Home » Public Forums » GMCnet » [GMCnet] Vacuum / Knocking / Timing?
| [GMCnet] Vacuum / Knocking / Timing? [message #104744] |
Mon, 01 November 2010 03:58  |
980
Messages: 192
Registered: July 2010
Location: United States
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The coach has been running pretty well but I had an interesting problem
tonight. I just replaced the intake manifold gasket with the blocked
exhaust crossover version. While in there I straightened out a bunch of
vacuum lines. (It used to whistle when I'd coast at speed!) and the
distributor was loose so we tightened that down and shot the timing.
Most of my trips have been somewhat short since then and it ran great
even over a big hill between here and Pacifica. But, when I made it
over 20 miles or so and started climbing a hill, the timing went way out
and the engine started knocking whenever I tried to get power out of
it. Especially on hills.
We pulled it over and checked the vacuum lines. We moved some related
diaphragms and checked out the rotor cap, which looked fine.
We though we'd freed a jammed advance linkage and it drove great for a
while. Until about 15 miles later, then it started knocking again but
not as much as before. We made our destination, but about 20 miles on
the return trip it came back. I now think that we didn't unjam anything
but perhaps the engine had just cooled down enough. So far my two
theories are that the "Christmas tree" looking vacuum valve on the front
of the intake manifold isn't right when it gets hot or that the timing
needs to be shot differently to cope with the vacuum advance on a GMC.
Going after it again in the morning. Your insights appreciated.
...The pumpkin made it fine and was a big hit!
Desmond
--
1977 GMC ex-Palm Beach, 26-3
Treasure Island, CA
KC6VHG, KAG0675 "980"
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104749 is a reply to message #104744] |
Mon, 01 November 2010 07:56  |
Mr ERFisher
Messages: 7117
Registered: August 2005
Karma: 2
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here is some info on timing
http://gmcmotorhome.info/engine.html#curve
gene
On Mon, Nov 1, 2010 at 1:58 AM, Desmond's GMC <980@opg.org> wrote:
> The coach has been running pretty well but I had an interesting problem
> tonight. I just replaced the intake manifold gasket with the blocked
> exhaust crossover version. While in there I straightened out a bunch of
> vacuum lines. (It used to whistle when I'd coast at speed!) and the
> distributor was loose so we tightened that down and shot the timing.
>
> Most of my trips have been somewhat short since then and it ran great
> even over a big hill between here and Pacifica. But, when I made it
> over 20 miles or so and started climbing a hill, the timing went way out
> and the engine started knocking whenever I tried to get power out of
> it. Especially on hills.
>
> We pulled it over and checked the vacuum lines. We moved some related
> diaphragms and checked out the rotor cap, which looked fine.
>
> We though we'd freed a jammed advance linkage and it drove great for a
> while. Until about 15 miles later, then it started knocking again but
> not as much as before. We made our destination, but about 20 miles on
> the return trip it came back. I now think that we didn't unjam anything
> but perhaps the engine had just cooled down enough. So far my two
> theories are that the "Christmas tree" looking vacuum valve on the front
> of the intake manifold isn't right when it gets hot or that the timing
> needs to be shot differently to cope with the vacuum advance on a GMC.
>
> Going after it again in the morning. Your insights appreciated.
>
> ...The pumpkin made it fine and was a big hit!
>
> Desmond
>
> --
> 1977 GMC ex-Palm Beach, 26-3
> Treasure Island, CA
> KC6VHG, KAG0675 "980"
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Gene Fisher -- 74-23,77PB/ore/ca
“Give a man a fish; you have fed him for today --- give him a URL and
-------
http://gmcmotorhome.info/
Alternator Protection Cable
http://gmcmotorhome.info/APC.html
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104770 is a reply to message #104744] |
Mon, 01 November 2010 14:22 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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Desmond:
You realy should set the distributor mechanical advance with the help of a timing light. Also if you were to take the cap off you should be able to twist the rotor and watch it return by it self that will tell you if it is stuck advanced. If you you think there is a problem with the vacuum lines which shouldn't cause any difference when you are climbing a steep hill as the vacuum would be very low. Take the vacuum hose loose at the distributor and go up the hill with it disconnected the vacuum advance will be nonexistent while disconnected.
