1981 GS750L Surging Idle

[NickB]

Hello everyone,

I'm having a bit of a problem with my 1981 GS750L surging as it idles. When I start it up (full choke), it initially idles fine (around 1200RPM) after a while it's idle ramps all the way up to 4000RPM+ and stays there for about 15 seconds. After which, it calms down again and promptly dies. Here's what I've tried to do to fix the problem:

1. Played around with the idle screw on the back of the carbs; mostly no affect, sometimes it will idle normally for a full minute but then ramp up and down and die.

2. Checked the engine for air leaks; Didn't find any.

3. Adjusted the throttle cable; Resulted in a sustained high idle (around 5000RPM)

4. Checked for air leaks in the air box and the rubber connectors from the carbs to the air box; None found.

5. Checked that my gas line wasn't being pinched; It wasn't.

6. Checked that I wasn't leaking fuel out of the carbs; It wasn't.

7. Slapped the gas tank as hard as I could; No affect.

Now most responses I've seen for normal idling issues is its either A) clogged carbs, or B)A fuel/air issue. Mine seems to be a bit more unique, or possibly a combination of several problems? I've had this bike for two years and have yet to ride it for longer than 10 minutes. I've been doing repair after repair (yes that includes cleaning the carbs) and I still seem to have gotten nowhere. So what I'm trying to say is that I am really at my wit's end with this machine and I'm a couple of degrees away from achieving cranial combustion... or maybe just selling it and getting something that runs better. So here's hoping that someone else had this problem and fixed it (preferably with not spending a huge chunk of money).

 

[tom203]

Most likely you got airleak- did you replace those o-rings at carb boot/cylinder interface. Tired ones have a nasty tendency to behave as you describe-erratic idle especially when warm. head to www.cycleorings.com for explanation and new ones.
Yeah, you have to remove carbs but after Two years and ten minutes of riding this is a minor detail!
Update your signature (user CP in top left of this page) to show bike/model/year. If you also show your location, there might be a member nearby to lend a hand.

 

[Andrew Vanis]

did you replace those o-rings at carb boot/cylinder interface. Tired ones have a nasty tendency to behave as you describe

This model has rubber boots on both sides of the carb. No O-rings (except those inside the carbs).

I've been doing repair after repair (yes that includes cleaning the carbs)

Must ask. Did you do the Nessism/BassCliff cleaning? http://www.mtsac.edu/~cliff/storage/gs/Mikuni_BS-CV_Carburetor_Rebuild_Tutorial.pdf

I have this same model/year and have just spent this spring/summer getting it roadworthy (or at least running swell enough that I want to ride it. The carb cleaning made a ton of difference. I had similar issues to yours when I put it in storage a few years ago.

There are others smarter at this stuff than me, but I?m happy to field your questions since I have familiarity with this specific one. The ?L?s are a little different in some ways.

or maybe just selling it

We?ll help you and we hope you don?t get to this point, but if you do, keep me in mind as a buyer.

 

[almarconi]

Couple items to check:
1.Whats your voltage going to the coils? Does idle change when headlight is turned on/off?
2.Is your choke working properly, rpm should increase when you pull up on the knob.When you push down, engine should return to idle.
3. Where are your mixture screws set, how many turns out from lightly seated?
4. Knob on bottom of carbs should change idle when screwed in. Take carbs off bike and verify that all butterflys are moving when knob is turned. There should be a spring on the knob,if missing idle will wander.
5. When you blip throttle, rpm should rise and fall quickly back to base line. If it doesn't adjust your mixture screws to richen or lean it as needed.
6. Have you synched the carbs both on the bench and with vacuum gauge?

 

[Guest]

When was the last time the valves have been adjusted?