| Backfire [message #264047] |
Sun, 12 October 2014 15:26  |
MikeT
Messages: 225
Registered: November 2009
Location: Marine City, Michigan
Karma: 0
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Hello all, nice 600 mile weekend trip from Detroit to Indianapolis. All went well until the last 90 miles home. Coach with 455 carb started to backfire at speed above 50 mph nearing Toledo. Pulled over with engine running i checked that all plug wires were connected and distributor was tight. Ran Ok for next 10 miles and then started to backfire again above 50 mph. Nursed it home luckily making it over the steep Rouge River bridge just south of Detroit cresting at 35 mph.
I will assume its either plug wires or distributor cap or rotor. I did mange to accelerate, at times, to 65 mph and the engine sounded good and ran good until I pushed the gas pedal a little more; then it would backfire.
Any advice on where else to possibly look? I wouldn't think it needs a carb rebuild since it runs great below that acceleration point.
Thanks for any and all advice.
Update: got it in the driveway and now it won't start at all. Getting fuel as I can see the two streams shooting in to the carb as I depress the gas pedal. Well, I have an extra cap, rotor and ignition module. I'll try that next although the cap and rotor looked good.
Mike Thomas
Marine City, MI
77 ex Palm Beach
[Updated on: Sun, 12 October 2014 16:19] Report message to a moderator |
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| Re: Backfire [message #264055 is a reply to message #264047] |
Sun, 12 October 2014 16:12  |
cbryan
Messages: 451
Registered: May 2012
Location: Ennis, Texas
Karma: 3
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If I'm not mistaken, again, this is a classic symptom of a clogged fuel filter, backfiring because of a lean mixture. Could be that simple. Note it didn't return until the filter got clogged enough running after the debris "relaxed" and fell off after turn off.
Hope I'm right here. Could be very cheap.
Best,
Carey
Carey from Ennis, Texas
78 Royale, 500 Cadillac, Rance Baxter EFI.
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| Re: [GMCnet] Backfire [message #264065 is a reply to message #264047] |
Sun, 12 October 2014 18:15 |
Nelson
Messages: 120
Registered: August 2014
Karma: 0
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Mike,
I had the same problem and it turned out to be the carb was loose on the intake manifold. Check the 4 bolts holding the carb on to make sure they are snug but don't over tighten them.
HTH,
Nelson Wright
Orlando FL
78 Royale rear bath
On Oct 12, 2014, at 4:26 PM, Thomas Mike wrote:
> Hello all, nice 600 mile weekend trip from Detroit to Indianapolis. All went well until the last 90 miles home. Coach with 455 carb started to
> backfire at speed above 50 mph nearing Toledo. Pulled over and checked that all plug wires were connected and distributor tight. Ran Ok for next 10
> miles and then started again to backfire above 50 mph. Nursed it home luckily making it over the steep Rouge River bridge just south of Detroit
> cresting at 35 mph.
>
> I will assume its either plug wires or distributor cap or rotor. I did mange to accelerate, at times, to 65 mph and the engine sounded good and ran
> good until I pushed the gas pedal a little more; then it would backfire.
>
> Any advice on where else to possibly look? I wouldn't think it needs a carb rebuild since it runs great below that acceleration point.
>
> Thanks for any and all advice.
> --
> Mike Thomas
> Troy, MI
> 77 ex Palm Beach, 77 Royale (rear bath)
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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| Re: Backfire [message #264067 is a reply to message #264047] |
Sun, 12 October 2014 18:59 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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Perhaps the "Ken H coil" syndrome. Fails when hot. That is why after your pause it runs better just to make you think you fixed it, but then under load it fails. Pickup coils and modules can also do this. .
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Backfire [message #264069 is a reply to message #264047] |
Sun, 12 October 2014 19:34 |
kerry pinkerton
Messages: 2565
Registered: July 2012
Location: Harvest, Al
Karma: 15
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MikeT wrote on Sun, 12 October 2014 15:26...Update: got it in the driveway and now it won't start at all. Getting fuel as I can see the two streams shooting in to the carb as I depress the gas pedal. Well, I have an extra cap, rotor and ignition module. I'll try that next although the cap and rotor looked good.
