Home » Public Forums » GMCnet » What's special about the GMC carb? (versus the Revcon's modified Toronado carb)
| What's special about the GMC carb? [message #64501] |
Mon, 16 November 2009 14:03  |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
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Subject says it all- apparently the Revcon uses a modified toronado carb. I don't understand carbs well enough to guess what this modificatiosn might be.
What did GMC do different? Is the MH carb totally unique?
Bias aside, would I be a lot happier if i put a GMC carb on my Revcon? (72 Revcon flatnose with Olds 455)
Thanks
Dave
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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| Re: [GMCnet] What's special about the GMC carb? [message #64509 is a reply to message #64501] |
Mon, 16 November 2009 14:41  |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
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Dave,
Here's a copy of a post Emery Stora sent in awhile back.
GM used seven different carbs for the GMC motorhome.
73 and 74 years had a carb #7043254
75 and 76 and early 77 had 7045254 except for California emission carbs
which were 7045554 77 coaches with the 403 engine had 17057254 and
California 17057559 78 had 17058254 and California 1758559
These carb numbers can be found on the left side of the carb, stamped
on the flat surface close to the secondary throttle shaft.
There is also a date code (4 digits) stamped in the same area.
73 to 76 coaches had a number starting with 70
77 & 78 carbs start with 170.
Some parts for these carbs are quite different on the 170 from the
70. DO NOT INTERCHANGE parts between these two series.
One example is the primary metering rods. They are .080" shorter in the 170
series, they had a different taper to the rods and these carbs used
different sizes of primary jets. The rods for the 70 series carbs will be
stamped D-50. (7046338) I am told that this was used ONLY in the GMC
motorhome carbs. The 170 series are different. All GMC carbs used the same
secondary metering rods. These will be stamped with a CJ. The primary jets
for the GMC at your elevation should be .070". (7031970)
The proper power spring for the GMC is #7036019 which has an operating range
from 4" to 8" of vacuum.
One problem that some people have had is the use of gaskets from an unknown
kit. There are changes in the air passages in the carbs and it is easy to
install a gasket that covers an air passage hole and that will make the carb
perform improperly.
The Delco overhaul kit for the 73-74 is 7046812. For the 75-76 (non Calif)
it is 17051886
Emery Stora
77 Kingsley
Santa Fe, NM
Here's a link to some more info on Gene Fishers website:
http://gmcmotorhome.info/engine.html#carb
Don't know how we could tell you if you would be happier with a carb off a
GMC on your Revcon as we have no frame of reference as to how your Revcon
runs.
Since you've asked this question I assume the carb off the 1974 GMC you're
parting out is gone?
Regards,
Rob Mueller
Sydney, Australia
AUS '75 Avion-The Blue Streak TZE365V100428
USA '75 Avion-Double Trouble TZE365V100426
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of dave silva
Sent: Tuesday, 17 November 2009 7:03 AM
To: gmclist@temp.gmcnet.org
Subject: [GMCnet] What's special about the GMC carb?
Subject says it all- apparently the Revcon uses a modified toronado carb. I
don't understand carbs well enough to guess what this modification might be.
What did GMC do different? Is the MH carb totally unique?
Bias aside, would I be a lot happier if i put a GMC carb on my Revcon? (72
Revcon flatnose with Olds 455)
Thanks
Dave
--
1972 Revcon 250- the other front wheel drive motorhome.
http://www.davesilva.com/revcon
Parting out 1974 GMC 26- http://www.davesilva.com/gmc
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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: [GMCnet] What's special about the GMC carb? [message #64512 is a reply to message #64509] |
Mon, 16 November 2009 15:01 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
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| Robert Mueller wrote on Mon, 16 November 2009 14:41 |
Don't know how we could tell you if you would be happier with a carb off a
GMC on your Revcon as we have no frame of reference as to how your Revcon
runs.
