Mixture question

[GS750GUY]

Greetings! GS750Guy here again.
I've just recently done a tune up on my 78GS750EC. Changed plugs, points, condensors, and adjusted the valve clearance.
When replacing the plugs I noticed the old #2 plug appeared to be running rich so I tweaked the fuel screw a quarter turn to the lean and went for a ride. Upon returning I noticed the #2 pipe at the head had turned blue and the other three show no signs of discoloration at all. Does this mean I'm now a little too lean and running hot on #2 and need to richen it up a little? Prior to this all pipes had no discolloration.
Thanks,
GS750GUY

[tkent02]

Might try putting the screw back where it was and do some plug chops to look at the new plugs.

[hampshirehog]

Blue pipes usually means a weak mixture though can mean the chrome on the pipe is susceptible to blueing (don't ask me why). Try plug chops at different throttle openings to see where you're weak.

[GS750GUY]

Sorry for being so stupid..........but what are plug chops?
GS750Guy

[hampshirehog]

Plug chop = checking your plug colours after they have been running at a fixed throttle opening. The way to do this is (with a fully warmed up motor).

1. Let it idle for a couple of minutes (make sure it doesn't overheat - a fan may be necessary) and stop the engine. Remove the plugs and see if you're rich or weak. Adjust as necessary pilot circuit. (You can make this session a lot easier with a colortune).

2. Run at 1/2 to 3/4 throttle for a minute on the road - a nice empty road with an uphill makes this easier / safer and more legal! Hit the kill switch and after stopping check the plugs. You're now checking the mixture as determined by the needle position.

3. Repeat above but at full throttle. This is for checking your main jet.

However, I completely misread your post (I'm allowed to get confused at my age!) and if your bike is running a standard filter and exhaust you shouldn't need to alter needle nor main jet so 2 and 3 are for info only. (Though worth checking that nobody has altered anything before you bought the bike)

I would wind your fuel screw back to where it was and check float height first. If that's ok set your airscrews (given as 'mixture screw setting) to the factory settings given in:

http://www.thegsresources.com/_forum/showthread.php?t=73723&page=5

Then adjust your fuel screws as necessary to get a good colour on a colurtune or plug chop. You can tweak this using the highest revs method.

[Greg O.]

Correct me if I am wrong, but the fuel screws, underneath the carbs, are more for the idle circuit, and really shouldn't affect high speed riding that much. But at any rate you need to do some plug chops to see what is going on. Try to richen the air pilot jets and see what that does. To richen them turn them in. Keep record of how much you turn them so you can set them back if it doesn't work out.

Greg O