Roy
Roy Keen
Minden,NV
76 X Glenbrook
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104853 is a reply to message #104845] |
Tue, 02 November 2010 11:42 |
980
Messages: 192
Registered: July 2010
Location: United States
Karma: -1
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Got some time to monkey with it yesterday. I thought that we'd just set the timing too advanced. We retarded the timing a bit and when I started it, it was mostly gutless for about the first ten minutes or so then came on fine. So I drove it, got it good and warm with some traffic and about 10 miles and got it to start pinging.
Pulled into a lot and it idled poorly. Kept running but not smoothly, just like before.
I retarded the timing a bit and got it to run smoothly and well. It started test driving in the lot fine but just after that when I put some pressure on it, the pinging was still there, though not as much as before. Tried tweaking it a little more, but couldn't make a big difference.
It's like the coach has two levels of warmup, One when the thermostat kicks and another after you've been using it a bit.
DC
On Nov 2, 2010, at 7:21 AM, Manuel Barje wrote:
>
>
> Just to rule out some faults and save some time, hook up a vacuum gauge and look at this troubleshooting chart,
> http://www.earlycuda.org/tech/vacuum2.htm
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104860 is a reply to message #104744] |
Tue, 02 November 2010 12:36 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
Karma: 0
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if you have a vacuum gauge set your timing for max vac. then back it off a bit. if it still pings back off a bit more. it is real possible that the timing marks on the harmonic balancer have shifted. happens often and will give you a false timing reading.
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104929 is a reply to message #104879] |
Wed, 03 November 2010 12:53 |
fred v
Messages: 999
Registered: April 2006
Location: pensacola, fl.
Karma: 0
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| Robert Mueller wrote on Tue, 02 November 2010 19:20 |
Fred,
It seems to me that if I were Desmond I would:
1) remove both wheel wells
2) remove the spark plugs
3) rotate the engine counter clockwise and get No 1 on TDC - Compression
4) check and note the position of the harmonic balancer mark in relation to
the engine timing plate
5) remove the distributor cap and rotate the engine clockwise and note how
far the crankshaft can be rotated before the distributor rotor moves
These two checks would determine if the harmonic balancer and timing chain
were OK.
Once that was done I'd time the engine with a timing light as per the
Maintenance Manual.
Regards,
Rob M.
USAussie
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of fred veenschoten
Sent: Tuesday, November 02, 2010 12:36 PM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Vacuum / Knocking / Timing?
if you have a vacuum gauge set your timing for max vac. then back it off a
bit. if it still pings back off a bit more. it is real possible that the
timing marks on the harmonic balancer have shifted. happens often and will
give you a false timing reading.
--
Fred V
'77 Royale RB 455
P'cola, Fl
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yes, he should do that too. it may be that he has an automobile distributor in there too and the advance curves are wrong.
lots of stuff to look at.
Fred V
'77 Royale RB 455
P'cola, Fl
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104939 is a reply to message #104937] |
Wed, 03 November 2010 15:15 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Desmond,
If it ran fine before the vacuum line changes -- I would only be concerned about what type of distributor I had as it relates to setting the timing. Which hoses/lines did you change -- any chance you plumbed them differently when you replaced them? Who set the timing most recently -- before the issues began? Do I recall that you had JimK install the block off plates (did they reset timing then?) Perhaps the bad lines had someone set the timing to compensate and now that is the problem.
Give us more detail.
Thanks.
Dennis Sexton
73 GMC
Germantown, TN
USA
-----Original Message-----
From: Desmond's GMC <980@opg.org>
To: gmclist@temp.gmcnet.org
Sent: Wed, Nov 3, 2010 2:55 pm
Subject: Re: [GMCnet] Vacuum / Knocking / Timing?
How can I tell if I have the wrong distributor? Is there a marking on it?
btw. I didn't have this issue before I repaired bad vacuum hoses.
DC
yes, he should do that too. it may be that he has an automobile distributor in
here too and the advance curves are wrong.
lots of stuff to look at.
