something hot enough inside the exhaust system to ignite a combustible mixture of gasoline and oxygen. If you get rid of either one of the three things I just mentioned, you will stop the banging. I would concentrate upon the gasoline source if it were me. High engine rpm on deceleration creates a very high vacuum with nearly closed throttle plates. If they are open a bit too far, then air and fuel can enter the exhaust system unburned, where it is ignited by hot exhaust components. Air can also enter via worn throttle shaft bushings, even with throttle plates closed. Time for a very close inspection of your carburetor. Just what I would do. Jim Hupy Salem, Oregon On Tue, Sep 14, 2021, 4:36 PM Bill Van Vlack <bill.van.vlack@gmail.com> wrote: > With a couple EFI systems I have, you can turn on or off 'Deceleration > Enleanment' and allow/disallow that burble. I used to turn off/on the > ignition > Dad's Dodge while coasting. BANG! I don't suppose that helped the muffler > much. > > -- > Bill Van Vlack > '76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, > Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o > mid > November 2015. > _______________________________________________ > GMCnet mailing list > Unsubscribe or Change List Options: >
T
twlldeen@gmail.com
Wed, Sep 15, 2021 2:17 AM

James Hupy wrote on Tue, 14 September 2021 17:21

I did that same thing with my 1938 Oldsmobile.  Blew out the muffler, and
broke a couple of valve springs. That ended that engine, and allowed me to
swap in a 303 Olds V-8. And Headers, and dual exhaust.
You have something hot enough inside the exhaust system to ignite a
combustible mixture of gasoline and oxygen. If you get rid of either one of
the three things I just mentioned,  you will stop the banging. I would
concentrate upon the gasoline source if it were me. High engine rpm on
deceleration creates a very high vacuum with nearly closed throttle plates.
If they are open a bit too far, then air and fuel can enter the exhaust
system unburned, where it is ignited by hot exhaust components. Air can
also enter via worn throttle shaft bushings, even with throttle plates
closed. Time for a very close inspection of your carburetor. Just what I
would do.
Jim Hupy
Salem, Oregon

Thanks Jim, will investigate.

Larry

--
Larry - Victoria BC -

1977 Palm Beach 40,000 miles, PO said everything working but forgot the word NOT. New Atwood fridge, water heater & furnace. New SS exhaust system,
6000w Onan, Iota Converter, R134A A/C, New fuel lines & heat exchange hoses

James Hupy wrote on Tue, 14 September 2021 17:21 > I did that same thing with my 1938 Oldsmobile. Blew out the muffler, and > broke a couple of valve springs. That ended that engine, and allowed me to > swap in a 303 Olds V-8. And Headers, and dual exhaust. > You have something hot enough inside the exhaust system to ignite a > combustible mixture of gasoline and oxygen. If you get rid of either one of > the three things I just mentioned, you will stop the banging. I would > concentrate upon the gasoline source if it were me. High engine rpm on > deceleration creates a very high vacuum with nearly closed throttle plates. > If they are open a bit too far, then air and fuel can enter the exhaust > system unburned, where it is ignited by hot exhaust components. Air can > also enter via worn throttle shaft bushings, even with throttle plates > closed. Time for a very close inspection of your carburetor. Just what I > would do. > Jim Hupy > Salem, Oregon Thanks Jim, will investigate. Larry -- Larry - Victoria BC - 1977 Palm Beach 40,000 miles, PO said everything working but forgot the word NOT. New Atwood fridge, water heater & furnace. New SS exhaust system, 6000w Onan, Iota Converter, R134A A/C, New fuel lines & heat exchange hoses
JR
John R. Lebetski
Wed, Sep 15, 2021 1:18 PM

Larry, idle mixture setting is a bit like tuning a guitar. Instead of a strobe tuner you use a tach needle. Since we have divided manifolds, each idle
mixture screw feeds 4 cylinders discretely. They don’t blend like with an open plane racing manifold. With engine at full 195 temp you slowly adjust
each mixture screw to get maximum idle speed.  Each screw is only contributing to 4 cyls so it’s a bit of a back and forth process.  If they were
set wrong, you will have to go and adjust the idle speed screw to get it to 650 so you are in the idle circuit range. The screws only work in a
limited range. Never bottom them with any force, and opening past about 3 turns does nothing further. Once you find the max RPM center spot about an
1/8 turn range, stop there.  Then turn each screw in very slightly to where it would just want to start losing RPM but does not.  You are then at the
best lean edge of the best range.  The GM lean drop setting would mean then turn one screw in watching tach to lose 25 RPM.  Then turn in other screw
for a 25 RPM drop. Then go back to the speed scree and add the 50 RPM you lost to get back to the 650. Technically this should be done with the air
cleaner in place and the AutoThermAC pre heat active so carb idle air is in the 117F range, but you need to have a flexible carb mixture tool to do it
correctly. If your carb throttle shaft is worn as Jim H suggests could be, you will be adjusting to a moving target making it less accurate.