Since you've asked this question I assume the carb off the 1974 GMC you're
parting out is gone?
|
I know the question was bit vague. Near as I can tell both engines run pretty well. I was happy with the Revcons performance on the 200 mile drive home. But I know the Toro carb was modified to move a 9000 lb coach and for the GMC they started from scratch. I assume that would yield better results.
I'm keeping the Revcon until it stops being fun but the more GMC supportable parts i have on it the better.
dave
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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| Re: [GMCnet] What's special about the GMC carb? [message #64520 is a reply to message #64501] |
Mon, 16 November 2009 15:45 |
tmaki
Messages: 200
Registered: September 2005
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dave silva wrote:
>
> Subject says it all
I would pay money for the answer to that question, with
exact and specific differences re: the nine parts that are
not common to the 7043250 (used on 350 Cutlass, Omega,
etc.), 7043252 (used on Toronado), 7043254 (used on GMCMH),
and 7043259 (marine/commercial applications).
Those nine parts are (applies to the above numbered
carburetors only, although numbers outside these may use the
same parts):
Air horn assembly
Secondary metering rod (easily changed out to motorhome spec)
Choke thermostat cover gasket
Choke housing to bowl seal
Primary jet (easily changed out to motorhome spec)
Power piston assembly (easily changed out to motorhome spec)
Power piston spring (easily changed out to motorhome spec)
Primary metering rod (easily changed out to motorhome spec)
Throttle body assembly
The overhaul kit, the power carburetor kit and the gasket
set is the same for all those carburetors.
I have not been able to get a clear answer to what
specifically is different about the air horn assembly, and
the throttle body assembly. From the view of the unaided
eye, there appears to be no significant differences even
where there are observable differences. If there are
differences measured in tens of thousandths of an inch,
those are not readily observable. There may be significant
differences, but I haven't been able to find them.
Although it may be heretical to say so, I think there is a
lot of mythology involved in the mystery. Unless someone can
provide exact and specific engineering data from an
authoritative source (GM/Delco/Rochester comes to mind) to
support the myth of the differences that can't be easily
changed (rods, jets, etc.), I'll remain unconvinced that a
well-built and maintained 3250 or 3252 with changed jets,
rods, and power piston/spring can't and won't perform
acceptably well in the motorhome.
When I re-did my carburetor last fall, I did all the
research, mixed and matched and modded. Mine is made from a
3252 bowl assembly, 3250 throttle body assembly, 3250 air
horn assembly, primary jets and PP spring from Edelbrock,
primary rods from my collection, PP from a junkyard model I
can't remember, and a rebuild kit from The Carburetor Doctor
in Stony Plain, Alberta, Canada.
I'm able to go up hills with the rest of the traffic, the
engine starts easily, idles smoothly, doesn't diesel, and I
can go faster than my guardian angel can fly.
> Is the MH carb totally unique?
Totally? Nope. As far as I can tell, it's just a matter of a
few parts.
>
> Bias aside, would I be a lot happier if i put a GMC carb on my Revcon?
A LOT happier? What would it take to make you a lot happier?
How unhappy are you now, and why?
For some folks (Larry, for one) just getting the engine to
run even near to right is ecstasy. For others, throwing wads
of cash is the path to happiness. Find a decent 3252 or
3250, rebuild it exceptionally well with the
motorhome-specific small parts, and try it. You won't be out
much money. It would be sad to think that bliss is only a
soak in carburetor cleaner and a couple of hours at a work
bench away.
All the standard disclaimers apply.
Toby Maki
'73 Glacier 230
Riverside, CA
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| Re: [GMCnet] What's special about the GMC carb? [message #64521 is a reply to message #64512] |
Mon, 16 November 2009 15:57 |
scott cowden
Messages: 170
Registered: February 2004
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Dave;
Funny you should ask that question, as I was talking to Dick Paterson about this today. I'm lucky in that Dick lives about 5 miles from me.
