Rochester jet/rod HELP
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Joined: Sep 2005
Posts: 83
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From: Woodbridge VA
Rochester jet/rod HELP
My latest issue....... I have a 71 base motor, with the original Quadrajet carb. 7041213. I rebuilt the motor with flat top pistons, 327/350 cam, balanced and kept the rest of it basically stock. Cast iron manifold, stock exhaust manifolds, etc. I feel the carb is underperforming for the motor and need some help on jet/rod combinations.
It had #74 main jets and # 44 rods. With this combination I could not get the idle screws to have any impact on tuning. I put #76 jets in ( with the stock #44 rods) and now I can get an idle screw adjustment, but still not feeling the correct power. I took it to an inspection station for an emissions test and at 2500rpm, they said I had the emissions of a 85 k-car. They thought I could go up another 2 jet sizes. Looking at the jet charts by Lars Grimsrud, I might need to go a couple sizes lower on the rods, not the jets.
Any Help?
It had #74 main jets and # 44 rods. With this combination I could not get the idle screws to have any impact on tuning. I put #76 jets in ( with the stock #44 rods) and now I can get an idle screw adjustment, but still not feeling the correct power. I took it to an inspection station for an emissions test and at 2500rpm, they said I had the emissions of a 85 k-car. They thought I could go up another 2 jet sizes. Looking at the jet charts by Lars Grimsrud, I might need to go a couple sizes lower on the rods, not the jets.
Any Help?
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
74/44 is what your carb came with from the factory. I'd leave it for now. That cam (the old "151" cam,..good chioice) could have your primary throttle blades too open at idle rendering your idle mix screws useless. That can be fixed, but first,..
Tell us more:
...about the carb?
Has it been rebuilt recently?
With the original parts?
With the correct kit?
Float level ok?
Is the power piston stuck?
Have you adjusted the rate of opening of the secondaries (air valves) to your mill?
Tell us more about "not getting correct power". What's the engine doing?
Lastly, many a carb problem has fixed by correctly setting the timing. Tell us what's going on there.
Tell us more:
...about the carb?
Has it been rebuilt recently?
With the original parts?
With the correct kit?
Float level ok?
Is the power piston stuck?
Have you adjusted the rate of opening of the secondaries (air valves) to your mill?
Tell us more about "not getting correct power". What's the engine doing?
Lastly, many a carb problem has fixed by correctly setting the timing. Tell us what's going on there.
Last edited by 73, Dark Blue 454; Jul 11, 2011 at 11:35 PM.
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
Worked on several Q-Jets, and nothing comes close to these on the street. Agree the power piston doesnt take much a piece of grit to hang it up, but an easy check. Not a bad idea pulling the distributor, and checking for a gummed up advance shaft, as very common. Also, places like H-O Entrprises package custom Q-Jet tuning kits.
Thread Starter
Advanced
Joined: Sep 2005
Posts: 83
Likes: 0
From: Woodbridge VA
Carb is stock (correct) that came on the car from the factory. I had it professionally rebuilt and it has about 20 miles on it since the rebuild. The stock motor was the base motor with 270HP and with #74 jets, #44 primary rods, and AR secondary rods. The only change I have made is to change the main jets to #76. I double checked the float level and power piston when I changed the jets.
Before changing the jets, I could get no response from the idle screws, now with the new jets I can get a response from the screws. It runs great at idle and cruise (no hesitation). I just don't feel that I am getting the power that I should. I have not adjusted the secondaries, since I have not gotten that far to really open it up.
The emissions test I just got proved that it is really lean at 2500rpm.
Before changing the jets, I could get no response from the idle screws, now with the new jets I can get a response from the screws. It runs great at idle and cruise (no hesitation). I just don't feel that I am getting the power that I should. I have not adjusted the secondaries, since I have not gotten that far to really open it up.
The emissions test I just got proved that it is really lean at 2500rpm.
Last edited by HunterPayne; Jul 15, 2011 at 12:17 PM.
