1980 GS450L trailing throttle question and more

[NW Rider]

Hi,

New to the forum, but not new to biking. I picked up a used GS450L from a friend for a song. It had been lightly used over the past couple of years, and after an hour or two of PM I got it on the road.

I searched the forum and didn't see a post like this.

The bike starts fine, accelerates fine and idles fine. The probllem I'm experiencing is that on a light or slightly trailing throttle like a quarter mile down grade, it seems to act like it's out of fuel or dying or kinda surgy almost like you get when your just on the edge of switching to reserve. But when I twist the throttle it comes back no surging. 20K on the odometer if that helps. Plugs both look tan. I took a temp reading of both head pipes with my infrared thermometer and they and the heads on each side are at the same temps, so as a rough observation it doesn't appear one side is running significently leaner.

The second question is related to the fuel tank. I'm not wild about the EZ Rider fuel tank mounting. I haven't taken the tank off to check, but can it be mounted with the front down on the frame a bit. I think it would look a bit more conventional. Throw in a pair of touring bars and it would be pretty nice to look at...

Hoping to short cut the diagnostic process...Thanks in advance.

NW Rider

[bwringer]

I've experienced the same thing when the air filter gets a little dirty or is over-oiled. (My GS850 uses an oiled foam filter.) I don't know what the 450 uses, but the principle is the same.

Even if it's a paper filter, if it's old and dirty, clogged with blowby, or has gotten wet and shrunk, it might be the problem.

[GearHead]

Being that its early in the am here, im kinda foggy. But check out your idle circuit. At light throttle settings all carburated engines opperate off the idle side of the carb. As you apply more throttle you start to use the main metering circuit. So i would start with the idle circuit. Basicaly check for trash in the carb, is it set to factory specs or has the po changed something? Like adding an aftermarket aircleaner.
Hope this helps.
Gearhead

[Charlie G]

My 450e did a simular thing regardless of the grade of the road or throttle position. It did seem to do it more often when I had a 100 miles plus on a tank of gas. It turned out to be that vaccum petcock was bad. My bike is a 1980 and has about 20k miles on it. Food for thought.

[NW Rider]

Thanks for the help.

Thanks to all, for the pointers on where to start. Since the carbs are clean, it idles perfectly, has a factory airbox with new OEM filter and new plugs. I'm going to focus the vacuum fuel valve. I've seen a number of posts on this and it seems to be one of those things which can give you problems.

I don't think I tried riding it in the "prime" setting, and wonder if that would help diagonse the problem? I'll give it a try next ride and see if that makes any difference.

Before I replace or rebuild the fuel valve, I'll check carb sync. Since it's a twin, I built a very simple differential vacuum indicator so I can actually see what is happning while riding, so I'll verify any vacuum difference under the light throttle condition, to rule out a vacuum leak on one side.