Given this, it sure points to ignition. Even if the carb was loose or the fuel filter partially plugged and leaning out, if you can see gas shooting out, it would at least hit. If you're getting nothing check for spark and timing.
Kerry Pinkerton - North Alabama
Had 5 over the years. Currently have a '06 Fleetwood Discovery 39L
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| Re: Backfire [message #264072 is a reply to message #264047] |
Sun, 12 October 2014 20:58 |
Gadabout
Messages: 124
Registered: March 2013
Location: Edmonton
Karma: 2
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Was the backfire through the carb or exhaust ?
Any chance it started shortly after you filled up with gas (bad gas ?)
Check vacuum lines , PCV , timing, etc.
A sticking intake valve will cause the symptoms you describe. If all the simple checks don't show a problem , pull the spark plugs an spin the engine over with the starter while holding your finger over each plug hole. If you have a valve problem you will notice that hole does not have good compression.
Good luck....
Carl Harr : Driver NASCAR Pro Series #2
1978 Gadabout-Restoring
1978 Palm Beach
1976 Glenbrook
Prevost Featherlite H3-45
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| Re: Backfire [message #264078 is a reply to message #264072] |
Sun, 12 October 2014 21:49 |
MikeT
Messages: 225
Registered: November 2009
Location: Marine City, Michigan
Karma: 0
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Backfire was through the carb. Filled up 200 miles prior with 1/4 tank remaining. I'll check all you recommended. Thanks
Mike Thomas
Marine City, MI
77 ex Palm Beach
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| Re: Backfire [message #264083 is a reply to message #264078] |
Sun, 12 October 2014 22:28 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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MikeT wrote on Sun, 12 October 2014 20:49Backfire was through the carb. Filled up 200 miles prior with 1/4 tank remaining. I'll check all you recommended. Thanks
Sounds like fuel but I suspect your ignition module.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: Backfire [message #264088 is a reply to message #264083] |
Sun, 12 October 2014 22:55 |
mrgmc3
Messages: 210
Registered: September 2013
Location: W Washington
Karma: 2
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Bob de Kruyff wrote on Sun, 12 October 2014 22:28MikeT wrote on Sun, 12 October 2014 20:49Backfire was through the carb. Filled up 200 miles prior with 1/4 tank remaining. I'll check all you recommended. Thanks
Sounds like fuel but I suspect your ignition module.
I'd suspect the fuel filter at the carb inlet. When it becomes partially or mostly plugged it will still flow enough fuel to support idle and low speed operation. It will stumble and backfire through the carb due to leaning of the mixture at higher flows. When stopped you'll have plenty of fuel in the float bowl so it looks normal.
It's a 5 minute job to replace but you must have a 1" and 5/8" wrench. No adjustable wrenches, and preferably use a 5/8 line wrench. The carb fitting and line strips very easily. Hand tighten both for at least 3-4 threads before putting a wrench on them.
Chris Geils - Twin Cities / W Wa
1978 26' Kingsley w/ very few mods; PD9040, aux trans cooler, one repaint in stock colors, R134a, Al rad, Alcoas, 54k mi
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| Re: Backfire [message #264144 is a reply to message #264047] |
Mon, 13 October 2014 13:11 |
MikeT
Messages: 225
Registered: November 2009
Location: Marine City, Michigan
Karma: 0
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Update:
Changed the ignition module and rotor. Still no start. Pulled a plug and cranked and it had spark but still no start. Changed the cap with coil and it started right up. Drives great again except for the exhaust manifold leak.
Thanks everyone for your input. Now I have to purchase spare cap, rotor and ignition module.
Mike Thomas
Marine City, MI
77 ex Palm Beach
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| Re: [GMCnet] Backfire [message #264745 is a reply to message #264179] |
Thu, 23 October 2014 16:28 |
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Broham
Messages: 204
Registered: March 2013
Location: Detroit, Michigan
Karma: 0
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Glad you made it back home safely!
Let me know if go to any rallies in the spring ... maybe we will ride together.
roll easy Mike
Todd
Owner of a 1976 Eleganza II 26’
other toys:
93 Vette
84 Goldwing
and gone but not forgotten
72 CB750 Chopper
96 Caprice Classic
34 Ford roadster
94 Fleetwood
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