Motorcycle Repair: idle troubles..., vacuum leaks, yamaha virago 750

Question
I rebuilt my carbs on my 82 yamaha virago 750 and now it idle hard and my float levels don't match my bench levels (lower). What am I doing wrong?

Answer
Hi Sara,

Most carb troubles are caused by air leaks around
the rubber intake mounts and old fuel.

The float levels will change depending on
the fuel, the atmosperic pressure and if
the bike is stopped or running.

The float level is usually just a ballpark figure.
If you set it close to specs and everything looks
clean then idle problems may be due to other
factors. Idle jets have to be very clean as
well as carb passageways.

There can be no vacuum leaks around the carb mounts.
Air leaks can cause the idle to fluctuate.
The carbs should be synced to close and open in unison.

Check if each cylinder is running, maybe one cylinder
is not firing due to fuel or spark problems.
Are both pipes the same temperature?

Tight valve clearance can also cause bad idle.

Carbs need to be soaked in Yamaha carb cleaner
to get them totally clean. If you didn't soak them
they may still have plugged fuel circuits.
Yamaha makes a good carb cleaner.

Viragos need a good battery or the spark can be weak.

Check your plug color, black is too rich with fuel.
See if it runs better with no choke or partial choke.
This can help diagnose it's needs and the possible
problem.

I recently had a bike that just had bad fuel and weak
battery and it would not idle no matter what.

I flushed the tank, added inline fuel filters, new fuel,
cleaned the carb, again and used a new battery.
It idles like a rock now.

Sometimes the Virago pilot idle mixture screws are set too lean
and are covered with a lead plug to prevent tampering.
They can be drilled a tiny bit and pry the lead plug out.
Then the pilot screws can be turned out about 2.5 turns which
helps.
http://www.viragoownersclub.org/Dr%20Piston/hitachi_carbs.htm

Double check everything as sometimes it seems like
you are going in circles but usually there is something being missed.
It might be minor but will even make professionals crazy at times.

Let me know if you need some detail on something in particular
you suspect.

It doesn't hurt to check your charging voltage as well.
14 volts at 2500 rpm is good.

Wayne S
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