> what's up with this motorcycle?

what's up with this motorcycle?

Posted at: 2015-04-14 
it's a 1980 Yamaha Maxim 650. Cylinders 1 & 4 wont fire until the bike is completely warmed up. whats wrong?

Heat can affect electrical connections and seals (metal expands when hot) and fuel mixture (air expands when hot). In hot air the engine naturally runs richer (i.e., will run with less fuel... it has to do with Bernoulli's Principle and the larger volume of hot expanded air through the carbs). Maybe 1 & 4 are set too lean? Maybe 1 & 4 the carb jets are kinda dirty and not squirting enough fuel? Old bike. (Viagra won't help.) Lots of possibilities. Ya gotta like check stuff dude. I would get a Haynes or Chalmers shop book, pull the carbs, rebuild all of them, check all the rubber parts, check the electrical connections, adjust the valves or do a top end overhaul and put in new plugs. While you're at it, replace the fuel filter(s), swish out the tank and check the finger strainers, if it has them, and replace the fuel lines as needed. Oh yeh... and change every single fluid (forks, engine oil, brakes). But that's me wanting to like, really get it running right. A good winter project.

also check & reset float levels precisely - give the carbies a clean while you're at it - strong probability that at some time a bike that old has been left sitting for months with fuel in the carbies.

Sticky valves maybe ? Bad springs ? ,,,, consider doing a compression + leakdown test .

Edit : Coils don't get better by getting hotter , just some food for thought .

Failing coil, try swapping them over, see if the missing cylinders will fire.
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