John Lebetski
Woodstock, IL
77 Eleganza II

Larry, idle mixture setting is a bit like tuning a guitar. Instead of a strobe tuner you use a tach needle. Since we have divided manifolds, each idle mixture screw feeds 4 cylinders discretely. They don’t blend like with an open plane racing manifold. With engine at full 195 temp you slowly adjust each mixture screw to get maximum idle speed. Each screw is only contributing to 4 cyls so it’s a bit of a back and forth process. If they were set wrong, you will have to go and adjust the idle speed screw to get it to 650 so you are in the idle circuit range. The screws only work in a limited range. Never bottom them with any force, and opening past about 3 turns does nothing further. Once you find the max RPM center spot about an 1/8 turn range, stop there. Then turn each screw in very slightly to where it would just want to start losing RPM but does not. You are then at the best lean edge of the best range. The GM lean drop setting would mean then turn one screw in watching tach to lose 25 RPM. Then turn in other screw for a 25 RPM drop. Then go back to the speed scree and add the 50 RPM you lost to get back to the 650. Technically this should be done with the air cleaner in place and the AutoThermAC pre heat active so carb idle air is in the 117F range, but you need to have a flexible carb mixture tool to do it correctly. If your carb throttle shaft is worn as Jim H suggests could be, you will be adjusting to a moving target making it less accurate. -- John Lebetski Woodstock, IL 77 Eleganza II
MC
Matt Colie
Wed, Sep 15, 2021 1:49 PM

boybach wrote on Tue, 14 September 2021 20:18

JohnL455 wrote on Tue, 14 September 2021 13:27

Mine will “burble” during long engine braking, but I have my exact  GM copy exhaust system installed so it is barely noticeable.
I suspect your idle mixture may be adding extra fuel causing this to be more pronounced.  If you have 3” stainless this may exaggerate it.
Correct mixture is balanced best idle just at the lean drop edge.  “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean
drop.

I have no way to check or test emissions, should I adjust the carburetor? How do I do that?

BTW, I have no idea what this statement means  :d "Correct mixture is balanced best idle just at the lean drop edge."

Larry

While John's description is good, it does not make two points.

There are two idle screws in the carburetor base that must be balanced.  I find the "around the corner" special screw driver and a mirror to be
essential.

The lean drop edge is when the mixture is lean and if it goes any leaner at all, there is a precipitous drop in the idle speed.  Without a tachometer
and possibly a vacuum gauge, I don't know how one finds this.  I did know one man that could, but he passed years back.

Matt

Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit

boybach wrote on Tue, 14 September 2021 20:18 > JohnL455 wrote on Tue, 14 September 2021 13:27 > > Mine will “burble” during long engine braking, but I have my exact GM copy exhaust system installed so it is barely noticeable. > > I suspect your idle mixture may be adding extra fuel causing this to be more pronounced. If you have 3” stainless this may exaggerate it. > > Correct mixture is balanced best idle just at the lean drop edge. “Emissions” setting would be 25 RPM lean drop per side to get 50 RPM lean > > drop. > > I have no way to check or test emissions, should I adjust the carburetor? How do I do that? > > BTW, I have no idea what this statement means :d "Correct mixture is balanced best idle just at the lean drop edge." > > > Larry While John's description is good, it does not make two points. There are two idle screws in the carburetor base that must be balanced. I find the "around the corner" special screw driver and a mirror to be essential. The lean drop edge is when the mixture is lean and if it goes any leaner at all, there is a precipitous drop in the idle speed. Without a tachometer and possibly a vacuum gauge, I don't know how one finds this. I did know one man that could, but he passed years back. Matt -- Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms SE Michigan - Near DTW - Twixt A2 and Detroit