Exhaust bang bang bang

boybach

boybach

Well-known member
Dec 9, 2020
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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
 
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 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
Sent: Tuesday, September 14, 2021 3:27 PM
To: gmclist
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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Unsubscribe or Change List Options:
 
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.
 
> 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
 
> 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
 
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
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:
>
 
> 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
 
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
 
> > 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
 
Matt, it takes many years of setting "lean drop" to do it without a tach
and vacuum gage. (TacVac).
It requires a fully warmed- up engine, and 8 cylinders that are within
10% compression pressure of each other, really tuned to factory ignition
specs, good wires, plugs, rotor, cap, etc.
In other words, a textbook perfect engine. If anything has been
monkeyed with, like pcv, or vacuum advance, or air cleaner, etc. Then all
bets are off.
Then, the best a person can hope for is a low CO reading at the
tailpipe, and a somewhat steady state idle at 650 rpm + or - 100 rpm either
way.
I try to set idle speed so that no dieseling or "running on" happens
on a hot engine when the ignition is switched off with the transmission in
neutral, and the A/C system off as well.
Real world stuff applies here, most of these GMC'S are high mileage
with leaky headers, etc., etc. Makes idle tuning a challenge for sure.
Then you take in humidity, altitude, barometric pressure, etc.
Challenge for sure.
Jim Hupy
Salem, Oregon

> > > 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
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
Well after reading the above I am both encouraged and discouraged! My coach isn't really very high mileage at all, but it is, like the others, old -
so maybe things have decomposed. I don't know. In the last 10 or 12 years it was only driven on three trips until I started camping this year.

Trip one was from Bakersfield, California to BC then across Canada to Belleville Ontario. Long haul. Second was from Belleville Ontario to Scotch
Creek BC. In the interim the coach spent 10 years indoors at a Belleville school bus fleet heated garage. Trip 3 was the delivery journey I made in
March of this year, Scotch Creek to Victoria BC. Those trips added about 8500 miles and the odometer just turned 40,000 when I got the coach to the
island.

Engine idles and runs smoothly, roars up hills with no problem and doesn't diesel on shutdown. If I were to describe the engine's behavior, I'd have
to say she "runs like a top".

So ...do I start messing with things or just leave it be and put up with the popping exhaust on steep downgrades? :?

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
 
Have you tried switching fuels?

Sully
Bellevue wa

> Well after reading the above I am both encouraged and discouraged! My
> coach isn't really very high mileage at all, but it is, like the others,
> old -
> so maybe things have decomposed. I don't know. In the last 10 or 12 years
> it was only driven on three trips until I started camping this year.
>
> Trip one was from Bakersfield, California to BC then across Canada to
> Belleville Ontario. Long haul. Second was from Belleville Ontario to Scotch
> Creek BC. In the interim the coach spent 10 years indoors at a Belleville
> school bus fleet heated garage. Trip 3 was the delivery journey I made in
> March of this year, Scotch Creek to Victoria BC. Those trips added about
> 8500 miles and the odometer just turned 40,000 when I got the coach to the
> island.
>
> Engine idles and runs smoothly, roars up hills with no problem and doesn't
> diesel on shutdown. If I were to describe the engine's behavior, I'd have
> to say she "runs like a top".
>
> So ...do I start messing with things or just leave it be and put up with
> the popping exhaust on steep downgrades? :?
>
> 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
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
> Have you tried switching fuels?
>
> Sully
> Bellevue wa

No, I just run regular ...are you suggesting running higher octane? I know higher octane has less oomph than regular but I don't ping or diesel ATM
...there isn't much difference in brands up here AFAIK, so hate to throw another tankful or two in her at 20ȼ a litre (about 50ȼ a US gallon) for
"premium" 8o

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
 
Try filling from half full with premium gas.
I usually alternate premium to regular when filling from about half full.
Seems to run better that regular only.
--
1977 Kingsley 455 as stock as it gets except lots of Ragusa parts
 
Matt I think I went into detail to state to use a tach and how and why both mixture screws must both be adjusted. If you have a good ear (retired
audio eng) you can do it by pitch as that correlates to a finite RPM. When trimming for the absolute edge, eyes and ears are best together.
Larry. Start by just checking base idle speed. A 50 RPM difference could minimize your issue.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
I mean non ethanol vs ethanol.

Sully
Bellevue wa

> > Have you tried switching fuels?
> >
> > Sully
> > Bellevue wa
>
> No, I just run regular ...are you suggesting running higher octane? I know
> higher octane has less oomph than regular but I don't ping or diesel ATM
> ...there isn't much difference in brands up here AFAIK, so hate to throw
> another tankful or two in her at 20ȼ a litre (about 50ȼ a US gallon) for
> "premium" 8o
>
> 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
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
> Matt I think I went into detail to state to use a tach and how and why both mixture screws must both be adjusted. If you have a good ear (retired
> audio eng) you can do it by pitch as that correlates to a finite RPM. When trimming for the absolute edge, eyes and ears are best together.
> Larry. Start by just checking base idle speed. A 50 RPM difference could minimize your issue. That’s the throttle stop screw on accelerator
> linkage drivers side of Qjet.

Thanks John, I hear you :lol:

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
 
> I mean non ethanol vs ethanol.
>
> Sully
> Bellevue wa

We don't have much of a choice in that dept., I think only the top Chevron grade is ethanol free and that's some expensive stuff.

Thanks for the tip though

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
 
If you *want* to try it - there are a few other choices locally that are supposed to be ethanol free:

Chevron 94
CO-OP premium
Costco Premium
Shell V-Power 91

https://www.pure-gas.org/index.jsp?stateprov=BC

Rob
76 Royale Twin Beds, Dry Bath
Victoria, BC

>

>> I mean non ethanol vs ethanol.
>>
>> Sully
>> Bellevue wa
>
> We don't have much of a choice in that dept., I think only the top Chevron grade is ethanol free and that's some expensive stuff.
>
> Thanks for the tip though
>
>
> 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
 
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