Qjet tuning - primary jet and rod combinations
09-27-2002, 04:44 PM
#1
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Qjet tuning - primary jet and rod combinations
I have an Edelbrock #1901 Qjet on my 68 bigblock. I also have a 68 Qjet that is close to the original carb, tho not the "correct" number. I am interested in setting up the Edelbrock to get more power.
The original carb for my 68 bigblock (4spd) came with 71 primary jets and 45 primary rods. This combo gives a metering area of 2.37 at cruise.
The Edelbrock came with 69 primary jets and 36 primary rods. The slightly smaller jets are paired with much smaller rods to give a much larger metering area at cruise - 2.72, nearly 15% greater than the stock combo. Vette ran fine with this combo, even tho it was a lot richer (right?) at cruise. The slightly smaller metering area at WOT with the 69 jets produces a little less peak performance.
I am looking at different combinations of jets/rods to try (I have already changed the fat secondary rods to AX, which were the factory setup).
First I just changed the jets from 69 to 71 without changing the primary rods. This new combination, 71/36 provides a cruise metering area of 2.941 which is 24% more than the stock combo. Probably too rich?
Two other combinations I will try are 74/45, which gives a cruise metering area of 2.71, almost the same as the Edelbrock 1901 out-of-the box.
Then, there is 77/45 which was used in the 72 454 vette (with 4spd), but the metering area of 3.066 is 30% greater than the stock setup for the 68 L36 engine! Seems that would be too rich compared to original 68, but that is how they came in 72... :confused:
77/49 was used in the 71 454 vette, but I dont have a set of 49 primary rods.
============= but enough about my carbs !! :lol:
I am a bit fuzzy about the jet/rod combinations that were selected by GM for various applications. In 1968/69 the 427 with 4spd had 71 jets and 45 rods. In 71, they stepped up the 454 vette to 77/49. What surprised me was that the 77 (big) jets were used in 76 - 79 smallblock vettes (of course fatter rods, 48 - 53 were used also) during those "low power" years. Just seems strange to me that you have 71 jets in a 68 427, while you have 77 jets in a smallblock 76. Any insight on jet/rod selection is appreciated! :) MJ
The original carb for my 68 bigblock (4spd) came with 71 primary jets and 45 primary rods. This combo gives a metering area of 2.37 at cruise.
The Edelbrock came with 69 primary jets and 36 primary rods. The slightly smaller jets are paired with much smaller rods to give a much larger metering area at cruise - 2.72, nearly 15% greater than the stock combo. Vette ran fine with this combo, even tho it was a lot richer (right?) at cruise. The slightly smaller metering area at WOT with the 69 jets produces a little less peak performance.
I am looking at different combinations of jets/rods to try (I have already changed the fat secondary rods to AX, which were the factory setup).
First I just changed the jets from 69 to 71 without changing the primary rods. This new combination, 71/36 provides a cruise metering area of 2.941 which is 24% more than the stock combo. Probably too rich?
Two other combinations I will try are 74/45, which gives a cruise metering area of 2.71, almost the same as the Edelbrock 1901 out-of-the box.
Then, there is 77/45 which was used in the 72 454 vette (with 4spd), but the metering area of 3.066 is 30% greater than the stock setup for the 68 L36 engine! Seems that would be too rich compared to original 68, but that is how they came in 72... :confused:
77/49 was used in the 71 454 vette, but I dont have a set of 49 primary rods.
============= but enough about my carbs !! :lol:
I am a bit fuzzy about the jet/rod combinations that were selected by GM for various applications. In 1968/69 the 427 with 4spd had 71 jets and 45 rods. In 71, they stepped up the 454 vette to 77/49. What surprised me was that the 77 (big) jets were used in 76 - 79 smallblock vettes (of course fatter rods, 48 - 53 were used also) during those "low power" years. Just seems strange to me that you have 71 jets in a 68 427, while you have 77 jets in a smallblock 76. Any insight on jet/rod selection is appreciated! :) MJ
09-27-2002, 06:16 PM
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Re: Qjet tuning - primary jet and rod combinations (MNJack)
I've been doing some swapping out of primary rods and jets on my 383. I'm almost settling down. I've looked through the carb listings and have had the same questions you are asking. I have never seen an answer.
