Editing Q-jet with dual capacity accelerator pump

==Overview== The question came up on what- if anything- is required to use a late model NON CCC/NON electronic Q-jet carb that has the dual capacity accelerator pump. Because these carbs are often mistaken for being full electronic/CCC-type feedback carbs, many who have them either don't know what they have, or if they do they don't know whether they can be used as-is or not. [[File:Q jet dual cap acc pump.jpg|300px|thumb|left|Arrow points to Dual Capacity Accelerator Pump plug; circled is where the TPS plug ''would be'' on an electronic or computer controlled Q-jet. The TPS plug '''must be absent''' for a Q-jet carb to be used on the street without a computer, however the feedback type Q-jet can be used in race only applications by removing the metering rods and changing the size of the primary jets.]] [[File:Qjet with CCC.jpg|300px|thumb|left|1981-up Q-jet with CCC emissions/computer control]] ==How to ID a dual capacity accelerator pump Q-jet== The dual capacity accelerator pump carbs can be ID'd by their solenoid plug mounted on the passenger side top front of the airhorn/carb top, along w/the LACK of a plug for the TPS (throttle position sensor) that's present on electronic feedback-type Q-jets used during the same years on vehicles using the "CCC" (Computer Command Control)emissions system. These NON CCC/NON electronic dual capacity accelerator pump Q-jet carbs are found on 1985-'86 GM trucks and vans, and have desirable features- adjustable top-accessed APT, electric choke, correct linkage geometry for OD transmission TV cable, large 800 cfm casting, and generally less use/abuse/rebuilds on them than carbs from the '60s and '70s. ==How to make it work== Cliff Ruggles verified that basically nothing needs to be done to have the carb perform just like the earlier single capacity accelerator pump carbs. The plug and solenoid can be left intact, the default setting (with '''no''' external wiring or electrical source supplied to the plug) is such that the accelerator pump will deliver a normal pump shot. Or if desired the cavity under the solenoid can be filled with molten lead to prevent any chance of a leak.<br style="clear:both"/> ==Other considerations== *Whenever possible, get the factory cable bracket. Even if you're using an aftermarket intake, it might come in handy for making up your own bracket. And regardless if using an aftermarket intake or not, the bracket can be used to measure the correct cable/carb relationship for setting up whatever bracket that's used. *Make note of the engine displacement/engine VIN code. These carbs were used on 4.3L V6 through 7.4L V8 engines, so knowing what it came from gives a leg up on what may potentially need changed. *If the Q-jet is to be used with an automatic transmission that uses a detent (kick down) or TV (throttle valve) cable, be sure the throttle arm is correct (below, right). Many manual transmission-equipped vehicles are missing the lower part of the throttle arm (below, left); this is where the detent/TV cable ordinarily attaches. {| |[[File:TYPICAL MANUAL TRANS Q-JET THROTTLE ARM1.jpg|250px|Manual trans throttle arm]] |[[File:LINKAGE TH350 CARB WITH AT.jpg|215px|Automatic trans throttle arm]] |}<br> ==Possible use for dual capacity pump== In cases where a large pump shot is needed, like with big cams, single plane intakes and/or heavy or high geared vehicles, an interesting idea would be to set the accelerator pump up with the larger pump shot volume needed for best acceleration when in the default mode. Then with the dual capacity accelerator pump solenoid ''activated'', have a reduced shot volume for the street. The accelerator pump position could be selected using a simple toggle switch. ==Carb numbers== The carb number will be '''17085''' or '''17086'''. The 6th number will be '''1''', '''2''', '''4''', or '''5'''. The 7th number will ''usually'' be '''0''', '''1''', or '''2''', but ''there are carbs made 1981 and later that do not follow the traditional numbering system''. For that reason, there are Chevy/GMC carbs with '''4''' and '''5''' in the 7th position. If the 8th (final) digit is an even number it sometimes denote an automatic transmission application, odd for manual. This is not an absolute. ===Some numbers to look for:=== *17085210 *17085206 *17085215 *17085226 *17085227 *17086046 (ALT, AT, dual acc. pump) *17086048 (ALT, AT, dual acc. pump) *17085227 (AT, dual acc. pump) *17086054 (AT, dual acc. pump) *17085292 (700R4, 160hp 5.7, dual acc. pump) ==Resources== *[[Carb vacuum port ID]] *[[How to rebuild a Rochester Quadrajet 4MV carburetor]] *[[Rochester Quadrajet carburetors: Identification and decoding]] *[[Quadrajet]] [[Category:Engine]] [[Category:Carburetors]] [[Category:Good articles]] [[Category:GM]]

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