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Discussion Starter · #1 ·
Hi All,

I just had my 396 rebuilt. I upgraded to a 454 crank and bored 90 over to make it a 434. I also threw in a moderate cam. While the engine was being rebuilt, I picked up a used Edelbrock 1901 Quadrajet carb and rebuilt it with an Edelbrock rebuild kit (P/N 1920). Now that it’s installed on the car, it runs great off the primaries and allowed me to break the engine and cam in but, it doesn’t seem to have the power I was expecting. My engine builder said it should roast the tires off my 69 chevelle when I punch the gas but it doesn’t.

when I drive it, It seems like the secondaries take forever to kick in. If I mash the pedal to the floor, I’ll get about halfway through whatever gear I’m in and then it puts you back in the seat.

Any ideas why this might be? Could it be the accelerator pump? I have the rod in the hole furthest from the pump (closest to the end of the lever). I already backed the spring tension off the air doors a little. If I rev it up quick by hand I can see them open maybe a half inch.

Could it be something else unrelated to the carb (the carb is just where my mind went first)?
 

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1970 SS 396 25-C, 1966 SS R-R
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See if it has an adjustment screw for the spring that holds the secondary top butterflies shut. If it does turn the screw to weaken the spring pressure on the butterflies. Also with the engine warm and off have someone press the gas pedal, then look down in the carburetor to see if you are getting full throttle front and rear.
 

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Discussion Starter · #3 ·
See if it has an adjustment screw for the spring that holds the secondary top butterflies shut. If it does turn the screw to weaken the spring pressure on the butterflies. Also with the engine warm and off have someone press the gas pedal, then look down in the carburetor to see if you are getting full throttle front and rear.
Thanks for the reply. I already checked the blades. The secondary mechanical blades do open up all the way with the pedal to the floor.

I already took some tension off the air door springs. Do you think I need to take more tension off them?
 

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Excessive loosening of the air door spring is a recipe for disaster. Also, having the link on the accelerator pump arm in the farthest out hole makes the pump shot less than when it's in the inner hole. Advancing initial timing might help out a bunch in your situation. One moar thing.... don't make a bunch of changes all at once. If you do, there's no telling which change helped and which one didn't.
 
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Discussion Starter · #5 ·
Excessive loosening of the air door spring is a recipe for disaster. Also, having the link on the accelerator pump arm in the farthest out hole makes the pump shot less than when it's in the inner hole. Advancing initial timing might help out a bunch in your situation. One moar thing.... don't make a bunch of changes all at once. If you do, there's no telling which change helped and which one didn't.
Thanks for the info. I’ll leave the spring tension on the air doors alone for now. I’ll start with moving the accelerator pump linkage. If that doesn’t do it, ill try to advance the timing.
 

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You went the wrong way on the air doors, if anything you should have gone with more tension. You should start by tightening them about 1/4 turn and road test until you feel power pick up (just don't go so far as to distort the spring).
Once upon a time I chased a Q jet problem for a couple of days, it ran fine on the primary's but fell apart on the secondary's. After checking ignition/timing and taking the carb apart and reinspecting everything, several times I was stumped I ultimately brought my truck to work and installed my carb, the car ran great telling me it had to be the carb. I did find the issue was in that hanger for the secondary metering rods was bent, so half of the engine was getting a lot more fuel than the other. I now make that one of the things I check when rebuilding a Q Jet.
As mentioned before make sure you have enough timing in it or all the fuel in the world won't make it run.
 

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Discussion Starter · #7 ·
You went the wrong way on the air doors, if anything you should have gone with more tension. You should start by tightening them about 1/4 turn and road test until you feel power pick up (just don't go so far as to distort the spring).
Once upon a time I chased a Q jet problem for a couple of days, it ran fine on the primary's but fell apart on the secondary's. After checking ignition/timing and taking the carb apart and reinspecting everything, several times I was stumped I ultimately brought my truck to work and installed my carb, the car ran great telling me it had to be the carb. I did find the issue was in that hanger for the secondary metering rods was bent, so half of the engine was getting a lot more fuel than the other. I now make that one of the things I check when rebuilding a Q Jet.
As mentioned before make sure you have enough timing in it or all the fuel in the world won't make it run.
Before taking some of the tension off the air doors I rarely ever got that put you in your seat feeling. Since loosening the air door tension, it puts you back in the seat. I assume that feeling is the secondaries opening up. It is happening consistently but just way too late.

