Weber Tuning Guide by Patrick (vw1s)
Mar 20, 2015 15:15:39 GMT -5
Post by thatmacguy on Mar 20, 2015 15:15:39 GMT -5
Hey Guys,
Patrick (vw1s) was nice enough to write-up this excellent step-by-step guide and send it to me over a year ago. Last fall I used it to tune-up my dual carbs on my Type 4 motor and it worked like a charm. Still wish for better gas-mileage, but this guide did get the car running better and with a slightly leaner overall setup. Drove the car all winter with zero problems and no cold-weather starting problems. Also had no carb-freezing this winter, although how that could be due to the tune I'm not sure! He gave me permission to post the guide in hopes it will help a few others:
Here's how I do my setup and adjustment.
Engine off.
Linkage unhooked from both carbs.
Idle stop screw about 1/8 to 1/4 turn past touching, or, as listed in the weber book, about .004" between the throttle plate and the wall of the carb, measured on the barrels closest to the stop screw. (they assume this will give you a slightly high idle, which will be adjusted later) This adjustment is to ensure that the throttle plates are aligned properly with the progression holes. The air bypass screws (the smaller of the two screws on the side, it has a jam nut) should be 1/2 turn out from gently bottomed. Mixture screws (the larger of the two screws on the side, it has a spring) 2 turns out from gently bottomed. (This ensures you don't run lean while tuning, assuming your idle jet is big enough)
Start the engine. If the idle is too high, turn all 4 air bypass screws in 1/8turn at a time, until the idle is at factory spec, or around 800-1000 rpm if spec is not available. If idle is too low, turn the air bypass screws out another 1/8 turn on all 4. If it won't idle even with the bypass screw out 3 turns, turn the stop screw in another 1/8 and try again. If it idles too high even with the bypass screws all the way in, adjust air bypass screws back out to 1/2 turn from bottomed and then turn the stop screw out 1/8 turn at a time until it is at factory spec or 800-1000 rpm.
Place sync tool on the left carb, barrel closest to stop screw. If you are using a uni-syn type, adjust the wheel until the ball is at the lowest mark. If you are using a snail, take note of the reading. Place sync on opposite carb, barrel closest to the stop screw. Adjust the stop screw until you have the same reading as the first carb.
Remove sync
Starting with #3 cylinder and working counter clockwise, adjust the mixture screws in until you hear that cylinder start to die out, then back out 1/8 turn at a time until it comes back and sounds good, waiting 10-15 second after any adjustment for the engine to respond. All four mixture screws should be very close in their adjustment, if not, check any odd barrel for a partially plugged idle jet, a vaccum leak, or a buggered adjustment screw. The mixture screws should be around 1/2 to one full turn from bottomed when properly adjusted, any more than 1-1/2 turns out, you need bigger idle jets, any less than 1/2 turn out, you need smaller idle jets. If turning the mixture screw makes no difference, it's either a plugged idle jet, or a vacuum leak.
This is where you get everything "dialed in"
Place your sync back on the left carb, barrel closest to the stop screw. Adjust the uni-syn to the first mark, or, take note of the reading on the snail. Move sync to second barrel on same carb, adjust the air bypass screw until reading matches first barrel, repeat on opposite carb, adjusting the air bypass screws to match the reading from the first barrel.
If the idle speed varies, go back to the first barrel and adjust the air bypass screw, in to lower or out to raise, then repeat syncing on other 3 barrels until all read the same and idle speed is at factory spec or 800-1000 rpm.
Patrick (vw1s) was nice enough to write-up this excellent step-by-step guide and send it to me over a year ago. Last fall I used it to tune-up my dual carbs on my Type 4 motor and it worked like a charm. Still wish for better gas-mileage, but this guide did get the car running better and with a slightly leaner overall setup. Drove the car all winter with zero problems and no cold-weather starting problems. Also had no carb-freezing this winter, although how that could be due to the tune I'm not sure! He gave me permission to post the guide in hopes it will help a few others:
Here's how I do my setup and adjustment.
Engine off.
Linkage unhooked from both carbs.
Idle stop screw about 1/8 to 1/4 turn past touching, or, as listed in the weber book, about .004" between the throttle plate and the wall of the carb, measured on the barrels closest to the stop screw. (they assume this will give you a slightly high idle, which will be adjusted later) This adjustment is to ensure that the throttle plates are aligned properly with the progression holes. The air bypass screws (the smaller of the two screws on the side, it has a jam nut) should be 1/2 turn out from gently bottomed. Mixture screws (the larger of the two screws on the side, it has a spring) 2 turns out from gently bottomed. (This ensures you don't run lean while tuning, assuming your idle jet is big enough)
Start the engine. If the idle is too high, turn all 4 air bypass screws in 1/8turn at a time, until the idle is at factory spec, or around 800-1000 rpm if spec is not available. If idle is too low, turn the air bypass screws out another 1/8 turn on all 4. If it won't idle even with the bypass screw out 3 turns, turn the stop screw in another 1/8 and try again. If it idles too high even with the bypass screws all the way in, adjust air bypass screws back out to 1/2 turn from bottomed and then turn the stop screw out 1/8 turn at a time until it is at factory spec or 800-1000 rpm.
Place sync tool on the left carb, barrel closest to stop screw. If you are using a uni-syn type, adjust the wheel until the ball is at the lowest mark. If you are using a snail, take note of the reading. Place sync on opposite carb, barrel closest to the stop screw. Adjust the stop screw until you have the same reading as the first carb.
Remove sync
Starting with #3 cylinder and working counter clockwise, adjust the mixture screws in until you hear that cylinder start to die out, then back out 1/8 turn at a time until it comes back and sounds good, waiting 10-15 second after any adjustment for the engine to respond. All four mixture screws should be very close in their adjustment, if not, check any odd barrel for a partially plugged idle jet, a vaccum leak, or a buggered adjustment screw. The mixture screws should be around 1/2 to one full turn from bottomed when properly adjusted, any more than 1-1/2 turns out, you need bigger idle jets, any less than 1/2 turn out, you need smaller idle jets. If turning the mixture screw makes no difference, it's either a plugged idle jet, or a vacuum leak.
This is where you get everything "dialed in"
Place your sync back on the left carb, barrel closest to the stop screw. Adjust the uni-syn to the first mark, or, take note of the reading on the snail. Move sync to second barrel on same carb, adjust the air bypass screw until reading matches first barrel, repeat on opposite carb, adjusting the air bypass screws to match the reading from the first barrel.
If the idle speed varies, go back to the first barrel and adjust the air bypass screw, in to lower or out to raise, then repeat syncing on other 3 barrels until all read the same and idle speed is at factory spec or 800-1000 rpm.