Rochester Quadrabog
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Rochester Quadrabog (Rhys)
Chris and I have been "chatting" a little about this subject via e-mail, so I thought I'd post some of our discussions here for all to see. I thought some of the trouble-shooting techniques and procedures might come in handy for others experiencing similar situations...:
From Chris:
Hey Lars. You suggested that I check my airvalve spring that it is no more than 7/8 wound. Well, I do not know which it is. Also,
When I am running at like 2000-3000 rpm and I punch it, I get a 1 second hesitation and a bog. How can I check that the secondaries are opening on my Rochester?
Reply from Lars:
In terms of checking to see if the Q-Jet secondaries are operating, I have never seen a Q-Jet with inoperable secondaries. The only thing that can prevent the secondaries from opening is the secondary lockout lever which is controlled by the choke. This lever is located on the passenger side of the carb, right down at the bottom at the secondary throttle shaft, It's an "L"-shaped device that locks out the secondary shaft by hooking itself over the shaft pin. Take a look at it and just make sure it retracts all the way when the engine is warm. You can manually open the throttle all the way (engine off, of course...) and observe if the secondary shaft is rotating: poke your finger into the secondary airvalve to open it up and look straight down the secondary side. You should be able to see the secondary throttle plates open up as you move the throttle to the wide open position. Usually, if you're getting a bog, they're working. Unfortunately, they're working a little TOO good and coming in too soon. The airvalve spring controls this.
Look at the passenger side of the carb, at the rear, up at the top. you have a piece of linkage running from your choke pulloff diaphragm to a lever arm right at the top of the secondaries. This lever arm is attached to the secondary airvalve. If you look at the side of the carb right where this lever arm is located, you'll see a small slotted-head screw. This screw is the adjustment screw for the secondary airvalve windup spring. If you use a mirror and look up underneath the area where the screw is, you'll see a small allen-head screw and you'll be able to see the spring and the lever that the spring contacts. The allen-head screw is a locking screw for the slotted head adjusting screw.
To adjust, use your mirror to see what's going on. Place a small screwdriver in the slotted head adjustment screw and hold on. Then, loosen the allen lock screw up about 1/4 turn to unlock the slotted screw. With the allen screw loosened, you will be able to turn the slotted screw with the screwdriver. While watching the spring in your mirror, let the slotted screw unwind from the spring and count how far it unwinds before you see the spring come out of contact with the pin lever. Note if it was less than 7/8 turn. Now, turn the slotted screw until you can see the spring JUST BARELY make contact with the lever. Then turn the screw 7/8 turn, hold it there, and tighten the allen lock screw to lock the slotted screw in this position. It helps to use all three hands while doing this....
Let me know if you need any assistance, and let me know if this fixes the problem. If it doesn't, I have all kinds of other thngs you can check...
From Chris:
Hi again Lars. I have spent too much time screwing with my Q-jet. It has been rebuilt and I have adjusted several things as you stated. It is still operating very strangely.
Do you think I should send it to you or just buy a new one?
Reply from Lars:
Chris -
Thanks for the note - I just pulled up your thread and read through everything.....
The thing that concerns me is that it appears that you can get the carb to run right by pulling the entire air cleaner, including the base, off the car. Thus, you don't have a carb-only problem - you have a system integration problem.
First, one of the Forum members suggested that you take a look at the hookup to your vacuum advance and make sure it's in the forward driver's side port. Note that several carbs of your vintage have a reverse vacuum port setup, where the forward driver's side port is actually direct manifold vacuum, and the ported vacuum is actually located at the port that comes out of the lower throttle plate on the forward passenger side at a 45-degree angle out the front. Check these two ports, and identify which one has virtually no vacuum at idle. The hook your distributor vac advance up to the one with little/no vacuum at idle.
