1981 gS650g carbs?

[keso151]

,Hello all, Purchased last fall an original owner 1981 gs650g with only 6000 miles...needed fuel system go thru so we did flushed tank(very clean),replaced all lines,o rings, intake boots, went thru carbs with all new parts after ultrasonically cleaning them. Now it runs perfect til higher rpm..then it fall flat. Plugs are black...does anyone have a direction we can go?

[Nessism]

You might want to check the Newbie Mistakes and carb rebuild tutorial as linked in my signature to see if anything applies.

[tom203]

Hopefully you saved the old carb jets,etc......if you used aftermarket carb rebuild kits , as opposed to OEM mikuni, you might have problem. Did you check the diaphragms for pinholes leaks when you had carbs apart? Did you get the diaphragm orientation correct in slide during reassembly?
At what rpm/loading point does it fall flat?

[keso151]

Seems to be around 7500...

[keso151]

Wow nice laid out carb tutorial..wish I would have seen this before...Have to check that the slide diaphrams were replaced ..cant remember...we did everything that was listed..so am removing them again to check over.

[keso151]

Ed, Still trying to sort out this bike..little more info..all stock..6000 mile orig. owner..rebuilt carbs to stock with redo of all fuel system..floats set to 21.5...main jets in sumo kit were 110. The problem we are having is it runs/starts perfect from idle to 4-5000 rpm then bogs down..any ideas where to look?

[Nessism]

Is the airbox and air filter installed?

[tom203]

Assuming you got the stock airbox / filter setup....make sure you didn't over oil the filter. If you might have,as a test, try peeling element back slightly to see if anything improves. Your mid range will suffer but > 6000 , things might improve.

[keso151]

Tom, Yes to stock airbox...when I get bike back I'll try the element. Thanks

[tom203]

I read your PM.....I sympathize with your friend....with the stock airbox setup these carbs are no fun getting off/on. Since it runs good up to 5k and then falls flat investigate two easy things first
1: peel back the airfilter element a bit, in case it's restricting airflow and go for ride
2: check to make sure the ignition advance mechanism is working properly. This is behind that little round cover on right side lower crankcase. Grab the tab projection on crank end and twist ...it should move about 30 degrees and spring back when you release it. The timing needs to advance fully by about 4K rpms

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[keso151]

Just got the bike back tonite...my guy cant seem to get it ..first one in 20 yrs! Slow and steady acceleration to 5k is good after that it falls flat...tried 105 jets no help...used a different fueling source no help...adjusted floats 4 times..no help. The carb kits put in were sumo brand..never had a problem before. Tried air filter trick and checked ignition advance ..he said it looked fine. This was his brothers bike originally and with only 6800 miles we both are stumped!. At least I get to store it for winter in a heated warehouse where we ride a few miles a week to keep things lubed..Any ideas out there?

[tom203]

This critter wakes up at 4K rpm with stock setup, not fall flat. Are you sure that your petcock is delivering enough fuel??
If yes, then I'm blaming the sumo main jet , so let's get some stock mikuni 110 and go from there .

[Steve]

Any ideas out there?

Yeah, just based on your last post.

... tried 105 jets no help.

Stock jetting is 110. Reducing the mains by two sizes will definitely reduce power.

... adjusted floats 4 times..no help.

From all reports, the 650 carbs are not easy to remove, your patience level must be EXTREMELY high.
Are you checking both sides of the float? In post #6, you mention setting floats to 21.5. That is on the rich side of the range, but if the float assembly is bent, and the other side is around 24, that carb will be running lean. I try to keep the two sides within 0.5mm, and use the larger number as my "set" level. It is also advisable to check the actual fuel level with a tube adapted to a modified drain screw.

At least I get to store it for winter in a heated warehouse where we ride a few miles a week to keep things lubed.

Do your bike a favor and just store it properly. "Lubing" it by riding it inside a warehouse will actually hurt it because it does not get up to full temperature. By not warming up fully, condensation will build in the crankcase and exhaust system. When the riding season is over, just treat the gas in the tank with stabilizer, ride it a few miles to ensure that treated fuel gets through all circuits in the carbs, then park the bike. Change the oil and filter. Run the engine for a couple of minutes to distribute the new oil. About once a month, top off the battery charge. When riding season starts again (or you just get a day that is warm enough to go for a ride), check the tire pressure, go for a ride.

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