My carb works well, but the casting is starting to sag after 35 years of being baked by heat and overtorqued by enthusiastic mechanics, shade tree and otherwise. I've been considering trying one of those Edelbrock QJet clones. I think they're about to or have gone out of production now and figured that if I was going to try this, now would be the time and take the jets and metering rods ans such from my old carb and put it in the new casting.
We were talking about carb stuff today and I asked him about adapting another Olds carb or QJet to work on a motor home. Dick said that he had tried on 2 or 3 occasions to make one work and had not been able to get it set right.
He wondered if some of the passageways and things that can't be seen are different, but had not been able to come up with a definitive answer as to why he'd not been successful at modifying one to work.
I suppose that, in your case, if you could find an original carb in good shape and try it, you'd have your answer.
Scott '74 Glacier Orillia, ON, Canada
> To: gmclist@temp.gmcnet.org
> From: gmc@davesilva.com
> Date: Mon, 16 Nov 2009 15:01:48 -0600
> Subject: Re: [GMCnet] What's special about the GMC carb?
>
>
>
> Robert Mueller wrote on Mon, 16 November 2009 14:41
> > Don't know how we could tell you if you would be happier with a carb off a
> > GMC on your Revcon as we have no frame of reference as to how your Revcon
> > runs.
> >
> > Since you've asked this question I assume the carb off the 1974 GMC you're
> > parting out is gone?
>
>
> I know the question was bit vague. Near as I can tell both engines run pretty well. I was happy with the Revcons performance on the 200 mile drive home. But I know the Toro carb was modified to move a 9000 lb coach and for the GMC they started from scratch. I assume that would yield better results.
>
> I'm keeping the Revcon until it stops being fun but the more GMC supportable parts i have on it the better.
>
>
> dave
>
>
>
>
> --
> 1972 Revcon 250- the other front wheel drive motorhome. http://www.davesilva.com/revcon
> Parting out 1974 GMC 26- http://www.davesilva.com/gmc
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| Re: [GMCnet] What's special about the GMC carb? [message #64531 is a reply to message #64520] |
Mon, 16 November 2009 16:33 |
klassic kampers
Messages: 93
Registered: July 2008
Location: greer,s.c./ellijay,ga
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Toby, I'm with you on the GMC carburetor not being a nessessity for good motorhome performance and economy.......I do Q-JETS for a lot of local GM trucks and have been able(lucky?)to calibrate any Q-JET to any GM truck.........my GMC has a 1978 trans am carb on it and starts great hot or cold, idles great hot or cold, runs as fast as your nerves will allow and gets decent economy(9-10) pulling the miata...........mostly it's understanding how the internal circuits inside a carburetor work and modifying them to the application....also ( in my opinion) no two GMC motor home applications are exactly the same due to camshaft opening and closing numbers being subject to wear, compression differences, exhaust system efficiency , coolant temp , ambient air temp, humidity of the coach's home region, average barometric pressure and coach weight......I do not feel a carburetor can be 100% or near without adjusting for these variances........on a recent trip to a local car show an fellow GMCer followed me to our favorite eating establishment and commented how I made him run his coach hard having to get into his secondary circuit.......I was only using about 1/4 throttle on my GMC and do not know if his GMC was well tuned or not..........this is not meant to influence anyone to modify their motorhome from stock in any way or offend any of our good parts(carburetor) suppliers... it's just one person's experiences and opinions and should be taken as such.....I'm not a "know it all" and are just here to offer helpful(hopefully) info and please delete if offensive............
Mike Stewart
1973 GMC 26' Canyonlands /
1973 B.S.A. B50 street tracker-----
Greer,S.C/Ellijay,Ga
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| Re: [GMCnet] What's special about the GMC carb? [message #64561 is a reply to message #64546] |
Mon, 16 November 2009 19:18 |
tmaki
Messages: 200
Registered: September 2005
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Matt Colie wrote:
>
> <Matt's good commentary snipped...>
(I'm encompassing Mike's commentary, too.)