Also, if your carb has been given a "performance" rebuild by somebody in the past, it is common for some of the speed shops to drill out the idle mixture orifices in the carb body. This will often produce an idle mixture which is too rich, and the car will bog on acceleration due to too much fuel. This becomes aggravated with the air cleaner installed, since this richens it up even more. You can check to see if this is the problem very easily: Simply knock the roll pin out of the accelerator pump lever on the top of the carb and remove the lever to inactivate the accelerator pump. I do this with the engine running so I don't have any trouble starting the engine without the pump hooked up. With the accel pump disconnected, rev the engine and/or drive the car. If the bog goes away or performance improves, your idle mixture is too rich and needs to be corrected through a jetting change/rod change. If the situation gets worse, you're running lean, and need to re-jet for a richer condition.
Try this out and let me know what results you get. Based on this, we can determine which route to go. I would be more than glad to take a look at the carb for you, however I do not have a Chevy engine in my inventory to actually run it on right now. I can run it on one of my Pontiacs with a little work if need be, but try the accel pump trick first and let me know what happens.
From Chris:
Sounds good. I can check this tonight. The thing that still concerns me is that the fuel leaks out around the accelerator pump rod (top of the carb). I can even smell it when I floor it while driving the car.
The advance is connected to the driver side port. I think I measured almost 0 vacuum at idle and 18 at 3000 rpm. I will double check tonight though.
Reply from Lars:
Hmmmm.... It is not unusual for fuel to "seep" a little from around the accel pump rod, since there is not a good, solid seal between the pump rod and the fuel bowl. Thus, on hard braking, sharp right hand corners, and very bumpy roads is possible to get a little "splash" seeping from around the rod area. However, if you're getting fuel seepage in this area while the engine is at idle or during normal, flat driving, it is indicative of the float level being too high, either caused from an incorrect float setting, a fuel-saturated float, or from a faulty needle/seat. A high fuel level, caused from any one of these factors, will cause the symptoms you describe due to a rich condition. It is not uncommon for carb builders to set the Q-Jet float level to the old '60s-era performance spec of 1/4". This is too high for your vintage carb, and it will cause a bog. Your carb needs a float level of .400" - .420" in order to perform correctly. If you're proficient with popping the top off of a Q-Jet, you can easily check this. Either way, it sounds like you're experiencing a rich condition, and the float level would be the prime suspect at this point based on your descriptions. Let me know if you need any assistance.
From Chris:
Hey Lars. You suggested that I check my airvalve spring that it is no more than 7/8 wound. Well, I do not know which it is. Also,
When I am running at like 2000-3000 rpm and I punch it, I get a 1 second hesitation and a bog. How can I check that the secondaries are opening on my Rochester?
Reply from Lars:
In terms of checking to see if the Q-Jet secondaries are operating, I have never seen a Q-Jet with inoperable secondaries. The only thing that can prevent the secondaries from opening is the secondary lockout lever which is controlled by the choke. This lever is located on the passenger side of the carb, right down at the bottom at the secondary throttle shaft, It's an "L"-shaped device that locks out the secondary shaft by hooking itself over the shaft pin. Take a look at it and just make sure it retracts all the way when the engine is warm. You can manually open the throttle all the way (engine off, of course...) and observe if the secondary shaft is rotating: poke your finger into the secondary airvalve to open it up and look straight down the secondary side. You should be able to see the secondary throttle plates open up as you move the throttle to the wide open position. Usually, if you're getting a bog, they're working. Unfortunately, they're working a little TOO good and coming in too soon. The airvalve spring controls this.
Look at the passenger side of the carb, at the rear, up at the top. you have a piece of linkage running from your choke pulloff diaphragm to a lever arm right at the top of the secondaries. This lever arm is attached to the secondary airvalve. If you look at the side of the carb right where this lever arm is located, you'll see a small slotted-head screw. This screw is the adjustment screw for the secondary airvalve windup spring. If you use a mirror and look up underneath the area where the screw is, you'll see a small allen-head screw and you'll be able to see the spring and the lever that the spring contacts. The allen-head screw is a locking screw for the slotted head adjusting screw.