The car has an HEI distributor. Could the lag in power be due to the mechanical timing advance coming in too late? If I advance the timing anymore the car wont start.
 

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Before taking some of the tension off the air doors I rarely ever got that put you in your seat feeling. Since loosening the air door tension, it puts you back in the seat. I assume that feeling is the secondaries opening up. It is happening consistently but just way too late.

The car has an HEI distributor. Could the lag in power be due to the mechanical timing advance coming in too late? If I advance the timing anymore the car wont start.
The secondary's are mechanical they open whenever you mash the throttle given the linkage is in proper adjustment. The air doors on the top control only mixture tighter=richer, looser=leaner. Generally speaking richer on the secondary's on a Q jet is your friend.
 

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Discussion Starter · #9 ·
[QUOTE="skier207, post: 11639406, member: 171129"
The air doors on the top control only mixture tighter=richer, looser=leaner.
[/QUOTE]

@skier207
Can you break this down for me? I’m having trouble understanding this. Isn’t the air to fuel ratio constant since the metering rods are lifted out of the jets by the air doors? For example if the air doors are open 50% isn’t the same amount of air and fuel being delivered regardless of how tight the spring is? How does the spring tension change the ratio between air and fuel to make it leaner or richer?
 

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[QUOTE="skier207, post: 11639406, member: 171129"
The air doors on the top control only mixture tighter=richer, looser=leaner.
@skier207
Can you break this down for me? I’m having trouble understanding this. Isn’t the air to fuel ratio constant since the metering rods are lifted out of the jets by the air doors? For example if the air doors are open 50% isn’t the same amount of air and fuel being delivered regardless of how tight the spring is? How does the spring tension change the ratio between air and fuel to make it leaner or richer?
[/QUOTE]
Think of it this way the air door operates somewhat as a choke, the harder the engine has to pull to open the air valves the more fuel it will get relative to the air it gets. This works up to a point then it starts cutting into total CFM which isn’t a big deal on smaller lower perf engines but in the BB territory it can become an issue. By increasing the tension it is more of a test to see if the engine will respond to a richer mixture. If so you may want to change the secondary metering rods they are marked and different ones are available.
One of the quick fixes I have successful used is to place a thin wire about .020 (a small paperclip sometimes works) bend it into a tight U shape and install it under the secondary metering rod hanger this lifts the metering rods a little higher out of the jets hence giving you more fuel per secondary air blade opening. The hanger is the piece that is right behind the main airhorn, (there's a picture of one in the attached link) between the secondary air valves held in with one small screw a straight slot on older carbs. You don’t have to take the carb apart just take off the air cleaner it is on top on the outside of the carb. While you have the hanger out check to make sure the “arms” are not bent I had one that was bent it had me going for quite a while before I found it.
Lastly make sure you have adequate fuel supply, Quadrajets have a very small float bowl and run out of fuel quickly in one case I had to install a larger orifice needle seat to keep up with demand.

I don’t know if this makes any sense but it is the best way I can explain it.

Here is a good link for buying parts such as different metering rods. Quadrajet primary and secondary metering rods and hangers Page (carburetion.com)
 

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This is what a performance Pontiac Q-jet looks like. Those slots in the rear flaps enrich the rear barrels. That trick worked for me many years ago.


 

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The car has an HEI distributor. Could the lag in power be due to the mechanical timing advance coming in too late? If I advance the timing anymore the car wont start.
Yup. You do really need to set the timing and TIMING CURVE before you fiddle anymore with the Qjet. Truly. Unless th eengine builder set it, and then I would NEVER ASSUME and check it again. 16-18 initial + another 9 degrees at the distributor ( 18 at crank) = 36 total @ 2500 rpm ( or set to your cruise rpm) is a good baseline. ALSO, some oem HEI units throw WAY TOO MUCH vacuum advance like 20 degrees worth, so check that as well. Fine for an 8.5:1 1985 smog engine, not so much with your pistons !

