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Adjusting the Pilot Circuit

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Adjusting the Pilot Circuit

Contributed by Dan Abbott

The backfire is caused by too lean a setting of the pilot circuit. This causes hotter exhaust gas temps. which ignite any unburned mixture in the exhaust pipe. On a cold day, these temps would be lowered by the ambient temp enough to prevent backfiring. This is no mystery, it just confirms the too lean pilot circuit.
So, how to fix your backfire problems in the 99 venture?

1 - Why

The Backfire is caused when coming off or going on the pilot circuit of the Mikuni 32bds carbs. In other words, coming of idle or decelerating with the throttle closed. The pilot circuit mixture is preset to a too lean position at the factory. Too lean means that the fuel air ratio is more air than fuel, resulting in high combustion and exhaust gas temperatures. When the super heated exhaust hits any unburned fuel mixture which is in the exhaust pipe/muffler, it ignites it and your bike says Pop POW Bang etc.

2 - How to fix it

a. Fire your mechanic and learn to fix it yourself.
b. Get a service manual and 4 tube mercury manometer for syncing carbs available in any city or by mail order from any one who sells motorcycle tools (try Vance and Hines or motorcycle accessory warehouse.)
c. Read and understand the simple procedure for synchronizing the carbs. This can be done without removing the tank seat or anything except the two rubber caps covering the intake manifold vacuum access tubes on cylinders 2 and four and the two vacuum hoses on cylinders 1 and three.
d. Synchronize the carbs. You should do this annually. Note: YOU CAN FIX THE BACKFIRE WITHOUT SYNCHRONIZING THE CARBS WHILE WAITING FOR YOUR SERVICE MANUAL BY DOING THE FOLLOWING PROCEDURE FIRST. BUT DO THE SYNCHONIZATION AT THE FIRST CONVIENIENT TIME.
Now for the good stuff, which is not in the service manual, but which every mechanic who is worth a rat's behind should know.
e. Here is how you change the mixture in the pilot circuit quick and dirty.
The pilot mixture screws are hidden behind removable brass caps which look like an upside down paper cup with a 0.1mm hole in the bottom of the cup. These are about the size of a pencil eraser. These are located on the insides of each carb (toward the center of the bike). You can see and access the ones on the right side carburetors #3 and #4 by looking through from the left side of the bike, and access the left carbs #1 and #2 by looking through from the right side of the bike.
Using a long slotted screw driver, pry off the brass covers (they come off very easily). Throw the covers away, you won't be replacing them. You now will be able to see the pilot screws with a cross pattern on each head.
Turn each pilot screw counter clockwise exactly 1/4 turn out (90 degrees) Note the original position of the screw and be sure you turn it only 1/4 turn.
This will fix your problem.
The complex way to adjust the pilot screw is to use an exhaust analyzer probe on each cylinder and adjust for 2 to 2.5 % CO and 600 to 800 PPM hydrocarbons. But since the factory gets it close, but too lean, the 1/4 turn method will work fine for you.

Update by Venturer TAC 9/24/07

Pilot screw locations. Carbs are removed from bike in first two pictures to better show locations. It is not neccessary to remove carbs or fuel bowls to access. The brass caps do have to be removed to uncover the screws if they have not been previously removed.

The view with carbs on bike.

Last update: 07:26 PM Monday, September 24, 2007

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