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Motorcycle Repair/carburetor help


Hi I have 1985 honda shadow vt700. My problem is when I let go the throttle the rear cylinder only pops out of the exhaust.  It has air cutoff valve on it but I don't think it works like it should and discontinued part.  I looked inside of it and the rubber diagram is dry and has tears. I tried to plug the ports and stopped the popping but then my rear spark plugs turn black but when I ride with the ports open with popping the spark plugs are tan. Would the poping hurt the engine if I drive it like it is? Thanks joe

Hi Joe,

Some minor popping in the exhaust is not too bad but
it might help if you can richen the idle or pilot
screws by turning them out a bit.

The popping is caused by the fuel mixture being
a bit lean when the throttle is released.
This means the fuel mixture has more air than
fuel at that time.

The cutoff valves are designed to add extra fuel
during decelleration to prevent lean popping.

If the idle pilot screw on the front side of the
carb was turned anti-clockwise a bit it might
help to richen the idle and may reduce the popping

Also make sure your exhaust header gaskets are not
leaking.  Any air getting into the hot exhaust pipe
can cause popping too.

If you are running open pipes then you will have
little luck in stopping the backfires completely.
The stock exhaust helps a bit in slowing down
the exhaust gas flow which can draw unburned
fuel into the pipes.

Wayne S

Motorcycle Repair

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Wayne S.


Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.


Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

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