Anyway, I've run many combinations of 73, 74, 75, and 77 jets with 48 and 50 rods. I do have a later Edelbrock Q-Jet which has the "truck" rods, so the tips are .036" and not .026" like the rest. I do have an O2 sensor in the driver's side collector and have been tuning for weeks now. I am currently using a 74/48 combo which gives me 2.491/3.283 cruise/wot metering areas. I'm a little rich at cruise, but I'm going lean under light (barely even touching the pedal) acceleration. I'm now starting to play with power piston springs and once I get that setup, I'll probably go down to a 73/50 jet/rod combo which will give me 2.22/3.17 cruise/wot metering. I had run with this for a while, but it was too lean on slight accel for me to be comfortable with.
Granted, I do not have a big block, but my 383 is fairly healthy.
Anyway, I've run many combinations of 73, 74, 75, and 77 jets with 48 and 50 rods. I do have a later Edelbrock Q-Jet which has the "truck" rods, so the tips are .036" and not .026" like the rest. I do have an O2 sensor in the driver's side collector and have been tuning for weeks now. I am currently using a 74/48 combo which gives me 2.491/3.283 cruise/wot metering areas. I'm a little rich at cruise, but I'm going lean under light (barely even touching the pedal) acceleration. I'm now starting to play with power piston springs and once I get that setup, I'll probably go down to a 73/50 jet/rod combo which will give me 2.22/3.17 cruise/wot metering. I had run with this for a while, but it was too lean on slight accel for me to be comfortable with.
Granted, I do not have a big block, but my 383 is fairly healthy.
09-27-2002, 06:48 PM
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Re: Qjet tuning - primary jet and rod combinations (MNJack)
Mark -
The Edelbrock model 1901 is a great carb - an exact reproduction of the early Q-Jet built off the original tooling and molds. It can be jetted and set up to produce an excellent level of performance.
The 1901, as you noted, is jetted 69/36/DR (Main jet/main rods/sec rods). The "DR" rods are .057" in diameter at the power tips, with short tips - quite lean.
The 1901 in the stock configuration is jetted for a 1969 Chevy Truck. Not too glamorous... The trucks had very mild cams, pulled lots of manifold vacuum under no-load conditions, and were designed for pulling things. Thus, it was essential to jet the truck carbs to produce low exhaust gas temps under moderate continuous loads (therefore the rich mixture at cruise), and a rich mixture at WOT was not required due to the short-duration truck cam. Obviously, this is not the optimum setup for a Vette.
As a side note - you cannot compare jetting configurations of pre-'75 carbs to post-75 carbs (ref one of the posts above) - they are completely different. Also of note, the pre-'75 carbs do not have "truck rods" in the truck carbs. Post-'75 carbs can have truck rods (designated by the "M" stamped into the rods). Truck rods in post-75 carbs are not suitable for Vettes due to their fat power tips. This is not a problem in the early carbs, such as the EDL-1901. The "B" rods are the standard rods.
The stock '68 427 High Perf engine came jetted 71/45/AX. This runs the carb very lean and crisp at cruise, richens it up just slightly when the power piston pops up, and then goes massively rich when the secondaries pop open (AX rods are only .040" diameter - about as rich as they came). A nice compromise to this is the following combo:
Try 73/44/DA. This gives you a 2.66 metering area at cruise - quite a bit leaner than the Edelbrock jetting, but slightly richer than the stock '68 spec. This will give you a very nice throttle "feel" off idle and at cruise with minimal gas mileage penalty. The #73 jets will richen you up nicely before hitting the secondaries, and the "DA" rods (part number 7046010 still available from GM and from GMPartsDirect) at .0443" diameter will compliment this very nicely (richer than the EDL rods, but not quite as abrupt as the stoc "AX" rods). If this doesn't provide a great setup for you, it will certainly give you a solid starting point. My experience is that this setup will usually rip your head off.
Please post up your results of your tuning, and let me know if you try this combo and what you think of it.
Have fun!!!
[Modified by lars, 4:51 PM 9/27/2002]
[Modified by lars, 7:18 PM 9/27/2002]
The Edelbrock model 1901 is a great carb - an exact reproduction of the early Q-Jet built off the original tooling and molds. It can be jetted and set up to produce an excellent level of performance.
The 1901, as you noted, is jetted 69/36/DR (Main jet/main rods/sec rods). The "DR" rods are .057" in diameter at the power tips, with short tips - quite lean.
The 1901 in the stock configuration is jetted for a 1969 Chevy Truck. Not too glamorous... The trucks had very mild cams, pulled lots of manifold vacuum under no-load conditions, and were designed for pulling things. Thus, it was essential to jet the truck carbs to produce low exhaust gas temps under moderate continuous loads (therefore the rich mixture at cruise), and a rich mixture at WOT was not required due to the short-duration truck cam. Obviously, this is not the optimum setup for a Vette.
