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GS450 idle adjustment?

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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I have a 1980 GS450 that I recently rescued from the back of someone's garage. I've been tinkering with it (first motorcycle!) and brought it back to life.

I'm having a weird idle issue, though. I start the bike on full choke, and that's all well and good. Once I run it for about 30 seconds I can slide the choke off. It seems to idle fine. However, if I ride it for only a short distance (and let the motor really warm up, I guess) the idle speed increases steadily. After a few minutes it's idling at 3500rpm and doesn't go higher than that.

I have a Clymer's manual but I cannot find any information about idle adjustment. Can anyone offer up some help? I believe it uses twin Mikuni BS 34 carbs. I recently had them cleaned up. The shop guy said they were "ready to go", so this idle issue is puzzling.
 
Congrats on your new GS450. I have an '80 GS450ST and had a similar problem. Sounds like your pilot jets are out of adjustment. Mine would idle at 2500rpm and not go down to 1200 unless I put it in gear and put some clutch drag on (front brake held on) to force the rpm down to 1200, then it would stay. As soon as I blipped the throttle, back up to 2500. Adjusting the throttle stop screw (big knurled thumb screw between the carbs, points down, easy to burn your hand on the starter cover if the engine is warm), adjusting this down to where it wouldn't idle at 2000rpm caused it to almost die instead. No happy medium. Turns out my pilot screws (small brass screws on the sides of the carbs toward the front) were adjusted to 1 1/2 turns out. Setting them to 2 1/2 turns out did the trick. I could then use the throttle stop screw to fine tune the idle to 1200 rpm. Your pilot jet screws may be sealed up, in which case you would have to know exactly where to drill to access them. I can send you a pic from my factory service manual if you need it. The EPA is to blame for this setup, the factory was trying to meet emissions regulations. The bike mag reviews complained about this when the bikes were first tested. Another common mod is to raise the main needles to richen the mixture back up. I think this was already done to my bike. I no longer have any problems with throttle response, hesitations or surging of any kind. I can pull away on a "whiff of throttle" as the UK guys say. The way it is set up now it starts without any choke at all, but needs to warm up for about two or three minutes before it will idle. I have a throttle lock "cruise control" on the grip and set this to make the bike idle at 1500 or so while I put on my boots and lid. By the time I'm ready to go the cruise control can be flipped off and the bike will idle at around 1000rpm. After a few miles of warming up the bike then idles at 1200rpm where it should. Your bike sounds like the throttle stop is cranked way up to make the bike idle while it is still cold but idle way too fast after warmup...
 
As you may already know, the enrichener (choke) raises the idle. You might be able to set your idle screw down to 1200 when warm, and use the enrichener to hold the idle up when cold.
 
In sunny SoCal, my bike won't run more than about 10 seconds with the choke on before it starts to bog down. I might need to adjust the linkage so the idle is raised sooner, 'cause nothing happens until the choke is almost fully closed. Needs longer to warm up than that, about 2-3 minutes before it will idle at 1000rpm or so then it takes about 3-4 miles before the oil temp gets to 150F, at which time it will idle at 1200rpm. I read somewhere that a cheap throttle lock can be fabricated using a large o-ring or rubber band or strip of inner tube to create the small amount of friction it takes to be able to set the throttle where you want it. I'm not comfortable riding around town with it on, though.
 
For what it's worth, mine ('80 450 L) bumps up the idle gradually, starting with just a little choke. I've never bothered to look and see how it works, though. Seems one oughta be able to adjust it, you think?
 
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