To adjust, use your mirror to see what's going on. Place a small screwdriver in the slotted head adjustment screw and hold on. Then, loosen the allen lock screw up about 1/4 turn to unlock the slotted screw. With the allen screw loosened, you will be able to turn the slotted screw with the screwdriver. While watching the spring in your mirror, let the slotted screw unwind from the spring and count how far it unwinds before you see the spring come out of contact with the pin lever. Note if it was less than 7/8 turn. Now, turn the slotted screw until you can see the spring JUST BARELY make contact with the lever. Then turn the screw 7/8 turn, hold it there, and tighten the allen lock screw to lock the slotted screw in this position. It helps to use all three hands while doing this....
Let me know if you need any assistance, and let me know if this fixes the problem. If it doesn't, I have all kinds of other thngs you can check...
From Chris:
Hi again Lars. I have spent too much time screwing with my Q-jet. It has been rebuilt and I have adjusted several things as you stated. It is still operating very strangely.
Do you think I should send it to you or just buy a new one?
Reply from Lars:
Chris -
Thanks for the note - I just pulled up your thread and read through everything.....
The thing that concerns me is that it appears that you can get the carb to run right by pulling the entire air cleaner, including the base, off the car. Thus, you don't have a carb-only problem - you have a system integration problem.
First, one of the Forum members suggested that you take a look at the hookup to your vacuum advance and make sure it's in the forward driver's side port. Note that several carbs of your vintage have a reverse vacuum port setup, where the forward driver's side port is actually direct manifold vacuum, and the ported vacuum is actually located at the port that comes out of the lower throttle plate on the forward passenger side at a 45-degree angle out the front. Check these two ports, and identify which one has virtually no vacuum at idle. The hook your distributor vac advance up to the one with little/no vacuum at idle.
Also, if your carb has been given a "performance" rebuild by somebody in the past, it is common for some of the speed shops to drill out the idle mixture orifices in the carb body. This will often produce an idle mixture which is too rich, and the car will bog on acceleration due to too much fuel. This becomes aggravated with the air cleaner installed, since this richens it up even more. You can check to see if this is the problem very easily: Simply knock the roll pin out of the accelerator pump lever on the top of the carb and remove the lever to inactivate the accelerator pump. I do this with the engine running so I don't have any trouble starting the engine without the pump hooked up. With the accel pump disconnected, rev the engine and/or drive the car. If the bog goes away or performance improves, your idle mixture is too rich and needs to be corrected through a jetting change/rod change. If the situation gets worse, you're running lean, and need to re-jet for a richer condition.
Try this out and let me know what results you get. Based on this, we can determine which route to go. I would be more than glad to take a look at the carb for you, however I do not have a Chevy engine in my inventory to actually run it on right now. I can run it on one of my Pontiacs with a little work if need be, but try the accel pump trick first and let me know what happens.
From Chris:
Sounds good. I can check this tonight. The thing that still concerns me is that the fuel leaks out around the accelerator pump rod (top of the carb). I can even smell it when I floor it while driving the car.
The advance is connected to the driver side port. I think I measured almost 0 vacuum at idle and 18 at 3000 rpm. I will double check tonight though.
Reply from Lars:
Hmmmm.... It is not unusual for fuel to "seep" a little from around the accel pump rod, since there is not a good, solid seal between the pump rod and the fuel bowl. Thus, on hard braking, sharp right hand corners, and very bumpy roads is possible to get a little "splash" seeping from around the rod area. However, if you're getting fuel seepage in this area while the engine is at idle or during normal, flat driving, it is indicative of the float level being too high, either caused from an incorrect float setting, a fuel-saturated float, or from a faulty needle/seat. A high fuel level, caused from any one of these factors, will cause the symptoms you describe due to a rich condition. It is not uncommon for carb builders to set the Q-Jet float level to the old '60s-era performance spec of 1/4". This is too high for your vintage carb, and it will cause a bog. Your carb needs a float level of .400" - .420" in order to perform correctly. If you're proficient with popping the top off of a Q-Jet, you can easily check this. Either way, it sounds like you're experiencing a rich condition, and the float level would be the prime suspect at this point based on your descriptions. Let me know if you need any assistance.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Rochester Quadrabog (lars)
From Chris:
How do I measure and set the float level? I have never taken a carb (for a automobile) apart, but I am mechanically inclined enough to handle it if I know how.
