Q-jet ?
Thread Starter
Racer
Joined: May 2011
Posts: 373
Likes: 6
From: Dover Delaware
Q-jet ?
To any and all Q-jet gurus....
The accelerator pump linkage has two holes..
What hole is the linkage supposed to be attached to?
My set-up has it connected to the hole closest to the carb..
mk's78
Mike
The accelerator pump linkage has two holes..
What hole is the linkage supposed to be attached to?
My set-up has it connected to the hole closest to the carb..
mk's78
Mike
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
The further in hole will increase the pump shot - some applications call for the outer hole vs the inner hole. A typical rebuild kit will enumerate these.
Last edited by Shark Racer; Sep 15, 2015 at 01:12 AM.
Melting Slicks
Joined: Sep 2004
Posts: 3,225
Likes: 667
From: Northeast
I was going to ask this question.
Looking in the 78 chassis manual for pump adjustment, in the pic, it points to the two holes and says "rod in specified hole of pump lever". I could not find any write up where it is specified.
Original 78's that have been posted here show the inside hole like you have now. Checked out some other pics of carbs and the rod has been in either.
Out of curiosity I checked my 78 and the rod is inserted on the end hole. The engine compartment has been changed and the previous owner said he rebuilt the carb. I have no complaints about how my engine runs.
It appears that by having it in the end hole the stroke would be longer.
Edit" I was typing while you were answering. Sorry shark racer. But what would be the purpose of increasing the pump shot.
Looking in the 78 chassis manual for pump adjustment, in the pic, it points to the two holes and says "rod in specified hole of pump lever". I could not find any write up where it is specified.
Original 78's that have been posted here show the inside hole like you have now. Checked out some other pics of carbs and the rod has been in either.
Out of curiosity I checked my 78 and the rod is inserted on the end hole. The engine compartment has been changed and the previous owner said he rebuilt the carb. I have no complaints about how my engine runs.
It appears that by having it in the end hole the stroke would be longer.
Edit" I was typing while you were answering. Sorry shark racer. But what would be the purpose of increasing the pump shot.
Last edited by BKarol; Sep 14, 2015 at 05:57 PM.
Instructor
Joined: Dec 2011
Posts: 189
Likes: 55
From: Virginia
This is write-up I found in one of my Rochester carburetor books:
Accelerator Pump Action - There are two pump-rod holes in some pump levers which establish pump travel. One at the end of the lever is used for normal driving in moderate climates. The inside hole, standard location on some models, gives a larger pump shot for racing or cold weather.
Accelerator Pump Action - There are two pump-rod holes in some pump levers which establish pump travel. One at the end of the lever is used for normal driving in moderate climates. The inside hole, standard location on some models, gives a larger pump shot for racing or cold weather.
Instructor
Joined: Dec 2011
Posts: 189
Likes: 55
From: Virginia
This is from:
Technical Information Bulletin Rev F 4-5-05
Q-Jet Problems I have Seen by Lars Grimsrud (Lafayette, CO)
Accelerator Pump Rod Installed in Wrong Hole in Pump Arm
Results in: Off-idle stumble or hesitation.
Comments: There are two holes in the accelerator pump lever arm for the lever arm rod to engage into: Inner and Outer. The outer hole produces a leaner pump shot, and can cause a lean stumble on engines requiring a robust pump shot. Corvettes and performance cars always used the rich, inner pump arm hole.
Technical Information Bulletin Rev F 4-5-05
Q-Jet Problems I have Seen by Lars Grimsrud (Lafayette, CO)
Accelerator Pump Rod Installed in Wrong Hole in Pump Arm
Results in: Off-idle stumble or hesitation.
Comments: There are two holes in the accelerator pump lever arm for the lever arm rod to engage into: Inner and Outer. The outer hole produces a leaner pump shot, and can cause a lean stumble on engines requiring a robust pump shot. Corvettes and performance cars always used the rich, inner pump arm hole.
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
I was going to ask this question.
Looking in the 78 chassis manual for pump adjustment, in the pic, it points to the two holes and says "rod in specified hole of pump lever". I could not find any write up where it is specified.
Original 78's that have been posted here show the inside hole like you have now. Checked out some other pics of carbs and the rod has been in either.
Out of curiosity I checked my 78 and the rod is inserted on the end hole. The engine compartment has been changed and the previous owner said he rebuilt the carb. I have no complaints about how my engine runs.
It appears that by having it in the end hole the stroke would be longer.
Edit" I was typing while you were answering. Sorry shark racer. But what would be the purpose of increasing the pump shot.
Looking in the 78 chassis manual for pump adjustment, in the pic, it points to the two holes and says "rod in specified hole of pump lever". I could not find any write up where it is specified.
Original 78's that have been posted here show the inside hole like you have now. Checked out some other pics of carbs and the rod has been in either.
Out of curiosity I checked my 78 and the rod is inserted on the end hole. The engine compartment has been changed and the previous owner said he rebuilt the carb. I have no complaints about how my engine runs.
It appears that by having it in the end hole the stroke would be longer.
Edit" I was typing while you were answering. Sorry shark racer. But what would be the purpose of increasing the pump shot.
Generally, you want more pump shot as power goes up - the engine will change RPM more quickly(in response to the sudden throttle angle change) than the circuits can keep up, so to speak.
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