Its a 99 intruder 800 the front carb opens before
Chances are there is a crack in the rear carb diaphram. CV carbs, ( constant velocity carbs ), depend on a good seal to work properly. A crack, tear, or pin hole in the soft rubber will cause the problem you have described. Also, do a compression check. The valves may need to be re-seated. Read the following >
THE VENTURI EFFECT
When air is still or moving slowly, it is dense and at a standard atmospheric pressure. But when air is forced to speed up faster than the surrounding air, it becomes thinner and less dense. This is known as a Low Pressure Area. And where a low pressure area exists, air at higher atmospheric pressure wants to rush in to equalize the pressure. This phenomenon of low pressure versus ambient atmospheric air pressure is basic to the functioning of CV carburetors.
How do we get air to move through a carburetor? When the piston goes down, it drastically increases the volume inside the cylinder and creates a vacuum. Open the intake valve, and new air will rush in to fill this vacuum. Throw a carburetor in that path and the descending piston now "sucks" air through the carb. And that's how we get an air flow through the carb. The vacuum, (created by the downward travel of the piston), is actually a low pressure area inside the cylinder, but we will call it a "vacuum" to differentiate it from the low pressure area we will be discussing which occurs inside the carb.
If the air path in the carburetor bore were the same size all the way through, outside air could rush through the bore quite easily to fill the vacuum being created by the downward travel of the piston. It would therefore take high piston speeds (lots of pumping) to achieve enough air speed through the carb bore to thin the air enough to create a low pressure condition inside the carb. However, if an obstruction, (such as a carburetor slide), is placed in the carb bore the air path is now much smaller. The air stream has to speed up greatly to get through this bottleneck. In this way, the air can be speeded up and a low pressure area can be created inside the carburetor bore.
In carb lingo, this bottle neck is called a "Venturi". As the incoming air speeds up to get past the Venturi, it thins out and loses density. We now achieve a "venturi effect" which is a high speed air flow creating low air pressure at the point of the restriction.
How is the venturi effect used in the carb? We know that, at the point of the low air pressure, outside air would love to rush in and equalize the pressure. For example we could drill a hole to the outside air at the point of the venturi and outside air would rush in. But the clever carb guys instead drill a hole which goes down into a bowl of fuel with outside air above it. The higher pressure outside air can't get up through the hole, (it is blocked by the fuel in the bowl), but it does try, such that it pushes down on the fuel in the bowl, thus forcing some of that fuel up a pipe and into the depressed air stream flying by the venturi. The low pressure created by the venturi assists in drawing in even more fuel. That plume of fuel coming out of the pipe mixes with the air rushing by, and that is how we get an air/ fuel mixture to feed the cylinders.
