Lars, carb question....Q jet especially....

[mrvette]

LONG time ago I had a Q rebuilt by a fellow on the west coast....Brad Urban, Ontario Carb shop fame.....I hear he passed away......

so, the big mystery for me was why the problem with tip in stumble....

he explained that two of those brass tubes in the carb top actually fed a thing called the off-idle transfer circuit,...they involve the two little slots in the leading edge area of the primary bores....feed up through those two tubes and meter extra gas, so they need richening if EGR is not used...apparently....

so the question is....why is that entire circuit necessary, and why is that not picked up with the accel pump action?? and how does just enrichening the primary jets necessarily cover that action?? and why all the different metering rod shapes/springs?? how in hell to know one from another....

these type questions were answered years ago, but I can't remember all of them.....except that OI xfer thing....that is because it was one over the top to me....

:banghead::beer:

 

[lars]

The transition circuit, as you noted, supplies fuel to vertical slots in the primary throttle bores. The fuel for the cricuit is supplied by the idle fuel circuit, except that the fuel to the transition slots does not get metered through the idle mixture screws - its only metering point is the size of the IFR (idle fuel restrictor, i.e. the hole in the idle restrictor tubes).

At low engine power settings, there is very little airflow through the carb venturi. In fact, there is so little flow that the primary booster venturi does not function well enough to produce fuel discharge from the primary discharge oriface. For this reason, the idle fuel discharge point is located below the throttle blades, pulling idle fuel from the carb due to manifold vacuum. This fixed fuel delivery only works with one throttle opening setting, since fuel flow through this system is not variable. As the throttle is opened a little more, the mixture would go lean if additional fuel was not delivered continuously (the accel pump only provides a momentary enrichment). So the vertical slots in the transition circuit come into play: They provide additional fuel for larger throttle openings prior to the main discharge circuit becoming effective. At light throttle cruise, there is so little air actually passing through the carb that the majority of the fuel metering is done by the transition circuit - not the main metering circuit. On the higher-end Holley and BG carbs, the "low speed" circuit (which is the transition curcuit) can be adjusted for mixture by changing the IFR or the low speed air bleeds. This is the best control for cruise mixture. On a Q-Jet, transition fuel (cruise) can be altered by changing the bleed hole sizes and the size of the holes in the ends of the idle restrictor tubes. This has a huge effect on off-idle response and cruise mixture control.

Lars

 

[mrvette]

The transition circuit, as you noted, supplies fuel to vertical slots in the primary throttle bores. The fuel for the cricuit is supplied by the idle fuel circuit, except that the fuel to the transition slots does not get metered through the idle mixture screws - its only metering point is the size of the IFR (idle fuel restrictor, i.e. the hole in the idle restrictor tubes).

At low engine power settings, there is very little airflow through the carb venturi. In fact, there is so little flow that the primary booster venturi does not function well enough to produce fuel discharge from the primary discharge oriface. For this reason, the idle fuel discharge point is located below the throttle blades, pulling idle fuel from the carb due to manifold vacuum. This fixed fuel delivery only works with one throttle opening setting, since fuel flow through this system is not variable. As the throttle is opened a little more, the mixture would go lean if additional fuel was not delivered continuously (the accel pump only provides a momentary enrichment). So the vertical slots in the transition circuit come into play: They provide additional fuel for larger throttle openings prior to the main discharge circuit becoming effective. At light throttle cruise, there is so little air actually passing through the carb that the majority of the fuel metering is done by the transition circuit - not the main metering circuit. On the higher-end Holley and BG carbs, the "low speed" circuit (which is the transition curcuit) can be adjusted for mixture by changing the IFR or the low speed air bleeds. This is the best control for cruise mixture. On a Q-Jet, transition fuel (cruise) can be altered by changing the bleed hole sizes and the size of the holes in the ends of the idle restrictor tubes. This has a huge effect on off-idle response and cruise mixture control.

Lars

:bonkers::crap: THERE right there lies the key to my misunderstumbling years ago.....where were YOU about 25 years ago???

:crap:

 

[lars]

25 years ago I was in a Super Stock car running NHRA SS/KA and teaching carbs at the GM Training Center... no... wait... that was 30 years ago... or was it 35..?

Lars

 

[mrvette]

25 years ago I was in a Super Stock car running NHRA SS/KA and teaching carbs at the GM Training Center... no... wait... that was 30 years ago... or was it 35..?

Lars

U 2, CRS???

:lol::crap:

 

[lars]

I don't remember if that was the diagnosis.