1981 GS550 Wont rev above 6-7k unless running out of fuel

[octusis]

I've looked all over the place and can't find anything on it. I've adjusted the floats to what I believe is factory spec. The bike seems to rev fine in neutral or just off-load in general but as soon as its under any kind of load it just falls on its face unless its running out of gas by shutting off the petcock.

I'm stumped at this point if anyone has any ideas on what it could be I would really appreciate it

[Easy-X]

Hmm, not getting max airflow at full throttle perhaps? (Restricting the fuel would lean it out to balance this.)

Worth checking the air filter and exhaust for blockages.
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[jaffa90]

Sounds like a rich mixture and improves with less fuel (lowering the fuel level in the carbs).
Are the main jets standard?
The spark plugs colours will tell a tale.

[-]

I’m thinking like Easy-X. If it’s over fuelling, it could be due to “under airing”. What’s the air filter like?
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[jaffa90]

The O.P. says it is ok for a mo after turning the fuel tap off.
So how can it be the air intake change?

[-]

[chickenstrip]

1981 bike? I wonder what condition all the carb seals and o rings are in?
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[jaffa90]

Another thought, does it have a vacuum operated fuel tap?
If so is there any fuel in that pipe going into the intake manifold?

[octusis]

[octusis]

[octusis]

[octusis]

[octusis]

[Damon]

Have you tried lowering the float level a tad?

[octusis]

[jaffa90]

What happens when you leave the fuel tap on when not using the bike?

[Robby]

Got confused and left the choke on?

[octusis]

[octusis]

[jaffa90]

[temeluchus]

You'll need to at least replace all the seals and float needles/valves in the carbs.

Don't try and cheap out, it might end up with the bike on fire.
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[octusis]

[octusis]

[chickenstrip]

If this is anything more than a hobby bike, I'd be thinking about getting shot of it for something else tbh. Getting a bit too long in the tooth, and I doubt parts are easy to come by if you need anything other than wear-and-tear.

If it's a hobby bike, I'd be looking at replacing anything in the carbs I could get new replacements for. What state is the inside of the fuel tank? If it's feeding crap into the carbs, it might cause fuel leaks/flooding/god knows what. Likewise with fuel lines. It doesn't take much.
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Chickenystripgeezer's Biking Life (Latest update 19/10/18) Belgium, France, Italy, Austria tour 2016 Picos de Europa, Pyrenees and French Alps tour 2017 Scotland Trip 1, now with BONUS FEATURE edit, 5/10/19, on page 2 Scotland Trip 2 Luxembourg, Black Forest, Switzerland, Vosges Trip 2017
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[octusis]