advice for first time tuning my quick fuel DP carb
08-04-2016, 08:34 PM
#1
advice for first time tuning my quick fuel DP carb well im nearing getting my car drivable and would like some advice on initial setup . my car is a full steel 70 chevelle lq4 , 225-225 581-581 113 cam, 243s,rpm intake,1 3/4 headers through full 3in exhaust, quick fuel ss dp 750 carb, aem wideband, 3:31 gears th400 3200 stall. just want to know if you have any carb tuning tips that will get me going as far as should I tune it with a vacuum gauge ? change jets? squirters? thanks for any info im told im going to have major bogging issues with this setup but I guess we will see
08-05-2016, 05:36 AM
#2
Tune it right and it won't bog. The wideband is the ticket to really dialing it in.
A vac gauge is one way to set the idle mixture screws. The wideband is a better way.
Just keep good records of everything you do. that way you can back out a change if things get worse.
A vac gauge is one way to set the idle mixture screws. The wideband is a better way.
Just keep good records of everything you do. that way you can back out a change if things get worse.
08-05-2016, 02:30 PM
#4
Wideband has taken the place of reading plugs. A total guru probably looks at a bunch of things but just the wideband readings will get you where you want to be.
Obviously the whole process is rather involved and takes multiple steps. Not sure I'm up to writing a step by step procedure, not sure I'm the best person for the task either, but there are quite a number of guides out there.
Try these videos from Innovate. They push there product but the idea is the same.
http://www.innovatemotorsports.com/resources/lm101.php
Obviously the whole process is rather involved and takes multiple steps. Not sure I'm up to writing a step by step procedure, not sure I'm the best person for the task either, but there are quite a number of guides out there.
Try these videos from Innovate. They push there product but the idea is the same.
http://www.innovatemotorsports.com/resources/lm101.php
08-10-2016, 11:11 AM
#5
Launching!
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Run it as is and see how it does. My guess is it will be rich at WOT and also at idle (carb suppliers always play it safe). Wideband will help with getting WOT correct. It wont be much help at idle with a bigger cam with overlap.
09-21-2016, 01:07 PM
#6
leaving in two weeks for the ocean city car show , I pretty much have the car wrapped up but now need to play with the carb . The wideband is hooked up but at idle it's all over the place ( 13.5-15.2) im going to put a vac gauge on it and go from there as it was running really rich so I back the screws in and it doesn't want to run at idle or when in drive unless I give it gas. It has a off idle stumble and when you stab the pedal and get off the gas real quick it will die in gear . Any advice as to what I can do just to make this thing drivable for the car show will be a big help or if there is some kind of kit I should but to aid in tuning . Thanks
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09-22-2016, 11:31 AM
#9
09-22-2016, 12:27 PM
#11
09-22-2016, 12:58 PM
#13
i have one , it reads all over the place at idle . Basically do air bleeds typically need changed ? Or squirters?... Or anything else when tuning these kind of carbs for the street
09-22-2016, 02:13 PM
#14
At idle, with that camshaft, you should be able to maintain a good quality idle between 14:1-14.5:1 no problem. This is to be checked with the car in gear, parking brake set and the wheels blocked. BE CAREFUL! Do not run over yourself or anyone/thing else! If it ends up needing high 13s at idle, no big deal really. When the car is cold, it will typically read leaner and get richer as it warms up. Without using the choke, the car should be a bit fussy when cold until it warms up if the idle calibration is lean enough. You should be able to achieve this with the idle mixture screws. If not, bleed changes could be needed.
At WOT it should be between 12.2 and 12.8:1. If not change of the main jets is necessary. Street carburetors with a power valve in the front and none in the rear are the most common arrangement. If this is your arrangement, make note of the jet spread e.g.: 72 front 82 rear. When changing jets, make sure you change all 4 jets equally. e.g: 2 jet sizes leaner would be 70 front and 80 rear.
