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| Vacuum question [message #288278] |
Tue, 06 October 2015 14:02  |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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This spring I pulled the exhaust manifolds and the intake manifold for a gasket changes. The exhaust manifolds are cracked so headers are in the near future, the cracks are n the flange so the remflex gaskets have taken care of them for the short term, I used the intake gasket from Applied with the stainless exhaust crossover block off plates. It seamed to go together well but I'm wondering if I didn't get a good seal on the intake manifold because I'm only getting 15 inches of vacuum at idle and the right exhaust sounds funny compared to the left.
A little history, Fuel tanks were full of crud, they have been cleaned an reinstalled including all new lines, Carb has been rebuilt by me as I don't have the proper carb numbers to have one of the experts do it. I got the kit form Cliffs high performance who said he knew the GMC carbs and could make this one work.
I added a wideband oxygen sensor to aid in tuning the carb which I haven't started yet because of the low vacuum readings. My fuel mixture on my last trip runs from 9 to 13 spending most of the time in the 9-10 range even cruising, the other problem I see is I can't adjust the idle mixture the screws seem to have little effect and it is bubbling gas out of the primary venturies even at idle. So I'm wondering if the low vacuum is letting the metering rods actuate before they should and could that cause the over rich mixture.
The only vacuum actuated items hooked up are the brake booster (which may be a source of a leak, haven't checked it out yet) and the vacuum modulator. the distributor is hooked to the carb ported vacuum I think.
Any ideas on how to track down the source of the low vacuum would be appreciated, hoping it's not internal engine problems, the coach has 81000 miles on it and has a rebuilt engine, I don't know anything about the rebuild I just found the heat disk that rebuilders use glued to one of the freeze plugs and it wasn't melted.
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: Vacuum question [message #288282 is a reply to message #288278] |
Tue, 06 October 2015 15:36  |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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bpimm wrote on Tue, 06 October 2015 12:02This spring I pulled the exhaust manifolds and the intake manifold for a gasket changes. The exhaust manifolds are cracked so headers are in the near future, the cracks are n the flange so the remflex gaskets have taken care of them for the short term, I used the intake gasket from Applied with the stainless exhaust crossover block off plates. It seamed to go together well but I'm wondering if I didn't get a good seal on the intake manifold because I'm only getting 15 inches of vacuum at idle and the right exhaust sounds funny compared to the left.
Any ideas on how to track down the source of the low vacuum would be appreciated, hoping it's not internal engine problems, the coach has 81000 miles on it and has a rebuilt engine, I don't know anything about the rebuild I just found the heat disk that rebuilders use glued to one of the freeze plugs and it wasn't melted.
Vacuum can be elusive and I am no expert. If the rebuilder put in a cam with any kind of valve overlap it could be the best the engine is going to do.
I would check the timing as well. Retarded timing causes the engine to lug and opening the throttle plates via the idle screw will correct it but lower vacuum.
Here is my "shade tree" intake manifold leak test. (If it is leaking it is probably from the valley and not from above as you would see and hear it leaking.)
1. Pull the PCV valve off at the carburetor and plug the tube going into the carburetor.
2. Make sure your oil fill is tight and dipstick is set in place.
3. Open the left side valve cover breather and set it aside.
4. Start the engine and look at the breather hole in the top of the left valve cover. With no PCV suction you should see a bit of blowby whiffing out.
5. Find a plastic shopping bag or other light weight plastic.
6. Cover the breather hole with the bag. Hang on tight! Really, hang on.
If the intake is sucking out of the valley the bag will be sucked into the hole quite vigorously. If it is neutral or puffing out a tiny bit, I say, no intake leak in the valley. Any inward suction has to be coming from somewhere and PCV is the only suction from the valley that is supposed to be happening.
If #4 shows "puffing" like a steam train, that is a sign of a hole in a piston or some other major compression leak such as broken ring landings.
If it passes the above it may be time to check the compression. Leaky intake/s valves will cause lower vacuum. If you pull the batt wire at the coil and crank the engine you can sometimes hear a weak compression cylinder. If the starter has a point when it suddenly speeds up as if there is no resistance in the crank, that can mean a bad cylinder. Time for a compression gauge.
Another check, for missing: go to the tail pipe at idle and put your shoe over the end. If you foot is being sucked and batted away with a regular pulse it may be missing due to an exhaust valve. This would be a slow action... not the regular 8 cylinder throb.
Like I said, some of the is shade tree stuff we have been doing for 50 years. Probably does not met with approval from real mechanics. "On the other hand she wore a green glove."
