GL1200 carb work

[Oldmannewwinger]

Hello, I appreciate all the careful answers to questions here. I have searched the site pretty well and still have a question.
Can someone explain the relationships between the vacuum operated needle jets, vs. main jet size vs. low rpm jet size vs. float level vs. idle screw adjustment? My 1985 Aspencade runs pretty well overall but smells a bit rich when idling and gets a little lower gas mileage than what I'm expecting. There is no hesitation ever and it has pretty good power through all RPMs. The idle is fairly consistent but slightly rough and maybe a little labored. It's all stock, but I'm considering installing smaller main jets. But maybe I should adjust the slider needle somehow?? The mixture screws are slightly less open than the manual calls for. The float levels are by the book.
BTW, I get the feeling that the 39 year old ignition system may not be optimally profiled. It has new coils, plugs and wires but I question the advance mechanism. As far as I know, it is all done by that 39 year old module with a vacuum line attached to it. Perhaps the mileage and performance would pick up if there was a way to optimize all that???
Thanks,

[Whiskerfish]

Really doubt that changing jets will resolve issues on a stock setup. Are thew carbs really really clean inside? What kind of fuel economy are you experiencing?

[robin1731]

Really doubt that changing jets will resolve issues on a stock setup. Are thew carbs really really clean inside? What kind of fuel economy are you experiencing?

This^^^^^^^^^^^^^^^^^^^

And they all run a little rich at idle. What do the plugs look like after idling 5 minutes or so? What do the plugs look like after cruising at 60mph for several miles and then shutting the engine off? Is the bottom of the plenum dry? Were the carbs rebuilt? If so were jets changed?

[Rat]

Anticipated mileage is low to mid 40s …

Gord(subject to wrist twist)Jones

[Oldmannewwinger]

Took me a while to get back to this. Thank you for your answers and questions. My mileage hits high 30's on a long ride and mid to high 20's on shorter rides, in town and stuff. The plugs don't look too bad. They have a bit of sooty blackness on them. The tail pipes are black inside as well. The carbs are very clean. Don't know if they need to be cleaner yet. Someone suggested that the needles might be worn??
The carbs have been messed with a lot, but the slow jets are still #35 and the main jets are still #108.
The plenum, I assume is the basin under the air cleaner? It is dry.

[CYBORG]

Have you replaced th air filter?

[Whiskerfish]

high 30's on the road is not far off the mark. If the carbs are clean and stock I would look to other "Tune up" items. Fresh plugs, air filter as Doug stated. Maybe check for brakes releasing properly

[ericheath]

Did you check the jets to see that they hadn’t been drilled out? Who would do that? ( Sheepishly raises his hand.)

You can get new keihin jets from JetsRus.

Unfortunately the timing is all mysteriously hidden in the black box. It uses gear selection and the vacuum to aid in timing advance. As best as I recall the vacuum isn’t a factor until 4th and 5th gear. Shouldn’t affect your rich idle though.

High fuel levels in the bowls could cause this.

Another item on those carbs is the cold start valves. Check the tangs and ensure they’re fully closing on all four. The lever at the handle bar doesn’t necessarily mean they’re all closing.

[rcmatt007]

MPG is right wrist dependent