Idle adjusting screw set too low. Drain the float bowls ditto. Vacuum line from the petcock to intake manifold cracked. If there's still a good spark it's safe to say that there will also be a good spark under pressure in the combustion chamber,speaking of which you should also do a compression test,anything over 100psi should be ok. Is there good exhaust when it runs or maybe that's plugged somehow. Fuel line pinched or kinked. With a helper see if you can watch the choke cable move the choke lever on the carbs when you move the choke lever to full choke.
The only other thing to consider is the timing. What have you done to it since then? Then play with the choke: does it start and run a bit with the choke on? If it runs for 5-10 seconds then stops you've got some sort of fuel starvation problem , could be dirt in the carb,blocked idle jet ,or air passage or even the needle could have come off it's clip,believe me with an old bike like yours anything could be worn and loose inside that carby. If all the abouve are correct and true it should fire up. Cables do break and or stretch. Sep 25, 2014 Hi, Anonymous and the usual suspects are: 1. Multi carburetors out of sync and need to be balanced. Good Luck Mar 27, 2011.
Idle port, transfer ports, slow air jet clogged. Hope this helps to solving your worries. If that is the case then reconnect, replace, or adjust as necessary. Good luck and have a wonderful day. You say good spark,so i'm presuming you are doing the old sight test.
Change the gas, it could have water or something in it. Try putting a small amount of fuel directly into the combustion chamber ,tighten up the spark plug ,install the plug cap and. That means it's running lean. Good luck and have a wonderful day. If the choke lever and carb choke system is moving correctly then the problem may be in the carburators themselves. .
Faulty auto choke or wiring connection. . Air fuel mixture screws not properly adjusted. Choke valve not seated or leaking. Try bending the arm on a know good sparkplug about 5-6mm,earth the plug and kick'er in the guts crank it over Be very carefull , high voltage danger.
Fuel cap or fuel tank is not venting properly. There may be a good spark but is it happening at the right time inside the engine???. Float needle and seat sticking. Contaminated or plugged low-speed circuit. The choke cable may be detached or far out of adjustment. . .
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