Anyone ever come across a twisted fuel filter in their carb? The filter i removed from my carb during rebuild had the end twisted and skewed sideways a bit [which was possibly the cause of my off idle bog/amost stall]... after my rebuild or refresh the carb is lean [no noticeable vacuum leaks by normal tests and appears to be running out of fuel in the mid throttle position. I haven't verified yet but i would bet a beer the filter is twisted again and its preventing full fuel flow from entering the carb under low pump RPM.

Also their has been talk on the Vette forums about a sintered bronze filter instead....anyone know of such a beast for the motorhome?

Have noticed twisted filter. Attributed it to installation (figured available surface area really didn’t change muck)..
What is your fuel pump situation? Added electric pump?

Sir, is your filter pointing forward toward radiator or sideways toward valve cover? You may be using wrong filter.there should be room for a filter and a spring.



6cuda6 wrote on Mon, 22 June 2020 20:14

Anyone ever come across a twisted fuel filter in their carb? The filter i removed from my carb during rebuild had the end twisted and skewed sideways a bit [which was possibly the cause of my off idle bog/amost stall]... after my rebuild or refresh the carb is lean [no noticeable vacuum leaks by normal tests and appears to be running out of fuel in the mid throttle position. I haven't verified yet but i would bet a beer the filter is twisted again and its preventing full fuel flow from entering the carb under low pump RPM.

Also their has been talk on the Vette forums about a sintered bronze filter instead....anyone know of such a beast for the motorhome?

If an off idle flat spot is the issue I doubt that the bowl level being low due to restricted filter is the issue. Usually the level recovers during low demand. You can prove it’s not the filter If it pulls hard at full throttle and does not go flat after a few seconds. Hard to say how much throttle you are giving it but my guesses are
1) float level set a little too low
2) bad or wrong P/N accelerator pump in carb
3) stuck closed (down) power piston on primaries.
4) clogged pullover slots circuit on primaries
5) non operative or miss patched vac advance. Factory would be no vac advance at idle, then vac advance in effect right where your problem is off idle. Without this added timing it would be flat off idle.

Why is there so much concern about idle speed and mixture? Set up your
basic idle somewhere between 600 -750 rpm. Adjust for lean/rich, correct
for speed, and leave it be. You should not idle your engine any more than
absolutely necessary. Doesn't get enough upper end lube at idle speed.
Start the engine when cold, run until the choke opens, put it in gear and
drive it slowly until the coolant is warm, then drive normally. When you
stop, shut it off.
Jim Hupy
Salem, Oregon

On Tue, Jun 23, 2020, 7:34 AM John R. Lebetski via Gmclist <
gmclist@list.gmcnet.org> wrote:

> Ok sounds like you can eliminate item 2 on my list but the other items
> need to be confirmed. To set the idle mixture, on warm engine find max RPM
> by
> adjusting each screw slowly to the center point of max RPM. Then lean each
> side just to the point where RPM wants to start to drop but does not. You
> may have to reset idle speed mid process. You can leave it there at max
> RPM just before drop, or do it by the book, which is turn in one screw for
> 25
> RPM drop and other screw for 25 RPM. You want 50 RPM total drop which
> should only be 1/8 turn in or so if you were at the lean edge before. If
> yours
> is way off it could be a problem at tip in as the idle circuit still
> contributes and transitions out not just a hard On/Off thing.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
>
>
> _______________________________________________
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A corvette is not a GMC. Flat spot off idle (low speed tip-in) is USUALLY
caused by one of two things. Excessively LEAN MIXTURE, or vacuum leak.
Could also be contributed to, by retarded spark timing, low float bowl
level, or warped carb base (very, very common in Quadrajet Carburetors.)
Cause? Overtightening the 4 corners. There is a Phillip's head screw in the
center of the iron throttle body casting that screws into the pot metal
float bowl. Often times, it is quite loose. The quadrajet carb is not
anywhere as complex as many believe it to be. Be very sure that yours is
the exact part number for your GMC. Close does not count. Dick Paterson
won't even rebuild a carb if it is not an OEM one, and I don't blame him
one bit.
Jim Hupy
Salem, Oregon

On Tue, Jun 23, 2020, 9:06 AM 6cuda6--- via Gmclist
wrote:

> Jim, i don't idle the engine normally any more than i have to but when one
> is trying to diagnose operational problems their really is not much choice
> in the matter.
>
> This coach has always fallen flat on its face to the point of stalling
> when you get into the throttle from a stop and double pumping the pedal for
> the
> extra shot of fuel appon start of accel is the only way to get moving. I
> rebuilt the carb and the problem still exists [along with the added possible
> vacuum leak or lean mixture]...the only visible issue when i tore the carb
> down was the twisted filter and a little bit of junk in the bowl, so trying
> to find out why the twisted filter and upon research the same diving
> conditions have happened to a few of the corvette guys and the culprit was a
> twisted filter.
>
>
> James Hupy wrote on Tue, 23 June 2020 11:22
>> Why is there so much concern about idle speed and mixture? Set up your
>> basic idle somewhere between 600 -750 rpm. Adjust for lean/rich, correct
>> for speed, and leave it be. You should not idle your engine any more than
>> absolutely necessary. Doesn't get enough upper end lube at idle speed.
>> Start the engine when cold, run until the choke opens, put it in gear and
>> drive it slowly until the coolant is warm, then drive normally. When you
>> stop, shut it off.
>> Jim Hupy
>> Salem, Oregon
>>
>> On Tue, Jun 23, 2020, 7:34 AM John R. Lebetski via Gmclist > gmclist@list.gmcnet.org> wrote:
>>
>>> Ok sounds like you can eliminate item 2 on my list but the other items
>>> need to be confirmed. To set the idle mixture, on warm engine find
> max RPM
>>> by
>>> adjusting each screw slowly to the center point of max RPM. Then lean
> each
>>> side just to the point where RPM wants to start to drop but does not.
> You
>>> may have to reset idle speed mid process. You can leave it there at
> max
>>> RPM just before drop, or do it by the book, which is turn in one
> screw for
>>> 25
>>> RPM drop and other screw for 25 RPM. You want 50 RPM total drop which
>>> should only be 1/8 turn in or so if you were at the lean edge before.
> If
>>> yours
>>> is way off it could be a problem at tip in as the idle circuit still
>>> contributes and transitions out not just a hard On/Off thing.
>>> --
>>> John Lebetski
>>> Woodstock, IL
>>> 77 Eleganza II
>>>
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
> --
> 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
>
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No, the carbs are not the same.
1. There is the one fitted to the 73 - late 76
455.
2. There is the one fitted to the late 76-78 49 states (excluding
California). 403
3. Then, there is the one fitted to the 76 California 403.
4. There is one fitted to the 77 California 403.
5. There is one fitted to the 78 California 403.
There may be others, as well. All the part numbers can be found in either
the 73-76 Shop manual, or the Supplemental manual for the late 76 - 78, as
well as California. Look in the emissions section, not the fuel system
section. Takes some sorting, but it's all in there.
Do not rely upon anyones memory of the correct part numbers. Check the
books, and compare the number you have on your carb to verify that you have
the correct carb.

