Need Diagnostic help - Carb Problems

H

hawk ii

New member
Dec 21, 1998
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Hi Group

I had a carb problem for a while now with my '73 XPD

Here's what's happening -- and it seems to be getting
worse.

First some history --
. I've had the heads done about 1 month ago.
reason--a burnt valve-- down to 20 psi on a
compression test - but coach ran great
-good mileage - nice brown plugs -- but slight miss
at idle.
. Changed from Mallory non vacuum ignition
to a SpringField HEI from Dick Patterson
reason-- the Mallory springs were weak (advancing
way
too soon and way too far-may have been the reason I
burnt a valve.

Results
. Compression went from avg. 155 psi to 170 psi
- -(guess
they shaved the heads a wee bit.
. idle Vacuum is very nice now -19-20 (at an initial
setting of 10 degrees and no vac)
. vacuum pulls in an additional 10 degrees (even at
idle)

BUT NOW

. The engine almost stalls with any fast depression of
the gas pedal.
. If you play with it (pump it madly or just ease
slow)
the coach will run at 60 fine - but the moment you
press down on the pedal - it starts to die.

Circumvention
. If I cock the primary choke plate a little(about 1/8
travel) - everything works - I do this by putting a
wad of paper under the external choke mech.

. It has gotten worse over the past day - I did pull
the carb about 3 weeks ago - and dismantled it -
(i was going to install a set of larger Mondello
main jets - but decided not to,based on the fact
that
my plugs looked great - and we still have real gas
here in Florida.
-I did put back the carb using the old air horn
gasket . I didn't have a new one handy.
-I did re-torgue and re-sequence the air horn bolts
again and reseated/retorqued the 4 carbs bolts
-all vacuum hoses are new and intact

Questions --
. My guess is there is something wrong in the "power
Circuit" - A piston device that responsd to vac &
raises and lowers the main needle - (if I
understand the manual correctly. what do you think??
. Could my Vacuum be TOO HIGH
. Will a re-build kit help.
. do the re-build kits key on carb# ???
. I see that there are different carb # for HEI and
non HEI (have I shot myself in my foot by changing
to HEI (I don't think so -- what do you guys think)
. How much different are the car carbs (different jet
/needles ??
. any and all suggestions -- I was planning to go
to a Civil War re-Enactment this coming weekend.
so I need help as soon as possible
. Will a re-built carb from Autozone($160-210) work
if cannot fix this one?


THANKS IN ADVANCE GUYS









=====
Pete Papas-->http://members.tripod.com/~mehawk_ii/index.html
. ___________ (MY TOYS) (904)672-0571
./_][__][] []| 1973 GMC M/H 1976 Bi-Centennial Harley M/C
.*O-------OO-* Painted Desert 1979 280ZX Datsun (1 of 1000)
__________________________________________________
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Emery you wrote :

"Sounds as if the secondaries are not cutting in."

I get the problem -- with a good depression of the gas
pedal -- starting from a light -- I'm not into the
secondaries at that point -- my problem happens - even
before the secondaries become involved

Thanks

=====
Pete Papas-->http://members.tripod.com/~mehawk_ii/index.html
. ___________ (MY TOYS) (904)672-0571
./_][__][] []| 1973 GMC M/H 1976 Bi-Centennial Harley M/C
.*O-------OO-* Painted Desert 1979 280ZX Datsun (1 of 1000)
__________________________________________________
Do You Yahoo!?
Talk to your friends online with Yahoo! Messenger.
http://im.yahoo.com
 
sounds like a vacuum problem to me. You have a vacuum line connected wrong.

>

>
> press down on the pedal - it starts to die.
> >>
>
>Sounds as if the secondaries are not cutting in. When you floor it you are
>probably not getting the additional fuel that you need and therefore too lean
>to let you rev up. You indicate that you took the carb apart three weeks
>ago. I suspect that something is not put together correctly and either
>binding the mechanism or the linkage or other mechanism is somehow
>interfering with the opening of the throttle plate for the secondary.
>
>Emery Stora
>77 Kingsley
>Santa Fe, NM
>
>
 
Tom Wrote

"sounds like a vacuum problem to me. You have a vacuum
line connected wrong."

I am not using the thermostatic value so a have what I
believe is ported vacuum directly to the dist.

I had always assumed that at idle - there would be no
vacuum on the ported connector (the highest one on the
carb (facing foward and slightly to the passenger
side)

But this is not true on my coach ???? I get full vac
advance (10 degrees at 800 rpm idle) --

as a diagnostic - I removed the line to the vac
advance
- -plug it - a tried again --- still stumbled badly from
a stop.

The fact that i get 19-20 lbs at idle would lead me to
think that the other vac line (which are as the always
have been ) are in the right place - i did double
checked with the manual today.

Thanks

=====
Pete Papas-->http://members.tripod.com/~mehawk_ii/index.html
. ___________ (MY TOYS) (904)672-0571
./_][__][] []| 1973 GMC M/H 1976 Bi-Centennial Harley M/C
.*O-------OO-* Painted Desert 1979 280ZX Datsun (1 of 1000)
__________________________________________________
Do You Yahoo!?
Talk to your friends online with Yahoo! Messenger.
http://im.yahoo.com
 
Arch wrote

"... You should see fuel being squirted into the
ventures. If you cant then you have a problem with the
acceleration pump...."

Thanks - but that was the very first thing I did -
should have mentioned that in my first post

I firmly believe I will find the problem in the power
circuit -- either a stuck piston or bad/missing
spring.

=====
Pete Papas-->http://members.tripod.com/~mehawk_ii/index.html
. ___________ (MY TOYS) (904)672-0571
./_][__][] []| 1973 GMC M/H 1976 Bi-Centennial Harley M/C
.*O-------OO-* Painted Desert 1979 280ZX Datsun (1 of 1000)
__________________________________________________
Do You Yahoo!?
Talk to your friends online with Yahoo! Messenger.
http://im.yahoo.com
 
Hi Pete,

Did you check the air horn adjustment? The air valve on top of the
secondaries, they should not be able to be pushed in easily. The adjustment
is on the left side IIRC but its facing the Manifold. hope I said that
right.

Ron

- ----- Original Message -----
From: "Pete Papas"
To:
Sent: Tuesday, January 11, 2000 9:38 PM
Subject: Re: GMC: Need Diagnostic help - Carb Problems

> Arch wrote
>
> "... You should see fuel being squirted into the
> ventures. If you cant then you have a problem with the
> acceleration pump...."
>
> Thanks - but that was the very first thing I did -
> should have mentioned that in my first post
>
> I firmly believe I will find the problem in the power
> circuit -- either a stuck piston or bad/missing
> spring.
>
>
>
> =====
> Pete Papas-->http://members.tripod.com/~mehawk_ii/index.html
> . ___________ (MY TOYS) (904)672-0571
> ./_][__][] []| 1973 GMC M/H 1976 Bi-Centennial Harley M/C
> .*O-------OO-* Painted Desert 1979 280ZX Datsun (1 of 1000)
> __________________________________________________
> Do You Yahoo!?
> Talk to your friends online with Yahoo! Messenger.
> http://im.yahoo.com
 
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