- secondary's not opening - more info

J

john r. lebetski

New member
Dec 20, 2006
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When you rev it in park the secondary air valve should only slightly open visually. If it is flopping wide open the front dash pot is bad, vac hose
to it is bad or spring on air valve set too lose. The front dashpot has a less than pin hole orifice to hold back the opening time after the vacuum
to the pot drops. It should open gradually in gear under load as you floor it. Fuel is heavy compared to air an takes time to match the more rapidly
accelerating air. If air valve opens too soon you get bog. If the front dashpot and hose to it are good, you can increase spring wind up tension on
the air valve. You hold the flat blade scrre with a small driver, loosen the locking allen and slightly turn the screw for more tension, then tighten
allen lock screw.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Also, if you "Stomp" the accelerator clean to the floor at too low of an
RPM, it will bog. Feed the throttle in gradually. If that works, then the
problem is not in the Carb, but in your foot. If it still bogs, then you
have a problem with too weak of a spring on the secondaries. There are
various springs for different applications on GM vehicles. Lastly, If you
are still running the 3:07 to 1 differential ratio, a solution might be to
go to a 3:55 or 3:70 to 1 final drive ratio.
Jim Hupy
Salem, Oregon

On Fri, Jul 23, 2021 at 3:26 PM John R. Lebetski
wrote:

> When you rev it in park the secondary air valve should only slightly open
> visually. If it is flopping wide open the front dash pot is bad, vac hose
> to it is bad or spring on air valve set too lose. The front dashpot has a
> less than pin hole orifice to hold back the opening time after the vacuum
> to the pot drops. It should open gradually in gear under load as you floor
> it. Fuel is heavy compared to air an takes time to match the more rapidly
> accelerating air. If air valve opens too soon you get bog. If the front
> dashpot and hose to it are good, you can increase spring wind up tension on
> the air valve. You hold the flat blade scrre with a small driver, loosen
> the locking allen and slightly turn the screw for more tension, then tighten
> allen lock screw.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
>
 
> Also, if you "Stomp" the accelerator clean to the floor at too low of an
> RPM, it will bog. Feed the throttle in gradually. If that works, then the
> problem is not in the Carb, but in your foot. If it still bogs, then you
> have a problem with too weak of a spring on the secondaries.

I've taken a few test drives to articulate this problem.
 
Is your distributor advance working properly?
Or is it frozen in place with corrosion?

D C "Mac" Macdonald
Amateur Radio K2GKK
Since 30 November '53
USAF and FAA, Retired
Member GMCMI & Classics
Oklahoma City, OK
"The Money Pit"
TZE166V101966
'76 ex-Palm Beach
k2gkk + hotmail dot com
________________________________
From: dave silva
Sent: Sunday, July 25, 2021 06:43
To: gmclist
Subject: [GMCnet] Re: - secondary's not opening - more info

> Also, if you "Stomp" the accelerator clean to the floor at too low of an
> RPM, it will bog. Feed the throttle in gradually. If that works, then the
> problem is not in the Carb, but in your foot. If it still bogs, then you
> have a problem with too weak of a spring on the secondaries.

I've taken a few test drives to articulate this problem.
 
Maybe check for blockage in the exhaust system .

Sent from my iPhone
Nelson Wright

Orlando Fl.

>
> Is your distributor advance working properly?
> Or is it frozen in place with corrosion?
>
> D C "Mac" Macdonald
> Amateur Radio K2GKK
> Since 30 November '53
> USAF and FAA, Retired
> Member GMCMI & Classics
> Oklahoma City, OK
> "The Money Pit"
> TZE166V101966
> '76 ex-Palm Beach
> k2gkk + hotmail dot com
> ________________________________
> From: dave silva
> Sent: Sunday, July 25, 2021 06:43
> To: gmclist
> Subject: [GMCnet] Re: - secondary's not opening - more info
>

