Home » Public Forums » GMCnet » [GMCnet] Spitting, Stumbling, Dying 403
| [GMCnet] Spitting, Stumbling, Dying 403 [message #321699] |
Sun, 06 August 2017 13:27  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
|
Senior Member |
|
|
We're having trouble turning a recently overhauled 403. It has a freshly
rebuilt carburetor, new plug wires, and a well-functioning distributor.
The symptoms are that when cold (
Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321700 is a reply to message #321699] |
Sun, 06 August 2017 13:46  |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
|
Senior Member |
|
|
Off hand it sounds like ignition - check the plug wires for correct order, then timing.
Dennis
Ken Henderson wrote on Sun, 06 August 2017 13:27We're having trouble turning a recently overhauled 403. It has a freshly
rebuilt carburetor, new plug wires, and a well-functioning distributor.
The symptoms are that when cold (
Dennis S
73 Painted Desert 230
Memphis TN Metro
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321701 is a reply to message #321699] |
Sun, 06 August 2017 13:43 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
|
Senior Member |
|
|
Ken, sounds lean to me. But, only because of the off-idle stumble. Things
that stand out to me are, recent overhaul, different carb. Does it still do
it when fully warmed up? Have you sprayed something like WD-40 around the
carb base to check for vacuum leaks? Have you checked running engine
vacuum? Machinist could have a tight valve guide. Especially on the intake
side. Possible cracked intake manifold? Lastly, plug wires, distributor
cap carbon tracking or ignition coil failure.
Hard to tell from long distance.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Aug 6, 2017 11:29 AM, "Ken Henderson" wrote:
> We're having trouble turning a recently overhauled 403. It has a freshly
> rebuilt carburetor, new plug wires, and a well-functioning distributor.
>
> The symptoms are that when cold ( the engine to spit, stumble, and, if the throttle's not released, die.
> Once babied past that first-starting situation, the engine spits and
> stumbles on the 1-2 shift, and may even die there with persistent throttle
> opening.
>
> We've checked timing, vacuum operation, and free centrifugal advance. All
> of the hoses were clearly marked before they were removed for the overhaul
> and the connection of all of them (now new) is correct.
>
> The carburetor has been checked for proper choke adjustment (both cold and
> hot), acceleration pump functioning, and both cold operation and high
> vacuum lock-out of the secondaries.
>
> The symptoms occur with or without the electric fuel boost pump being used.
>
> Occasionally, even after warmed up, there will be an off-idle stumble.
> We've had one report of long, steep grade stumbling which was cured by
> turning on the electric pump.
>
> We're baffled and looking for new ideas.
>
> Ken H.
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
|
|
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321711 is a reply to message #321700] |
Sun, 06 August 2017 17:39 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
|
Senior Member |
|
|
Ken
My first reply was to the cryptic message on the forum.
I see you had actually written more.
So - based on a somewhat similar experience with another vehicle I would not rule out a bad new spark plug. I have seen them misfire when cold or under load.
Dennis
Dennis S wrote on Sun, 06 August 2017 13:46Off hand it sounds like ignition - check the plug wires for correct order, then timing.
Dennis
Ken Henderson wrote on Sun, 06 August 2017 13:27We're having trouble turning a recently overhauled 403. It has a freshly
rebuilt carburetor, new plug wires, and a well-functioning distributor.
The symptoms are that when cold (
Dennis S
73 Painted Desert 230
Memphis TN Metro
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321719 is a reply to message #321699] |
Sun, 06 August 2017 19:58 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
|
Senior Member |
|
|
If you are sure it's not ignition or fuel I would make sure no valves are sticking in the guides and the cam is timed correctly to the crankshaft on rare occasion this has happened.
Roy Keen
Minden,NV
76 X Glenbrook
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321726 is a reply to message #321711] |
Sun, 06 August 2017 20:31 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
|
Senior Member |
|
|
We spent a little more time on the stumbling 403 today. We'd already
checked all the suggestions you've offered so far. Most of the items were
discounted on the basis of the 19 in Hg idle vacuum and the very smooth
running when warm.
Today we pulled the hatch cover so one of us could watch the carb
continuously. Before cranking up, John checked the spark plugs and found
that the mixture is too lean and the timing slightly too advanced (for the
mixture they're seeing). WIth many years of racing engine building, he
reads plugs like I read a newspaper. He reset the choke a little richer
than normal as a partial temporary compensation.
Cold, the engine ran much better, with little stumbling and dying -- not
totally cured, but definitely better. When the engine got warm, I began to
drive more aggressively and then found that I could kill the engine with a
throttle burst. Watching the secondary air doors we saw that they were not
opening. Further investigation showed that the secondary butterflies
weren't even opening!
