Home » Public Forums » GMCnet » [GMCnet] 1978 GMC 403 backfire through the carb
| [GMCnet] 1978 GMC 403 backfire through the carb [message #127545] |
Thu, 26 May 2011 13:17  |
rallymaster
Messages: 662
Registered: February 2004
Location: North Plains, ORYGUN
Karma: -4
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Jim Hupy or anybody with suggestions.
Just went out to move the coach to a level spot, so I could light the
fridge, preparatory to a long weekend trip. It started as usual, but I
could hear a faint pop, pop, pop.
I moved the coach and turned on the furnace and fridge, then took off the
engine cover.
Yup, pop, pop, etc. I revved up the engine, and it got louder. Sounded
like it came from the carb, so I killed the engine and lifted the air
cleaner cover. Restarted the engine, and there it was again. I could
also see a flash in the left side of the carb. Shut it off, put the air
cleaner and hatch covers back on.
Checked my GMC notes and found this:
Backfire
The old basics still hold true.Backfire through the carb = fuel problem.
Backfire through the tailpipe = ignition problems.
I realize it's not always this simple but this is a reliable point of
departure for beginning troubleshooters.
A couple of things.Assuming it idles and runs alright without missing
other than the backfire through the carb.
1. Are the vacuum lines all connected correctly?
2. distributer advance working freely?
3. Timing correct when you set it?
Make sure the ground at the distributer coil has a good ground.
tom
checked internet for oldsmobile backfire through carb and found opposite
comment:
backfire through carb was ignition problem, backfire through exhaust was
fuel.
this sounds more logical, but I'm not an engine person.
Help
RonC
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Ron & Linda Clark
North Plains, ORYGUN
78 Eleganza II
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127553 is a reply to message #127545] |
Thu, 26 May 2011 14:54  |
GeorgeRud
Messages: 1380
Registered: February 2007
Location: Chicago, IL
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I always heard that a backfire through the carb is either a too-lean condition or too-advanced timing. A sticking valve could certainly be a culprit if the atomized fuel is not reaching the cylinder and staying in the manifold.
George Rudawsky
Chicago, IL
75 Palm Beach
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127561 is a reply to message #127545] |
Thu, 26 May 2011 15:49 |
Richard Brown
Messages: 281
Registered: May 2009
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I'm not sure about the Toronado intake manifold, but most intakes are dual-plane, so if it's popping in the left bore, it's probably a cylinder in the right bank. The left side of the carb feeds the right side of the engine & visa versa...
Richard & Carol Brown
1974 Eleganza SE
"DILLIGAF"
Lindale, Tx. 75771
903-881-0192
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Richard & Carol Brown
1974 Eleganza SE
1174 Hickory Hills Dr.
Murchison, TX. 75778
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127562 is a reply to message #127561] |
Thu, 26 May 2011 16:07 |
emerystora
Messages: 4442
Registered: January 2004
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On May 26, 2011, at 2:49 PM, Richard Brown wrote:
> I'm not sure about the Toronado intake manifold, but most intakes are dual-plane, so if it's popping in the left bore, it's probably a cylinder in the right bank. The left side of the carb feeds the right side of the engine & visa versa...
>
> Richard & Carol Brown
Not true for the Olds 455.
The right side of the carb feeds cylinders 2, 3, 5, and 8. The left side feeds cylinders 1, 4, 6 and 7.
Emery Stora
77 Kingsley
Santa Fe, NM
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127566 is a reply to message #127562] |
Thu, 26 May 2011 17:03 |
emerystora
Messages: 4442
Registered: January 2004
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Someone asked me if the 403 is the same as I just know about the 455? Richard's statement might be true for the 403 but I'm not sure.
Emery Stora
On May 26, 2011, at 3:07 PM, Emery Stora wrote:
>
> On May 26, 2011, at 2:49 PM, Richard Brown wrote:
>
>> I'm not sure about the Toronado intake manifold, but most intakes are dual-plane, so if it's popping in the left bore, it's probably a cylinder in the right bank. The left side of the carb feeds the right side of the engine & visa versa...
>>
>> Richard & Carol Brown
>
>
> Not true for the Olds 455.
> The right side of the carb feeds cylinders 2, 3, 5, and 8. The left side feeds cylinders 1, 4, 6 and 7.