On Tue, Jun 23, 2020, 10:18 AM 6cuda6--- via Gmclist <
gmclist@list.gmcnet.org> wrote:

> Jim,
> Were the 77 403 and 455 carbs the same? The reason i ask is you said 3
> lower plate screws, mine only had 2.....
>
>
>
> James Hupy wrote on Tue, 23 June 2020 12:29
>> A corvette is not a GMC. Flat spot off idle (low speed tip-in) is USUALLY
>> caused by one of two things. Excessively LEAN MIXTURE, or vacuum leak.
>> Could also be contributed to, by retarded spark timing, low float bowl
>> level, or warped carb base (very, very common in Quadrajet Carburetors.)
>> Cause? Overtightening the 4 corners. There is a Phillip's head screw in
> the
>> center of the iron throttle body casting that screws into the pot metal
>> float bowl. Often times, it is quite loose. The quadrajet carb is not
>> anywhere as complex as many believe it to be. Be very sure that yours is
>> the exact part number for your GMC. Close does not count. Dick Paterson
>> won't even rebuild a carb if it is not an OEM one, and I don't blame him
>> one bit.
>> Jim Hupy
>> Salem, Oregon
>>
>> On Tue, Jun 23, 2020, 9:06 AM 6cuda6--- via Gmclist
>> wrote:
>>
>>> Jim, i don't idle the engine normally any more than i have to but
> when one
>>> is trying to diagnose operational problems their really is not much
> choice
>>> in the matter.
>>>
>>> This coach has always fallen flat on its face to the point of stalling
>>> when you get into the throttle from a stop and double pumping the
> pedal for
>>> the
>>> extra shot of fuel appon start of accel is the only way to get
> moving. I
>>> rebuilt the carb and the problem still exists [along with the added
> possible
>>> vacuum leak or lean mixture]...the only visible issue when i tore the
> carb
>>> down was the twisted filter and a little bit of junk in the bowl, so
> trying
>>> to find out why the twisted filter and upon research the same diving
>>> conditions have happened to a few of the corvette guys and the
> culprit was a
>>> twisted filter.
>>>
>>>
>>> James Hupy wrote on Tue, 23 June 2020 11:22
>>>> Why is there so much concern about idle speed and mixture? Set up
> your
>>>> basic idle somewhere between 600 -750 rpm. Adjust for lean/rich,
> correct
>>>> for speed, and leave it be. You should not idle your engine any more
> than
>>>> absolutely necessary. Doesn't get enough upper end lube at idle
> speed.
>>>> Start the engine when cold, run until the choke opens, put it in
> gear and
>>>> drive it slowly until the coolant is warm, then drive normally. When
> you
>>>> stop, shut it off.
>>>> Jim Hupy
>>>> Salem, Oregon
>>>>
>>>> On Tue, Jun 23, 2020, 7:34 AM John R. Lebetski via Gmclist >
> gmclist@list.gmcnet.org> wrote:
>>>>
>>>> > Ok sounds like you can eliminate item 2 on my list but the other
> items
>>>> > need to be confirmed. To set the idle mixture, on warm engine find
>>> max RPM
>>>> > by
>>>> > adjusting each screw slowly to the center point of max RPM. Then
> lean
>>> each
>>>> > side just to the point where RPM wants to start to drop but does
> not.
>>> You
>>>> > may have to reset idle speed mid process. You can leave it there at
>>> max
>>>> > RPM just before drop, or do it by the book, which is turn in one
>>> screw for
>>>> > 25
>>>> > RPM drop and other screw for 25 RPM. You want 50 RPM total drop
> which
>>>> > should only be 1/8 turn in or so if you were at the lean edge
> before.
>>> If
>>>> > yours
>>>> > is way off it could be a problem at tip in as the idle circuit
> still
>>>> > contributes and transitions out not just a hard On/Off thing.
>>>> > --
>>>> > John Lebetski
>>>> > Woodstock, IL
>>>> > 77 Eleganza II
>>>> >
>>>> >
>>>> > _______________________________________________
>>>> > GMCnet mailing list
>>>> > Unsubscribe or Change List Options:
>>>> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>> >
>>>> _______________________________________________
>>>> GMCnet mailing list
>>>> Unsubscribe or Change List Options:
>>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>
>>>
>>> --
>>> 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
>>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
> --
> 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
>
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