This is just a trouble shooting measure. To make sure you don't have issues with corroded or shorting kill functions, disconnect the Black/white strip wire from the CDI under the tank. This will remove external kill functions completely. If you still have the exact same symptoms then this is NOT your problem, hook it back up.

The small backfire on deceleration is usually a lean mixture. Could be clogged jets or passages, could be the decel enrichner circuit is not working. The left carb has a round cover plate on the side of the carb. This houses a diaphragm that when activated causes a richer mixture on decel. The hose that routes over the secondary carb brings the vacuum from the intake of the cylinder that operates the decal valve diaphragm. Make sure the hose is NOT leaking. It would be a good idea to just replace the vacuum diaphragm and brass valve. These are available from Yamaha. Use only OEM parts for best results.

Adjust the fuel screw at the bottom of the primary carb at the front (cylinder side) using a small flat blade screwdriver. Adjust it for a peak iidle RPM. This sets the mixture. Starting place is 1 1/2 turns out. Since this is a fuel screw, inward is leaner, outward is richer. When done readjust your idle speed. This is good to fine tune if you change to a very different altitude. At 1200' amsl mine is set at about 1.5 turns out, at 12000 feet amsl last year in Colorado I set it at about 1/2 turn. It ran fine. When returning back home bike would not start until I remembered to screw the fuel screw back to 1.5 turns out. The started normally.

Pull you plug, hook up to spark wire to plug, ground plug. Kick hard, you should see a bright blue spark, it is red or spindly then it is weak. Could be the CDI or Coil.

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

hi and thanks steveloomis i will try disconecting the black white wire at the week end , ive looked at the coasting enricher diaframe tonight it seems good the plunger is free but the housing/compartment was oily /dirty so have cleaned it all out and put it back together , so will see what it is like going to work tomorrow . zigzag