All of the discussion on carburetors--at least as far as I'm
concerned--brings us around to the root issue. I believe
that issue has several components:
1. These coaches are over 30 years old.
2. They use components that are difficult, if not impossible
to find.
3. Many of the coaches have been changed in ways unknown to
the present owner.
4. Not everybody can, either because of financial or
regulatory constraints, convert to fuel injection.
5. There is a very limited population of recyclable parts
that is dwindling everyday.
6. No one is manufacturing the exact application specific
replacement for the motorhome carburetor.
7. The likelihood of finding someone to work on them is
small, especially when that someone is informed that the
"motorhome carburetor is special".
...and probably a few more.
My thinking is that given these things, and the hue and cry
that goes up "from the people" that these coaches need to be
maintained and kept alive into perpetuity, why not discover
a workaround?
If more common and readily available components can be
pressed into acceptable service, why not get as much
specific information and experience as to what CAN be done
with COTS parts, instead of crying, moaning and wringing
hands about how "special" parts can't be found and "Oh, what
am I going to do now"?
It has been proven by several that the OEM GMC carburetor,
albeit ideal, is not an "11th Commandment" kind of thing. It
is not required to get a GMC with a 455 engine running and
running well.
We are made aware of stories everyday of GMCs that won't
start, don't run right, can't get out of their own way, or
scores of other performance complaints. Without thorough
troubleshooting with eyes, hands and ears there is no way to
know specifics. But when enough information is passed around
for opinion, and the opinion mill stops turning, out pops
the little slip of paper that reads "Yep, you need to get
the right carburetor on that thing or she'll never run
right. Yep...umm-hmmm...yep." In the words of Colonel
Potter, "Horse hockey!"
Here's the thing:
Unless GM intentionally misinformed its service organization
years ago with the spec sheets, parts lists, etc., we have
accurate information as to the state of the design
originally. Finding commonality is easy. Finding specific
differences is harder, but not impossible in most cases.
Where it seems impossible, make an informed decision to
depart from the sacred mythology and create something
new/better/at least functional.
There are probably a lot of GMCs sitting in driveways,
garages, storage lots, etc. with disheartened owners that
could be back on the road if only the owners could be
convinced that the solution to a poor running engine is just
four bolts away. The owners are being held captive by fear
that there is no solution, or the solution is so expensive
that they might as well abandon hope. I think that's sad.
I started on this quest to dispel the myth a year ago after
my engine caught fire, I disassembled the carburetor on the
galley counter by flashlight, fixed the stuck float valve,
put it back together in the same flashlight and drove 100
miles home. I didn't have secondaries (the plastic cam
melted), so going up the Camarillo grade was a little
time-consuming, but on the flat, the engine ran good--after
all the fire grief.
After doing some studying, I concluded that there is no
sacred or magic smoke in the GMC carburetor. I decided to
not pay any more attention to that man behind the curtain.
I believe that with a little effort, acceptable and maybe
even superior workarounds are possible. It might be a good
way to get a few more GMCs back on the road, and a few more
owners having the fun that they bought the thing to have.
Toby Maki
'73 Glacier 230
Riverside, CA
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| Re: [GMCnet] What's special about the GMC carb? [message #64571 is a reply to message #64561] |
Mon, 16 November 2009 20:03 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
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The "GMC only carb" is a much overplayed myth. Sure they were different in several aspects but so were thousands of other part numbers. The same goes for distributors, shocks, brake components, etc. I think out of the 8000 or so GMC's still on the road, many folks have been able to enjoy their coaches with readily available junk yard components--albeit with a bit of ingenuity.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] What's special about the GMC carb? [message #64578 is a reply to message #64520] |
Mon, 16 November 2009 20:47 |
Dennis S
Messages: 3046
Registered: November 2005
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| tmaki wrote on Mon, 16 November 2009 15:45 |
dave silva wrote:
>
> Subject says it all
I would pay money for the answer to that question, with
exact and specific differences re: the nine parts that are
not common to the 7043250 (used on 350 Cutlass, Omega,
etc.), 7043252 (used on Toronado), 7043254 (used on GMCMH),
and 7043259 (marine/commercial applications).
