Hooking two carbs together can result in a situation where the linkage can cause the carbs to get stuck in the wide open position if the geometry of the linkage is off. Some carbs have removable throttle arms that can be installed in any position, if they are not installed in the correct position they can cause the linkage to go past center resulting in the throttle being stuck in the wide open position. Make sure if you do have removable throttle arms that they are installed at the correct angle and they are in the same position on each carb.
Next, make sure that when the gas pedal is mashed to the floor that both carbs are opening all the way up. Disconnect the pedal linkage and have somebody hold the pedal to the floor while you hold the carbs wide open. Adjust the pedal linkage so it perfectly lines up with the carb’s mounting ball. We want to make sure the carbs open all the way but we don’t want them to do it before the pedal reaches the floor.
Before adjusting the carbs make sure the timing is correct. Always adjust timing first before adjusting the carbs. Adjusting the timing after you have set the carbs will mess-up all your carb adjustments. Set the point gap (dwell) first and then the timing, Dwell changes timing, timing does not effect the dwell. Remember set the dwell first, then the timing and finally the air fuel mixture always in this order.
Before adjusting the carbs make sure the float levels are correct. Check the throttle shafts for excessive play. Check that the choke plates are in the fully open position, just because the choke cable is all the way in does not mean they are completely open.
Lightly bottom out all the air fuel mixture screws, over tightening them can damage the needles and seats. Paint a mark on them at the 12 o’clock position. Back each screw out a ½ to a full turn. Does not matter as long as they are all equal. This is just to get it running so you can synchronize the throttle plate openings at idle. Adjust the idle speed screws so they are just slightly off their stops.
You need to buy a uni-sync tool which is a device that sits on top of the carb. It measures how much air is going through the carb at an idle. I ordered mine from Speedway for around $25.00. You set the tool on top of the carb. The tool has a little plastic ball inside a clear tube. Engine vacuum raises the ball up into the tube, the more air flowing through the carb the higher the ball will rise up the tube. You want the same amount of air going through each carb. When the ball is at the same spot in the tube on each carb you know that the throttle plates are opened exactly the same amount. To do this you will need to disconnect the linkage between the two carbs so that the throttle plates can be adjusted separately. If the carbs were hooked together adjusting one carb would cause the other one to open so the linkage needs to be disconnected. Install a throttle return spring on each carb, otherwise vacuum could pull a carb fully open and the engine will head to 10,000 rpm before you can shut it off. You want return springs on each carb to make sure both carbs are fully closed against their idle stops. You will need to go back and forth several times adjusting the idle speed screw on each carb while observing the uni-sync. Changing the throttle opening on one carb effects the overall intake manifold vacuum which has an effect on the other carb. When you have the uni-sync reading the same on both carbs and the engine idling at the correct speed you are ready to start adjusting the air fuel mixture. Go ahead and leave the linkage between the carbs disconnected for now.
If the idle speed is too fast the engine will start pulling fuel out of the other carburetor delivery circuits and you will not be able to adjust the air fuel ratio properly. We want the only source of fuel to be coming from the idle mixture screws. That means the idle speed must be correct before attempting to adjust the air fuel misture. Turning the screws in reduces the fuel and backing them out gives the engine more fuel. I like to use a little propane bottle to check the air fuel adjustment. I turn the bottle on just a little and pass it over the carbs. If the idle goes up I know that the engine is too lean, it needs more fuel. I back each idle A/F screw out an 1/8 of a turn and then check it again with the propane. Remember to turn all the screws equally on both carbs using your paint marks as a guide. When the idle speed no longer changes with the addition of the propane I know I’m really close to the ideal air fuel ratio. If the idle goes down with addition of the propane that tells me the engine is already too rich. Then I do the opposite and start turning the screws in an 1/8 of turn at a time to reduce the fuel. In theory both carbs should have the A/F mixture screws adjusted exactly the same but there are going to be slight differences between the carburetors and the design of the intake manifold. Adjusting the screws evenly on both carbs is a good starting point. You will probably endup with slightly different adjustments between the two carbs. Just make sure the screws on each carb are set exactly the same. Once you get the idle mixture screws adjusted to where the addition of the propane has no effect you can back the screws out slightly, maybe an 1/8 of a turn. This is a good idea because atmospheric conditions are never the same two-days in a row. Remember we don’t have computers on these antique engines adjusting for altitude, temperature and barometric pressure like modern automobiles do, so we tune it slightly to the rich side to compensate for these factors. It is better to be a little too rich than too lean.
Adjusting the air fuel mixture probably changed your idle speed. Now you have to go back with the uni-sync and reset your idle speed. That will change your air fuel ratio slightly. You will go back and forth playing this game until the idle speed is where you want it and the mixture is adjusted correctly. When your air fuel ratio is correct and you have the idle speed where you want it connect your linkage back between the two carbs. With the carbs against their idle stops adjust the length of the linkage connecting the two carbs so it will slip over the balls on the arms without moving either carb off of their throttle stops.
A word about those nostaligic little chromed air filters you see on flatheads. Most of them are absolutely junk and should not be used under any circumstances. After adjusting flathead carbs I have installed these little air cleaners only to have the idle speed go down just from installing the air cleaners. This means that these air cleaners cannot flow enough air even at an idle.
Something else to look for. When you snap the throttle open you could end up with a hesitation caused by too much fuel being squirted by the two accelerator pumps. The accelerator pump was designed to richen up the mixture momentarily when the throttle is opened suddenly. Since it takes a split second for the fuel to be drawn out of the carb when you first open the throttle the accelerator pump mechanically injects a squirt of fuel. Two accelerator pumps squirting fuel might be too much fuel and momentarily flood the engine. Many of the carbs have adjustable accelerator pumps where you can move the linkage to different holes which will change how much fuel they squirt. If you have a hesitation try shortening the stroke on the pumps so they don’t deliver too much fuel.
I have attached a picture of my uni-sync and propane bottle. The long hose on the propane bottle allows me to cruise down the road while adding propane to the air cleaners. If the engine power increases I know I’m a little on the lean side and need to increase the jet sizes. This is my poor man’s dyno.
Here is a link to a Youtube video. This is one of the best sounding flatheads I have ever heard. It shows the engine being fired up and the carbs being adjusted using a uni-sync.
http://www.youtube.com/watch?v=keexe2G2F_Q
flathead fever, thats a whole lot of great info, and yes that is one great sounding flathead.