Quadra-Jet Question
Thread Starter
Melting Slicks
Joined: Feb 2003
Posts: 2,529
Likes: 27
From: Auburndale Florida
2025 C2 of the Year ('64-'66) Finalist - Unmodified
St. Jude Donor '03-'05-'06-'07-'09
Quadra-Jet Question
Helping a friend out with his 71 SB, auto, AC coupe. Here's the back ground. Recent frame off, has about 500 miles, idle like crap and low speed about the same. Originally a 350 base engine, rebuilt with a 350 HP cam, guessing the cam specs are somethng like 222*/222* duration @ .050 with .450 lift on a 112-114 LSA, running a stock torque convetor. He has tried 3 different carbs with the same disapponting results.
So it dawned on me, when he and the many others who have worked on the car, have to order parts. They order for a stock 350 on such things like carbs, torques convertor, and so on.
With the lower idle vaciuum, due to the cam, would this affect the operation of the carb? Vacuum at 1,000 idle, in neutral, is 14*hg.
Is there any differences in Hi-Perf Q-Jets?
thanks, Mark
So it dawned on me, when he and the many others who have worked on the car, have to order parts. They order for a stock 350 on such things like carbs, torques convertor, and so on.
With the lower idle vaciuum, due to the cam, would this affect the operation of the carb? Vacuum at 1,000 idle, in neutral, is 14*hg.
Is there any differences in Hi-Perf Q-Jets?
thanks, Mark
Last edited by mbeeman350; Apr 6, 2009 at 11:53 PM.
Melting Slicks
Joined: Mar 2006
Posts: 2,465
Likes: 0
From: Orange Park Fl
With vacuum at 14, the stock Quadrajet should not give any problems if it was rebuilt correctly. There are things that could be done to fine tune it for the modifications, but that car is not far enough from stock to REQUIRE it to be altered. My guess is you are getting bad commercial rebuilds.
There are modifications that can be made to Quadrajets that will allow them to perform optimally with higher performance engines than they were originally set up for. And the true beauty of the Quadrajet is that it can still be optimally set up for the street at the same time.
There are people around who understand these carbs well and can make them work for you. Also, most people are capable of doing the needed research and learning how to at least do a stock rebuild for themselves. Either way, avoid the "carb factories".
God bless, Sensei
There are modifications that can be made to Quadrajets that will allow them to perform optimally with higher performance engines than they were originally set up for. And the true beauty of the Quadrajet is that it can still be optimally set up for the street at the same time.
There are people around who understand these carbs well and can make them work for you. Also, most people are capable of doing the needed research and learning how to at least do a stock rebuild for themselves. Either way, avoid the "carb factories".
God bless, Sensei
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
It's possible the power piston is pulling the rods up at idle "believing" that it's under a mild load with 14* depending on what power piston spring is in play. Or the rods could fluttering or floating in and out if 14 hg is in the transition zone for the carb's set up.
But the fact you couldn't get any one of three carbs (all q-jets?) would lead me to believe the problem may lie elsewhere. Tell us more about your timing. Also how does it run otherwise (cruise, under a load, etc).
Any other symptoms?
But the fact you couldn't get any one of three carbs (all q-jets?) would lead me to believe the problem may lie elsewhere. Tell us more about your timing. Also how does it run otherwise (cruise, under a load, etc).
Any other symptoms?
Last edited by 73, Dark Blue 454; Apr 6, 2009 at 11:31 PM.
Thread Starter
Melting Slicks
Joined: Feb 2003
Posts: 2,529
Likes: 27
From: Auburndale Florida
2025 C2 of the Year ('64-'66) Finalist - Unmodified
St. Jude Donor '03-'05-'06-'07-'09
It's possible the power piston is pulling the rods up at idle "believing" that it's under a mild load with 14* depending on what power piston spring is in play. Or the rods could fluttering or floating in and out if 14 hg is in the transition zone for the carb's set up.
But the fact you couldn't get any one of three carbs (all q-jets?) would lead me to believe the problem may lie elsewhere. Tell us more about your timing. Also how does it run otherwise (cruise, under a load, etc).
Any other symptoms?
But the fact you couldn't get any one of three carbs (all q-jets?) would lead me to believe the problem may lie elsewhere. Tell us more about your timing. Also how does it run otherwise (cruise, under a load, etc).
Any other symptoms?
Tried maniflod and ported vacumm for the advance, adjusted idle air screws, set initial timing at 0* to 30* and not much change.
To me it acts like it has a dead cylinder, 2 of the plugs were dark almost wet. Could be wires. I picked up some plug wires and I going to try the new wires on the suspect plugs and replace the plugs Car has all the shielding. So I am going to McGiver the wires before I pull them through the block shielding and motor mounts.
If this does not improve anything. I am going pull all the plugs and ck compression. I do have a Lars build Q-jet off my 72 I could test on the car. This carb ran fine on my car and my cam is just a little stronger. The current plugs are AC Rapid Fires. i also ckd the valve lash!!
thanks
Le Mans Master
Joined: Aug 2006
Posts: 7,423
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From: mount holly NC
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2019 C3 of Year Finalist (appearance mods)
You said the distributor side play is marginal-use a dwell meter and see if it is steady at 29 to 31 degrees as the RPM goes up.
Timing- try 10* BTDC and then reconnect the vacuum advance can to manifold vacuum, timing should go to about 26* at idle.
How are the springs and weights on the centrifugal advance- are they stock or have they been changed?
After 3 carb changes I doubt that they all had the wrong power piston spring. PG.
Timing- try 10* BTDC and then reconnect the vacuum advance can to manifold vacuum, timing should go to about 26* at idle.
How are the springs and weights on the centrifugal advance- are they stock or have they been changed?
After 3 carb changes I doubt that they all had the wrong power piston spring. PG.
