ok,
So I have one rich and one perfect plug on one side...
I will use the idle mixture screw to adjust this condition...
What is the air bypass screw for, where do you guys set it, and how does it change the way the carb tunes??
I was having some sooty exhaust, so I checked the floats..perfect..if not just a tad lean...11mm which is ok with me...
Rich
Full Version: weber tuning
use the idle air bypass screw to adjust the airflow of that particular stack. Using a unisyn get the stacks in each carb to flow the same. then get the flow bank to bank adjusted, then adjust idle mixture
The air bypass screws are there to balance the airflow between the two throats on one carb. Ideally, one screw on each carb should be completely closed, with the other opened just enough to give equal airflow through both throats. Balance the two carbs using the throttle stop screws, and all four should flow the same.
The air bypass screws add air so you can get each cylinder bank to sync front to rear. I leave mine closed. Unless the sync is way off it is better to leave them closed.
| QUOTE (TimT @ Feb 6 2006, 04:54 PM) |
| use the idle air bypass screw to adjust the airflow of that particular stack. Using a unisyn get the stacks in each carb to flow the same. then get the flow bank to bank adjusted, then adjust idle mixture |
AAAAAAAHHHH
Sorry, let me get this straight.
I use the air bypass the get the carb to balance between both stacks...
Ok, assuming that is done...
I can use the idle stop screw to get both sides the same and balanced...eventually the linkage will do this...
So, the Idle screw...how do I make sure this is right?
Do I have to just read the plugs, or is there a way to test it with the synchrometer???
I know it was running to rich at idle,,,light cough up the carb...rear plug was black front was brown and perfect.
This is a beetle....in order for me to remove the plugs to read them, I have to remove the carb and manifold (I did not design it...just have to live with it)
These are the CB offset manifolds...pissing me off...I dont want to have to keep taking off the carb to check the plugs...
I want to figure out how to adjust the idle screw so its correct,,,and know its correct...
But, if its just plug reading, so be it.
Rich
Once you have the airflow balanced play with the mixture screws, turn each in until that cylinder stumbles, do this procedure one cylinder at a time, then back it out till the cylinder fires again, repeat with with mixture screw on each of the other cylinders. Then go around again and refine the adjustment
Rich;
I was going through all of this just a week back. According to the Redline Weber site the Air by-pass screws are not used to intially tune the carb (what the hell are they there for then?)
However an article on the Aircooled.net site did describe how to use the Air-bypass when sychronizing your carbs. It's more for the fine tuning of the individual barrels. If all 4 cylinders have equal compression and volume, then you most likely not need to adjust the Air-Bypass. Your issue sounds more like either lean idle adjustment or possibly a clogged idle jet. Float setting sounds correct.
Some Articles I found helpful:
How to Sync Dual Carbs
Redline Lean Idle Procedures
Jetting Theory
IDF Set Up
IDF tuning
Weber Tuning for Porsches
A couple of quotes:
"Loosen the 8mm wrench size nuts on the “air bleed” screws, turn in the air screws until it seats then tighten the nut."
"Most Critical! Be sure for initial carburetor set up all air by-pass screws should be in closed position. These are not commonly used in standard carburetor adjustment."
I was going through all of this just a week back. According to the Redline Weber site the Air by-pass screws are not used to intially tune the carb (what the hell are they there for then?)
However an article on the Aircooled.net site did describe how to use the Air-bypass when sychronizing your carbs. It's more for the fine tuning of the individual barrels. If all 4 cylinders have equal compression and volume, then you most likely not need to adjust the Air-Bypass. Your issue sounds more like either lean idle adjustment or possibly a clogged idle jet. Float setting sounds correct.
Some Articles I found helpful:
How to Sync Dual Carbs
Redline Lean Idle Procedures
Jetting Theory
IDF Set Up
IDF tuning
Weber Tuning for Porsches
A couple of quotes:
"Loosen the 8mm wrench size nuts on the “air bleed” screws, turn in the air screws until it seats then tighten the nut."
