Motorcycle Repair: 1982 Honda Ascot Carburettor, air fuel mixture, honda ascot

Question
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Followup To
Question -
Hi,
When my bike is at operating temp and cruising when I rev to overtake then close the throttle it backfires continuously almost cutting out and when I slow down, low speed performance less than 3000rpm is terrible. I jave set the idle mix screw as per manual to no avail. Have changed fuel, plug, filter and checked rubber mounts for leaks. Help
Answer -
Gordon, I think that the air cut valve diaphragm has failed. It is located under a small plate (2 screws) on the side of the carburetor body. Go to:
http://www.powersportspro.com/partsfish/displayfiche.asp
Register for free, sign in and select a vehicle... Honda M/C then the model will appear, select FT500 and then look at the carburetor section... #10 shows the whole assembly at $81!
Go to Ebay and check the vendors there who sell carb kits and carb parts. They can supply a new air cut valve for a lot less $$$... either from K&L or KEYSTER, perhaps.

Other things that cause backfiring are any air leaks in the exhaust system from the head to the tip of the muffler.

An aftermarket muffler with little or no backpressure or one that makes excessive noises.


Bill Silver

Thanks Bill,
What does the air cut valve actually do?
Regards,
Gordon


Answer
Learn more here....

http://www.4strokes.com/tech/carbover.asp

An air-cut valve is used to prevent popping in the exhaust system during deceleration.  The valve enriches the air/fuel mixture during deceleration.  A diaphragm in the air cut valve is activated whenever high manifold vacuum is present, such as during deceleration.  The movement of the diaphragm causes a partial blockage of the air bleed system in the low speed circuit.  This reduces the aeration to the low speed jet which creates a richer mixture.  The air-cut valve is factory pre-set and should not be altered.  It may have to be replaced eventually, because the rubber diaphragm in the valve may deteriorate over time.  If the diaphragm is deteriorating, you may notice leanness or a popping in the exhaust during deceleration.



Bill Silver