Bike starts, then dies if I don't keep giving it throttle

[Anonymous]

Hi guys,

I'm having the problem above with a new (to me) bike. It ran fine for a while, sat for maybe a month while I was re-doing the rear brakes and too busy to ride. 1980 GS750LT. I thought it was a gas problem at first as it wouldn't start at all, but I took apart the petcock and cleaned it up, seems to be ok. This is what happens:

Fires up, choke all the way out, it will die within about 5 seconds if I don't keep giving it gas. It warms up, same problem. The problem is actually worse if I push the choke in. Of course since the bike is warm and the choke is out, I'm getting some smoke for an over-rich mixture, but I want it to idle properly.. I hear there are fuel mixture screws, but they are typically not to be touched.

Does it sound like air is getting in somewhere from all of my monkeying around? What's the best way to check for air leaks?

Thanks.

-Warren

[hoyterb]

http://www.thegsresources.com/images...20LOCATION.jpg carb idle screw loc.
http://www.thegsresources.com/images...ir%20screw.jpg air screw
http://www.thegsresources.com/images...ir%20screw.jpg pilot jet plug
http://www.thegsresources.com/images...%20removed.jpg pilot jet
http://www.thegsresources.com/images...%20removed.jpg pilot jet out
Hi Warren Sounds like your pilot jet is plugged.When you turned the carbs upside down a bunch of crap plugged the holes in jet.The url's at top are from the "Carb Cleanup Series" on the main page.This will give you a good idea of where to look.It isn't getting fuel when the throttle is closed or nearly so.You may as well clean the carbs completely.They WILL give you problems if you don't.

[Anonymous]

Well, I didn't turn the carbs upside down.. In fact, I really didn't touch them other than removing and installation the tank & petcock. It seemed to be running pretty well when I got it, which is strange.

Anyway, I was hoping it wasn't going to come to the carb rebuild. What are people's opinions of simply cleaning out my carb rather than buying rebuild kits (4 at ~$20 each). Are they worthwhile? Are the seals usually not re-usable?

Thanks

-Warren

[Anonymous]

It does sound like plugged pilot jets. You could try running some carb cleaner like gumout through the tank, but it isn't as effective as removing the carbs and cleaning them. It will also run like crap untill the gumout is gone. As far as not touching fuel mixture screws, I'm glad I touched mine. Two of them were factory set all the way in. A ridiculously lean situation just to satisfy EPA regulations.

[Don Lobacz]

Rebuild kits are very seldom needed. A film of varnish is all it takes to plug a pilot jet or 2, but that is fairly easy to clean up. A month or 2 is usually not enough to clog the jets, so it may be possible that you have simply fouled a plug when you started it. Check those and change them if they are black.

[Anonymous]

Hi all,
Are the air screws mentioned in the carb clean up series really PILOT SCREWS? I'm confused as to what they are and it makes a difference in which way I should turn them to richen my carbs depending on what they are.

Thanks, Steve

[nabrams]

Steve,

If you've got CV carbs (I believe you do, because your bikes are newer than 1979) then there should be only one "mixture" screw per carb. I'm pretty sure this controls fuel flow (not air), so turning them "out" (ccw) will richen your mixture.
My 1980 GS450 (has CV carbs) was running lean at idle (plugs were nearly white) and when I turned the mixture screws out (had to remove tamper-proof caps) it improved things. My understanding is that the factory sets these screws to very lean position.
You can be certain one way or the other by simply going to either extreme, let the bike idle for an hour, and check the plugs. Nothing like knowing for sure through empirical evidence.

[Jay B]

Another thing to check before going into the carb clean up is the fuel filter and fuel lines. You mentioned that the tank was off. You could have stirred up a lot of gunk that pluged the filter or fuel line. Also check to make sure that the line isnt pinched or kinked.