"Most Critical! Be sure for initial carburetor set up all air by-pass screws should be in closed position. These are not commonly used in standard carburetor adjustment."
thanx...that will work...
I hope...kinda hard getting a screwdriver in there...
Could I potentially set the carbs (the air bypass screw) on the bench with a vacuum cleaner hooked to the bottom of the manifold..??
I know that may sound stupid...but I would have a hard time doing otherwise...
Would a vacuum create enough suction to produce the correct effect??
Rich
I hope...kinda hard getting a screwdriver in there...
Could I potentially set the carbs (the air bypass screw) on the bench with a vacuum cleaner hooked to the bottom of the manifold..??
I know that may sound stupid...but I would have a hard time doing otherwise...
Would a vacuum create enough suction to produce the correct effect??
Rich
| QUOTE (hmeeder @ Feb 6 2006, 05:07 PM) |
| Rich; I was going through all of this just a week back. According to the Redline Weber site the Air by-pass screws are not used to intially tune the carb (what the hell are they there for then?) However an article on the Aircooled.net site did describe how to use the Air-bypass when sychronizing your carbs. A couple of quotes: "Loosen the 8mm wrench size nuts on the “air bleed” screws, turn in the air screws until it seats then tighten the nut." "Most Critical! Be sure for initial carburetor set up all air by-pass screws should be in closed position. These are not commonly used in standard carburetor adjustment." |
Ive read all the same stuff also...
What is the point of that screw if not for adjusting something...
One guy says seat it all the way, dont touch it...
Another says use it to tune the carbs...
I agree with Tim, it makes sense that it is for tuning the carb to itself, one stack to the other...
Then it stays set that way...
Amazing how much dribble is out there when it comes to tuning these carbs...lots of completely opposed instructions...
Rich
Ive been tuning Webers 3bbl weber for over 25 years, Ive found there is alot of info floating around that needlessly complicates what is really a pretty simple procedure.
the way I do it:
disconnect throttle linkage
raise idle to about 15-1800 rpm
adjust airbleeds (close them, check airflow, adjust to highest flowing stack)
reduce idle to desired rpm via idle adjustment screws (check bank to bank flow with unisyn)
adjust idle mixture screws (one cylinder at a time), turn in until cylinder stumbles, then turn back out till it fires again.
go over the idle mixture screws a few times fine tuning
install throttle linkage such that there is no preload
turnoff engine, check that butterflies open when go pedal is fully depressed, adjust linkage as needed
Once you set the air bleed screws you should never have to touch them again
thats the way I do it, its a distillation of all the other methods Im sure, but its simple to me, and it works
the way I do it:
disconnect throttle linkage
raise idle to about 15-1800 rpm
adjust airbleeds (close them, check airflow, adjust to highest flowing stack)
reduce idle to desired rpm via idle adjustment screws (check bank to bank flow with unisyn)
adjust idle mixture screws (one cylinder at a time), turn in until cylinder stumbles, then turn back out till it fires again.
go over the idle mixture screws a few times fine tuning
install throttle linkage such that there is no preload
turnoff engine, check that butterflies open when go pedal is fully depressed, adjust linkage as needed
Once you set the air bleed screws you should never have to touch them again
thats the way I do it, its a distillation of all the other methods Im sure, but its simple to me, and it works
Coughing through the carb means that cylinder is lean.
Air bypass screws might be useful on a triple throat but on a dual throat keep them closed. I have tried it both ways.
Air bypass screws might be useful on a triple throat but on a dual throat keep them closed. I have tried it both ways.
I've copied this from a post I made in Sept 04, for Trekkor. Tim T has pretty much covered this too, but there's a little more info
Assumptions: the carbs have the optimum jet and venturi package (good luck on this one), the float level in the carbs is correct, the cams are correctly timed, the valves are properly adjusted, the ignition timing is dead on, you have the proper spark plugs for your engine, the linkage is good, the fuel is good, the engine is good.
