Home » Public Forums » GMCnet » mechanical fuel poump question for a 78 403.
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Re: mechanical fuel poump question for a 78 403. [message #149297 is a reply to message #149296] |
Thu, 10 November 2011 12:44 |
Dennis S
Messages: 3046 Registered: November 2005
Karma: 2
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Bob,
That is a replacment fuel pump that has a return to the fuel tank as a pressure relief. While blocking it off is ok -- I suppose it could be a source of fuel leak.
Dennis
rssbob wrote on Thu, 10 November 2011 12:36 | I am replacing the valve cover gaskets because they leak and drip down onto the intake manifold YUK. In doing the passenger side I had to remove the AC compressor and mounting bracket to get the valve cover off. (WHEW!) When I looked down, I noticed there was what looks like a vacuum port blocked off on my fuel pump. Anyone know what it should be connected to? I could not find anything in the 7525 manual,Thanks.
http://www.gmcmhphotos.com/photos/showphoto.php?photo=41434&title=dsc01053&cat=3798
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Dennis S
73 Painted Desert 230
Memphis TN Metro
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Re: [GMCnet] mechanical fuel poump question for a 78 403. [message #149299 is a reply to message #149296] |
Thu, 10 November 2011 12:50 |
Ken Henderson
Messages: 8726 Registered: March 2004 Location: Americus, GA
Karma: 9
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Bob,
What you have is a fuel pump intended for a Toronado with air conditioning.
The plugged fitting is for a return line to the fuel tank. As long as you
monitor it for leaks, there's no reason to be concerned about it. Then
next time you buy a fuel pump, the NAPA numbers are one digit different for
the pump with and without that fitting -- IIRC, it's NAPA M6109 with the
return and M6108 without.
Ken H.
On Thu, Nov 10, 2011 at 1:36 PM, Bob Sobrito <rssbob@cox.net> wrote:
>
>
> I am replacing the valve cover gaskets because they leak and drip down
> onto the intake manifold YUK. In doing the passenger side I had to remove
> the AC compressor and mounting bracket to get the valve cover off. (WHEW!)
> When I looked down, I noticed there was what looks like a vacuum port
> blocked off on my fuel pump. Anyone know what it should be connected to?
> I could not find anything in the 7525 manual,Thanks.
>
>
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=41434&title=dsc01053&cat=3798
> --
>
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Ken Henderson
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www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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Re: [GMCnet] mechanical fuel poump question for a 78 403. [message #149301 is a reply to message #149297] |
Thu, 10 November 2011 14:01 |
rssbob
Messages: 259 Registered: January 2004 Location: La Mesa, Ca. (San Diego a...
Karma: 0
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Thank you Ken & Dennis.
On Nov 10, 2011, at 10:44 AM, Dennis Sexton wrote:
>
>
> Bob,
>
> That is a replacment fuel pump that has a return to the fuel tank as a pressure relief. While blocking it off is ok -- I suppose it could be a source of fuel leak.
>
> Dennis
>
> rssbob wrote on Thu, 10 November 2011 12:36
>> I am replacing the valve cover gaskets because they leak and drip down onto the intake manifold YUK. In doing the passenger side I had to remove the AC compressor and mounting bracket to get the valve cover off. (WHEW!) When I looked down, I noticed there was what looks like a vacuum port blocked off on my fuel pump. Anyone know what it should be connected to? I could not find anything in the 7525 manual,Thanks.
>>
>> http://www.gmcmhphotos.com/photos/showphoto.php?photo=41434&title=dsc01053&cat=3798
>
>
> --
> Dennis S
> 73 Painted Desert 230
> Germantown, TN
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Bob Sobrito
78 Palm Beach
La Mesa, Ca
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Re: [GMCnet] mechanical fuel poump question for a 78 403. [message #149307 is a reply to message #149301] |
Thu, 10 November 2011 17:26 |
emerystora
Messages: 4442 Registered: January 2004
Karma: 13
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Yes. That is a return line. Back to the fuel tank. Just blocking it
with a rubber cap could be a big fire hazard if the rubber
deterioates. It could be crimped and brazed or soldered but the right
fuel pump is readily available. Check the GMCMI parts booklet for
numbers.
Emery Stora
On Nov 10, 2011, at 1:01 PM, robert sobrito <rssbob@cox.net> wrote:
> Thank you Ken & Dennis.