-
977 GMC ex-Palm Beach, 26-3
reasure Island, CA
C6VHG, KAG0675 "980"
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MCnet mailing list
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Dennis S
73 Painted Desert 230
Memphis TN Metro
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104944 is a reply to message #104939] |
Wed, 03 November 2010 15:31 |
980
Messages: 192
Registered: July 2010
Location: United States
Karma: -1
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Senior Member |
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It ran "fine" before the vacuum line repairs. It needed those, it was whistling under the hood.
I had Applied change my intake manifold gasket to solve an exhaust leak, which it did, but I forgot to ask them to use a crossover blocking gasket, so they used a regular one.
I replaced that gasket with a crossover blocking one when I was repairing damaged vacuum hoses. I have a very experienced mechanic working with me while we're doing these. He knows the vacuum setup pretty well. The coach ran GREAT after the first timing, but after a couple of days this pinging problem started up.
I'm not at the coach right now but I'm thinking that the coolant issue sounds like a great place to start. I watch the temp gauge but I don't' think that it works totally right. What do most of you consider a normal reading on your OEM gauges? Mine gets up to about 25% but no further.
I also think that maybe the vacuum advance is sticking on. It only happens when I've got it really warmed up. Takes about 15 miles to happen.
I'll check those as soon as I can get to it. I appreciate all of your help.
DC
On Nov 3, 2010, at 1:15 PM, Dennis S wrote:
>
> Desmond,
> If it ran fine before the vacuum line changes -- I would only be concerned about what type of distributor I had as it relates to setting the timing. Which hoses/lines did you change -- any chance you plumbed them differently when you replaced them? Who set the timing most recently -- before the issues began? Do I recall that you had JimK install the block off plates (did they reset timing then?) Perhaps the bad lines had someone set the timing to compensate and now that is the problem.
> Give us more detail.
> Thanks.
>
>
> Dennis Sexton
> 73 GMC
> Germantown, TN
> USA
>
>
>
> -----Original Message-----
> From: Desmond's GMC <980@opg.org>
> To: gmclist@temp.gmcnet.org
> Sent: Wed, Nov 3, 2010 2:55 pm
> Subject: Re: [GMCnet] Vacuum / Knocking / Timing?
>
>
> How can I tell if I have the wrong distributor? Is there a marking on it?
> btw. I didn't have this issue before I repaired bad vacuum hoses.
> DC
> yes, he should do that too. it may be that he has an automobile distributor in
> here too and the advance curves are wrong.
> lots of stuff to look at.
>
>
>
> -
> 977 GMC ex-Palm Beach, 26-3
> reasure Island, CA
> C6VHG, KAG0675 "980"
> _______________________________________________
> MCnet mailing list
> ist Information and Subscription Options:
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>
> _______________________________________________
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> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104947 is a reply to message #104937] |
Wed, 03 November 2010 15:38 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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If I interpret your statement correctly, you did not have the pinging before
you repaired the vacuum hoses, and if I recall correctly, some mention was
made of the temperature actuated multiport vacuum manifold that some refer
to as the christmas tree. If I recall correctly, on non California equipped
engines, the correct routing goes from the "G" port of the carb, to the
vacuum delay valve, to the vacuum control valve, then to the solenoid valve
to port #1 on the distributor thermal vacuum switch. Port # 1 is the closest
to the bottom of the switch. Port # 2 is connected to the distributor vacuum
advance cannister. How it is supposed to function is to allow no advance
when the engine is cold to force the engine to warm up more quickly. As the
engine warms up, the valve gradually opens, allowing vacuum to be applied to
the distributor. You have said that the engine pings when fully warmed up. I
would start there and check if all components of that system are operating
as they should. A good explanation of this system is in the 1977 & 1978
Maintenence manual supplement part # x-7725 brginning on page 6T-1
Incidentally, the 1977 455 Cu in engine should have Dist # 1112893, and the
vacuum control should be # 1973523. The 77 & 78 403 cu in engine should have
the 1103267 dist for the Federal engines and #1103309 for the California
equipped ones. The federal vacuum control should be #1973609 and the
California one should be 1973634. If you need the specs, they are also in
the same manual. Hope this helps more than confuses.
