jet size
Pro
Joined: Sep 2004
Posts: 595
Likes: 0
From: Boston MA
Picking the jet size is not that easy.
From what I learned from Lars, you want to start by figuring out the number on your carb. Your goal is to determine the metering area that is associated with your jets and rod. You want it to be between 2.8 and 3.0 for performance. When selecting the jet and rod combo, there should be about a difference of 30. For example 45 rods and 75 jets.
The thing is that you cannot just take any jets and rods and put then in.
Instead, get your number off the carb and figure out the correct combo for that carb. Lars has a chart that will tell you this info. Once you know the baseline, see if it is lean, which would be a metering area of 2.4 to 2.6 or rich over 3.0. Then go from there to decide on the correct jet and rod combo.
Stay away from the M truck secondary rods, they have a much thicker tip.
1. Start by finding out what the carb is supposed to be by emailing lars at V8Fastcars@msn.com (I think) with the number.
2. open the carb and see what the combo is currently setup.
3. While you have it open, check the needle. It should not be in the hole on the float. It should be on the inner edge where it floats on a small half circle.
4. Also check to make sure that the tip on the power piston is not cut off, that is common on Holley rebuilt and other rebuilt carbs.
5. Check the float level. It is different for carbs before and after 1975.
I can't remember the exact setting, you will have to check lar's paper.
This is all available in the quadrajet paper by lars. You can get more details from that paper; if you email him, he will most likely send it to you.
Thanks,
Joe
From what I learned from Lars, you want to start by figuring out the number on your carb. Your goal is to determine the metering area that is associated with your jets and rod. You want it to be between 2.8 and 3.0 for performance. When selecting the jet and rod combo, there should be about a difference of 30. For example 45 rods and 75 jets.
The thing is that you cannot just take any jets and rods and put then in.
Instead, get your number off the carb and figure out the correct combo for that carb. Lars has a chart that will tell you this info. Once you know the baseline, see if it is lean, which would be a metering area of 2.4 to 2.6 or rich over 3.0. Then go from there to decide on the correct jet and rod combo.
Stay away from the M truck secondary rods, they have a much thicker tip.
1. Start by finding out what the carb is supposed to be by emailing lars at V8Fastcars@msn.com (I think) with the number.
2. open the carb and see what the combo is currently setup.
3. While you have it open, check the needle. It should not be in the hole on the float. It should be on the inner edge where it floats on a small half circle.
4. Also check to make sure that the tip on the power piston is not cut off, that is common on Holley rebuilt and other rebuilt carbs.
5. Check the float level. It is different for carbs before and after 1975.
I can't remember the exact setting, you will have to check lar's paper.
This is all available in the quadrajet paper by lars. You can get more details from that paper; if you email him, he will most likely send it to you.
Thanks,
Joe
Last edited by 68coupe; Sep 25, 2005 at 05:36 PM.
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
This was for another thread but it applies here also.
Get the ignition setup first. Get a set of springs in the distributer that achieve their full advance (whatever that may be) around 2800 RPM. Set the initial advance (with vacuum advance disconnected) so that initial + centrifugal = about 36 degrees at 2800 RPM. Don't forget to reconnect the vacuum advance.
As far as the carburetor goes, it will take some experimentation to arrive at the proper setup.
First, set the float correctly.
Second, get the idle circuit setup properly. You can not tune an engine that will not idle.
Third is the off idle circuit.
Now at this point some will try to tune the accelerator pump. If the engine will go to WOT without "too" much trouble, I like to leave the accelerator pump tuning for later because it tends to cover up problems in the power circuit.
Forth comes the power circuit. This is the time to set the primary main jet size. To do this, you need to make timed runs at WOT with the secondaries disconnected. Whatever jet size gives the lowest time between, say 45 and 55 MPH is the one you want. I cannot give a specific recommendation here because I do not know what is in the carburetor to begin with. You will need to find out what the initial setup is (jet and rod sizes and power piston springs) and go from there. I will say this, whatever the jet size, get at least the next three sizes richer. If your time continues to drop as you jet up you may need to get more jets.
Fifth on the list is the cruise metering circuit. This is where the rods and power piston springs come in. Again, I cannot give you a magic number here but if you had to go three or four sizes richer on the main jets you may very well have to go leaner on the rods. The object here is to run as lean as you can without the engine surging or rattling at steady state cruising speeds. If the engine is sluggish during moderate acceleration you may need to go a little stronger on the power piston springs.
Sixth is the accelerator pump. You want the engine to go cleanly from idle to WOT without any hesitation or sagging and without blowing black smoke out the tailpipe.
Last is the secondaries. You'll probably want to go to the strip to tune these. Set the air valve opening so there is no sagging when the valve opens. Keep going richer on the secondary metering rods until your times stop improving.
Having the ignition setup properly is VERY important when tuning the idle, off idle and accelerator pump systems of the carburetor because retarded timing can look like a lean carburetor circuit.
BigBlockk
Later.....
Get the ignition setup first. Get a set of springs in the distributer that achieve their full advance (whatever that may be) around 2800 RPM. Set the initial advance (with vacuum advance disconnected) so that initial + centrifugal = about 36 degrees at 2800 RPM. Don't forget to reconnect the vacuum advance.
As far as the carburetor goes, it will take some experimentation to arrive at the proper setup.
First, set the float correctly.
Second, get the idle circuit setup properly. You can not tune an engine that will not idle.
Third is the off idle circuit.
Now at this point some will try to tune the accelerator pump. If the engine will go to WOT without "too" much trouble, I like to leave the accelerator pump tuning for later because it tends to cover up problems in the power circuit.
Forth comes the power circuit. This is the time to set the primary main jet size. To do this, you need to make timed runs at WOT with the secondaries disconnected. Whatever jet size gives the lowest time between, say 45 and 55 MPH is the one you want. I cannot give a specific recommendation here because I do not know what is in the carburetor to begin with. You will need to find out what the initial setup is (jet and rod sizes and power piston springs) and go from there. I will say this, whatever the jet size, get at least the next three sizes richer. If your time continues to drop as you jet up you may need to get more jets.
Fifth on the list is the cruise metering circuit. This is where the rods and power piston springs come in. Again, I cannot give you a magic number here but if you had to go three or four sizes richer on the main jets you may very well have to go leaner on the rods. The object here is to run as lean as you can without the engine surging or rattling at steady state cruising speeds. If the engine is sluggish during moderate acceleration you may need to go a little stronger on the power piston springs.
Sixth is the accelerator pump. You want the engine to go cleanly from idle to WOT without any hesitation or sagging and without blowing black smoke out the tailpipe.
Last is the secondaries. You'll probably want to go to the strip to tune these. Set the air valve opening so there is no sagging when the valve opens. Keep going richer on the secondary metering rods until your times stop improving.
Having the ignition setup properly is VERY important when tuning the idle, off idle and accelerator pump systems of the carburetor because retarded timing can look like a lean carburetor circuit.
BigBlockk
Later.....
Last edited by BigBlockk; Sep 25, 2005 at 07:50 PM.