Chris -
On your Q-Jet, proceed as follows:
Once the air cleaner is off:
1. Remove the air cleaner attach stud
2. Knock the roll pin out of the accel pump level using a small finish nail, a pop rivet stem, a small pin punch, or whatever else you have laying around. When you tap the pin out of the lever (you will tap in it towards the choke air horn wall), don't tap it so far in that it hits the choke air horn: leave just a little gap between the end of the roll pin and the choke air horn so you can get a screwdriver blade between the pin and the air horn in order to pry the pin back into place again later. Remove the lever.
3. Remove the 2 screws that attach the choke pulloff diaphragm assembly to the carb air horn and remove the diaphragm with its secondary airvalve rod.
4. Remove the screw attaching the secondary rod hanger and remove the hanger with the secondary rods.
5. Remove the screw attaching the choke rod lever to the passenger side of the choke and remove the lever and the connecting rod. The connecting rod will need to be rotated and cocked slightly to disengage from its actuating lever inside the carb. If your carb has a simple clip on the choke rod and not a screw on the choke lever, simply remove the clip and remove the rod. Most later-model QJets use the screw-on lever.
6. Remove the 9 screws holding the carb air horn (the "top" of the carb) to the float bowl: Two long screws at the back, 2 screws on either side of the secondary air valves, 2 screws right behind the choke air horn, 2 small screws down inside the air horn righ by the venturis, and 1 screw right at the front edge. The 2 screws inside the air horn are designed with heads on them large enough that they cannot drop down through the venturi and into the intake manifold, but I prefer to not tempt fate and test their size, so use a pair of needlenose to fish them out of the air horn once they're loose.
7. Lift the carb air horn straight up until the air emulsion tubes clear the gasket and the accel pump rod is clear of the air horn.
8. Remove the accel pump
9. Carefully remove the air horn gasket by freeing it from around the power piston.
10. Remove the power piston and the primary metering rids by depressing the piston and "flicking" it up against the spring pressure. This will pop it out of the bore.
11. Remove the float bowl filler block.
You can now check and set the float level:
Using one finger to firmly seat the spring pin float hinge into the bowl, lightly press down on theend of the float over the needle to seat the needly lightly in the seat. Measure the distance from the top of the float bowl to the top of the rear edge of the float. In your vintage carb, this should be .400" - .420". To adjust, remove the float with the attached needle and bend it carefully at the notched area of the float arm. When you re-install the float with the needle clipped to it, note that the little clip does not hook into the float through the holes in the float arm: the clip slides over the half-round protrosion at the rear edge of the actuating surface without going through either of the holes. Insall the float and needle and re-check.
With the air horn off the carb, practice installing the choke actuator rod:
Take the rod and note the position/orientation of the mating actuating lever inside the carb. Practice inserting the rod into the actuating arm by lower the rod, slightly twisted, engaging the hole, and twisting the rod to engage it. You will be doing this somewhat blindly once the air horn is installed, so get the technique down now while you can see what you're doing...
To re-assemble, reverse the procedure. A few notes:
After dropping the float bowl filler block back in, you need to install the power piston and metering rods. It is a little difficult to get the orods to hit the jets and engage them, but make sure that the rods insert the jets and that the power piston is free to move up and down against its spring pressure with the locking collar holding it all in place.