As a side note - you cannot compare jetting configurations of pre-'75 carbs to post-75 carbs (ref one of the posts above) - they are completely different. Also of note, the pre-'75 carbs do not have "truck rods" in the truck carbs. Post-'75 carbs can have truck rods (designated by the "M" stamped into the rods). Truck rods in post-75 carbs are not suitable for Vettes due to their fat power tips. This is not a problem in the early carbs, such as the EDL-1901. The "B" rods are the standard rods.
The stock '68 427 High Perf engine came jetted 71/45/AX. This runs the carb very lean and crisp at cruise, richens it up just slightly when the power piston pops up, and then goes massively rich when the secondaries pop open (AX rods are only .040" diameter - about as rich as they came). A nice compromise to this is the following combo:
Try 73/44/DA. This gives you a 2.66 metering area at cruise - quite a bit leaner than the Edelbrock jetting, but slightly richer than the stock '68 spec. This will give you a very nice throttle "feel" off idle and at cruise with minimal gas mileage penalty. The #73 jets will richen you up nicely before hitting the secondaries, and the "DA" rods (part number 7046010 still available from GM and from GMPartsDirect) at .0443" diameter will compliment this very nicely (richer than the EDL rods, but not quite as abrupt as the stoc "AX" rods). If this doesn't provide a great setup for you, it will certainly give you a solid starting point. My experience is that this setup will usually rip your head off.
Please post up your results of your tuning, and let me know if you try this combo and what you think of it.
Have fun!!!
[Modified by lars, 4:51 PM 9/27/2002]
[Modified by lars, 7:18 PM 9/27/2002]
09-27-2002, 07:39 PM
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Re: Qjet tuning - primary jet and rod combinations (lars)
man oh man oh man... Lars, you gotta come to Chicago!
off to find 73/44/DA for my carb.... :yesnod:
off to find 73/44/DA for my carb.... :yesnod:
09-27-2002, 08:59 PM
#6
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Re: Qjet tuning - primary jet and rod combinations (BlueL36)
Hey lars, where can I learn what you know?? More realistically, can somebody lead me to an intro to carb tuning? It's one of several dark voids in my car knowledge. appreciated!
09-27-2002, 09:35 PM
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Re: Qjet tuning - primary jet and rod combinations (RUXperienced)
Blue -
I used to go out to Chicago pretty frequently, but haven't been out there in a while. If I get a chance to head out that way, I'll be sure to post up a notice so we can do some tuning out that way.
For the jetting, keep in mind that all car/engine combos react a little differently to various jetting combos. There is no one magic combination that will be the one and only "best" combo for all cars. I always recommend that people start out with the stock jetting setup for their car, and verify that everything on the carb is correct - you'd be amazed at how many people attempt to re-jet and adjust their Q-Jets when, in fact, the carb has major problems with various parts and systems. No jetting or adjusting will compensate for bent power piston arms, sunk floats, jammed secondary airvalves, misadjusted throttle plates, and plugged idle metering circuits. I also have a standard observation that "most carb problems are ignition problems." In other words, before you ever start playing with the car, make darned sure your distributor is set up right with a properly functioning centrifugal advance system, a functioning vacuum advance, properly set total timing, and a good coil. If you don't do this before playing with the carb, you'll be in for a frustrating, tail-chasing experience.
RU -
The stuff I know about Q-Jets has been gathered over the last 30 years of actually working on these carbs. I have gathered GM parts books, service manuals, service bulletins, been to GM Training School, taught at the GM Training Center, raced Q-Jet equipped cars in NHRA class racing, and rebuilt hundreds of these things to gain some insight into the problems and solutions. I still don't know it all. The books I've seen are informative and factual - very good resources. But until you've torn into a bunch of carbs with a bunch of different problems, it's tough to learn all the little tricks. Unfortunately, I'm finding that a bunch of the people who really knew and understood these carbs back in the '60's and '70s are now either retired, dead, or no longer interested - it's really becoming a lost art.
[Modified by lars, 7:36 PM 9/27/2002]
I used to go out to Chicago pretty frequently, but haven't been out there in a while. If I get a chance to head out that way, I'll be sure to post up a notice so we can do some tuning out that way.
For the jetting, keep in mind that all car/engine combos react a little differently to various jetting combos. There is no one magic combination that will be the one and only "best" combo for all cars. I always recommend that people start out with the stock jetting setup for their car, and verify that everything on the carb is correct - you'd be amazed at how many people attempt to re-jet and adjust their Q-Jets when, in fact, the carb has major problems with various parts and systems. No jetting or adjusting will compensate for bent power piston arms, sunk floats, jammed secondary airvalves, misadjusted throttle plates, and plugged idle metering circuits. I also have a standard observation that "most carb problems are ignition problems." In other words, before you ever start playing with the car, make darned sure your distributor is set up right with a properly functioning centrifugal advance system, a functioning vacuum advance, properly set total timing, and a good coil. If you don't do this before playing with the carb, you'll be in for a frustrating, tail-chasing experience.