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: [GMCnet] Vacuum question [message #288285 is a reply to message #288278] |
Tue, 06 October 2015 16:32 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Brian,
I assume this is the gasket set you used:
http://www.appliedgmc.com/prod.itml/icOid/827
That gasket set is from Dick Paterson and IIRC he recommends coating the interface between the heads and the gasket and the gasket
and the manifold with Permatex Ultra Black RTV:
Tiny URL:
http://tinyurl.com/kxcy4qt
Full URL:
http://www.permatex.com/products-2/product-categories/gasketing/gasket-makers/permatex-ultra-black-maximum-oil-resistance-rtv-silico
ne-gasket-maker-detail
Regards,
Rob M.
The Pedantic Mechanic
-----Original Message-----
From: Brian Pimm
This spring I pulled the exhaust manifolds and the intake manifold for a gasket changes. The exhaust manifolds are cracked so
headers are in the near future, the cracks are n the flange so the remflex gaskets have taken care of them for the short term, I
used the intake gasket from Applied with the stainless exhaust crossover block off plates. It seamed to go together well but I'm
wondering if I didn't get a good seal on the intake manifold because I'm only getting 15 inches of vacuum at idle and the right
exhaust sounds funny compared to the left.
A little history, Fuel tanks were full of crud, they have been cleaned an reinstalled including all new lines, Carb has been rebuilt
by me as I don't have the proper carb numbers to have one of the experts do it. I got the kit form Cliffs high performance who said
he knew the GMC carbs and could make this one work.
I added a wideband oxygen sensor to aid in tuning the carb which I haven't started yet because of the low vacuum readings. My fuel
mixture on my last trip runs from 9 to 13 spending most of the time in the 9-10 range even cruising, the other problem I see is I
can't adjust the idle mixture the screws seem to have little effect and it is bubbling gas out of the primary venturies even at
idle. So I'm wondering if the low vacuum is letting the metering rods actuate before they should and could that cause the over rich
mixture.
The only vacuum actuated items hooked up are the brake booster (which may be a source of a leak, haven't checked it out yet) and the
vacuum modulator. the distributor is hooked to the carb ported vacuum I think.
Any ideas on how to track down the source of the low vacuum would be appreciated, hoping it's not internal engine problems, the
coach has 81000 miles on it and has a rebuilt engine, I don't know anything about the rebuild I just found the heat disk that
rebuilders use glued to one of the freeze plugs and it wasn't melted.
--
Brian
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Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: Vacuum question [message #288286 is a reply to message #288282] |
Tue, 06 October 2015 16:53 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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Senior Member |
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[quote title=George Beckman wrote on Tue, 06 October 2015 13:36
Vacuum can be elusive and I am no expert. If the rebuilder put in a cam with any kind of valve overlap it could be the best the engine is going to do.[/quote] I hadn't thought of a cam I just assumed a stock rebuild but that's not what I would have done.
[quote title=George Beckman wrote on Tue, 06 October 2015 13:36
I would check the timing as well. Retarded timing causes the engine to lug and opening the throttle plates via the idle screw will correct it but lower vacuum.[/quote]I'll check it again but my HF timing light puts it at about 10* advanced but it's bouncing around a bit.
[quote title=George Beckman wrote on Tue, 06 October 2015 13:36
Here is my "shade tree" intake manifold leak test. (If it is leaking it is probably from the valley and not from above as you would see and hear it leaking.)
1. Pull the PCV valve off at the carburetor and plug the tube going into the carburetor.
2. Make sure your oil fill is tight and dipstick is set in place.
3. Open the left side valve cover breather and set it aside.
4. Start the engine and look at the breather hole in the top of the left valve cover. With no PCV suction you should see a bit of blowby whiffing out.
5. Find a plastic shopping bag or other light weight plastic.
6. Cover the breather hole with the bag. Hang on tight! Really, hang on.
If the intake is sucking out of the valley the bag will be sucked into the hole quite vigorously. If it is neutral or puffing out a tiny bit, I say, no intake leak in the valley. Any inward suction has to be coming from somewhere and PCV is the only suction from the valley that is supposed to be happening.
If #4 shows "puffing" like a steam train, that is a sign of a hole in a piston or some other major compression leak such as broken ring landings.
If it passes the above it may be time to check the compression. Leaky intake/s valves will cause lower vacuum. If you pull the batt wire at the coil and crank the engine you can sometimes hear a weak compression cylinder. If the starter has a point when it suddenly speeds up as if there is no resistance in the crank, that can mean a bad cylinder. Time for a compression gauge.[/quote]Compression looks good, I should have put that in the original post. 1:157,2:165,3:165,4:163,5:160,6:165,7:165,8:165
[quote title=George Beckman wrote on Tue, 06 October 2015 13:36
Another check, for missing: go to the tail pipe at idle and put your shoe over the end. If you foot is being sucked and batted away with a regular pulse it may be missing due to an exhaust valve. This would be a slow action... not the regular 8 cylinder throb.
