top end rebuild of a 1978 GS1000

[Big T]

What screws are you turning and why?

[superawesome]

Hi,

I had the engine idle at about 2500rpm.
Well, there is a locknut and a screw (see pic below). I loosen the locknut and at the same time turn the screw to adjust. Or am I wrong here?

Carb settings are
fuel screw: 7/8 turns out
air screw: 1 3/4 turns out

WP_20180302_15_17_17_Pro.jpg
(this is an old picture, carbs are spotless clean now )

[superawesome]

Rob,

can you (or somebody) briefly explain why a re-shim changes the vacuum and makes it necessary to sync the carbs? Clearance changes only fractions of mm, is that enough to affect the vacuum that much?

[hannibal]

Don't think there's much to explain. You adjust the valve clearance in 0.05 mm increments and it's enough to affect carb balance.

[Big T]

Hi,

I had the engine idle at about 2500rpm.
Well, there is a locknut and a screw (see pic below). I loosen the locknut and at the same time turn the screw to adjust. Or am I wrong here?

Carb settings are
fuel screw: 7/8 turns out
air screw: 1 3/4 turns out

[ATTACH=CONFIG]56668[/ATTACH]
(this is an old picture, carbs are spotless clean now )

You are correct.

[azr]

so that brings us back to the dilemma of why his vacuum is not higher than it is when synching....hmmm. When this has happened to me I remove the carbs and start over. Bench synch them, use a very thin, paper clip thickness, piece of wire to set them. You may have them set too high and compensating with your main idle?

[superawesome]

I vacuum-synced with carbtune on Saturday, but still somehow had unstable idle. So got the colortune out of the box (my fav toy ) and discovered that #3 was not firing in idle. Had a spark but only fired when I pulled the throttle. Ran out of time, left it for the day, pursuing other tasks (You probably know that feel when you do your shopping, stand in the produce area, and your mind wanders off, it's all about air leaks, you're muttering "check the obvious first, check the obvious first, ...").

Also a friend said, whenever it acts weird, check for air leaks, so that's what I did (again). Finally found one.
My assumption was that it should be something between carbs and engine, as only #3 was affected, so I concentrated on that area. Brake cleaner had no effect so I check further backwards. Eventually I found that one of the big clamps between air filter and airbox was not tightened. I remember loosening it when I pushed the airbox onto the carbs, but must have forgotten to tighten it again. So there was my obvious leak!

I assumed that air leaks near the air filter can't have a big effect (other than the ones between carbs and engine), but I was proven wrong and take that as a lesson.

I did not have time for a test ride yet, but the idle was really smooth at the end of my (carb- and color-)tuning. She idled stable at 1300rpm, where before I could not get a stable idle below 1500rpm. Carbtune said vacuum's about 22 cmHg, but I am not worried since it sounds smooth now. And this is roughly what I had before.

[superawesome]

How about a picture of your bike from the back showing the tank, I think you have that cool pin stripping down the tank.

Here it is. I am actually amazed that those golden stripe stickers stayed on there for 40 years...

Here's the tank while carb-syncing...


and on the bike