Remember that the mixture and air bypass adjustment screws are precision needle valves, not head gaskets. Use your fingers to tighten them, not your fist.
Start and warm up the engine.
Make sure the two drop links for the throttle linkage are exactly the same length, and disconnected. You can use a 8mm thin igntion wrench to snap them off.
Turn the mixture screws all the way in and then 5 half turns out.
Turn the air bypass screws all the way in.
Turn the idle speed screws out til it just touches, and then in 5 half turns.
Put on your hearing protection and start the car.
Use your STE and find the barrel that pulls the most. We'll call this one baseline.
Balance the barrel in the other carb that pulls the most with the idle speed screw. (if you have a Uni-syn, give it to someone you don't like and purchase a STE airflow meter)
Go back to the other carb, with the baseline barrel. You will have one all the way in, then use the air bypass screws and balance the other two barrels.
Go to the other carb and do the same thing.
Snug the jamb nuts on the air bypass screws.
All six barrels should pull the same amount of air at this point, if not repeat air adjustment proceedure.
Snap the throttle linkage drop links back on the carbs. If the idle changes then you need to barely adjust the linkage mounts so snapping the drop links on, doesn't change the side to side idle balance.
Use the hand throttle or a vice grip and rag to lock the linkage between 1400 and 1800 rpm.
Start back at the baseline barrel and adjust the mixture screw in or out, to get the smoothest running and highest rpm, then turn it in 1/4 turn.
Do the same with the five other mixture screws.
If you have to turn the mixture screws more than two turns either way, you've got the wrong jets.
Recheck side to side and individual air balance, adjust as needed.
Road test the car.
If you get snapping and poping out the intake, it's generally a lean condition.
If you get heavy exhaust fumes, or pboofing out the exhaust it's probably too rich.
If you get a flat spot or popping out the intake at between 2800 and 3200 rpm, you probably need larger idle jets.
That's a rough, five minute draft of my carb tuning proceedure, hope it helps!
If it goes good it should take about 45 minutes, if not about three years.
PK
Another good tip: If you think you have a lean cylinder (idle jet plugged) you can back out the jet carrier a half to one full turn and the engine should stumble, if it doesn't change theres a good chance the jet is clogged. This primarily applies to the 3 barrel carbs, as the 2 barrel IDF's have a different type idle jet set up (right Tim?)
Assumptions: the carbs have the optimum jet and venturi package (good luck on this one), the float level in the carbs is correct, the cams are correctly timed, the valves are properly adjusted, the ignition timing is dead on, you have the proper spark plugs for your engine, the linkage is good, the fuel is good, the engine is good.
Remember that the mixture and air bypass adjustment screws are precision needle valves, not head gaskets. Use your fingers to tighten them, not your fist.
Start and warm up the engine.
Make sure the two drop links for the throttle linkage are exactly the same length, and disconnected. You can use a 8mm thin igntion wrench to snap them off.
Turn the mixture screws all the way in and then 5 half turns out.
Turn the air bypass screws all the way in.
Turn the idle speed screws out til it just touches, and then in 5 half turns.
Put on your hearing protection and start the car.
Use your STE and find the barrel that pulls the most. We'll call this one baseline.
Balance the barrel in the other carb that pulls the most with the idle speed screw. (if you have a Uni-syn, give it to someone you don't like and purchase a STE airflow meter)
Go back to the other carb, with the baseline barrel. You will have one all the way in, then use the air bypass screws and balance the other two barrels.
Go to the other carb and do the same thing.
Snug the jamb nuts on the air bypass screws.
All six barrels should pull the same amount of air at this point, if not repeat air adjustment proceedure.
Snap the throttle linkage drop links back on the carbs. If the idle changes then you need to barely adjust the linkage mounts so snapping the drop links on, doesn't change the side to side idle balance.
Use the hand throttle or a vice grip and rag to lock the linkage between 1400 and 1800 rpm.