Like I said, some of the is shade tree stuff we have been doing for 50 years. Probably does not met with approval from real mechanics. "On the other hand she wore a green glove."
[/quote]
I'll try the shade tree tests and report back. Thanks
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: Vacuum question [message #288288 is a reply to message #288278] |
Tue, 06 October 2015 17:29 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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Senior Member |
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With the PCV disconnected and the breather out there is no visible blow by and the bag test shows no vacuum and a very minor amount of positive pressure, barely pushed the bag out from the grommet and the bag is plenty strong to seal it off.
no exhaust suction just evenly spaced pulses.
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: [GMCnet] Vacuum question [message #288290 is a reply to message #288287] |
Tue, 06 October 2015 17:44 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Brian,
In my opinion installing the OEM intake manifold is a two man job (even if they're 30)!
When Chuck Boyd and I installed Dan Gregg's OEM manifold at a GMCMI Convention we used two guide studs, one in the driver side front
bolt hole and one in the driver side rear bolt hole.
We coated the heads with Permatex Black RTV and then the gaskets. We used the Permatex that came in the pressurized can. That makes
it easy to lay a nice even bead at the front and back of the manifold to seal the lifter valley.
We had a hell of a time aligning it even though we had the guide studs and at the last instant it dropped a bit. When Dan drove home
from the convention he reported that he had a bit of oil leakage out the front and rear of the lifter valley. He cleaned the areas
with a bit of solvent and smeared additional RTV over where the leaks were and that problem was solved.
I believe when the manifold dropped a bit it affected the seals at the front and rear of the manifold. This is why I say installing
the manifold is a two man job!
Regards,
Rob M.
The Pedantic Mechanic
-----Original Message-----
From: Brian Pimm
Yes that is it, I did use the RTV as per the instructions, If it is messed up it's from trying to sit it down evenly on the block,
even with a guide bolt that manifold is to heavy for my bad back to handle alone, I should have had help sitting it on... live and
learn, guess I'm not 30 anymore. :d
Brian
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: Vacuum question [message #288292 is a reply to message #288288] |
Tue, 06 October 2015 18:02 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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Senior Member |
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bpimm wrote on Tue, 06 October 2015 15:29With the PCV disconnected and the breather out there is no visible blow by and the bag test shows no vacuum and a very minor amount of positive pressure, barely pushed the bag out from the grommet and the bag is plenty strong to seal it off.
no exhaust suction just evenly spaced pulses.
I would eliminate the intake gasket. You did well with that, it seems. We had one engine just about take the bag away. I would say the intake and pistons are in pretty good shape. I guess the next is the timing and the compression test. Could have valves leaking.
If you have not timed a GMC get ready for fun. You have to practically stand on your head and you "still can't see anything". So, use white paint or whiteout on the cut on the damper. At the same time, clean it off at that point until you can see the damper and inner ring. THere are two punched dots that should be lining up. If not you need a new damper. You can try to get your hand up high, about 11 o'clock on the driver's side and clean off the little plate that with the degrees. Don't try to read the numbers. Count points and notches and time it as per the manual.
I also own my timing light so several years ago I took the clips off and replaced them with a cigar lighter bullet. Now I can just plug it into the dash and get the cord out of the way of the fan and belts. (Dick recommended this at Rapid CIty and I thought, wow, I actually thought of something Dick does and I thought of it all by myself. There may be hope.) If you have long hair or loose clothing, get it out of the way because you are looking down between the water hose, power steering pump, water pump and alternator belt and you might be able to see something. For me when I get the light in there there is no room for my head. *smile* Meanwhile the fan and belts are whipping around.
For guys thinking of doing an intake manifold:
If you ever do have to do an intake again, the guide bolt was not something I understood until Dick Paterson showed me. I tried to let it down onto the guides. Dick demonstrated: The guides go on the right side of the engine... two of them. You put the black RTV on nice and easy, "Like you were putting cream on a baby's bottom," according to Dick. "Don't goo it all up." Of course different RTV for the water ports at the four corners.
Get the China wall ends done up nicely, sit the edge of manifold down on the two studs and then swing it down, just like closing a door. Still heavy with an original but easier than lowering the whole thing like a helicopter.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: Vacuum question [message #288294 is a reply to message #288292] |
Tue, 06 October 2015 18:21 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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Senior Member |
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Yes timing is a b***h, I need to check the the 2 dimples if I can find them. I like the Cig lighter plug Idea.
