Surging Idle

[Keith]

1997 Bandit 1200 - recently purchased, unsure of milage as the speedo cable was missing when I got it, but my guess is 35,000 miles.
My bike has an issue that I will try to describe:
It runs great, starts easy in the morning, 40+ MPG etc.  Idles a bit over 1,000rpm.  I commute about 45 miles at about 50 - 75 MPH.  When I get off the freeway, the RPM's begin to surge when in neutral (or clutch in).  By surge, I mean the engine runs up to about 3,000 rpm, then will drop/bounce to 2,000rpm and back to 3k.  Very annoying.

Any Help?

Thanks much,
Keith

[pmackie]

Surging at idle is usually an indication of a lean condition. This can usually be corrected by adjusting the air screws, which mean drilling out the plugs first.

There are a couple of other things to check first:
1. Look for any cracks or splits in vacuum lines, which can cause air to suck.
2. Ensure the carbs are secure in the intake boots, that the clamps are tight, etc.
3. Make sure the boots (intake manifold) is tight on the engine.
In effect, look for sources of an air leak.
4. Possibly some built up crud in the carbs, blocking or partially blocking an idle jet circuit.

So, I would suggest you remove the carbs for a full cleaning, drill out the air screw plugs, check the boots/manifold, reinstall and then adjust air screws.

[Keith]

Thanks much, I will check all the vacume lines and carb/intake conections.

Does anyone have a scan of the service manual for pulling carbs?  Also a scan for the section on a Valve adjustment?  I have not yet bought my own manual.

thanks

[Keith]

OK,  I pulled and cleaned the carbs.  They were not too bad, but a bit gummy.  Hopefully that does the trick as I did not see any source for potential air leaks.  I will reinstall the carbs this evening and hope to run it tomorrow.

One bitch about buying used, is that you have no idea what has been done before.  The stock main jets were in place, but the A/F plugs were removed and the needles were shimmed.  All part of the fun I guess.

thanks,

[Keith]

Got the carbs back in but same problem still.     I have the air screws out about 3 turns.

It is weird.  If I blip the trottle, the revs fall quickly but then build back up to over 2K rpm.  The choke seems to cause it to rev higher.

Could out of sinc carbs cause this?

[Keith]

OK,  Here is my prognosis:  The fuel line from the petcock to the carb was kinked and when the fuel level got low, it was starving for gas at idle.  I straightened it out and all seems to be OK.
Tomorrow I should get a chance to get her on the road and find out for sure.

[rocketman1]

I have not had that problem with a Bandit but my Honda XR was doing the same thing, it was water in the base of the carby bowl, just enough to buzz around and get temporarily in the way of the fuel.
I cleaned the carb and emptied the brown gravy/ water in the fuel tank and now goes well.

[Red01]

M-M-M-M, brown gravy!  

[rocketman1]

Rusty water slopping around in petrol the bottom of a white plastic tank looks just like....