PS , all those rods are TAPERED, so as they lift, an increasing amount of fuel is let loose by the shrinking cross sectional area as the taper diminishes coming out of the jets.
 
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Before taking some of the tension off the air doors I rarely ever got that put you in your seat feeling. Since loosening the air door tension, it puts you back in the seat. I assume that feeling is the secondaries opening up. It is happening consistently but just way too late.

The car has an HEI distributor. Could the lag in power be due to the mechanical timing advance coming in too late? If I advance the timing anymore the car wont start.
Sounds like you are going in the right direction. Adjust it a little further. For reference, if you go too far you will hear the secondaries flop open with a "buhWAHH" sound and after a delay feel the power come on strong as the air flow and fuel catches up. Go back the other direction until you get a smooth transition to WOT.

Regarding your timing, have you mapped the curve from idle to full advance to see where it is at? Measure from idle on up in 500rpm increments. That will help determine if that is a factor.
 

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Discussion Starter · #14 ·
@skier207
Can you break this down for me? I’m having trouble understanding this. Isn’t the air to fuel ratio constant since the metering rods are lifted out of the jets by the air doors? For example if the air doors are open 50% isn’t the same amount of air and fuel being delivered regardless of how tight the spring is? How does the spring tension change the ratio between air and fuel to make it leaner or richer?
Think of it this way the air door operates somewhat as a choke, the harder the engine has to pull to open the air valves the more fuel it will get relative to the air it gets. This works up to a point then it starts cutting into total CFM which isn’t a big deal on smaller lower perf engines but in the BB territory it can become an issue. By increasing the tension it is more of a test to see if the engine will respond to a richer mixture. If so you may want to change the secondary metering rods they are marked and different ones are available.
One of the quick fixes I have successful used is to place a thin wire about .020 (a small paperclip sometimes works) bend it into a tight U shape and install it under the secondary metering rod hanger this lifts the metering rods a little higher out of the jets hence giving you more fuel per secondary air blade opening. The hanger is the piece that is right behind the main airhorn, (there's a picture of one in the attached link) between the secondary air valves held in with one small screw a straight slot on older carbs. You don’t have to take the carb apart just take off the air cleaner it is on top on the outside of the carb. While you have the hanger out check to make sure the “arms” are not bent I had one that was bent it had me going for quite a while before I found it.
Lastly make sure you have adequate fuel supply, Quadrajets have a very small float bowl and run out of fuel quickly in one case I had to install a larger orifice needle seat to keep up with demand.

I don’t know if this makes any sense but it is the best way I can explain it.

Here is a good link for buying parts such as different metering rods. Quadrajet primary and secondary metering rods and hangers Page (carburetion.com)
[/QUOTE]
Thanks for the reply. I think I need to read this a few more times and hopefully it will click. One more clarification, when the air doors are closed, are the metering rods allowing any fuel to get by into the secondaries?
 

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Discussion Starter · #16 ·
Yup. You do really need to set the timing and TIMING CURVE before you fiddle anymore with the Qjet. Truly. Unless th eengine builder set it, and then I would NEVER ASSUME and check it again. 16-18 initial + another 9 degrees at the distributor ( 18 at crank) = 36 total @ 2500 rpm ( or set to your cruise rpm) is a good baseline. ALSO, some oem HEI units throw WAY TOO MUCH vacuum advance like 20 degrees worth, so check that as well. Fine for an 8.5:1 1985 smog engine, not so much with your pistons !

PS , all those rods are TAPERED, so as they lift, an increasing amount of fuel is let loose by the shrinking cross sectional area as the taper diminishes coming out of the jets.
Yeah, I’m not using the vacuum advance. My engine builder said that would probably damage the engine. We initially set the timing to about 34 deg somewhere around 3000 RPM and the engine was happy but after turning the engine off, it wouldn’t start again so we had to retard it. I’m going to move the accelerator pump linkage tomorrow and road test, my hopes are not high that will solve the problem but I think the linkage should be moved to the hole Edelbrock originally had it in regardless.
Then I’ll get my timing set and back to the air doors if need be.
 