RU -
The stuff I know about Q-Jets has been gathered over the last 30 years of actually working on these carbs. I have gathered GM parts books, service manuals, service bulletins, been to GM Training School, taught at the GM Training Center, raced Q-Jet equipped cars in NHRA class racing, and rebuilt hundreds of these things to gain some insight into the problems and solutions. I still don't know it all. The books I've seen are informative and factual - very good resources. But until you've torn into a bunch of carbs with a bunch of different problems, it's tough to learn all the little tricks. Unfortunately, I'm finding that a bunch of the people who really knew and understood these carbs back in the '60's and '70s are now either retired, dead, or no longer interested - it's really becoming a lost art.
[Modified by lars, 7:36 PM 9/27/2002]
09-27-2002, 10:16 PM
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Re: Qjet tuning - primary jet and rod combinations (MNJack)
Lars is on the money - start with his setup - I ended up with 72/47 on my 427
I used a O2 sensor to tell me what I was doing at cruse and it is still a little rich - like one or two rod sizes right now. - Be sure to check the primary rod rod height on your 1901 mine was WAY off causing the rod to be too high and the rod was out of the jet to far - Lars has an EXCELLENT document to follow for tuning which I think is much better than some of the books that is out there - also as Lars says - be sure to get your ignition in order as well - get some timing recommendations for the cam that you have.
Good luck
I used a O2 sensor to tell me what I was doing at cruse and it is still a little rich - like one or two rod sizes right now. - Be sure to check the primary rod rod height on your 1901 mine was WAY off causing the rod to be too high and the rod was out of the jet to far - Lars has an EXCELLENT document to follow for tuning which I think is much better than some of the books that is out there - also as Lars says - be sure to get your ignition in order as well - get some timing recommendations for the cam that you have.
Good luck
09-27-2002, 11:29 PM
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Re: Qjet tuning - primary jet and rod combinations (kostutsman)
lars, thanks again! :) I have a limited selection of jets and rods. I may have my dealer order the 44 primary and DA secondary rods... "rip your head off" is what I am trying for!! :lol: GMPartsdirect has them, as does http://www.carbs.net/index.htm but for just one or 2 parts shipping and handling gets expensive, so the dealer is handy for small stuff.
I think my ignition is in decent shape. I have Echlin points and condensor per lars recommendation, installed a lighter set of springs and set the timing also per lars method. The distributor setup gained me 3 mph in the 1/4 mile!
Kostutsman, I will check the rod height. Is that one of lars tuning tips? Xperienced, lars is extremely generous with his knowledge and has written several papers (available at http://www.corvettefaq.com ) Doug Roe's book, Rochester Carburetors is also a very useful reference. MJ
[Modified by MNJack, 10:30 PM 9/27/2002]
I think my ignition is in decent shape. I have Echlin points and condensor per lars recommendation, installed a lighter set of springs and set the timing also per lars method. The distributor setup gained me 3 mph in the 1/4 mile!
Kostutsman, I will check the rod height. Is that one of lars tuning tips? Xperienced, lars is extremely generous with his knowledge and has written several papers (available at http://www.corvettefaq.com ) Doug Roe's book, Rochester Carburetors is also a very useful reference. MJ
[Modified by MNJack, 10:30 PM 9/27/2002]
09-28-2002, 12:16 PM
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Re: Qjet tuning - primary jet and rod combinations (MNJack)
Mark -
The 1901's, or early model Q-Jets, do not have adjustable-height power pistons like the post-'75 models. However, it is common for the arms on the power piston to be bent a little, and this will affect how high the rods ride in the jets. When you pop the top off the carb and pop the piston out, take a look at it and make sure the rod hanger arms are completely horizontal and straight.
Once you get this setup, you can easily check to see if it's too rich for you (needing fatter primary rods): With the engine fully warmed up, press the pin out of the accel pump lever and disconnect the pump. Take the car out and drive it (don't go on busy streets with busy intersections in case the car stalls...!). If the car has a slight hesitation without the pump hooked up, your primary rod selection is good. If the car drives just fine, or even better, with the accel pump off, you need to go fatter on the primary rods. Make changes 2 sizes at a time to obtain a noticable difference. So you might want to pick up a set of 44's and 46's to try them out.