Once those two calibrations are done, you can explore part throttle and drivability changes.
NOTE: Before doing anything, make sure your float levels are set correctly. Once you have them set to the recommended levels, make sure they are always in the same place. If you change the float levels, it will mess with the tune. For Holley style carburetors with site glasses in the fuel bowls typically carb makers recommend a fuel level of 1/3 of the way up the site glass. In older Holley carbs that do not have sight windows, they recommend setting the fuel level so it just trickles out the fuel level inspection ports with the plugs removed. I use and recommend fuel pressure settings of 5 to 6 pounds for Holley style carburetors. Too much fuel pressure can cause overflowing fuel and fuel aeration. Too low fuel pressure can cause insufficient supply.
If you have a "tip in" stumble after these adjustments are made it is not uncommon.
This can be caused by insufficient accelerator pump enrichment or by too small of idle fuel restrictor in the low speed/idle circuits. Which is the problem is easy to determine. If the problem remains constant at light throttle under a load like cruising, then it is in the low speed/idle circuit. If it is a temporary stumble and then the engine recovers then it is in the accelerator pump system.
If you have a stumble that you suspect is in the accelerator pump system, the first thing to do is to make sure your accelerator pumps are adjusted correctly. There should be no slack in the arms. Remove all slack so there is some, but minimal preload on the system. If you still have a lean stumble after the arm are adjusted correctly, you can increase the size of the discharge nozzles. I like to do front and rear equally. It is not uncommon for people to need as much as .037"-.040" accelerator pump discharge nozzles on an LS engine. Don't be afraid to try it. In some cases the engine could need even more volume. In these cases it is necessary to change the front pump diaphragm assembly to a 50cc unit. Try that last.
If you have a stumble or off idle lean stumble that you suspect is in the low speed/idle circuit, try increasing the idle fuel restrictor's size. You can drill these out or replace the jet if you have a carburetor model with replaceable IFRs. I like to do this in .002" increments until the problem is solved.
Once all this is done, you will likely have a pretty decent carburetor. The only further tuning you choose to do is to make efforts to dial in your part throttle/cruise air fuel ratios. This is done by changing the primary only main jets until your cruise air fuel ratios are what you want and then changing the power valve control orifice (PVCR) to return the WOT to match the air fuel ratio that you had before changing the primary main jet. NOTE: This is the only time when you will change the primary to secondary jet spread. In this case it is okay because are going to compensate with changes to the PVCR.
At WOT it should be between 12.2 and 12.8:1. If not change of the main jets is necessary. Street carburetors with a power valve in the front and none in the rear are the most common arrangement. If this is your arrangement, make note of the jet spread e.g.: 72 front 82 rear. When changing jets, make sure you change all 4 jets equally. e.g: 2 jet sizes leaner would be 70 front and 80 rear.
Once those two calibrations are done, you can explore part throttle and drivability changes.
NOTE: Before doing anything, make sure your float levels are set correctly. Once you have them set to the recommended levels, make sure they are always in the same place. If you change the float levels, it will mess with the tune. For Holley style carburetors with site glasses in the fuel bowls typically carb makers recommend a fuel level of 1/3 of the way up the site glass. In older Holley carbs that do not have sight windows, they recommend setting the fuel level so it just trickles out the fuel level inspection ports with the plugs removed. I use and recommend fuel pressure settings of 5 to 6 pounds for Holley style carburetors. Too much fuel pressure can cause overflowing fuel and fuel aeration. Too low fuel pressure can cause insufficient supply.
If you have a "tip in" stumble after these adjustments are made it is not uncommon.
This can be caused by insufficient accelerator pump enrichment or by too small of idle fuel restrictor in the low speed/idle circuits. Which is the problem is easy to determine. If the problem remains constant at light throttle under a load like cruising, then it is in the low speed/idle circuit. If it is a temporary stumble and then the engine recovers then it is in the accelerator pump system.