>
>
> On Nov 10, 2011, at 10:44 AM, Dennis Sexton wrote:
>
>>
>>
>> Bob,
>>
>> That is a replacment fuel pump that has a return to the fuel tank
>> as a pressure relief. While blocking it off is ok -- I suppose it
>> could be a source of fuel leak.
>>
>> Dennis
>>
>> rssbob wrote on Thu, 10 November 2011 12:36
>>> I am replacing the valve cover gaskets because they leak and drip
>>> down onto the intake manifold YUK. In doing the passenger side I
>>> had to remove the AC compressor and mounting bracket to get the
>>> valve cover off. (WHEW!) When I looked down, I noticed there was
>>> what looks like a vacuum port blocked off on my fuel pump. Anyone
>>> know what it should be connected to? I could not find anything in
>>> the 7525 manual,Thanks.
>>>
>>> http://www.gmcmhphotos.com/photos/showphoto.php?photo=41434&title=dsc01053&cat=3798
>>
>>
>> --
>> Dennis S
>> 73 Painted Desert 230
>> Germantown, TN
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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Re: mechanical fuel poump question for a 78 403. [message #149311 is a reply to message #149296] |
Thu, 10 November 2011 18:19 |
Ken Burton
Messages: 10030 Registered: January 2004 Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Here are the good numbers.
Borg Warner 40713
AC 41566
NAPA M-6108
Carter M6108
AutoZone also has them but I do not know the number. I bought one from AZ about 6 years ago by just asking for a 75 Toro WITHOUT AC.
I have often wondered if connecting that unused fuel pump return port to a return line to the tanks might not help those GMCers in the Peoples Republic with their vapor lock problems. GM put them on AC equipped Toros for a reason.
Since you already have the pump why not run a return hose back to the front(reserve) tank. There will be next to no pressure on the return line because the open end of the line inside the tank.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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Re: mechanical fuel poump question for a 78 403. [message #149420 is a reply to message #149311] |
Sat, 12 November 2011 00:11 |
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John Heslinga
Messages: 632 Registered: February 2011 Location: Edmonton, Alberta, Canada
Karma: 4
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Bob and Others.
GM used that three port pump on many of it's V8 engines in 70s and 80s. It is not a pressure relief. The purpose is to flow a small amount of fuel back to the gas tank so that the fuel pump stays cool thus avoiding the vapour lock that many of you have. The port has quite a restriction in it so that the amount of fuel being returned to the tank is really quite small..
If you remember what vapour lock is, it will make a lot of sense. Vapour lock is where fuel gets stuck in the fuel pump while the carb is not needing any fuel.(Float valve closed) The surrounding heat from a recent heavy load or quite often when idling, starts to heat and vapourize the fuel in the pump and the incoming fuel lines. (That's why the toros had them - Idling with the Air Contitioning running in the Great Detroit and California traffic Jams) The valving in the fuel pump is not efficient with vapour and therefore it refuses to suck fuel into the pump. (Therefore not delivering the fuel to the carb when needed) Vapour in the pressure line going into the carb simply goes out the carb vents once the float valve is open. Keeping fuel going through the fuel pump at all times keeps the fuel, fuel pump, and fuel lines cool helping to avoid heat problems.
When I replaced my fuel pump, I did plumb the extra port to the tank at the fill pipe behind drivers wheel as I've seen some GMCrs with fuel injection use, and it fixed my vapour lock problems. (I do a lot of mountain driving in the Rockies) I simply ran a 1/4 inch fuel line parallel to the steel fuel line to the steel fill pipe.
If yours is blocked with a hose and plug, it will cause no problems because the port is on the pressure side of the pump valve. However: like some have already said it's just one more place to check for leaks. It could have 4 to 6 PSI. Keep your eye on it in the same way you would the rest of your fuel delivery system.
Best regards
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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Re: mechanical fuel poump question for a 78 403. [message #149422 is a reply to message #149296] |
Sat, 12 November 2011 00:37 |
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John Heslinga
Messages: 632 Registered: February 2011 Location: Edmonton, Alberta, Canada
Karma: 4
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There is another point I wanted to make about those pumps.