Those nine parts are (applies to the above numbered
carburetors only, although numbers outside these may use the
same parts):
Air horn assembly
Secondary metering rod (easily changed out to motorhome spec)
Choke thermostat cover gasket
Choke housing to bowl seal
Primary jet (easily changed out to motorhome spec)
Power piston assembly (easily changed out to motorhome spec)
Power piston spring (easily changed out to motorhome spec)
Primary metering rod (easily changed out to motorhome spec)
Throttle body assembly
I have not been able to get a clear answer to what
specifically is different about the air horn assembly, and
the throttle body assembly.
snip snip
Toby Maki
'73 Glacier 230
Riverside, CA
|
Toby et al,
My 73 Glacier came with a service replacement carb for a 76 Toronado. It was in pretty good condition except for bad gaskets and poor adjustments. I refurbished it and followed a "recipe" in Cliff Ruggles book -- and picked a few mods to apply. Changed the primary jets to .071, installed the "red" power spring, ground small half moons into the secondary air flaps under the pull-over-enrichment outlets and ground down the secondary air flap opening stop to the setting for greater cfm (probably not necessary).
I also tried to measure all the ports and pieces of the carb.
It started, idled and ran very well. But I have now sold that coach.
Later when I had the opportunity to take a brief look at a GMC MH carb I saw two differences. The Toronado primary throttle plates have holes bored in them, one of the air flaps on the GMC carb had a baffle (?) welded to the bottom side. I now have a GMC MH carb and plan to disassemble it in order to take measurements for comparisons in hopes of determining any differences.
From my limited experience it seems most carb problems are that someone has "adjusted" them and that several of the gaskets are bad. Also, the linkage often seems to out of adjust and does not allow full opening of the throttles.
What I think we want -- a strong idle, and a carb that for the most part runs on only the primaries -- so, a weak power spring so the secondaries are only called on when we have a real need (not on a gradual rolling road), good control of the opening time of the secondaries -- so they don't flop open rapidly, but open gradually.
Anyway -- my thoughts and all the standard disclaimers apply.
Dennis
Dennis S
73 Painted Desert 230
Memphis TN Metro
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| Re: [GMCnet] What's special about the GMC carb? [message #64592 is a reply to message #64590] |
Mon, 16 November 2009 23:15 |
LarryInSanDiego
Messages: 336
Registered: September 2006
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From fiddling with Pontiac Q-Jets years ago, I found out there is a lot more than just jets and metering rods.
I very carefully compared some high perf Pontiac Ram Air carbs and found some interesting detail differences, many in the air horn:
Air bleeds
Emulsion tubes
Pullover tubes
Secondary hanger
Secondary hanger cam
Secondary plate (in the main body), not to mention a kazillion variations of number, location, and heights of emulsion tubes and air bleeds. And we're not even talking about choke, evap, and vacuum fittings.
Also given the fact that vehicles with GVWR over 8500 lbs. had different standards than passenger cars, it's easy to see that there were well over 1000 different carb numbers. Some changes supposedly affect driveability, some emissions (both tailpipe and evaporative), and some for quicker warmups and better hot start performance.
Take a very CLOSE look, it amazes me how many different changes, and kinds of changes they made. Some mods were done in the main body were there just isn't any easy way to correctly duplicate exactly how Rochester/Carter made those carbs. There are orifices in the venturi cluster and base plate that are circular and some are vertical slots. How they made those, I don't know outside of EDM'ing. How critical they are and to what degree they do are above my pay grade. I'll assume if they went to the bother of making a change, there was probably a good reason.
