[Jtoll]

This is a 40-year old bike so everything relating to fuel delivery typically has issues. You may have a fuel restriction in the petcock, or it’s the carbs. I would start by replacing the petcock, flushing the tank and replace the fuel line/ vacuum lines. Look for signs of rust in the tank and do an acid flush (a strong citric/water solution for 12 hours) or have it treated professionally if needed. Next, the carburetors need to be rebuilt. I would buy 4 (K&L) carb kits and go with factory specs for now unless you are changing the exhaust. I would consider replacing the air box with clamp style air filters to boost performance. If not, replace the air box filter. It’s a good winter project. That thing will run like a champ. Also, always use non-oxy premium fuel and a good motorcycle grade oil for changes like Lucas, as wet clutches need it. Don’t ever put automotive oil in a motorcycle.

 

[TrialsRider]

... consider replacing the air box with clamp style air filters to boost performance....

On a bank of vacuum operated carburetors? would love for once to see a dyno chart or race performance results to support that claim. I wager the bikes performance falls flat on its face somewhere through the normal operating range or won't decelerate and return to idle for crap, but it will make a ton more induction noise that might be perceived as performance gains.

 

[speedrattle]

i'm not so sure that short open pipes are a bad idea. its all in the tuning-- if the pipes are short but that is also the correct length, then short is not a problem.

i run 34-inch open pipes on an old top-speed triumph LSR bike. that's some two feet shorter than the stock pipes and mufflers. but 34 inches is where the pipes resonate at the rpms i ride the bike at. two inches either way from 34 slow the machine four miles per hour.

people say that open pipes lead to an overrich condition at around mid-range or so. that is quite true. the same machine that benefits so much from short pipes also has a reversion issue at around 3800-4100 rpm that make it stumble. that's an rpm that you might use on the street, but is one that i pass through quickly during normal riding on this machine. so it's not a problem for the use that machine sees.

running a muffler is probably a better deal for the street unless you're a squid like me, but the short/long question is best resolved by just testing your own machine to see what works.

yes, you will have to jet the carbs to suit the pipes, but that's not a killer task

 

[Teazer]

If you would like to take a trip to Chicagoland sometime, I'd be happy to shopw you the dyno charts and 1/4 mile tickets that show that those carbs can work very well with good pods or short bellmouths or long velocity stacks.

In general, bike run sweetest with stock exhausts and intake systems, but with care they can work with aftermarket pipes. Japanese 4 cylinder bikes of that era certainly seemed to like much longer and more complete exhausts as a rule.

But what happened to the OP and how is he getting on with his problem solving.

 

[crazypj]

As usual, late to the party.
It is actually running on all 4 cylinders?
think carbs should be checked and cleaned properly.
If the cam timing isn't out by a tooth, the most common cause would be restricted emulsion tubes and probably damaged diaphragms.
The earlier slide carbs on GS responded much better to modifications in my experience.
At least you don't have 3 jet Kei-Hin on it
As for short open pipes, they work best at high rpm and usually need quite a lot of extra main jet,
'Streetable' if you have the experience but not good for a novice or someone used to modern fuel injected bikes (or someone who has only ridden small dirt bikes)