Jim Hupy
Salem, Or
78 Royale 403
On Wed, Nov 3, 2010 at 12:55 PM, Desmond's GMC <980@opg.org> wrote:
> How can I tell if I have the wrong distributor? Is there a marking on it?
>
> btw. I didn't have this issue before I repaired bad vacuum hoses.
>
> DC
> > yes, he should do that too. it may be that he has an automobile
> distributor in there too and the advance curves are wrong.
> > lots of stuff to look at.
> >
> >
>
>
> --
> 1977 GMC ex-Palm Beach, 26-3
> Treasure Island, CA
> KC6VHG, KAG0675 "980"
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104962 is a reply to message #104879] |
Wed, 03 November 2010 16:36 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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| Robert Mueller wrote on Tue, 02 November 2010 19:20 |
Fred,
It seems to me that if I were Desmond I would:
1) remove both wheel wells
2) remove the spark plugs
3) rotate the engine counter clockwise and get No 1 on TDC - Compression
4) check and note the position of the harmonic balancer mark in relation to
the engine timing plate
5) remove the distributor cap and rotate the engine clockwise and note how
far the crankshaft can be rotated before the distributor rotor moves
These two checks would determine if the harmonic balancer and timing chain
were OK.
Once that was done I'd time the engine with a timing light as per the
Maintenance Manual.
Regards,
Rob M.
USAussie
|
Here is the easy way to check if the timing mark has slipped on the vibration dampener.
http://www.gmcmhphotos.com/photos/showphoto.php?photo=10091&cat=3872
http://www.gmcmhphotos.com/photos/showphoto.php?photo=10090&cat=3872
If you can not find the two hash marks on the face of the two pieces, then note that the outer mark that you time to is lined up with the two hash marks on the face. Then a line through all three of the marks goes through the exact center of the large hole. Finally a line through all 4 of these goes through the center of the keyway holding the unit in position on the crankshaft.
On the question of the of the car distributor, The is no obvious difference from the outside. The biggest difference is the value of the vacuum advance used and the springs on the advance weights.
Our vacuum advance should only run 10 or 12 degrees at full vacuum. You can check this at the time you set the static vacuum. With the vacuum line to the distributor disconnected and plugged you need to set the static at somewhere between 8 and 12 BTDC (depending on the version of the distributor). Then connect the vacuum lead of the distributor to a static vacuum source on the intake manifold. The timing should jump to it's maximum designed value. If it is more than 10 or 12 degrees then you have the wrong vacuum advance.
Do not forget when you get done to return the vacuum advance hose back the the upper right ported vacuum port on the carburetor.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: [GMCnet] Vacuum / Knocking / Timing? [message #104963 is a reply to message #104944] |
Wed, 03 November 2010 16:43 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Senior Member |
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Desmond,
Normal operating temperature usually shows as 1/4 on the OEM gauge -- a function of the sender and some have replaced the sending unit to gain a different (higher) gauge reading for normal.
Dennis
| 980 wrote on Wed, 03 November 2010 15:31 |
It ran "fine" before the vacuum line repairs. It needed those, it was whistling under the hood.
I had Applied change my intake manifold gasket to solve an exhaust leak, which it did, but I forgot to ask them to use a crossover blocking gasket, so they used a regular one.
I replaced that gasket with a crossover blocking one when I was repairing damaged vacuum hoses. I have a very experienced mechanic working with me while we're doing these. He knows the vacuum setup pretty well. The coach ran GREAT after the first timing, but after a couple of days this pinging problem started up.
I'm not at the coach right now but I'm thinking that the coolant issue sounds like a great place to start. I watch the temp gauge but I don't' think that it works totally right. What do most of you consider a normal reading on your OEM gauges? Mine gets up to about 25% but no further.
I also think that maybe the vacuum advance is sticking on. It only happens when I've got it really warmed up. Takes about 15 miles to happen.
I'll check those as soon as I can get to it. I appreciate all of your help.
DC
|
Dennis S
73 Painted Desert 230
Memphis TN Metro
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