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My girlfriend's '81 GL500 Silverwing is bogging down at idle in neutral and dying in first gear due to low idle speed. Very new to motorcycles but thinking this has to be some kind of fuel issue. Is this a carb issue or does it have something do to with the what I believe is an aftermarket fuel filter or something I should check in the tank? Thanks in advance. Getting my '82 GL500 Silverwing Interstate later this week!
 

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Have you tried increasing the idle speed? When warmed up the bike should idle at about 1100 rpm.
 

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Welcome to the forum. Please add your location to your profile - there could be a forum member nearby that can help you in person (see Forum Settings link in my signature).

And welcome to the world of antique vehicle ownership (they own us, not the other way around). Your bike is about 4 decades old and the Previous Owners may or may not have done the maintenance necessary to keep it safe & reliable so it is highly recommended to download the Factory Shop Manual for your model (available through the CX Wiki - link in my signature) and go through all of the service procedures, regardless of whether your bike has reached the specified mileage.
I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. Check the date codes on your tires and replace them if they are over 5 years old no matter how good they look & feel because old rubber simply cannot flow around the irregularities in the asphalt well enough to grip, especially if it is cool or wet. If your bike still has the original rubber brake line(s) (should be replaced every 2 or 3 fluid changes = 5 or 6 years) I recommend shopping for modern stainless braided ones (they last practically forever and double the life of the fluid).

I'd try turning the idle up a but first too but if it idled OK before you need to find the real cause of the problem. I wouldn't discount the fuel filter. As you said, it isn't original; The original design relies on a filter over the petcock inside the tank, which is perfectly adequate if the inside of the tank is clean BUT a steel fuel tank can become pretty dirty & rusty over the course of almost 4 decades (the GL500/650 tank also has pockets at the bottom where water can collect and build up for years if the bike is stored with the tank full)(I always recommend draining the tank for storage) and particles of that can make their way through the petcock screen and foul the carbs. The proper solution is to clean the tank out and epoxy line it (some rad shops offer that service or you can buy the POR15 kit and deal with the mess & hazardous chemicals yourself) but many people opt for the band aid solution of adding an inline fuel filter. The problem is that they don't think about the orientation of the filter. It's outlet should be at least 1/2" lower than the inlet so that fuel will flow through it.
 

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Since no one has bothered to tell you, there's a black knob between the carburetors that can be turned to change the idle speed.
 
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