Start back at the baseline barrel and adjust the mixture screw in or out, to get the smoothest running and highest rpm, then turn it in 1/4 turn.
Do the same with the five other mixture screws.
If you have to turn the mixture screws more than two turns either way, you've got the wrong jets.
Recheck side to side and individual air balance, adjust as needed.
Road test the car.
If you get snapping and poping out the intake, it's generally a lean condition.
If you get heavy exhaust fumes, or pboofing out the exhaust it's probably too rich.
If you get a flat spot or popping out the intake at between 2800 and 3200 rpm, you probably need larger idle jets.
That's a rough, five minute draft of my carb tuning proceedure, hope it helps!
If it goes good it should take about 45 minutes, if not about three years.
PK
Another good tip: If you think you have a lean cylinder (idle jet plugged) you can back out the jet carrier a half to one full turn and the engine should stumble, if it doesn't change theres a good chance the jet is clogged. This primarily applies to the 3 barrel carbs, as the 2 barrel IDF's have a different type idle jet set up (right Tim?)
When adjusting the idle mixture screws, go slow. Turn 1/8th, wait a few seconds, repeat. There's some lag between the time to adjust and the time the engine note changes. So, turn in 1/8th at a time, pause, until the engine note changes, then back out 1/8th, pausing, until the engine note changes again. Repeat until you get it just rich of stumbling. Start each one about 3 turns out from lightly bottomed.
as stated. initially set all 4 of the bypass the same. closed, open 1/2 turn, just make sure they are the same, and foget them until latter in the tunning proceedure. set each idle/air mixture screw the same, try 2 1/2 or 3 turns out. don't get excited guys, this is just a common starting point. just try this...with everything in place, back one idle adjust screw off its stop, turn the idle up with the remaining one, and check the flow with the throats facing you on each carb. if they are not flowing equally, adjust the droplink on the non idle stopped carb to balance the flow. something often overlooked...there are little orings under the idle air mixture screw springs. check their condition. they get brittle and cracked. now...flowing the same? turn the idle down. gently seat each idle/air mixture screw, one at a time!!! the idle will fall off and you will hear a snapping out the exhaust. turn it back out til' the idle smooths out (at this point i turn it out the tiniest bit more). do the remaining 3 for a smooth idle. if you want to, you now check the flow rates for each throat. this is the point where you would equalize all 4 of them with the air by-pass screws. of course, your timing and valves are spot on right?
k
k
I got it.
See, I am hearing that the air bypass screw is for setting up the two stacks in any particular carb.....
Then I hear its for balancing all four carbs....
If I do this, tell me if this will work.
Sync each carb to itself using the airbypass screw
Set each idle stop screw to the same physical spot...measuring off the body of the carb.
Sync all four/or the two carbs...using the airbypass screw, with the idle set physically the same.
Do the idle mixture screw deal...
Then, and only then..install the cross bar linkage, and sync the carbs using that...
When I set up my cross bar linkage, I only use on idle stop screw..
Rich
See, I am hearing that the air bypass screw is for setting up the two stacks in any particular carb.....
Then I hear its for balancing all four carbs....
If I do this, tell me if this will work.
Sync each carb to itself using the airbypass screw
Set each idle stop screw to the same physical spot...measuring off the body of the carb.
Sync all four/or the two carbs...using the airbypass screw, with the idle set physically the same.
Do the idle mixture screw deal...
Then, and only then..install the cross bar linkage, and sync the carbs using that...
When I set up my cross bar linkage, I only use on idle stop screw..
Rich
| QUOTE |
Sync each carb to itself using the airbypass screw Set each idle stop screw to the same physical spot...measuring off the body of the carb. Sync all four/or the two carbs...using the airbypass screw, with the idle set physically the same. |
close, set one carb so it is flowing the same amount of air though each stack. then do the same for the other carb
Then set the idle, use the unisyn and fiddle with the idle stop screws, so you have the idle speed you want, and each carb is flowing the same amount of air.