Compression is
1:157
2:165
3:165
4:163
5:160
6:165
7:165
8:165
Maybe I can check the timing tomorrow, right now I have to go pick up all the moss I cleaned off the roof of the shop, probably 1-1.5 CY and I cleaned it 2 years ago. My metal roof on the house hasn't been cleaned in 15 years and looks great. I think a metal roof for the shop is a must.
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: [GMCnet] Vacuum question [message #288302 is a reply to message #288294] |
Tue, 06 October 2015 20:44 |
Bruce Hart
Messages: 1501
Registered: October 2011
Location: La Grange, Wyoming
Karma: 5
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If you have a spare 12 volt battery, place it in front of the drivers or
passenger seat and use it for the timing light. There is a picture of the
timing marks in the manual which shows 0 degrees, 4 degrees, 8 degrees etc.
At rapid city Dick Patterson said that the total timing ( vacuum and
mechanical) at 2500 rpm or so should be 39 degrees. You will need a an
adjustable timing light to accomplish this.
On Tue, Oct 6, 2015 at 5:21 PM, Brian Pimm wrote:
> Yes timing is a b***h, I need to check the the 2 dimples if I can find
> them. I like the Cig lighter plug Idea.
>
> Compression is
> 1:157
> 2:165
> 3:165
> 4:163
> 5:160
> 6:165
> 7:165
> 8:165
>
> Maybe I can check the timing tomorrow, right now I have to go pick up all
> the moss I cleaned off the roof of the shop, probably 1-1.5 CY and I cleaned
> it 2 years ago. My metal roof on the house hasn't been cleaned in 15 years
> and looks great. I think a metal roof for the shop is a must.
>
> --
> Brian & RaeDean
>
> 1973 26' #383
>
> Washougal WA
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
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>
--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
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Bruce Hart
1976 Palm Beach
1977 28' Kingsley
La Grange, Wyoming
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| Re: Vacuum question [message #288305 is a reply to message #288294] |
Tue, 06 October 2015 21:29 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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bpimm wrote on Tue, 06 October 2015 16:21Yes timing is a b***h, I need to check the the 2 dimples if I can find them. I like the Cig lighter plug Idea.
Compression is
1:157
2:165
3:165
4:163
5:160
6:165
7:165
8:165
Maybe I can check the timing tomorrow, right now I have to go pick up all the moss I cleaned off the roof of the shop, probably 1-1.5 CY and I cleaned it 2 years ago. My metal roof on the house hasn't been cleaned in 15 years and looks great. I think a metal roof for the shop is a must.
Good compression! Nothing there that should cause vacuum loss.
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: [GMCnet] Vacuum question [message #288310 is a reply to message #288305] |
Wed, 07 October 2015 05:12 |
Bruce Hart
Messages: 1501
Registered: October 2011
Location: La Grange, Wyoming
Karma: 5
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Senior Member |
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Brian,
What is your elevation? The higher you go the less vacuum you can
achieve. I live at 5,000 feet elevation and my vacuum is at16".
On Tue, Oct 6, 2015 at 8:29 PM, George Beckman wrote:
> bpimm wrote on Tue, 06 October 2015 16:21
>> Yes timing is a b***h, I need to check the the 2 dimples if I can find
> them. I like the Cig lighter plug Idea.
>>
>> Compression is
>> 1:157
>> 2:165
>> 3:165
>> 4:163
>> 5:160
>> 6:165
>> 7:165
>> 8:165
>>
>> Maybe I can check the timing tomorrow, right now I have to go pick up
> all the moss I cleaned off the roof of the shop, probably 1-1.5 CY and I
>> cleaned it 2 years ago. My metal roof on the house hasn't been cleaned
> in 15 years and looks great. I think a metal roof for the shop is a must.
>
>
> Good compression! Nothing there that should cause vacuum loss.
> --
> '74 Eleganza, SE, Howell + EBL
> Best Wishes,
> George
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
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--
Bruce Hart
1976 Palm Beach
Milliken, Co
GMC=Got More Class
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Bruce Hart
1976 Palm Beach
1977 28' Kingsley
La Grange, Wyoming
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| Re: Vacuum question [message #288323 is a reply to message #288292] |
Wed, 07 October 2015 13:38 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
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George Beckman wrote on Tue, 06 October 2015 16:02
THere are two punched dots that should be lining up. If not you need a new damper.
Can these be seen with the pulleys on? I cant see anything yet and haven't been able to find a picture that shows them so I know where to look.