Larry Engelbrecht
San Diego, CA
'73 26' ex-Glacier
TZE063V100319 03/07/73
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| Re: [GMCnet] What's special about the GMC carb? [message #64610 is a reply to message #64592] |
Tue, 17 November 2009 08:57 |
tmaki
Messages: 200
Registered: September 2005
Karma: 0
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Senior Member |
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Larry Engelbrecht wrote:
>
<some good cautionary points>
Rather than address each one, I'll counter by saying that in
the universe of Oldsmobile engines and 455s specifically
from 1973 through 1976, there is a comparatively small
number of applicable carburetors. And in most of those
applications, there are only a few parts that are different
between them.
If one were to build a new template for a carburetor that is
available, serviceable and effective, then determining the
differences and the reasons for them is less than a daunting
task given that we can reduce the 1000s and kazillions to
only nine parts. All of the significant variations can be
discovered. And if they can be discovered, they can be
documented and applied to the new template. And if one can
take any member of the 70432XX or 7045XXX families (and
perhaps some outside these families), lay the template down
over them, mix, match, and modify to the template, then the
goal will be met. We will have created offspring that may
actually be better than the parents, if you will.
We've had several of the list members indicate that they
made certain subtle and maybe not so subtle changes to other
than stock "GMC motorhome" carburetors, and they are getting
good if not outstanding performance. If those folks can be
persuaded to share those specifics, then an outline can
emerge to create a recipe for a new template.
A complete, original, sacrificial 3254 (5254 or 5554 for
'75-'76) or 3259 (commercial) would be a great find. I'm
still looking for one that has little hope of being revived
by even the wizards of carburetors.
I suppose in some folks' minds, it's all tilting at
windmills. But I figure that most of the country will be
belly button deep in winter for the next several months.
Other than trying to figure out what color to make new
upholstery, or how to connect a diode, what else ya gonna
do? A couple of note pads, an old carburetor or two, and
some precision measuring tools and optical aids sitting at a
nice warm bench instead of laying in the freeze-ass cold
squirting rust-release juice seems like a decent way to pass
the time.
Still carrying on at this end...
Toby Maki
'73 Glacier 230
Riverside, Ca
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| Re: [GMCnet] What's special about the GMC carb? [message #64627 is a reply to message #64546] |
Tue, 17 November 2009 14:28 |
Rick Denney
Messages: 430
Registered: January 2004
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Senior Member |
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Matt Colie writes...
> Another manufacturer that has tried to replicate the piece could
> very simply have made the changes or reverse engineered the wrong
> piece to begin with and everybody is happy.
Matt can confirm this:
My understanding of Rochester is that that GM worked with the carbs
during testing to get what they wanted, and then Rochester made them
that way. There was a production tolerance that was considered in the
design, but GM ordered them by part number and Rochester did what
Rochester did, even if it meant steps that customized the carb casting
to that application.
Holley, Edelbrock, and other aftermarket carburetor manufacturers have
a completely different business model. For them, each basic model must
accommodate a wide range of applications. So, instead of building a
factor OEM carb, they built a carb designed to be field-tuned to the
application. Unlike the Rochester, they provided relatively easy
access to jets, metering rods, freemistats, emulsion tubes, Johnson
bars, power-valve springs, whiketybacks, and all the other esoterica
of making a carburetor work. When I tuned my Weber 40DCOE side-draft
carbs on my Toyota race car back in the deeps of time, I could remove
the top cover--it was held down by one (1) *wingnut*--pull the
emulsion tubes, replace them or the jets inserted into the ends of
them, put them back, tighten down the cover, and go for another test
lap. It took 30 seconds. But forget getting a smooth transition from
idle to just off-idle, which would be expected for any street vehicle.
That was outside its design envelope (especially with the cam in that
car).