Once you have set the air bypass screws, you should not have to touch them again.... Id say forever, but lets just say for a very long time
The bypass screws you set once... this basically synchs the carb to itself. after the each carb has its airflow corrected.. then you synch the left and right carbs.. by idle stop NOT air bypass
Hope this didnt confuse things... its really very simple....hard to explain though?
Oh and when you install the linkage, dont touch the idle stop screws, make the linkage fit the carbs the way they are set
megasquirt megasquirt megasquirt 
i just pulled my carbs off a few days ago. just not my cup of tea.
i just pulled my carbs off a few days ago. just not my cup of tea.
FWIW heres a lil something my friend and I wrote for the local PCA magazine..no tech just reminiscing. Carbs are incredibly simple.. no voodoo or black magic..same story for EFI
Carbs
Carbs
When you are doing the intitial set-up, linkage is off and you are searching for the cylinder with the fastest air speed.
This becomes the "baseline".
The air by-pass screws will allow you to ajust the other barrels to "baseline".
After each adjust, snap the throttle and adjust the idle speed to keep it around 950rpms.
After you get it done it will seem simple to you.
KT
This becomes the "baseline".
The air by-pass screws will allow you to ajust the other barrels to "baseline".
After each adjust, snap the throttle and adjust the idle speed to keep it around 950rpms.
After you get it done it will seem simple to you.
KT
I love this...
I know how to do it...I just want to know which is the right way...
I got Tim saying (correct me if I am wrong here)
Use the air bypass screw to adjust each carb to itself...idle stop screw to adjust each carb to the other...
Then I get Trekkor (correct me if I am wrong)
Use the airbypass screw to find the fastest stack, adjust all stacks, both carbs to match...I assume idle stop screw would be adjusted physically the same so this works...
Then install linkage and fine tune linkage...
I see both methods working...as a matter of fact I have used Tim's method for the most part...
My problem was that the total setup would be perfect...all synced...blipped throttle all was fine...
Three or four days later,,,maybe a week or two, it was out of sync again...
Its my sons car, but I can hear it coming in the driveway..I can hear it running rough...
So I get out the synchromter...and lo and behold, they are off again...and not by a bit...but one side (both stacks match) is pulling 3 the other side is pulling 7
Yes valves are fine
Yes timing is fine
Motor is fresh rebuild by a pro.
So, this makes me think...mmm.. I must be doing this wrong...
So I am inclined to use the air bypass screw to set all four stacks the same speed.
Idle screw will be measured and set at the same spot for both carbs...I can even measure the throttle plate to guarantee that the screw is calibrated the same way on both carbs...
Once all of it is matching...then I will put the linkage back on...I figure the carbs may have been set incorrectly from the outset, so I need to start over..
Make sense?
Rich
I know how to do it...I just want to know which is the right way...
I got Tim saying (correct me if I am wrong here)
Use the air bypass screw to adjust each carb to itself...idle stop screw to adjust each carb to the other...
Then I get Trekkor (correct me if I am wrong)
Use the airbypass screw to find the fastest stack, adjust all stacks, both carbs to match...I assume idle stop screw would be adjusted physically the same so this works...
Then install linkage and fine tune linkage...
I see both methods working...as a matter of fact I have used Tim's method for the most part...
My problem was that the total setup would be perfect...all synced...blipped throttle all was fine...
Three or four days later,,,maybe a week or two, it was out of sync again...
Its my sons car, but I can hear it coming in the driveway..I can hear it running rough...
So I get out the synchromter...and lo and behold, they are off again...and not by a bit...but one side (both stacks match) is pulling 3 the other side is pulling 7
Yes valves are fine
Yes timing is fine
Motor is fresh rebuild by a pro.
So, this makes me think...mmm.. I must be doing this wrong...
So I am inclined to use the air bypass screw to set all four stacks the same speed.
Idle screw will be measured and set at the same spot for both carbs...I can even measure the throttle plate to guarantee that the screw is calibrated the same way on both carbs...
