Here's one for the savvy....

[terry1]

I just replaced my supply and vent lines, and didn't need to do the section forward of the torsion bar cross member. Used over 25 feet of 3/8 hose and
25 feet of 5/16 for vent lines (not including the vent to the filler neck, that's 3/8). I did not replace some of the vent line going to the canister
because it was already done and passed all the tests I could throw at it. Seems like the dividing line between good and bad hose was the T-Bar cross
member. Reworked the fuel pump and added filter and fixed several areas of dislike and previous owners' modifications so my measurements will not be
the same as anybody else on the planet. Also, I could not use the last 4 feet on the reel of 3/8 hose because it was collapsed from being wound so
tight. Would have had to get another reel (25 ft)if I didn't already have some extra pieces.
Jim Bounds says there is 56 feet of fuel line to be replaced down there...

When I saw the leak at the vent port, I immediately went under there to check the body mount pads and look for evidence of missing tank pads on the
floor. No missing tank pads, and the body mount pads still had a 1/2>>5/8 inch gap to the frame, just the size of the vent line hose going to the
vapor separator. Same thickness everywhere.

This is a poor design with no retention straps on top of the tank. How did that ever get by SAE? I could winch those tanks right up against and even
raise the floor with them if I didn't pay attention carefully on installation. A floating/hanging fuel tank does not pass muster for me(unless you
plan to eject it sometime in flight), so I used pads glued to the tank to hold it FIRMLY in place. Still not the preferred retention from the top
down, but the best I came up with without welding and drilling holes in the frame.

This was all to no avail, as I still have the random loss of power that feels like running out of fuel...
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.

 

[6cuda61]

Thanks for the info and lengths.

When it dies what happens when you check the accelerator pump function? Do you have fuel out the nozzels?

> I just replaced my supply and vent lines, and didn't need to do the section forward of the torsion bar cross member. Used over 25 feet of 3/8 hose
> and 25 feet of 5/16 for vent lines (not including the vent to the filler neck, that's 3/8). I did not replace some of the vent line going to the
> canister because it was already done and passed all the tests I could throw at it. Seems like the dividing line between good and bad hose was the
> T-Bar cross member. Reworked the fuel pump and added filter and fixed several areas of dislike and previous owners' modifications so my measurements
> will not be the same as anybody else on the planet. Also, I could not use the last 4 feet on the reel of 3/8 hose because it was collapsed from
> being wound so tight. Would have had to get another reel (25 ft)if I didn't already have some extra pieces.
> Jim Bounds says there is 56 feet of fuel line to be replaced down there...
>
> When I saw the leak at the vent port, I immediately went under there to check the body mount pads and look for evidence of missing tank pads on
> the floor. No missing tank pads, and the body mount pads still had a 1/2>>5/8 inch gap to the frame, just the size of the vent line hose going to
> the vapor separator. Same thickness everywhere.
>
> This is a poor design with no retention straps on top of the tank. How did that ever get by SAE? I could winch those tanks right up against and
> even raise the floor with them if I didn't pay attention carefully on installation. A floating/hanging fuel tank does not pass muster for me(unless
> you plan to eject it sometime in flight), so I used pads glued to the tank to hold it FIRMLY in place. Still not the preferred retention from the
> top down, but the best I came up with without welding and drilling holes in the frame.
>
> This was all to no avail, as I still have the random loss of power that feels like running out of fuel...

--
Rich Mondor,

Brockville, ON

77 Hughes 2600

 

[terry1]

It never dies, just won't get out of it's own way once in awhile. If I pull of to the side to check it out, it idles fine and the accelerator pump and
main metering circuit work just fine ( I mean it squirts AND will run high RPMs longer than just a few seconds on the side of the road when this
happens). Except for that, it feels like the carbs main fuel circuit runs out of fuel when climbing a mountain or sometimes just leaving a stoplight.
At one time, I figured it must be the Mallory ignition amp breaking down when hot, but disproved that in the winter.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.

 

[6cuda61]

Do you have a "battery voltage/amp gauge" on your dash or the stock dash? Have you checked charge current or your isolator recently? The reason i ask
is the same symptoms, happened to me on my way home from picking this coach up last year from the east coast. You'd be driving along and all of a
sudden it felt like you were running out of fuel even though i knew it wasnt.

Next time your out with the coach and that starts to happen switch the battery boost switch into the "boost" position momentarily and see if it cleans
it up...if so replace your isolator. You can test your isolator with a DVM as well instead if you would rather do that.....mine had a bad diode and
caused the mentioned condition and if left long enough when driving the coach would either die completely or overcharge the battery that it was
feeding all the current too.

Something to try as it seems you have gone down the fuel road already.....

> It never dies, just won't get out of it's own way once in awhile. If I pull of to the side to check it out, it idles fine and the accelerator pump
> and main metering circuit work just fine ( I mean it squirts AND will run high RPMs longer than just a few seconds on the side of the road when this
> happens). Except for that, it feels like the carbs main fuel circuit runs out of fuel when climbing a mountain or sometimes just leaving a
> stoplight. At one time, I figured it must be the Mallory ignition amp breaking down when hot, but disproved that in the winter.