A little tutorial may be in order here for some: The Quadrajet's
secondaries are operated by the throttle via a "slack link" between the
primaries and the secondaries. That is, the primaries can get almost all
the way open before there's any action on the secondaries. When the
secondaries are called for, it's through a spring, which can give without
moving the secondary butterfiles. And there are two mechanisms which may
lock those butterflies: 1. The secondary butterflies shaft extends out of
the right rear of the carb body. There's a pin through that protruding
shaft. The choke, when closed, positions a stop at the end of that pin so
the butterflies cannot open. That's what I call the "Cold Choke Secondary
Stop". 2. When the engine vacuum is high (I don't really know HOW high),
the vacuum control at the right front of the carburetor can also interpose
a block on secondary butterfly movement. When the vacuum drops enough that
lock is opened, allowing secondary operation.
Examining the "Cold Choke Secondary Stop" device, we found that it's very
loose and was not coming "Off" when the choke was fully open. So we
blocked it off with a small screwdriver. The secondaries then began to
open normally, accompanied by the typical Quadrajet "power moan, or roar".
WIth that solved, there's still a slight hesitation just before the
secondaries come in, but not bad. The interlock is temporarily
safety-wired OFF. It will have to be repaired or removed.
Tomorrow we'll take a look inside the Quadrajet. We'll check with the carb
rebuilder first, but it's almost certain that he left the same main jets in
it, and probably won't remember what they were. The PO, a very
conservative driver (warned the current owner that GMC's weren't meant to
be driven over 55 mph -- which was true back in Jimmy Carter days,
remember?). Given that fact, it's entirely possible that he installed jets
leaner than the OEM 67's. If not, we'll probably install 68's or 69's to
richen the mixture a little, as the plugs (and the hesitation) indicate it
needs.
The replaced engine died with #7&8 rod bearings beaten out. Recent
teardown of that engine showed that #1&2 were also going. Does that
indicate that the lean mixture was causing unnoticed pre-ignition??? We'll
never know, but it's certainly a possibility.
I'll let Y'All know what we find inside the carb.
Ken H.
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321731 is a reply to message #321726] |
Sun, 06 August 2017 21:30 |
Kosier
Messages: 834
Registered: February 2008
Karma: 1
|
Senior Member |
|
|
Ken,
It sounds like it is a little slow on the transition to power enrichment.
You might try one step stronger on the spring under the power piston.
Gary Kosier
77PB w/500Cad
Newark, Ohio
--------------------------------------------------
From: "Ken Henderson"
Sent: Sunday, August 06, 2017 9:31 PM
To: "GMC Mail List"
Subject: Re: [GMCnet] Spitting, Stumbling, Dying 403
> We spent a little more time on the stumbling 403 today. We'd already
> checked all the suggestions you've offered so far. Most of the items were
> discounted on the basis of the 19 in Hg idle vacuum and the very smooth
> running when warm.
>
> Today we pulled the hatch cover so one of us could watch the carb
> continuously. Before cranking up, John checked the spark plugs and found
> that the mixture is too lean and the timing slightly too advanced (for the
> mixture they're seeing). WIth many years of racing engine building, he
> reads plugs like I read a newspaper. He reset the choke a little richer
> than normal as a partial temporary compensation.
>
> Cold, the engine ran much better, with little stumbling and dying -- not
> totally cured, but definitely better. When the engine got warm, I began
> to
> drive more aggressively and then found that I could kill the engine with a
> throttle burst. Watching the secondary air doors we saw that they were
> not
> opening. Further investigation showed that the secondary butterflies
> weren't even opening!
>
> A little tutorial may be in order here for some: The Quadrajet's
> secondaries are operated by the throttle via a "slack link" between the
> primaries and the secondaries. That is, the primaries can get almost all
> the way open before there's any action on the secondaries. When the
> secondaries are called for, it's through a spring, which can give without
> moving the secondary butterfiles. And there are two mechanisms which may
> lock those butterflies: 1. The secondary butterflies shaft extends out
> of
> the right rear of the carb body. There's a pin through that protruding
> shaft. The choke, when closed, positions a stop at the end of that pin so
> the butterflies cannot open. That's what I call the "Cold Choke Secondary
> Stop". 2. When the engine vacuum is high (I don't really know HOW high),
> the vacuum control at the right front of the carburetor can also interpose
> a block on secondary butterfly movement. When the vacuum drops enough
> that
> lock is opened, allowing secondary operation.