Part numbers for the primary rods are:
7034844
7034846
Part numbers for some various jet sizes are:
7031970
7031971
7031972
7031973
7031974
7031975
The 1901's, or early model Q-Jets, do not have adjustable-height power pistons like the post-'75 models. However, it is common for the arms on the power piston to be bent a little, and this will affect how high the rods ride in the jets. When you pop the top off the carb and pop the piston out, take a look at it and make sure the rod hanger arms are completely horizontal and straight.
Once you get this setup, you can easily check to see if it's too rich for you (needing fatter primary rods): With the engine fully warmed up, press the pin out of the accel pump lever and disconnect the pump. Take the car out and drive it (don't go on busy streets with busy intersections in case the car stalls...!). If the car has a slight hesitation without the pump hooked up, your primary rod selection is good. If the car drives just fine, or even better, with the accel pump off, you need to go fatter on the primary rods. Make changes 2 sizes at a time to obtain a noticable difference. So you might want to pick up a set of 44's and 46's to try them out.
Part numbers for the primary rods are:
7034844
7034846
Part numbers for some various jet sizes are:
7031970
7031971
7031972
7031973
7031974
7031975
09-28-2002, 02:04 PM
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Re: Qjet tuning - primary jet and rod combinations (lars)
MNJack,
Yes lars does describe how to check the primary rod height in his paper - the detail is great - you can also get a good visual if the float is out and you have a really good light you can see how far the rod is in the jet. From lars' paper you will see that you want the rod in the jet almost up to the top ridge.
My 1901 had a plug that I had to drill out to get to the adjustment screw for the power piston height setting. I was real reluctant to change this adjustment but the 1901 was brand new and I really could not see any thing else to do but to make this change.
PS I think I am going to try the accelerator pump test Lars described a few minutes.
kos
[Modified by kostutsman, 12:07 PM 9/28/2002]
Yes lars does describe how to check the primary rod height in his paper - the detail is great - you can also get a good visual if the float is out and you have a really good light you can see how far the rod is in the jet. From lars' paper you will see that you want the rod in the jet almost up to the top ridge.
My 1901 had a plug that I had to drill out to get to the adjustment screw for the power piston height setting. I was real reluctant to change this adjustment but the 1901 was brand new and I really could not see any thing else to do but to make this change.
PS I think I am going to try the accelerator pump test Lars described a few minutes.
kos
[Modified by kostutsman, 12:07 PM 9/28/2002]
09-28-2002, 03:58 PM
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Re: Qjet tuning - primary jet and rod combinations (MNJack)
i am not familiar with the edelbrock carbs but different GM Q jets use different size air bleeds which effect the jet size. you may be comparing apples to oranges. :chevy
09-28-2002, 04:21 PM
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Re: Qjet tuning - primary jet and rod combinations (clem zahrobsky)
Clem -
You're absolutely right - you can't take the jetting from one model Q-Jet and transfer it to another to obtain equivalent levels of performance due to different bleeds and calibrations. In fact, I get this request quite frequently from people: "I have a 1973 truck Q-Jet, but I want it jetted to the 1968 427 spec..." It simply doesn't work. This is why I always tell people to start with the correct, stock jetting for the carb number they have, and to make smart changes from that point.
In making recommendations for the EDL 1901, these recommendations come from having played with a few of them on various Corvettes in my garage. Fattening up the mains, leaning out the pri rods, and richening up the sec side seems to wake these carbs up. In stock form, the EDL1901 feels "sluggish" and "mushy." (just my personal opinion and experience). The jet change noted has worked very well in improving off-idle response and WOT performance. I'll be curious to hear the results that Mark gets from this change (being at 5500 feet, my jetting differs slightly from sea level).
You're absolutely right - you can't take the jetting from one model Q-Jet and transfer it to another to obtain equivalent levels of performance due to different bleeds and calibrations. In fact, I get this request quite frequently from people: "I have a 1973 truck Q-Jet, but I want it jetted to the 1968 427 spec..." It simply doesn't work. This is why I always tell people to start with the correct, stock jetting for the carb number they have, and to make smart changes from that point.
In making recommendations for the EDL 1901, these recommendations come from having played with a few of them on various Corvettes in my garage. Fattening up the mains, leaning out the pri rods, and richening up the sec side seems to wake these carbs up. In stock form, the EDL1901 feels "sluggish" and "mushy." (just my personal opinion and experience). The jet change noted has worked very well in improving off-idle response and WOT performance. I'll be curious to hear the results that Mark gets from this change (being at 5500 feet, my jetting differs slightly from sea level).