Once all of it is matching...then I will put the linkage back on...I figure the carbs may have been set incorrectly from the outset, so I need to start over..
Make sense?
Rich
EDIT: one more thing I do before tweaking anything on my carbs is pop the air cleaner. put my hand over each velocity stack. engine should stumble. if when covering a stack and the engine don't change tune then the idle jet is plugged.
Here is one that I stumbled upon after fighting un-synched carbs after each Autocross.
Seems I was putting my fot down so hard that I was bending the whole cross bar linkage Piece of crap.
Because my pedal stop was lower than the stops on the linkage arms of the throttle shafts.
Now that my pedal stop is adjusted I have had less un-synch problems.
Now go tell your boy to stop street racing before he gets hurt.
I did that trick to my son's 914 way back when. pulled slack into the cable at the linkage arm. never got past 3/4 throttle.
Just recently he drove my car down the block and told me that this engine is WAY faster in this car than when it was in his.
I guess he will not grow up to be a good wrench. good thing he is going to College to be an Engineer of sumpin
Here is one that I stumbled upon after fighting un-synched carbs after each Autocross.
Seems I was putting my fot down so hard that I was bending the whole cross bar linkage Piece of crap.
Because my pedal stop was lower than the stops on the linkage arms of the throttle shafts.
Now that my pedal stop is adjusted I have had less un-synch problems.
Now go tell your boy to stop street racing before he gets hurt.
I did that trick to my son's 914 way back when. pulled slack into the cable at the linkage arm. never got past 3/4 throttle.
Just recently he drove my car down the block and told me that this engine is WAY faster in this car than when it was in his.
I guess he will not grow up to be a good wrench. good thing he is going to College to be an Engineer of sumpin
I think your getting sound advice on how to set up the carbs. People often have different ways of describing the same procedures.
If the carbs are becoming unsynched after a few days of driving, try this
synch the carbs again, then paint the threads of the idle stop screws with some nailpolish, any color will do. Let your kid drive the car for a few days then check the carbs, see if the idle stop screws have moved.
If you can see that the nailpolish isnt broken, the culprit to the carbs becoming unsynched isnt in the carbs its elswhere.
And as Joe R mentioned having a pedal stop in place is a must, a heavy foot can bend pieces of the linkage
If the carbs are becoming unsynched after a few days of driving, try this
synch the carbs again, then paint the threads of the idle stop screws with some nailpolish, any color will do. Let your kid drive the car for a few days then check the carbs, see if the idle stop screws have moved.
If you can see that the nailpolish isnt broken, the culprit to the carbs becoming unsynched isnt in the carbs its elswhere.
And as Joe R mentioned having a pedal stop in place is a must, a heavy foot can bend pieces of the linkage
The methods Tim, Perry describes and mine are all the same. 
Just worded differently.
I have not heard of any measurements of the throttle positions. Each will be what it is to provide equal air flow and a steady 950rpm's
The out of sync returning sounds like crud in the fuel lines clogging jets/passages to me.
Hang in there...
KT
Just worded differently.
I have not heard of any measurements of the throttle positions. Each will be what it is to provide equal air flow and a steady 950rpm's
The out of sync returning sounds like crud in the fuel lines clogging jets/passages to me.
Hang in there...
KT
we're making too much of this. if you are experiencing a out of balance issue (ie one barrel (closest to the drop links) on each carb) after a few days of driving, it leads me to think you have a linkage issue. perhaps one of the jam nuts on the drop links is coming lose. i don't quite get placing your hand over one of the venturies to find a plugged idle jet. you're shutting of the fuel and air supply to that cly entirely. if turning one of the idle/air mixture screws in has no effect , the idle jet for that barrel is more than likely plugged. pull the idle jet feeding that cylinder. when the jet is plugged, the idle circuit for that barrel is essentially non functional. really, really, really...buy, borrow, or steal, bob tomlinsons Weber Tech Manual.
k
k
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