Don't over-tighten the air horn screws. This will distort and chew up the top of your carb. Start by snugging up the screws in the center of the carb and work your way out. Re-check.
To install the accel pump lever, insert whatever tool you used to punch the pin out through the hole to make the lever line up. Then use a screwdrive and pry the pin back in by prying between the pin and the choke air horn wall.
Install all the other removed components and try it out.
Contact me with any questions.
Lars
-----Original Message-----
From: Chris [mailto:Chris@VetteFinders.com]
Sent: Monday, August 20, 2001 9:44 AM
To: Grimsrud, Lars
Subject: Re: Hesitation
How do I measure and set the float level? I have never taken a carb (for a automobile) apart, but I am mechanically inclined enough to handle it if I know how.
Chris -
On your Q-Jet, proceed as follows:
Once the air cleaner is off:
1. Remove the air cleaner attach stud
2. Knock the roll pin out of the accel pump level using a small finish nail, a pop rivet stem, a small pin punch, or whatever else you have laying around. When you tap the pin out of the lever (you will tap in it towards the choke air horn wall), don't tap it so far in that it hits the choke air horn: leave just a little gap between the end of the roll pin and the choke air horn so you can get a screwdriver blade between the pin and the air horn in order to pry the pin back into place again later. Remove the lever.
3. Remove the 2 screws that attach the choke pulloff diaphragm assembly to the carb air horn and remove the diaphragm with its secondary airvalve rod.
4. Remove the screw attaching the secondary rod hanger and remove the hanger with the secondary rods.
5. Remove the screw attaching the choke rod lever to the passenger side of the choke and remove the lever and the connecting rod. The connecting rod will need to be rotated and cocked slightly to disengage from its actuating lever inside the carb. If your carb has a simple clip on the choke rod and not a screw on the choke lever, simply remove the clip and remove the rod. Most later-model QJets use the screw-on lever.
6. Remove the 9 screws holding the carb air horn (the "top" of the carb) to the float bowl: Two long screws at the back, 2 screws on either side of the secondary air valves, 2 screws right behind the choke air horn, 2 small screws down inside the air horn righ by the venturis, and 1 screw right at the front edge. The 2 screws inside the air horn are designed with heads on them large enough that they cannot drop down through the venturi and into the intake manifold, but I prefer to not tempt fate and test their size, so use a pair of needlenose to fish them out of the air horn once they're loose.
7. Lift the carb air horn straight up until the air emulsion tubes clear the gasket and the accel pump rod is clear of the air horn.
8. Remove the accel pump
9. Carefully remove the air horn gasket by freeing it from around the power piston.
10. Remove the power piston and the primary metering rids by depressing the piston and "flicking" it up against the spring pressure. This will pop it out of the bore.
11. Remove the float bowl filler block.
You can now check and set the float level:
Using one finger to firmly seat the spring pin float hinge into the bowl, lightly press down on theend of the float over the needle to seat the needly lightly in the seat. Measure the distance from the top of the float bowl to the top of the rear edge of the float. In your vintage carb, this should be .400" - .420". To adjust, remove the float with the attached needle and bend it carefully at the notched area of the float arm. When you re-install the float with the needle clipped to it, note that the little clip does not hook into the float through the holes in the float arm: the clip slides over the half-round protrosion at the rear edge of the actuating surface without going through either of the holes. Insall the float and needle and re-check.
With the air horn off the carb, practice installing the choke actuator rod:
Take the rod and note the position/orientation of the mating actuating lever inside the carb. Practice inserting the rod into the actuating arm by lower the rod, slightly twisted, engaging the hole, and twisting the rod to engage it. You will be doing this somewhat blindly once the air horn is installed, so get the technique down now while you can see what you're doing...
To re-assemble, reverse the procedure. A few notes:
After dropping the float bowl filler block back in, you need to install the power piston and metering rods. It is a little difficult to get the orods to hit the jets and engage them, but make sure that the rods insert the jets and that the power piston is free to move up and down against its spring pressure with the locking collar holding it all in place.