>> Also, if you "Stomp" the accelerator clean to the floor at too low of an
>> RPM, it will bog. Feed the throttle in gradually. If that works, then the
>> problem is not in the Carb, but in your foot. If it still bogs, then you
>> have a problem with too weak of a spring on the secondaries.
>
>
> I've taken a few test drives to articulate this problem.
>
> From a dead stop it takes off very smoothly with a light touch on the throttle, if i leave my foot in the same spot it will very slowly accelerate to
> 45 mph or so, but if i give it any more gas it bogs no matter how gently i feather it.
>
> This problem came on over a short period of time.
>
> The coach had been completely gone over in 2013 with a new/rebuilt carb, distributor, wiring etc. It drove up here here to NC from the COOP and ran
> fine. Then it sat until 2020.
>
> When i first took it on there was a lot of cranking and pouring gas down it's throat before it sparked. Then i got it running pretty good with
> fresh fuel.
>
> At that point it behaved like the muscle car it is. When i stomped on the throttle it roared and launched like a 10,000 lb missile.
>
> Then it started having a power loss under load. I changed several dirty fuel filters but that seemed to have little effect. It was getting harder to
> start and had less power.
>
> Then it lost spark. I diagnosed the HEI, installed new components and it ran great, it starts instantly and idles smoothly but this power problem has
> not gone away.
>
> So it appears the ignition problem masked or imitated an intermittant fuel problem but when i got the spark right the fuel problem is still there.
>
> I think i've ruled out issues related to dirty fuel, clogged filter, bad pump, etc.
>
>
>
>
>
>
>
> --
> Dave & Ellen Silva
> Hertford, NC
>
> 76 Birchaven, 1-ton and other stuff
>
> Currently planning the Great american Road Trip Summer 2021
>
> It's gonna take a lot of Adderall to get this thing right.
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
 
Recent experience(july)

Coach showed up in my neck of the woods. Guy revived a coach, fixed a few things(fuel filters, put cherry bomb mufflers on because old ones were
blown, few other things including new batteries)

Took off from Pennsylvania and drove to mn. Had many other issues but ran strong and good power.

Then while here in MN for a week, he had power issues. Seemed like fuel. Replaced fuel filters again, and they were dirty already. Full
tune up. No improvement.

We swapped on a spare carb and that fixed it.

So from what your story is, i am leaning toward carb has some issues that are now shown up from bad gas.
--
Jon Roche
75 palm beach
EBL EFI, manny headers, Micro Level, rebuilt most of coach now.
St. Cloud, MN
http://lqqkatjon.blogspot.com/
 
I think Jon is on to something.
Maybe it's a plugged fuel filter
after sitting for several months the
gas may be dirty.
--
Bill Wevers GMC49ers, GMC Western States
1975 Glenbrook - Manny Powerdrive, OneTon
455 F Block, G heads
San Jose
 
When you mention filters, you haven't specifically mentioned the one at the carb. A carb swap would likely get that when it was possibly the only
problem.
--
Douglas & Virginia Smith,
dsmithy18 at gmail,
Lincoln Nebraska,
’73 “Sequoia” since ‘95: "Wanabizo";
Quadrabag/6 wheel disks/3:70 final/Paterson QuadraJet/Thorley’s/Alloy wheels/Sundry other
 
Need more info to solve. Is this issue happening in the first 1/3 of pedal travel (carb primaries only) or only when you push past that 1/3 pedal
travel and open the secondaries. You should be able to feel the resistance of pushing against the secondary return spring so you know when you are
starting to open the secondaries for reference.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Yes, i have changed the main filter and the filter on the carb.

I recently opened the distibutor and swapped out the control board and the coil. timing advance looks good.

Also, when i rev it with a timing light on i can se it advance.

It is possible the timing is messed up.

On the first 1/3 of pedal travel it runs great and accelerates very slowly.

It bogs down when i try to go past that, no matter how gentle i am.

This problem have been slowly getting worse over the last 200 miles. (all short test drives)

--
Dave & Ellen Silva
Hertford, NC

76 Birchaven, 1-ton and other stuff

Currently planning the Great american Road Trip Summer 2021

It's gonna take a lot of Adderall to get this thing right.
 
A very wise GMCer PM'd me suggesting it might be a cracked intake manifold.

That fits perfectly with the symptoms.

While the coach had a slew of problems related to sitting for a long time; each time i fixed one (fuel filter, bad gas, etc) I would get a slight
improvement but overall the problem kept getting worse no matter what else got better.

This seems to line up perfectly with a crack that is growing with each heat cycle.

I have not had a chance to back and test for this (ether?)

--
Dave & Ellen Silva
Hertford, NC

76 Birchaven, 1-ton and other stuff

Currently planning the Great american Road Trip Summer 2021

It's gonna take a lot of Adderall to get this thing right.
 
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