| Carb / Vacuum issue [message #288553] |
Mon, 12 October 2015 10:33  |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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I'm starting a new thread to continue this discussion, was vacuum question. I was loaned a known good carb from a local GMC'er to test mine against. At idle mine was developing 16" of vacuum, dribbling fuel out of the primaries and running mostly in the 10-12 AFM area. After switching carbs idle now developing 19" of vacuum, not dribbling out of the primaries and operating in the 11-15 AFM area plus it runs a great deal better. the replacement carb is a little lean coming off idle and into the primaries under load it can bog a bit if you don't feather the throttle so it's not perfect but it is much better. it is idling at 14 to 1 and doesn't load up if it idles for an extended period.
So now we know that the carb is the problem. what should I be looking for on the tear down. History is carb was rebuilt by me 2 years ago but I had fuel problems, gunk in the tanks plugging the filter, this has been resolved by cleaning the tanks and replacing all the fuel lines, including hard lines. I had the top off of the carb a few weeks ago to check the float level and everything looked clean inside but I didn't tear it down any further.
Problems are low vacuum, fuel dribbling out of the primaries and little or no effect from the idle screws, which is probably from the excess fuel being pulled from the primaries.
I'm not a carb expert although I have rebuilt and tuned a few through the years, mainly Weber side drafts.
any and all help is appreciated.
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: Carb / Vacuum issue [message #288559 is a reply to message #288553] |
Mon, 12 October 2015 11:03  |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Was the float level correct when you took the airhorn off? (Airhorn is the top of the carb)
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: Carb / Vacuum issue [message #288561 is a reply to message #288559] |
Mon, 12 October 2015 11:23 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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Bullitthead wrote on Mon, 12 October 2015 09:03Was the float level correct when you took the airhorn off? (Airhorn is the top of the carb)
Yes it was.
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: Carb / Vacuum issue [message #288562 is a reply to message #288559] |
Mon, 12 October 2015 11:27 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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Sorry, brain fog...the quad on the boat did this once after it was rebuilt...the cam that moves the primary metering rods was not being allowed to settle all the way down because the screw that holds the rod bracket was too long. I guess there was another one nearby when he rebuilt that carb, I could not find one that got swapped. So I ground (actually used a file and vice grips, at the dock)the screw a little shorter so it allowed the rods to settle completely and problem solved.
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: Carb / Vacuum issue [message #288564 is a reply to message #288563] |
Mon, 12 October 2015 11:33 |
Bullitthead
Messages: 1411
Registered: November 2013
Karma: 5
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A tiny lock washer under that screw might accomplish the same result if that is the culprit.
Terry Kelpien
ASE Master Technician
73 Glacier 260
Smithfield, Va.
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| Re: [GMCnet] Carb / Vacuum issue [message #288583 is a reply to message #288553] |
Mon, 12 October 2015 15:36 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Brian,
Have you considered sending the carb to Dick Paterson for a rebuild? If you do make sure you contact him before you send it so he
can tell you what to check to make sure it is a good core.
http://www.paterson-gmc.com/
Regards,
Rob M.
The Pedantic Mechanic
-----Original Message-----
From: Brian Pimm
I'm starting a new thread to continue this discussion, was vacuum question. I was loaned a known good carb from a local GMC'er to
test mine against.
At idle mine was developing 16" of vacuum, dribbling fuel out of the primaries and running mostly in the 10-12 AFM area. After
switching carbs idle now developing 19" of vacuum, not dribbling out of the primaries and operating in the 11-15 AFM area plus it
runs a great deal better. the replacement carb is a little lean coming off idle and into the primaries under load it can bog a bit
if you don't feather the throttle so it's not perfect but it is much better. it is idling at 14 to 1 and doesn't load up if it idles
for an extended period.