On GM Trucks with the fuel return pump and dual tanks, the switching valve had 6 ports in them 3 for the fuel and three for the return fuel. This was so that the fuel was returned to the same tank the pump was taking fuel from. Owners that put in a secondary fuel tank that was not factory, often would comment that the main tank would fill up with fuel when they used the secondary tank because they would not plumb in the return line and switching valve properly. The worst problem would be when they had both tanks full and then use the secondary tank first. (The main tank would overfill and spill out the fillerpipe. I've fixed a few of those in my early years.
This story would help to explain what could happen to a GMC Owner who installs the return to the filler pipe as I have. The fuel will return to the front tank only and when just using the rear tank, It will simply empty pretty fast while the front tank fills with the return fuel. This will be no concern while both tanks are above 1/3, ( They share the fill height for the top 2/3s, but the situation might suprise the unexpecting owner
Best regards
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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Re: mechanical fuel poump question for a 78 403. [message #149424 is a reply to message #149422] |
Sat, 12 November 2011 01:12 |
Ken Burton
Messages: 10030 Registered: January 2004 Location: Hebron, Indiana
Karma: 10
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Senior Member |
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John Heslinga wrote on Sat, 12 November 2011 00:37 | There is another point I wanted to make about those pumps.
On GM Trucks with the fuel return pump and dual tanks, the switching valve had 6 ports in them 3 for the fuel and three for the return fuel. This was so that the fuel was returned to the same tank the pump was taking fuel from. Owners that put in a secondary fuel tank that was not factory, often would comment that the main tank would fill up with fuel when they used the secondary tank because they would not plumb in the return line and switching valve properly. The worst problem would be when they had both tanks full and then use the secondary tank first. (The main tank would overfill and spill out the fillerpipe. I've fixed a few of those in my early years.
This story would help to explain what could happen to a GMC Owner who installs the return to the filler pipe as I have. The fuel will return to the front tank only and when just using the rear tank, It will simply empty pretty fast while the front tank fills with the return fuel. This will be no concern while both tanks are above 1/3, ( They share the fill height for the top 2/3s, but the situation might suprise the unexpecting owner
Best regards
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John,
My thought on this is the fuel should always be returned to the front tank. (I really do not know how much fuel we are talking about in the return line.) Since the two tanks are connected together at the filler, the fuel level in the two tanks will equalize any way no matter where you insert it. It is only after the tanks are way down that it makes any difference. The last 1/3 or so of the fuel in the tanks does not automatically transfer between them. Since you normally run on the rear tank, all of the fuel removed from the rear and returned will be in the front. When there is no excess flowing from front to the rear, the rear tank will eventually run dry. This still leaves you a reserve of around 7 gallons in the front when you push the reserve switch on the dash. This arrangement will give you maximum use of the total fuel available in both tanks.
Note: The rear tank is the main tank and the front is the reserve tank.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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Re: mechanical fuel poump question for a 78 403. [message #149448 is a reply to message #149296] |
Sat, 12 November 2011 10:53 |
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John Heslinga
Messages: 632 Registered: February 2011 Location: Edmonton, Alberta, Canada
Karma: 4
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Senior Member |
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Ken:
Yes! While I was writing, I was picturing the rear tank as auxiliary. I have never encountered a problem with my set up, because like many others, I seldom let my fuel levels go below 1/3. Any tank with more than 1/3 will spill over to the next. With the front being auxiliary, nobody will notice a thing.
The actual amount of fuel is really just a trickle however it is constant and does not change with load, speed, or rpm.
Best regards
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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Re: [GMCnet] mechanical fuel poump question for a 78 403. [message #149449 is a reply to message #149420] |
Sat, 12 November 2011 10:51 |
rssbob
Messages: 259 Registered: January 2004 Location: La Mesa, Ca. (San Diego a...
Karma: 0
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Senior Member |
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Thanks John and Ken for further insight on this seemingly extra fitting. I would like to utilize this fitting by connecting it to one of tanks to prevent vapor lock. Does anyone have a photo to show where one would connect to the front tank line?
Thanks again.
Bob
On Nov 11, 2011, at 10:11 PM, John Heslinga wrote:
>
>
> Bob and Others.
>
> GM used that three port pump on many of it's V8 engines in 70s and 80s. It is not a pressure relief. The purpose is to flow a small amount of fuel back to the gas tank so that the fuel pump stays cool thus avoiding the vapour lock that many of you have. The port has quite a restriction in it so that the amount of fuel being returned to the tank is really quite small..