If you have a stumble that you suspect is in the accelerator pump system, the first thing to do is to make sure your accelerator pumps are adjusted correctly. There should be no slack in the arms. Remove all slack so there is some, but minimal preload on the system. If you still have a lean stumble after the arm are adjusted correctly, you can increase the size of the discharge nozzles. I like to do front and rear equally. It is not uncommon for people to need as much as .037"-.040" accelerator pump discharge nozzles on an LS engine. Don't be afraid to try it. In some cases the engine could need even more volume. In these cases it is necessary to change the front pump diaphragm assembly to a 50cc unit. Try that last.
If you have a stumble or off idle lean stumble that you suspect is in the low speed/idle circuit, try increasing the idle fuel restrictor's size. You can drill these out or replace the jet if you have a carburetor model with replaceable IFRs. I like to do this in .002" increments until the problem is solved.
Once all this is done, you will likely have a pretty decent carburetor. The only further tuning you choose to do is to make efforts to dial in your part throttle/cruise air fuel ratios. This is done by changing the primary only main jets until your cruise air fuel ratios are what you want and then changing the power valve control orifice (PVCR) to return the WOT to match the air fuel ratio that you had before changing the primary main jet. NOTE: This is the only time when you will change the primary to secondary jet spread. In this case it is okay because are going to compensate with changes to the PVCR.
Last edited by speedtigger; 09-22-2016 at 02:19 PM.
09-22-2016, 04:56 PM
#15
At idle, with that camshaft, you should be able to maintain a good quality idle between 14:1-14.5:1 no problem. This is to be checked with the car in gear, parking brake set and the wheels blocked. BE CAREFUL! Do not run over yourself or anyone/thing else! If it ends up needing high 13s at idle, no big deal really. When the car is cold, it will typically read leaner and get richer as it warms up. Without using the choke, the car should be a bit fussy when cold until it warms up if the idle calibration is lean enough. You should be able to achieve this with the idle mixture screws. If not, bleed changes could be needed.
At WOT it should be between 12.2 and 12.8:1. If not change of the main jets is necessary. Street carburetors with a power valve in the front and none in the rear are the most common arrangement. If this is your arrangement, make note of the jet spread e.g.: 72 front 82 rear. When changing jets, make sure you change all 4 jets equally. e.g: 2 jet sizes leaner would be 70 front and 80 rear.
Once those two calibrations are done, you can explore part throttle and drivability changes.
NOTE: Before doing anything, make sure your float levels are set correctly. Once you have them set to the recommended levels, make sure they are always in the same place. If you change the float levels, it will mess with the tune. For Holley style carburetors with site glasses in the fuel bowls typically carb makers recommend a fuel level of 1/3 of the way up the site glass. In older Holley carbs that do not have sight windows, they recommend setting the fuel level so it just trickles out the fuel level inspection ports with the plugs removed. I use and recommend fuel pressure settings of 5 to 6 pounds for Holley style carburetors. Too much fuel pressure can cause overflowing fuel and fuel aeration. Too low fuel pressure can cause insufficient supply.
If you have a "tip in" stumble after these adjustments are made it is not uncommon.
This can be caused by insufficient accelerator pump enrichment or by too small of idle fuel restrictor in the low speed/idle circuits. Which is the problem is easy to determine. If the problem remains constant at light throttle under a load like cruising, then it is in the low speed/idle circuit. If it is a temporary stumble and then the engine recovers then it is in the accelerator pump system.
If you have a stumble that you suspect is in the accelerator pump system, the first thing to do is to make sure your accelerator pumps are adjusted correctly. There should be no slack in the arms. Remove all slack so there is some, but minimal preload on the system. If you still have a lean stumble after the arm are adjusted correctly, you can increase the size of the discharge nozzles. I like to do front and rear equally. It is not uncommon for people to need as much as .037"-.040" accelerator pump discharge nozzles on an LS engine. Don't be afraid to try it. In some cases the engine could need even more volume. In these cases it is necessary to change the front pump diaphragm assembly to a 50cc unit. Try that last.