So now we know that the carb is the problem. what should I be looking for on the tear down. History is carb was rebuilt by me 2
years ago but I had fuel problems, gunk in the tanks plugging the filter, this has been resolved by cleaning the tanks and replacing
all the fuel lines, including hard lines. I had the top off of the carb a few weeks ago to check the float level and everything
looked clean inside but I didn't tear it down any
further.
Problems are low vacuum, fuel dribbling out of the primaries and little or no effect from the idle screws, which is probably from
the excess fuel being pulled from the primaries.
I'm not a carb expert although I have rebuilt and tuned a few through the years, mainly Weber side drafts.
any and all help is appreciated.
--
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: Carb / Vacuum issue [message #288585 is a reply to message #288553] |
Mon, 12 October 2015 16:24 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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I'm betting a float/needle seat issue. 1) how do you know the spec height number you are using is correct for this application? 2) even if set correctly the needle seat can be slightly bad and cause flooding over. The primary rods properly in or stuck out in enrichment mode should not matter at base idle. Are you sure your float is not gas logged?
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Carb / Vacuum issue [message #288598 is a reply to message #288585] |
Mon, 12 October 2015 17:45 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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JohnL455 wrote on Mon, 12 October 2015 14:24I'm betting a float/needle seat issue. 1) how do you know the spec height number you are using is correct for this application? 2) even if set correctly the needle seat can be slightly bad and cause flooding over. The primary rods properly in or stuck out in enrichment mode should not matter at base idle. Are you sure your float is not gas logged?
Early on the float stuck down and flooded over big time, gas flowing out the vent tube and puddling on top of the engine, I rebuilt the carb put in new needle and seat and float and it seemed to be better no more flooding over, I think it was drawing fuel from the primaries at idle after the rebuild, I hadn't even looked at it before the great flood. After the rebuild is when I found the water in the gas tanks so I drained and flushed them, added a 12 micron water separator/fuel filter and used it for the summer last year, a few short trips. This last winter I pulled the tanks and rebuilt the whole fuel system.
I'm going to order another kit for it and see if the needle and seat may have been the culprit, I disassembled it today and they look new, nothing looks out of place or bad so far.
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: [GMCnet] Carb / Vacuum issue [message #288603 is a reply to message #288598] |
Mon, 12 October 2015 18:00 |
k2gkk
Messages: 4452
Registered: November 2009
Karma: -8
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Somebody else mentioned possible leaky float. Also the float height may be causing minor flooding!
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
~~ ~ D C "Mac" Macdonald ~ ~~
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~ ~ Since 30 November '53 ~ ~
~ ~ USAF and FAA, Retired ~ ~
~ Member GMCMI and Classics ~
~ ~ ~ Oklahoma City, OK ~ ~ ~
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~~ k2gkk + hotmail dot com ~~
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> Date: Mon, 12 Oct 2015 16:45:18 -0600
> To: gmclist@list.gmcnet.org
> From: brian@pimmlabs.com
> Subject: Re: [GMCnet] Carb / Vacuum issue
>
> JohnL455 wrote on Mon, 12 October 2015 14:24
>> I'm betting a float/needle seat issue. 1) how do you know the spec height number you are using is correct for this application? 2) even if set
>> correctly the needle seat can be slightly bad and cause flooding over. The primary rods properly in or stuck out in enrichment mode should not
>> matter at base idle. Are you sure your float is not gas logged?
>
>
> Early on the float stuck down and flooded over big time, gas flowing out the vent tube and puddling on top of the engine, I rebuilt the carb put in
> new needle and seat and float and it seemed to be better no more flooding over, I think it was drawing fuel from the primaries at idle after the
> rebuild, I hadn't even looked at it before the great flood. After the rebuild is when I found the water in the gas tanks so I drained and flushed
> them, added a 12 micron water separator/fuel filter and used it for the summer last year, a few short trips. This last winter I pulled the tanks and
> rebuilt the whole fuel system.
>
> I'm going to order another kit for it and see if the needle and seat may have been the culprit, I disassembled it today and they look new, nothing
> looks out of place or bad so far.