>
> Examining the "Cold Choke Secondary Stop" device, we found that it's very
> loose and was not coming "Off" when the choke was fully open. So we
> blocked it off with a small screwdriver. The secondaries then began to
> open normally, accompanied by the typical Quadrajet "power moan, or roar".
> WIth that solved, there's still a slight hesitation just before the
> secondaries come in, but not bad. The interlock is temporarily
> safety-wired OFF. It will have to be repaired or removed.
>
> Tomorrow we'll take a look inside the Quadrajet. We'll check with the
> carb
> rebuilder first, but it's almost certain that he left the same main jets
> in
> it, and probably won't remember what they were. The PO, a very
> conservative driver (warned the current owner that GMC's weren't meant to
> be driven over 55 mph -- which was true back in Jimmy Carter days,
> remember?). Given that fact, it's entirely possible that he installed
> jets
> leaner than the OEM 67's. If not, we'll probably install 68's or 69's to
> richen the mixture a little, as the plugs (and the hesitation) indicate it
> needs.
>
> The replaced engine died with #7&8 rod bearings beaten out. Recent
> teardown of that engine showed that #1&2 were also going. Does that
> indicate that the lean mixture was causing unnoticed pre-ignition???
> We'll
> never know, but it's certainly a possibility.
>
> I'll let Y'All know what we find inside the carb.
>
> Ken H.
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321732 is a reply to message #321731] |
Sun, 06 August 2017 21:37 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
|
Senior Member |
|
|
Sounds like somebody messed with that Quadra jet big time. I might be
tempted to go back to ground zero, find the correct carb, and swap them
just to rule out what you suspect might be wrong. But, that is just me. You
can tamper with what you have, but....
Jim Hupy
On Aug 6, 2017 7:30 PM, "Gary Kosier" wrote:
> Ken,
>
> It sounds like it is a little slow on the transition to power enrichment.
> You might try one step stronger on the spring under the power piston.
>
> Gary Kosier
> 77PB w/500Cad
> Newark, Ohio
>
> --------------------------------------------------
> From: "Ken Henderson"
> Sent: Sunday, August 06, 2017 9:31 PM
> To: "GMC Mail List"
> Subject: Re: [GMCnet] Spitting, Stumbling, Dying 403
>
> We spent a little more time on the stumbling 403 today. We'd already
>> checked all the suggestions you've offered so far. Most of the items were
>> discounted on the basis of the 19 in Hg idle vacuum and the very smooth
>> running when warm.
>>
>> Today we pulled the hatch cover so one of us could watch the carb
>> continuously. Before cranking up, John checked the spark plugs and found
>> that the mixture is too lean and the timing slightly too advanced (for the
>> mixture they're seeing). WIth many years of racing engine building, he
>> reads plugs like I read a newspaper. He reset the choke a little richer
>> than normal as a partial temporary compensation.
>>
>> Cold, the engine ran much better, with little stumbling and dying -- not
>> totally cured, but definitely better. When the engine got warm, I began
>> to
>> drive more aggressively and then found that I could kill the engine with a
>> throttle burst. Watching the secondary air doors we saw that they were
>> not
>> opening. Further investigation showed that the secondary butterflies
>> weren't even opening!
>>
>> A little tutorial may be in order here for some: The Quadrajet's
>> secondaries are operated by the throttle via a "slack link" between the
>> primaries and the secondaries. That is, the primaries can get almost all
>> the way open before there's any action on the secondaries. When the
>> secondaries are called for, it's through a spring, which can give without
>> moving the secondary butterfiles. And there are two mechanisms which may
>> lock those butterflies: 1. The secondary butterflies shaft extends out
>> of
>> the right rear of the carb body. There's a pin through that protruding
>> shaft. The choke, when closed, positions a stop at the end of that pin so
>> the butterflies cannot open. That's what I call the "Cold Choke Secondary
>> Stop". 2. When the engine vacuum is high (I don't really know HOW high),
>> the vacuum control at the right front of the carburetor can also interpose
>> a block on secondary butterfly movement. When the vacuum drops enough
>> that
>> lock is opened, allowing secondary operation.
>>
>> Examining the "Cold Choke Secondary Stop" device, we found that it's very
>> loose and was not coming "Off" when the choke was fully open. So we
>> blocked it off with a small screwdriver. The secondaries then began to
>> open normally, accompanied by the typical Quadrajet "power moan, or roar".
>> WIth that solved, there's still a slight hesitation just before the
>> secondaries come in, but not bad. The interlock is temporarily
>> safety-wired OFF. It will have to be repaired or removed.
