1968 360 viking only starts without exhaust off

Hello, link below is my taking over the 1968 viking from my father in law. Bike sat in his barn for 15 years. I cleaned the carb, freed the piston, and now it only starts with the exhaust OFF the bike, Put the exhaust pipe on and you can kick it a hundred times and it never catches. No smoke no nothing. Take the exhuast pipe off and it starts and runs for 5 seconds then dies. Any ideas? FEMSA flywheel and coil. Needle on second position. Bing 1/32/14 ultrasonic cleaned.

https://photos.app.goo.gl/NbjxxbJMQzGQEHfPA

I can put a garden hose straight through the pipe so I don’t think anything is blocked.

Try a new sparkplug and pull the air filter bell then squirt some fuel past the slide. Nice bike!!

Take the pipe off. See if something is blocking it up. Seen many mice nest or "food" in pipes. Then put pipe back on without the muffler. Try to start. If it runs. Which is what I'm betting on. Then try with the muffler. If not then you know something is blocking in the muffler.

Condenser is probably f####d!

Try to start it with the silencer removed.

The skyway silencer-spark arrestor is notorious for the screen catching carbon and reducing exhaust flow. I know you can run water through it but that doesn't mean its not restricted. Due to the age of the bike it probably has intake air leaks everywhere so by removing the pipe, i.e. the restriction, it runs. When the pipe is installed so is the restriction. Remove the silencer and see how much light you can see through it.

Hi Ethan and Happy Thanksgiving. I just read through your post on FB and am glad the new plug and a little spray past the carb slide got some fire. The poster that said to adjust the points gap and then move the backing plate is correct. You've got the correct flywheel puller, right! Thats right out of the manual. I had a problem with my 360 automatic that the head nuts and bolts got loose and it would fire every now and then and my foot is still recovering. So Check that! Might as well tigten the intake bolts too!! Your timing should be 1.8-2.0 mm before tdc. Moving clockwise while looking at the flywheel! You can put a peice of tape on the head and drop a straw into the top plug hole and move the piston to the very top, mark the straw at that point watching the tape edge. Pull the straw and add 2mm to the top above the line. Put the straw back into the plughole and bring the piston up to the first line, that should be 2mm btdc. At this point put a pencile mark on the flywheel and the case in line so you have a good refrence when you look thru the window on the flywheel and you should see the points opening at this point. That's your basic no kickback timing adjustment. Put a NEW plug in and trim the end of the coil wire and put the cap back on. Squirt some premix into the carb with the slide open and it should fire pretty easily. If it does!! Clean the carb 5 more times and you should be able to run the bike off a hanging bottle. If you are able to run it off a bottle , pull the tank and throw some nuts and bolts in it and shake it for 10 minutes.Clean the petcock thouroughly also!! Rinse the tank out and reinstall it and put an inline filter before the carb and ride the crap out of it!! Good luck and keep us posted!! Chris
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AND THANK YOU STEVE J