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: Vacuum question [message #288326 is a reply to message #288323] |
Wed, 07 October 2015 16:40 |
George Beckman
Messages: 1085
Registered: October 2008
Location: Colfax, CA
Karma: 11
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bpimm wrote on Wed, 07 October 2015 11:38George Beckman wrote on Tue, 06 October 2015 16:02
THere are two punched dots that should be lining up. If not you need a new damper.
Can these be seen with the pulleys on? I cant see anything yet and haven't been able to find a picture that shows them so I know where to look.
Brian,
I am the one who mouthed off about the punched dots and it was something Dick Paterson showed in a slide at Rapid City. I never knew they were there. I don't think you would see the inner mark with the pulley on. Maybe I drifted and was having a day dream. I certainly cannot find reference to them on the Internet.
I "think" the key in the crank is supposed to more or less line up with the timing mark on the damper. Not sure you can see the keyway either.
Guys pull #1 plug and put their finger over the hole while someone turns the engine. (Clockwise, key off, looking at the front of the crank) when it is blowing out air it is on compression stroke. I would assume that if it stopped blowing about the time the timing mark was in position the balancer is OK. Your engine has been rebuilt I think and the balancer should be good.
I guess the real way is to bust a spark plug and screw in a bolt that pokes down into the cylinder. I have never done this. THen they turn the engine this way and that noting when the piston comes against the bolt. THe timing mark should be 1/2 way between the two noted marks.
The only reason I started this part of the talk is because retarded timing can result in low vacuum. Listening to the engine on the video it does not sound like it is retarded and I assume you have checked the timing? The vacuum would go low if the idle was set up to compensate for retarded spark. The throttle plates would be open a bit reducing vacuum.
Did you get my private PM about the carburetor? gbeckman at pggp dot com
'74 Eleganza, SE, Howell + EBL
Best Wishes,
George
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| Re: Vacuum question [message #288328 is a reply to message #288278] |
Wed, 07 October 2015 16:43 |
Chris Tyler
Messages: 458
Registered: September 2013
Location: Odessa FL
Karma: 7
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Senior Member |
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I suspect your power piston is hanging up. 16" is normally enough to keep it pulled down.
It is possible to pull the airhorn without pulling the carb but I dont recommend it if you havent done it a lot
The piston gunks up at the bottom usually. Sometimes they hang up on the float baffle
Also check your fuel pressure and float settings. Too high of either would do it.
If you have access to the idle mixture screws and they arent damaged [overtightening can deform the tips] you should be able to lean it out until it barely runs
76 Glenbrook
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| Re: Vacuum question [message #288336 is a reply to message #288326] |
Wed, 07 October 2015 19:05 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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Senior Member |
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George Beckman wrote on Wed, 07 October 2015 14:40bpimm wrote on Wed, 07 October 2015 11:38George Beckman wrote on Tue, 06 October 2015 16:02
THere are two punched dots that should be lining up. If not you need a new damper.
Can these be seen with the pulleys on? I cant see anything yet and haven't been able to find a picture that shows them so I know where to look.
Brian,
I am the one who mouthed off about the punched dots and it was something Dick Paterson showed in a slide at Rapid City. I never knew they were there. I don't think you would see the inner mark with the pulley on. Maybe I drifted and was having a day dream. I certainly cannot find reference to them on the Internet.
I "think" the key in the crank is supposed to more or less line up with the timing mark on the damper. Not sure you can see the keyway either.
Guys pull #1 plug and put their finger over the hole while someone turns the engine. (Clockwise, key off, looking at the front of the crank) when it is blowing out air it is on compression stroke. I would assume that if it stopped blowing about the time the timing mark was in position the balancer is OK. Your engine has been rebuilt I think and the balancer should be good.
I guess the real way is to bust a spark plug and screw in a bolt that pokes down into the cylinder. I have never done this. THen they turn the engine this way and that noting when the piston comes against the bolt. THe timing mark should be 1/2 way between the two noted marks.
The only reason I started this part of the talk is because retarded timing can result in low vacuum. Listening to the engine on the video it does not sound like it is retarded and I assume you have checked the timing? The vacuum would go low if the idle was set up to compensate for retarded spark. The throttle plates would be open a bit reducing vacuum.
Did you get my private PM about the carburetor? gbeckman at pggp dot com
I re-timed it to 8*BTC. I agree it's running to good to be way off so I'm asuming that the balancer hasn't spun. it was at about 12* BTC before but wasn't pinging
Now idling at 600RPM the vacuum is 15-16 inches and the fuel/air mixture is 11-12. It is still pulling fuel from the primaries at idle.
I did get your message about the carb and responded, that would be a good test.
Brian & RaeDean
1973 26' #383
Washougal WA
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