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Hi All,

I just had my 396 rebuilt. I upgraded to a 454 crank and bored 90 over to make it a 434. I also threw in a moderate cam. While the engine was being rebuilt, I picked up a used Edelbrock 1901 Quadrajet carb and rebuilt it with an Edelbrock rebuild kit (P/N 1920). Now that it’s installed on the car, it runs great off the primaries and allowed me to break the engine and cam in but, it doesn’t seem to have the power I was expecting. My engine builder said it should roast the tires off my 69 chevelle when I punch the gas but it doesn’t.

when I drive it, It seems like the secondaries take forever to kick in. If I mash the pedal to the floor, I’ll get about halfway through whatever gear I’m in and then it puts you back in the seat.

Any ideas why this might be? Could it be the accelerator pump? I have the rod in the hole furthest from the pump (closest to the end of the lever). I already backed the spring tension off the air doors a little. If I rev it up quick by hand I can see them open maybe a half inch.

Could it be something else unrelated to the carb (the carb is just where my mind went first)?
May not be the carb.

Recommend reading the 'ignition 101' sticky in the ignition section of this forum.

Get/verify the timing is right before futzing with carb.

Pete
 

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For sure, you have to get the "foundation" set up before you can tackle the carb settings. If it was at 34 degrees and you drew it back to 28-26, this could be part of the problem. It also could indicate that the mechanical curve ( the weights that take it from the initial to total), is too SHORT. Most of us have to limit it using bushings or stops. You may need more actually. As I said a common set up is 16-18 initial with 34-38 total. That's near 18 crank degrees so simply 18+18 = 36.

Can you tell us what initial timing was when you set it to get 34 @ 3000 rpms>?
 

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Think of it this way the air door operates somewhat as a choke, the harder the engine has to pull to open the air valves the more fuel it will get relative to the air it gets. This works up to a point then it starts cutting into total CFM which isn’t a big deal on smaller lower perf engines but in the BB territory it can become an issue. By increasing the tension it is more of a test to see if the engine will respond to a richer mixture. If so you may want to change the secondary metering rods they are marked and different ones are available.
One of the quick fixes I have successful used is to place a thin wire about .020 (a small paperclip sometimes works) bend it into a tight U shape and install it under the secondary metering rod hanger this lifts the metering rods a little higher out of the jets hence giving you more fuel per secondary air blade opening. The hanger is the piece that is right behind the main airhorn, (there's a picture of one in the attached link) between the secondary air valves held in with one small screw a straight slot on older carbs. You don’t have to take the carb apart just take off the air cleaner it is on top on the outside of the carb. While you have the hanger out check to make sure the “arms” are not bent I had one that was bent it had me going for quite a while before I found it.
Lastly make sure you have adequate fuel supply, Quadrajets have a very small float bowl and run out of fuel quickly in one case I had to install a larger orifice needle seat to keep up with demand.

I don’t know if this makes any sense but it is the best way I can explain it.

Here is a good link for buying parts such as different metering rods. Quadrajet primary and secondary metering rods and hangers Page (carburetion.com)
Thanks for the reply. I think I need to read this a few more times and hopefully it will click. One more clarification, when the air doors are closed, are the metering rods allowing any fuel to get by into the secondaries?
[/QUOTE]

Yes
 

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Discussion Starter · #20 ·
For sure, you have to get the "foundation" set up before you can tackle the carb settings. If it was at 34 degrees and you drew it back to 28-26, this could be part of the problem. It also could indicate that the mechanical curve ( the weights that take it from the initial to total), is too SHORT. Most of us have to limit it using bushings or stops. You may need more actually. As I said a common set up is 16-18 initial with 34-38 total. That's near 18 crank degrees so simply 18+18 = 36.

Can you tell us what initial timing was when you set it to get 34 @ 3000 rpms>?
I can’t. We set the timing while breaking in the cam. Then we shut it down before taking a reading at idle. Then had to retard it to start it up again.

I’m going to put a mechanical advance recurve kit into it so that the timing comes in a little faster and hopefully that will allow me to keep the timing retarded enough to start and get the timing advanced soon enough for good performance.

I’ll let you all know how it goes today.
 
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