>
> If you remember what vapour lock is, it will make a lot of sense. Vapour lock is where fuel gets stuck in the fuel pump while the carb is not needing any fuel.(Float valve closed) The surrounding heat from a recent heavy load or quite often when idling, starts to heat and vapourize the fuel in the pump and the incoming fuel lines. (That's why the toros had them - Idling with the Air Contitioning running in the Great Detroit and California traffic Jams) The valving in the fuel pump is not efficient with vapour and therefore it refuses to suck fuel into the pump. (Therefore not delivering the fuel to the carb when needed) Vapour in the pressure line going into the carb simply goes out the carb vents once the float valve is open. Keeping fuel going through the fuel pump at all times keeps the fuel, fuel pump, and fuel lines cool helping to avoid heat problems.
>
> When I replaced my fuel pump, I did plumb the extra port to the tank at the fill pipe behind drivers wheel as I've seen some GMCrs with fuel injection use, and it fixed my vapour lock problems. (I do a lot of mountain driving in the Rockies) I simply ran a 1/4 inch fuel line parallel to the steel fuel line to the steel fill pipe.
>
> If yours is blocked with a hose and plug, it will cause no problems because the port is on the pressure side of the pump valve. However: like some have already said it's just one more place to check for leaks. It could have 4 to 6 PSI. Keep your eye on it in the same way you would the rest of your fuel delivery system.
>
> Best regards
> --
> John and Cathie Heslinga
> 1974 Canyonlands 260
> TC4W "Too Cool For Words"
> Retirement Projects Galore
> Edmonton, Alberta
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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Bob Sobrito
78 Palm Beach
La Mesa, Ca
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Re: mechanical fuel pump question for a 78 403. [message #149531 is a reply to message #149296] |
Sat, 12 November 2011 23:45 |
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John Heslinga
Messages: 632 Registered: February 2011 Location: Edmonton, Alberta, Canada
Karma: 4
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Senior Member |
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Bob:
Here are a couple of photos of my return line connection to the fill tank. It is not unique as I had seen the same type of photos from the Fuel injection Guys. This fuel line will not pump anywhere near the amount of fuel as a FI System because the FI System uses fuel return as part of pressure regulation. The small amount of fuel coming from the Carb Pump really has no pressure when it gets here.
http://www.gmcmhphotos.com/photos/showphoto.php?photo=41459&nocache=1
Best regards
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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Re: mechanical fuel poump question for a 78 403. [message #149532 is a reply to message #149448] |
Sun, 13 November 2011 00:38 |
Ken Burton
Messages: 10030 Registered: January 2004 Location: Hebron, Indiana
Karma: 10
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Senior Member |
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Your are probably correct. I have no idea how much return fuel we are talking about. If it is a minor amount then it really not worth the effort to get fuel to get all of the fuel to return to the front (reserve tank).
I never run low enough for it to make a difference.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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Re: [GMCnet] mechanical fuel pump question for a 78 403. [message #149534 is a reply to message #149531] |
Sun, 13 November 2011 01:27 |
rssbob
Messages: 259 Registered: January 2004 Location: La Mesa, Ca. (San Diego a...
Karma: 0
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Thanks John,
Is it the smaller line with the hose clamp on the end? If so, how did you penetrate the metal fill tube and how does the rubber line connect to it?
Thanks
On Nov 12, 2011, at 9:45 PM, John Heslinga wrote:
>
>
> Bob:
> Here are a couple of photos of my return line connection to the fill tank. It is not unique as I had seen the same type of photos from the Fuel injection Guys. This fuel line will not pump anywhere near the amount of fuel as a FI System because the FI System uses fuel return as part of pressure regulation. The small amount of fuel coming from the Carb Pump really has no pressure when it gets here.
>
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=41459&nocache=1
>
> Best regards
> --
> John and Cathie Heslinga
> 1974 Canyonlands 260
> TC4W "Too Cool For Words"
> Retirement Projects Galore
> Edmonton, Alberta
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
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Bob Sobrito
78 Palm Beach
La Mesa, Ca
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Re: mechanical fuel poump question for a 78 403. [message #149544 is a reply to message #149420] |
Sun, 13 November 2011 09:17 |
Carl S.
