| [GMCnet] Q Jet Primary Rods for 1976 GMC [message #142898] |
Fri, 09 September 2011 16:04  |
Arthur Mansfield
Messages: 290
Registered: April 2010
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I found a listing for the 1976 GMC Q Jet. If I read it correctly my carb (7045245) is supposed to have rods that are .038 but they are .050. The .038 would provide a.002714 area orifice which seems to rich. I have another Q Jet of off a car 455 and it has an area orifice area of .002299. The .050 rods (reads 50 and measures .050) would give a orifice area of .001963 which seems lean. If I go to .073 jets with the ,050 rods will give me .002222 orifice area which would seem better ut may be a little on the rich side.. I wonder what size primary rod the is used in the 1976 GMC RV? Is the 038 correct or is the .050 correct? The tip size is .036 which I believe is supposed to be correct for all 76 and latter carbs. I installed .073 jets and will give them a try.
Art & Doris
76 EL
Decatur Al
256 476 9723
amansfield1104@charter.net
1104agm@gmail.com
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| Re: [GMCnet] Q Jet Primary Rods for 1976 GMC [message #142912 is a reply to message #142907] |
Fri, 09 September 2011 20:52  |
Arthur Mansfield
Messages: 290
Registered: April 2010
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Thanks I found what I was looking for. I did transpose the digits. The 50D is the proper rod size for our carb. I found some other info that recommended that the jets may be changed to 71 or 72 and get good results. I will try the 73 since I have one.
Art
On Sep 9, 2011, at 8:27 PM, Emery Stora wrote:
>
> On Sep 9, 2011, at 5:04 PM, 1104agm wrote:
>
>> I found a listing for the 1976 GMC Q Jet. If I read it correctly my carb (7045245) is supposed to have rods that are .038 but they are .050. The .038 would provide a.002714 area orifice which seems to rich. I have another Q Jet of off a car 455 and it has an area orifice area of .002299. The .050 rods (reads 50 and measures .050) would give a orifice area of .001963 which seems lean. If I go to .073 jets with the ,050 rods will give me .002222 orifice area which would seem better ut may be a little on the rich side.. I wonder what size primary rod the is used in the 1976 GMC RV? Is the 038 correct or is the .050 correct? The tip size is .036 which I believe is supposed to be correct for all 76 and latter carbs. I installed .073 jets and will give them a try.
>>
>> Art & Doris
>>
>
> Art -- I do not think that your carb is a GMC motorhome carb -- unless you are transposing the last two digits.
>
> 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 emision 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 coachs 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
>
>
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