Don't over-tighten the air horn screws. This will distort and chew up the top of your carb. Start by snugging up the screws in the center of the carb and work your way out. Re-check.
To install the accel pump lever, insert whatever tool you used to punch the pin out through the hole to make the lever line up. Then use a screwdrive and pry the pin back in by prying between the pin and the choke air horn wall.
Install all the other removed components and try it out.
Contact me with any questions.
Lars
-----Original Message-----
From: Chris [mailto:Chris@VetteFinders.com]
Sent: Monday, August 20, 2001 9:44 AM
To: Grimsrud, Lars
Subject: Re: Hesitation
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Rochester Quadrabog (flood)
Flood -
You're much too kind....
I've been racing and running Q-Jets for 30 years now. Street racing back in the early 70's we had Q-Jets on all our cars. After you've managed to completely screw up a bunch of them, you eventually figure things out..! I shudder when I think about how many nice, late-sixties vintage Q-Jets I completely destroyed by trying all kinds of cutting, drilling and go-fast-mods Du Jour. Back then, you could get used Q-Jets virtually for free, so we didn't hesitate to try some pretty bizzarre stuff and then throw the carb in the trash can if it didn't work. I still have notes I took on these mods and jetting changes etc. from 30 years ago, and I keep a running log on everything I still do so I have some idea of what works and what doesn't. I know of more things that don't work than things that do work....
You're much too kind....
I've been racing and running Q-Jets for 30 years now. Street racing back in the early 70's we had Q-Jets on all our cars. After you've managed to completely screw up a bunch of them, you eventually figure things out..! I shudder when I think about how many nice, late-sixties vintage Q-Jets I completely destroyed by trying all kinds of cutting, drilling and go-fast-mods Du Jour. Back then, you could get used Q-Jets virtually for free, so we didn't hesitate to try some pretty bizzarre stuff and then throw the carb in the trash can if it didn't work. I still have notes I took on these mods and jetting changes etc. from 30 years ago, and I keep a running log on everything I still do so I have some idea of what works and what doesn't. I know of more things that don't work than things that do work....
Thread Starter
Platinum Supporting Dealership
Joined: Dec 1999
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From: Traveling the US
Re: Rochester Quadrabog (Nomad78SA)
As a follow-up. I disassembled the carb per Lars' instructions and set the float at .410. I started the car and revved the engine about 5 times. Gas started pooling around the accel pump rod (AGAIN!). Of course, since everything is always easy, I got a screw in my tire and it is now flat, so I can not drive the car to test for bogging. Regardless, I do not like the fuel leak.
Team Owner
Joined: May 2001
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From: Exiled to Richmond, VA - Finally sold my house in Murfreesboro, TN ?? Corner of "Bumf*&k and 'You've got a purdy mouth'."
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CI-VIII Burnout Champ
St. Jude Donor '06-'10, '13
Re: Rochester Quadrabog (Rhys)
If you DO switch to the Holley, I recommend the Holley Avenger 670 CFM.
It has a lot of fixes for the problems Holley has had before.
It has a blow out proof power valve and you can now change the vacuum secondary springs without major disassembly.
I too am running the QJet right now. When I swap motors in October, I will go to the 670 Holley. I can tune and adjust Holley's but have the WORST luck with QJets.
Not a problem with the QJets, just a problem with me !!
[Modified by BSeery, 5:55 PM 8/20/2001]
It has a lot of fixes for the problems Holley has had before.
It has a blow out proof power valve and you can now change the vacuum secondary springs without major disassembly.
I too am running the QJet right now. When I swap motors in October, I will go to the 670 Holley. I can tune and adjust Holley's but have the WORST luck with QJets.
Not a problem with the QJets, just a problem with me !!
[Modified by BSeery, 5:55 PM 8/20/2001]