Messages: 4186 Registered: January 2009 Location: Tucson, AZ.
Karma: 13
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Senior Member |
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John Heslinga wrote on Fri, 11 November 2011 23:11 | Bob and Others.
GM used that three port pump on many of it's V8 engines in 70s and 80s. It is not a pressure relief. The purpose is to flow a small amount of fuel back to the gas tank so that the fuel pump stays cool thus avoiding the vapour lock that many of you have. The port has quite a restriction in it so that the amount of fuel being returned to the tank is really quite small..
If you remember what vapour lock is, it will make a lot of sense. Vapour lock is where fuel gets stuck in the fuel pump while the carb is not needing any fuel.(Float valve closed) The surrounding heat from a recent heavy load or quite often when idling, starts to heat and vapourize the fuel in the pump and the incoming fuel lines. (That's why the toros had them - Idling with the Air Contitioning running in the Great Detroit and California traffic Jams) The valving in the fuel pump is not efficient with vapour and therefore it refuses to suck fuel into the pump. (Therefore not delivering the fuel to the carb when needed) Vapour in the pressure line going into the carb simply goes out the carb vents once the float valve is open. Keeping fuel going through the fuel pump at all times keeps the fuel, fuel pump, and fuel lines cool helping to avoid heat problems.
When I replaced my fuel pump, I did plumb the extra port to the tank at the fill pipe behind drivers wheel as I've seen some GMCrs with fuel injection use, and it fixed my vapour lock problems. (I do a lot of mountain driving in the Rockies) I simply ran a 1/4 inch fuel line parallel to the steel fuel line to the steel fill pipe.
If yours is blocked with a hose and plug, it will cause no problems because the port is on the pressure side of the pump valve. However: like some have already said it's just one more place to check for leaks. It could have 4 to 6 PSI. Keep your eye on it in the same way you would the rest of your fuel delivery system.
Best regards
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John,
Just to reinforce your position on this, I had a serious vapor lock problem on my coach when I first got it. It was so bad on the first drive home that I was wondering what I had gotten myself into. I was discussing this problem with one of our resident "experts", Dave Lensi, and he suggested doing exactly what you did. He was too busy, at the time, to look up the part number, and I never called him back to get the info.
I have taken a number of steps to correct this problem (heat dissipator under the carb, new tank valve, shielded line from pump to carb, electric pusher pump back at the tanks, painted the bottom of the tanks white, etc, etc.) But have not done the fuel pump return (yet). I still have vapor lock issues when it is hot, which it frequently is in Southern Arizona (where all the gas has "up to 10%" ethanol in it). My next steps to fix this will be to install crossover block-offs or a Rockwell manifold, and the bypass fuel pump.
Thank you for sharing the photos of how you tapped the fill pipe for the return fitting. That has removed the last obstical to doing this, for me.
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles, Solar battery charging. GMCSJ and GMCMI member
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Re: mechanical fuel poump question for a 78 403. [message #149603 is a reply to message #149296] |
Mon, 14 November 2011 00:19 |
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John Heslinga
Messages: 632 Registered: February 2011 Location: Edmonton, Alberta, Canada
Karma: 4
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Senior Member |
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Carl:
Glad that helped. If we understand that liquids under lower pressures (Suction cycle and side of fuel pump or the Air conditioning expansion valve and evaporator) boil at lower temperatures, and that liquids under higher pressures boil at higher temperatures (Engine cooling systems at 10 PSI) we can understand that the vapor lock problem is all about low pressure side of the fuel system. That's the area you need to focus on.
Here is another sample of return line connection to the fill tank that may be useful to you.
It looks like he used an epoxy metal product to help seal the 90 degree fitting
http://www.gmcmhphotos.com/photos/showphoto.php?photo=27749&title=return-line&cat=3
Best Regards
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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Re: mechanical fuel poump question for a 78 403. [message #149761 is a reply to message #149758] |
Wed, 16 November 2011 02:26 |
Ken Burton
Messages: 10030 Registered: January 2004 Location: Hebron, Indiana
Karma: 10
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Senior Member |
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I think the real way to figure out how much fuel actually moves through that line is to stick a hose on that port and run it to a gas can. Then start the engine and see how much flow you have.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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