--
Rich Mondor,

Brockville, ON

77 Hughes 2600

 

[Ken H.]

Excellent suggestion! Why? To understand what apparently happened, one
must understand the construction of the isolator: It's basically a piece
of aluminum with two metal-cased silicon diodes' cathodes (the "arrow" in a
diode symbol) welded to the piece of aluminum. There's a single stud
sticking up from the aluminum base plate and one on the anode (the "flat"
part of a diode symbol) of each diode -- the Alternator terminal, and the
two Battery terminals, respectively. Now that assembly is "potted" into
the finned aluminum heat sink/case with a thermally (but not electrically)
conductive plastic compound.

With the alternator connected to the two cathodes, when its output voltage
exceeds that of the connected batteries (by about 0.7 VDC), it sends
current to them. Conversely, when the diodes' anodes (connected to the
batteries) are more positive than the alternator terminal, no current
flows, therefore the two batteries are "isolated" from each other.

The most common failure mode for such an isolator, in my experience, is
that the continual thermal expansion and contraction of the potting
compound eventually breaks one of the diodes' weld to the base plate.
Since the chassis battery generally cycles more frequently through higher
current loads (from the starter depleting the battery) on the alternator,
that diode heats, cools, and therefore fails most frequently. When the
diode-base connection to the chassis battery is open, the alternator no
longer charges the chassis battery, but continues to monitor its voltage
and increases its output in attempting to do so. Meanwhile, the house
battery, with its diode still conducting, is absorbing that 18-25 VDC
maximum output from the alternator! By the time the chassis battery
voltage is too low to satisfactorily supply the ignition system, the house
battery may have been seriously damaged -- perhaps taking out a lot of
other electrical stuff too!

To complicate the troubleshooting, the chassis battery diode may only need
to cool down a couple of degrees to again make contact through the broken
weld. Intermittent connections like that can be extremely difficult to
identify.

Ken H.

On Sat, May 2, 2020 at 8:28 AM 6cuda6--- via Gmclist <

> Do you have a "battery voltage/amp gauge" on your dash or the stock dash?
> Have you checked charge current or your isolator recently? The reason i ask
> is the same symptoms, happened to me on my way home from picking this
> coach up last year from the east coast. You'd be driving along and all of a
> sudden it felt like you were running out of fuel even though i knew it
> wasnt.
>
> Next time your out with the coach and that starts to happen switch the
> battery boost switch into the "boost" position momentarily and see if it
> cleans
> it up...if so replace your isolator. You can test your isolator with a DVM
> as well instead if you would rather do that.....mine had a bad diode and
> caused the mentioned condition and if left long enough when driving the
> coach would either die completely or overcharge the battery that it was
> feeding all the current too.
>
> Something to try as it seems you have gone down the fuel road already.....
>
>
>

> > It never dies, just won't get out of it's own way once in awhile. If I
> pull of to the side to check it out, it idles fine and the accelerator pump
> > and main metering circuit work just fine ( I mean it squirts AND will
> run high RPMs longer than just a few seconds on the side of the road when
> this
> > happens). Except for that, it feels like the carbs main fuel circuit
> runs out of fuel when climbing a mountain or sometimes just leaving a
> > stoplight. At one time, I figured it must be the Mallory ignition amp
> breaking down when hot, but disproved that in the winter.
>
>
> --
> Rich Mondor,
>
> Brockville, ON
>
> 77 Hughes 2600
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[Matt Colie]

> It never dies, just won't get out of it's own way once in awhile. If I pull of to the side to check it out, it idles fine and the accelerator pump
> and main metering circuit work just fine ( I mean it squirts AND will run high RPMs longer than just a few seconds on the side of the road when this
> happens). Except for that, it feels like the carbs main fuel circuit runs out of fuel when climbing a mountain or sometimes just leaving a
> stoplight. At one time, I figured it must be the Mallory ignition amp breaking down when hot, but disproved that in the winter.

Terry,

I have a similar issue in hot places and finally tracked it down to the metal fuel line at the front cross member. That is the hottest place and the
lowest pressure in the system. I thought about verifying that with the IR gun, but since my hand said it was too hot, I just believed it. Crumpled
aluminum foil got us going reliably as a jury rig, and I keep looking for a simple and reliable solution. While the aluminum foil missed the last two
reassemblies, I am still considering:
Using the wrong fuel pump and sending the bleed back to the fill neck.
Or
Doing two low pressure (we are still carburetor) back at the tanks.

I have the wrong fuel pump on the shelf because that was all I could get one day.

We seldom go places that hot, and for sure won't this year. If Herr Governor ever releases us, there won't be much season left.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[terry1]

Thanks for that info Matt, maybe today will be a good day for a GMC ride and I'll check that out with the pyrometer at my destination stop and
definitely right after a power loss if it happens. Pyrometer is now standard GMC and trailering tool for the toolboxes. Cheap enough now to warrant
multiple units and also allows you to cross check them occasionally.
--
Terry Kelpien

ASE Master Technician

73 Glacier 260

Smithfield, Va.