>
> --
> Brian & RaeDean
>
> 1973 26' #383
>
> Washougal WA
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| Re: Carb / Vacuum issue [message #288606 is a reply to message #288553] |
Mon, 12 October 2015 19:09 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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You could simply adjust it to lower the level 1/4". I think if too low you will be more likely to have tip in sag, but that's better than washing the walls with gas. Worth a try cause it costs nothing to do but about 20 mins of time.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: Carb / Vacuum issue [message #288607 is a reply to message #288553] |
Mon, 12 October 2015 19:44 |
mercdave
Messages: 45
Registered: May 2014
Karma: 2
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When I used to do a lot of mechanical work in the carburetor days, when one behaved like yours, it was usually because the idle circuit was plugged or had an air leak causing a no idle condition. This would make you open the throttle plates until it was idling on the primaries. Follow your idle passages and make sure all of the new gaskets you used have the right holes in them. I always save the old gaskets from a rebuild until assembly and use them to identify which ones from the kit I should use.
Dave L
Lynnwood, Wa.
1976 GMC 26' Edgemonte Rear Twin
1973 GMC 23' Rear-Ended, Totaled
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| Re: Carb / Vacuum issue [message #288609 is a reply to message #288553] |
Mon, 12 October 2015 20:09 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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I may have found something... The primary throttle plates in the good carb have about 1/8" holes in them near the idle screws which would pass more air than the bad carb that has solid plates. So I'm guessing the idle setting has the throttle plates open to far drawing to much air through the primary venturies. Drilling those holes may allow the idle screw to be adjusted out closing the throttle plate a bit and stopping the fuel flow out of the primaries. With it being a single hole off center it maybe it won't activate the venturi.
Maybe this is the difference between small block carbs and big block carbs?
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: [GMCnet] Carb / Vacuum issue [message #288625 is a reply to message #288595] |
Tue, 13 October 2015 10:01 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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[quote title=bpimm wrote on Mon, 12 October 2015 15:31]USAussie wrote on Mon, 12 October 2015 13:36Brian,
Dick's website says he will only work on the proper number carbs, the one I have is not one of the proper numbers for the coach, so no I haven't. between the $600 rebuild and the $300 core, I can fuel inject it for about the same or less. Plus I don't know if there are any cores available with the right numbers. At this point I'm just trying to get this carb to work right to see if I can tune it good enough. 
I need to correct what I said here, I was operating off of my memory and apparently it's not worth a s*@t... Dick doesn't list pricing on his website or state that he will not work on off number carbs, I remember reading that on the forum somewhere and I should have stated it that way, I was going by pricing I found from a vendor on dick's rebuilds and again my memory was faulty.... I'm seeing a pattern here. 
My apologies to Dick for speaking out of turn
I think I'll go get another cup of coffee and see if I can get the taste of feet out of my mouth...
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: Carb / Vacuum issue [message #288650 is a reply to message #288553] |
Tue, 13 October 2015 14:07 |
bpimm
Messages: 211
Registered: June 2013
Location: Washougal Washington
Karma: 2
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Senior Member |
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Ok some success, I drilled the holes into the throttle plates and reassembled the carb. after reaching operating temp I readjusted everything and I have no primary dribble at idle and some adjustment with the idle screws so I think this must have been a carb from a smaller engine and needed to much air for the solid throttle plates. Now I'm just rich everywhere but idle.
Is there a list of the jets and rods that came in the correct carbs? I thought I had seen one but I can't locate it now.
Thanks
Brian & RaeDean
1973 26' #383
Washougal WA
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| Re: Carb / Vacuum issue [message #288652 is a reply to message #288650] |
Tue, 13 October 2015 14:28 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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Senior Member |
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If you check the carburator list # you should be able to determine what it came off of and the size engine
Roy Keen
Minden,NV
76 X Glenbrook
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