It may be possible to tune an aftermarket carb to fit the GMC
application, though I predict that it will be a big job. The GMC is
expected to be as smooth in all ranges as a passenger car, but still
be able to make big power on demand without bogging down or surging,
or going into power enrichment just driving down the highway. Finding
an aftermarket carb and going through that with a stock engine might
be a good project, except that it isn't that much more expensive to
put in the fuel injection system and most folks investing that sort of
time into the fuel system seem to be going that route.
The Rochester just wasn't designed to be field tunable in that way.
Rick "who, as a professional mechanic, handed the QJ's to the QJ Guru
in the shop to rebuild, using his guru voodoo magic" Denney
'73 230 Ex-Glacier "Jaws"
Northern Virginia
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'73 Glacier 230 "Jaws"
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| Re: [GMCnet] What's special about the GMC carb? [message #64628 is a reply to message #64627] |
Tue, 17 November 2009 14:45 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Senior Member |
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Rick,
My own ambition was not to replicate the GMC MH carb with aftermarket but to know the differences between the MH car and the Toronado. Once I think I know -- well then I may attempt to modify a T carb.
And, I agree that the Qjet seems to require more dismantling in order to modify -- especially jets.
Dennis
| Rick Denney wrote on Tue, 17 November 2009 14:28 |
Matt Colie writes...
> Another manufacturer that has tried to replicate the piece could
> very simply have made the changes or reverse engineered the wrong
> piece to begin with and everybody is happy.
Matt can confirm this:
My understanding of Rochester is that that GM worked with the carbs
during testing to get what they wanted, and then Rochester made them
that way. There was a production tolerance that was considered in the
design, but GM ordered them by part number and Rochester did what
Rochester did, even if it meant steps that customized the carb casting
to that application.
Holley, Edelbrock, and other aftermarket carburetor manufacturers have
a completely different business model. For them, each basic model must
accommodate a wide range of applications. So, instead of building a
factor OEM carb, they built a carb designed to be field-tuned to the
application. Unlike the Rochester, they provided relatively easy
access to jets, metering rods, freemistats, emulsion tubes, Johnson
bars, power-valve springs, whiketybacks, and all the other esoterica
of making a carburetor work. When I tuned my Weber 40DCOE side-draft
carbs on my Toyota race car back in the deeps of time, I could remove
the top cover--it was held down by one (1) *wingnut*--pull the
emulsion tubes, replace them or the jets inserted into the ends of
them, put them back, tighten down the cover, and go for another test
lap. It took 30 seconds. But forget getting a smooth transition from
idle to just off-idle, which would be expected for any street vehicle.
That was outside its design envelope (especially with the cam in that
car).
It may be possible to tune an aftermarket carb to fit the GMC
application, though I predict that it will be a big job. The GMC is
expected to be as smooth in all ranges as a passenger car, but still
be able to make big power on demand without bogging down or surging,
or going into power enrichment just driving down the highway. Finding
an aftermarket carb and going through that with a stock engine might
be a good project, except that it isn't that much more expensive to
put in the fuel injection system and most folks investing that sort of
time into the fuel system seem to be going that route.
The Rochester just wasn't designed to be field tunable in that way.
Rick "who, as a professional mechanic, handed the QJ's to the QJ Guru
in the shop to rebuild, using his guru voodoo magic" Denney
'73 230 Ex-Glacier "Jaws"
Northern Virginia
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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Dennis S
73 Painted Desert 230
Memphis TN Metro
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| Re: [GMCnet] What's special about the GMC carb? [message #64674 is a reply to message #64610] |
Tue, 17 November 2009 19:02 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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Senior Member |
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I have a friend that has a 33 Ford coupe with a 73 455 olds engine in it. He was having a problem with hesitation when he stabbed the throttle. I told him we need to replace the accelerator pump as the carb was in real good shape just a dried out accelerator from setting.It was a matching carb for his engine. He said nope pulled the carb gave it to me went to Summit and bought a Quadrojet for a Chevy bolted it on and said it is perfect now?? What can I say?
Roy
Roy Keen
Minden,NV
76 X Glenbrook
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