Dual Webers fine tuned
Posted: Fri Oct 12, 2018 3:30 pm
FYI:
I've dialed the Weber IDF's in further with the LM-2.
2056cc T-4 1972 Westy hardtop
Jetting is now:
47.5 idles
120 main jets
200 air corrector jets
F11 tubes
28mm vents
Float height - 10.5mm (ball not depressed gasket in place)
Float drop - 32mm (28-32mm is the range)
At WOT I'll see mid 12 then hold 13 or so
At cruise around town I'll see AFR with part/cracked throttle anywhere from 15 or so to 17-18
On the highway at speed the AFR with part throttle will hover mid 15 - 18 or so mostly. Get on it and it will richen to high 12 - low 13.
As for lean tune cruising it can only be achieved if you're using a vacuum advance distributor or a mapped EFI set up of some sort. No centrifugal only distributors allowed. You simply won't get enough added advance to burn the lean AFR at the right moment.
I'm using a Bosch SVDA that I've limited the advance in a few degrees. Out of the box it had 25 degrees of total centrifugal advance (not incl vac advance). Too much. I cleaned it well and lubed it with Super-Lube. Then checked the vac can with a might vac tester. It will begin movement at 3.25in and end at ~ 8-9in.
Once that was done you need to see if you'll get the vacuum pull needed at the carb. Hook a small vacuum hose up to the vac advance port that allows for ported vacuum (none at idle). On a warm engine blip the throttle and watch for vacuum gauge needle movement. It should flick quickly to a higher reading perhaps 7-11in range or so. The flicking of the needle is perfectly normal because the vac pull is at cracked/part throttle condition only. With the engine idling parked there is little to no load on the engine hence the quick gauge movement.
Look for how high the needle is going and get an average high. If you're getting 7+in you'll have no issue with using an SVDA. You'll only need to pull from (1) carb vac port (preferably #4) given it's closer to the distributor. I also use an anti-pulse valve situated as close to the carb port as you can get. Smooths the vacuum out.
As for the carbs....
Start by cleaning them well and checking the float height/drop (see above). No more than 3.5lbs fuel pressure for Weber carbs. Once the carbs are installed you'll start with the idle speed screws in 1/2 turn from first contact. No more. The A/F mixture screws will be roughly 2 turns or so out from lightly seated position.
A good jetting size to start with on a 2L engine is 50 idles, 120 mains, 200 air corrector, 28mm vents. This will get you in the ball park (those at or near sea level)
Linkage should be as close to sync as you can get initially (cold engine). Don't worry about perfect cause it will change slightly once the engine gets to normal operating temp. Once the engine is running (likely like a tractor) let it warm up a bit. When you feel it's warm....
Start by adjusting LBI on each air mixture screw (highest/smoothest running spot). Once done check your sync again with a snail. Then check your idle speed. Keep it around 800 or so for now. Bring a higher flowing carb down to the lower flowing carb initially. Now do your LBI again for each A/F screw.
When done, check your sync again. Good? Now you can again adjust to your desired idle speed by bringing each carb up a bit at a time. Lower speed is better IMO. Too high and you expose the progression ports. Once done you do LBI again on each A/F screw on each cylinder. If the idle speed changes (higher/lower) after adjustment you do another re-sync and LBI again.
Now that the engine is nice and warm, you'll check and confirm your carbs are coming off the throttle stops at the precise same time. Any adjustment here and you go back and check your sync and LBI again. Now drive it and pay close attention to what it's doing in different RPM ranges with a STEADY pedal. Any issue that arises at a given RPM will be a clue as to whether or not it's your idle circuit.
With regards to Weber IDF's....
A bigger air corrector jet will tip the main jet in sooner. Smaller and it will delay the main from coming in. The below graph shows the relation between EGT, CHT, Horsepower, Internal cylinder pressure, Brake specific fuel consumption
As you can see going leaner from 15:1 the CHT & EGT's go down. Stay out of the 14 - 15:1 range at WOT. Part throttle condition/s do not equate to load condition. Your throttle (foot) position dictates that.
The line at the bottom of the graph is 1/Brake Specific Fuel Consumption.....(Power from fuel used). Note the flat area of the curve and where in relation to the AFR it is and EGT/CHT....that should be your target 'cruise' range at part throttle condition.