If you have a stumble or off idle lean stumble that you suspect is in the low speed/idle circuit, try increasing the idle fuel restrictor's size. You can drill these out or replace the jet if you have a carburetor model with replaceable IFRs. I like to do this in .002" increments until the problem is solved.
Once all this is done, you will likely have a pretty decent carburetor. The only further tuning you choose to do is to make efforts to dial in your part throttle/cruise air fuel ratios. This is done by changing the primary only main jets until your cruise air fuel ratios are what you want and then changing the power valve control orifice (PVCR) to return the WOT to match the air fuel ratio that you had before changing the primary main jet. NOTE: This is the only time when you will change the primary to secondary jet spread. In this case it is okay because are going to compensate with changes to the PVCR.
At WOT it should be between 12.2 and 12.8:1. If not change of the main jets is necessary. Street carburetors with a power valve in the front and none in the rear are the most common arrangement. If this is your arrangement, make note of the jet spread e.g.: 72 front 82 rear. When changing jets, make sure you change all 4 jets equally. e.g: 2 jet sizes leaner would be 70 front and 80 rear.
Once those two calibrations are done, you can explore part throttle and drivability changes.
NOTE: Before doing anything, make sure your float levels are set correctly. Once you have them set to the recommended levels, make sure they are always in the same place. If you change the float levels, it will mess with the tune. For Holley style carburetors with site glasses in the fuel bowls typically carb makers recommend a fuel level of 1/3 of the way up the site glass. In older Holley carbs that do not have sight windows, they recommend setting the fuel level so it just trickles out the fuel level inspection ports with the plugs removed. I use and recommend fuel pressure settings of 5 to 6 pounds for Holley style carburetors. Too much fuel pressure can cause overflowing fuel and fuel aeration. Too low fuel pressure can cause insufficient supply.
If you have a "tip in" stumble after these adjustments are made it is not uncommon.
This can be caused by insufficient accelerator pump enrichment or by too small of idle fuel restrictor in the low speed/idle circuits. Which is the problem is easy to determine. If the problem remains constant at light throttle under a load like cruising, then it is in the low speed/idle circuit. If it is a temporary stumble and then the engine recovers then it is in the accelerator pump system.
If you have a stumble that you suspect is in the accelerator pump system, the first thing to do is to make sure your accelerator pumps are adjusted correctly. There should be no slack in the arms. Remove all slack so there is some, but minimal preload on the system. If you still have a lean stumble after the arm are adjusted correctly, you can increase the size of the discharge nozzles. I like to do front and rear equally. It is not uncommon for people to need as much as .037"-.040" accelerator pump discharge nozzles on an LS engine. Don't be afraid to try it. In some cases the engine could need even more volume. In these cases it is necessary to change the front pump diaphragm assembly to a 50cc unit. Try that last.
If you have a stumble or off idle lean stumble that you suspect is in the low speed/idle circuit, try increasing the idle fuel restrictor's size. You can drill these out or replace the jet if you have a carburetor model with replaceable IFRs. I like to do this in .002" increments until the problem is solved.
Once all this is done, you will likely have a pretty decent carburetor. The only further tuning you choose to do is to make efforts to dial in your part throttle/cruise air fuel ratios. This is done by changing the primary only main jets until your cruise air fuel ratios are what you want and then changing the power valve control orifice (PVCR) to return the WOT to match the air fuel ratio that you had before changing the primary main jet. NOTE: This is the only time when you will change the primary to secondary jet spread. In this case it is okay because are going to compensate with changes to the PVCR.
10-02-2016, 08:23 AM
#16
On The Tree
suggest calling QF tech line, they are great to deal with, fine tuning on these carbs is not bad, especially with the wideband, the power valve restrictions(under power valve) are used to tune and do a great job.