Engine upgrade won't start

[will wei]

Hey there,

Just upgrading my bike from a 49cc engine to a 66/80cc motor. The previous bike worked just fine, and I only replaced the motor/mounts, still using the same gas tank, carb, rear wheel sprocket, CDI, etc. My bike will not start or idle. The only way it will fire at all is if I leave the air filter off, keep the choke off, and go W.O.T., and even then if I let go of the throttle completely it will die immediately. I have good spark and compression, the fuel is getting into the carb bowl, and I don't think I have any leaks. Running carb needle on second pin from top currently, and using an NGK B6HS spark plug gapped at .028. If anyone has any ideas I'd appreciate it!

 

[Deleted member 12676]

unscrew the carb slide cap and take the carb off.
rinse off all the inside of the float bowl and install a fuel filter in your fuel line.
strip off the plastic from a twist-tie from a loaf of bread and use that wire to stick up into the main jet hole to clear out any obstruction. (and idle jet if it has one)
always carry that wire with you for ability to do the same on the side of the road.

make sure the float height is correct and that the float valve shuts off fuel flow when the floats are "up".
see http://www.dragonfly75.com/motorbike/jetting.html

 

[crassius]

if it won't start with air filter on, then something has happened to fuel delivery

 

[will wei]

Got to work on it this morning. I took the carb off again and noticed the float wasn't sitting at the right height, it was closer to 14mm. I adjusted that and tightened the idle screw, and now the bike is working well. My only concern now is that the chain sits on the rear wheel sprocket so that the sprocket teeth are close to one side of the chain, and it kind of sketches me out because I don't want the teeth to catch the chain and cause a hard break. Any ideas? The chain tensioner will not sit in a good position to hold the chain center on the sprocket, so I'm thinking either the rear wheel is off tilt or the motor is not straight vertical. I can get some pictures up later probably if that would help.

 

[crassius]

I often have to measure how far it is off, then take off tensioner and bend it in vise to the right spot - the bottom run of chain needs to be fed onto rear sprocket so that hits in middle of links.