>
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>
>
> _______________________________________________
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127570 is a reply to message #127566] |
Thu, 26 May 2011 17:41 |
klassic kampers
Messages: 93
Registered: July 2008
Location: greer,s.c./ellijay,ga
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I have two gmcs come in with bent pushrods caused by gunk from the fuel tank ..........the gunk stuck to the intake valve stems, the valves stuck, and the pushrods bent from missing the point of contact on the rocker arms.....all of this caused by old ethanol fuel blend..............just a thought.......
is it possible a couple of plug wires could have gotten mysteriously crossed?????......a thought...
also if old plug wires are in contact with each other they can "crossfire" ......another thought...
Mike Stewart
1973 GMC 26' Canyonlands /
1973 B.S.A. B50 street tracker-----
Greer,S.C/Ellijay,Ga
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127590 is a reply to message #127570] |
Thu, 26 May 2011 20:37 |
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mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
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| klassic kampers wrote on Thu, 26 May 2011 15:41 |
I have two gmcs come in with bent pushrods...
... and the pushrods bent from missing the point of contact on the rocker arms.....
|
I had a Ford motor that was making the same popping noise. A rocker adjustment nut had backed of an exhaust valve so it do not open. The pop was the compressed contents of that cylinder back-feeding into the intake when the intake valve opened.
Adjusted the rocker and locked down the adjustment... worked fine.
A bent exhaust push rod would have the same sounds. -but- Didn't I hear about the possibility of a lifter popping out if not held down by a push rod. IIRC: A lifter popping out will do BAD THINGS to your oil pressure.
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127621 is a reply to message #127545] |
Fri, 27 May 2011 01:17 |
rallymaster
Messages: 662
Registered: February 2004
Location: North Plains, ORYGUN
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Thanks, everybody, with all this help, we'll sort this mess out, yet.
RonC
On Thu, 26 May 2011 20:32:45 -0700 Jim Kanomata <jimkanomata@gmail.com>
writes:
> Your gasket between the carb and the intake are blown.
> Replace them.
> Too much air leaking in and leaning the mixture.
> Don't waste time taking compression.
>
>
>
>
> On Thu, May 26, 2011 at 12:24 PM, Larry Davick
> <ljdavick@comcast.net> wrote:
> > I would check compression on all cylinders to see if you might
> have a sticking intake valve.
> >
> > Larry Davick
> > Fremont, California
> > The Mystery Machine
> > '76 (ish) Palm Beach
> >
> > _______________________________________________
> > GMCnet mailing list
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> > http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
> >
>
>
>
> --
> Jim Kanomata
> Applied/GMC, Fremont,CA
> jimk@appliedairfilters.com
> http://www.appliedgmc.com
> 1-800-752-7502
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Ron & Linda Clark
1978 Eleganza II
North Plains, ORYGUN
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Ron & Linda Clark
North Plains, ORYGUN
78 Eleganza II
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127623 is a reply to message #127611] |
Fri, 27 May 2011 05:51 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
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For a 'gross' compression check, simply pull the power plug opff the HEI, and spin the engine on the starter. A low cylinder will cause a repetitive change in cranking speed. <For engines without cat converters ONLY. This will throw a bit of raw gas out the tailpipe, which converters don't like at all.>
--johnny
--- On Fri, 5/27/11, Jim Kanomata <jimkanomata@gmail.com> wrote:
From: Jim Kanomata <jimkanomata@gmail.com>
Subject: Re: [GMCnet] 1978 GMC 403 backfire through the carb
To: gmclist@temp.gmcnet.org
Date: Friday, May 27, 2011, 3:32 AM
Your gasket between the carb and the intake are blown.
Replace them.
Too much air leaking in and leaning the mixture.
Don't waste time taking compression.
On Thu, May 26, 2011 at 12:24 PM, Larry Davick <ljdavick@comcast.net> wrote:
> I would check compression on all cylinders to see if you might have a sticking intake valve.
>
> Larry Davick
> Fremont, California
> The Mystery Machine
> '76 (ish) Palm Beach
>
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--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
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| Re: [GMCnet] 1978 GMC 403 backfire through the carb [message #127671 is a reply to message #127545] |
Fri, 27 May 2011 17:11 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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If it is happening on every other rotation of the crank ( consistantly by sound) I'd vote for a sticking intake. A Lean mixture will be more erratic and not mechanically repetitive to one cyl. Weak ignition will be more erratic too. However, after you do the 'no ignition crank test' allready suggested, if that comes up fairly normal sounding, the next thing I would do is pop the cap off can check for cracks, tracking and deposits, or broken rotor along with a quick look at the wires for chafing, corrosion and ....with it running sparking.
John Lebetski
Woodstock, IL
77 Eleganza II
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