Pop noise when turning key

Currently restoring an 82 xj550 maxim, have fixed up old bikes before so it's not my first rodeo sort of speak but I'm at a loss. After the bike sits overnight and I turn the ignition I hear a popping noise nearly identical to a carb backfire coming from the right side of the bike and I smell a mix of oil and rubber, as if I have a short. But there is no sparks, no heat, no continuous power draw. Fuses are good and it only does it once the first time I turn the key. Bike operates as normal after, lights work, turns over, etc. it's as if there's pressure being released somewhere in the crankcase but I am at a loss here. The only way to reproduce the noise is to let the bike sit for several hours then turn the key again. Any ideas what could be causing it?

 

Just one pop?
If it does it every time it shouldn't be hard to eliminate one thing after another. I would start with disconnecting the tci.

 

Pull the tank so you can help locate the sound better. This is going to sound weird, by any chance one coil is firing and causing the pop sound from one of the cylinders? Flip the engine off switch on the right bar and then try and create the issue.

Weird that you hear and then smell an indication. Get a supper quiet area and maybe even a dark area and try to troubleshoot. Good luck and keep us posted.

 

Not that unusual as the TCI always fires once after the key is turned on and the 2 to 3 second delay occurs without starting the bike. If there is fuel in the cylinders from a carb overflow then you can expect a loud bang. The difference here seems to be lack of "damn near jumped out of my shoes" as that is what usually occurs in this situation.

Setting the kill switch to stop would be a good way to check as suggested - sit over night - kill set to stop - key on no noise?? - kill set to run - 2 to 3 seconds later noise occurs

 

This is the conclusion I came to aswell, just without the 2-3 second delay. Spark plugs where firing on key turn with a bit of leftover gas in the cylinder. A lot of things had to line up but I've been unable to reproduce the noise after removing gas tank and draining carbs

 

So you likely have two issues:

A petcock that does not shut the gas off when the bike is not running

And a carb overflowing just a bit - theoretically the carbs should not overflow even if the vacuum assisted petcock is flowing gas when the bike is off

Be sure to check you oil level and take a whiff for gas - you don't want to run it with fuel diluted oil

 

The gas tank hasn't been on the bike in 4 years, you are correct that the carbs did overflow. Fixed that this morning. The pop happened with the gas tank off and only the carbs installed with the bowls filled from a gas container I have rigged up for testing machinery.