>>
>> Tomorrow we'll take a look inside the Quadrajet. We'll check with the
>> carb
>> rebuilder first, but it's almost certain that he left the same main jets
>> in
>> it, and probably won't remember what they were. The PO, a very
>> conservative driver (warned the current owner that GMC's weren't meant to
>> be driven over 55 mph -- which was true back in Jimmy Carter days,
>> remember?). Given that fact, it's entirely possible that he installed
>> jets
>> leaner than the OEM 67's. If not, we'll probably install 68's or 69's to
>> richen the mixture a little, as the plugs (and the hesitation) indicate it
>> needs.
>>
>> The replaced engine died with #7&8 rod bearings beaten out. Recent
>> teardown of that engine showed that #1&2 were also going. Does that
>> indicate that the lean mixture was causing unnoticed pre-ignition??? We'll
>> never know, but it's certainly a possibility.
>>
>> I'll let Y'All know what we find inside the carb.
>>
>> Ken H.
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321800 is a reply to message #321699] |
Tue, 08 August 2017 08:43 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
|
Senior Member |
|
|
In my Qjet expeience, the secondary lockout that actually blocks the secondary lower throttle plates from rotating is what I call the "Chevy" type. The Olds type, from my experience, has the little shoe by the choke rod that swings back and blocks the secondary air valve only. Are you sure this is the correct carb? But the Chevy type would also have the sideways fuel inlet--- unless the carb was assembled from different upper middle lower sections in the past? Anyway- just armchairing here but that may explain wrong jetting. As far as secondary vac pulloff-- there is a very small orifice to act as a timing device. When you punch it, this times the air valve opening as vac drops and ariflow tries to open the air valve working against the vac pulloff and clock spring windup tension (closing forces) A spec of dirt in the orifice and cause a problem as can the wrong vac pulloff PN with different orifice size. Also the OEM RP black and cream colored ones seem to be long extinct and the only options are the bright white Chinese replacements and though I am told they "work" I have no experience.
John Lebetski
Woodstock, IL
77 Eleganza II
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #321803 is a reply to message #321800] |
Tue, 08 August 2017 09:35 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
|
Senior Member |
|
|
I bet carb developers and engineering departments at GM are rotating in
their graves when they hear how some of their best work is being messed
with. A Quadrajet is a lot more complex than many who tamper with them.
When they work the way they were designed to, you can just about forget
that they are under the hood. I had a 1961 Cadillac with the 390 cubic
inch, 345 horsepower option. When you got on the loud pedal, that thing
would be going 90 miles per hour on an on ramp. Loved the way that carb
worked. I try to make them all work that way. Not always an easy job with
all the carb tweakers out there.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
On Aug 8, 2017 6:44 AM, "John R. Lebetski" wrote:
> In my Qjet expeience, the secondary lockout that actually blocks the
> secondary lower throttle plates from rotating is what I call the "Chevy"
> type.
> The Olds type, from my experience, has the little shoe by the choke rod
> that swings back and blocks the secondary air valve only. Are you sure this
> is
> the correct carb? But the Chevy type would also have the sideways fuel
> inlet--- unless the carb was assembled from different upper middle lower
> sections in the past? Anyway- just armchairing here but that may explain
> wrong jetting. As far as secondary vac pulloff-- there is a very small
> orifice to act as a timing device. When you punch it, this times the air
> valve opening as vac drops and ariflow tries to open the air valve working
> against the vac pulloff and clock spring windup tension (closing forces)
> A spec of dirt in the orifice and cause a problem as can the wrong vac
> pulloff PN with different orifice size. Also the OEM RP black and cream
> colored ones seem to be long extinct and the only options are the bright
> white Chinese replacements and though I am told they "work" I have no
> experience.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
|
|
|
|
| Re: [GMCnet] Spitting, Stumbling, Dying 403 [message #322004 is a reply to message #321803] |
Sun, 13 August 2017 09:33 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
|
Senior Member |
|
|
Well, we seem to have finally gotten it running right. As I mentioned, the
plugs showed lean mixture, so we put in larger main jets and got them
looking better. That seemed to cure any hot running problems, but the cold
running was still plagued with stumbling. Richening the electric choke
setting cured that, but it caused the secondary butterfly interlock to stay
engaged even when the engine's warm. So, I locked that out -- a common
mod, it seems. As someone said, "...smart enough to not use full throttle
on a cold engine...".
Now I remember why I like my EFI/EBL so much! :-)
Ken H.
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
|
|
|
|
Goto Forum:
Current Time: Wed Nov 06 22:40:58 CST 2024
Total time taken to generate the page: 0.01375 seconds
|
GMCforum
Search
Help
Register
Login
Home