I've dialed the Weber IDF's in further with the LM-2.
2056cc T-4 1972 Westy hardtop
Jetting is now:
47.5 idles
120 main jets
200 air corrector jets
F11 tubes
28mm vents
Float height - 10.5mm (ball not depressed gasket in place)
Float drop - 32mm (28-32mm is the range)
At WOT I'll see mid 12 then hold 13 or so
At cruise around town I'll see AFR with part/cracked throttle anywhere from 15 or so to 17-18
On the highway at speed the AFR with part throttle will hover mid 15 - 18 or so mostly. Get on it and it will richen to high 12 - low 13.
As for lean tune cruising it can only be achieved if you're using a vacuum advance distributor or a mapped EFI set up of some sort. No centrifugal only distributors allowed. You simply won't get enough added advance to burn the lean AFR at the right moment.
I'm using a Bosch SVDA that I've limited the advance in a few degrees. Out of the box it had 25 degrees of total centrifugal advance (not incl vac advance). Too much. I cleaned it well and lubed it with Super-Lube. Then checked the vac can with a might vac tester. It will begin movement at 3.25in and end at ~ 8-9in.
Once that was done you need to see if you'll get the vacuum pull needed at the carb. Hook a small vacuum hose up to the vac advance port that allows for ported vacuum (none at idle). On a warm engine blip the throttle and watch for vacuum gauge needle movement. It should flick quickly to a higher reading perhaps 7-11in range or so. The flicking of the needle is perfectly normal because the vac pull is at cracked/part throttle condition only. With the engine idling parked there is little to no load on the engine hence the quick gauge movement.
Look for how high the needle is going and get an average high. If you're getting 7+in you'll have no issue with using an SVDA. You'll only need to pull from (1) carb vac port (preferably #4) given it's closer to the distributor. I also use an anti-pulse valve situated as close to the carb port as you can get. Smooths the vacuum out.
As for the carbs....
Start by cleaning them well and checking the float height/drop (see above). No more than 3.5lbs fuel pressure for Weber carbs. Once the carbs are installed you'll start with the idle speed screws in 1/2 turn from first contact. No more. The A/F mixture screws will be roughly 2 turns or so out from lightly seated position.
A good jetting size to start with on a 2L engine is 50 idles, 120 mains, 200 air corrector, 28mm vents. This will get you in the ball park (those at or near sea level)
Linkage should be as close to sync as you can get initially (cold engine). Don't worry about perfect cause it will change slightly once the engine gets to normal operating temp. Once the engine is running (likely like a tractor) let it warm up a bit. When you feel it's warm....
Start by adjusting LBI on each air mixture screw (highest/smoothest running spot). Once done check your sync again with a snail. Then check your idle speed. Keep it around 800 or so for now. Bring a higher flowing carb down to the lower flowing carb initially. Now do your LBI again for each A/F screw.
When done, check your sync again. Good? Now you can again adjust to your desired idle speed by bringing each carb up a bit at a time. Lower speed is better IMO. Too high and you expose the progression ports. Once done you do LBI again on each A/F screw on each cylinder. If the idle speed changes (higher/lower) after adjustment you do another re-sync and LBI again.
Now that the engine is nice and warm, you'll check and confirm your carbs are coming off the throttle stops at the precise same time. Any adjustment here and you go back and check your sync and LBI again. Now drive it and pay close attention to what it's doing in different RPM ranges with a STEADY pedal. Any issue that arises at a given RPM will be a clue as to whether or not it's your idle circuit.
With regards to Weber IDF's....
A bigger air corrector jet will tip the main jet in sooner. Smaller and it will delay the main from coming in. The below graph shows the relation between EGT, CHT, Horsepower, Internal cylinder pressure, Brake specific fuel consumption
As you can see going leaner from 15:1 the CHT & EGT's go down. Stay out of the 14 - 15:1 range at WOT. Part throttle condition/s do not equate to load condition. Your throttle (foot) position dictates that.
The line at the bottom of the graph is 1/Brake Specific Fuel Consumption.....(Power from fuel used). Note the flat area of the curve and where in relation to the AFR it is and EGT/CHT....that should be your target 'cruise' range at part throttle condition.