[mgevans]

hiya folks - first time on this forum.

I have a bandit 1200s that began stuttering this year. description of problem: throughout the RPMs the bike will stutter (feels like a mini-stall) for half a second or less. As the RPMs go up the stutter is less - i'm assuming because the next cylinder fires off and recovers the speed. Also as the bike is in higher gears the stuttering is not as noticable. The bike showed signs of stuttering at idle, not in gear, even if the choke was or was not engaged. When the bike was fully warmed up the stutter was a bit less.

Have had it in the ''zuki shop in leesport, pa - good guys. They found the caps in the carbs were cracked and replaced them. The carbs were balanced and while it was running the shop didn't appear to notice anything.

While the bike sat for a few days before I went to pick it up. When they rolled it out the bike wouldn't stayed fired. I sat around for an hour while they pulled the plugs and found at least one plug was fouled (left cylinder). The plugs were changed but it didn't appear to make a difference.

They kept it in the shop for another day to look at it again (no further labor charged). The mechanic found gas in the oil and gas/oil in the airbox. They cleaned out the airbox and changed the oil. If the bike sat for a while it might also produce white smoke when first starting.

the shop suspects the petcock was leaking slowly (i've ordered one - apparently the bandit has had a Rev update in the petcock).

While riding home (70 miles) the stutter did return, although not as pronounced (yet).

I'm looking for other ideas - thoughts anyone?
thanks!
Michael

 

[Dan631]

A faulty fuel petcock is not going to allow fuel to enter the cylinder and seep past the rings while the bike is off. The float, float needle and needle seat in the carb float bowl are the culprits. Before you try to diagnose a running issue, the leaking barbs should be fixed first. When fuel leaks into the engine by way of the carb leaking into the cylinders it acts in the same way as overfilling the engine with oil. There is a case vent tube that runs from the engine cases and vents into the air box. Oil and fuel are forced into the air box as the cases vent pressure resulting in a rich running condition.

 

[iamgumby]

Big Bandits have vacuum operated petcocks. When the bike is off - there is no vacuum and petcock should be off. Just pull a line and leave it routed into a container to check it. But yes, especially with a good load of fuel - the seeping petcock can 'overpower' the needle and seat (it just takes 1) and seep fuel into the crankcase. And yeah - them petcocks are known to go bad from time to time and do some strange stuff.

It's really not all that unusual to have some fuel and oil in the airbox from time to time. Have an owner's manual? There is a hose on the bottom side (3 but only 1 has a plug) with a plug that will drain the airbox - do it once a year - when inspecting the air filter. If the PO didn't drain it periodically - no telling how much could have been in there (condensation, too).

Did they take a good look at the plugs they took out? That gives a good idea of how everything is working - some clues. Hope you get it sorted out directly!

 

[mgevans]

good checks

Dan631:
I didn't mention it, but the floats were adjusted when they noticed the fuel in the oil. - I was wondering about the method that fuel would get into the airbox (i'm a computer nerd, but still have a lot to learn about engines )

iamgumby:
thanks for the confirmation on the petcock. the mechanic at the 'zuki shop did mention pulling (and leaving) the plug out of the air-box for a time to see if that helps. I did pull and check the air-filter (it had been replaced on a few dozen riding hours before), but I didn't check the box for fuel before taking it in for the stuttering help. They did have a look at the plugs and found the left cylinder plug (low side while on kick-stand) was fouled (they gave me the plugs that they pulled - there was a difference between the left, middle and right plugs.

Right now I'm putting a fuel clamp just below the petcock as soon as I get off of the bike. I'm also putting the bike on the center stand when I park for more than a couple minutes. The bike _is_ better off than before. A tiny bit of choke and the bike starts up - even if cold (the big boy is not happy when it is cold!).

So outside of a fuel issue flooding the bike before it starts - are there any other issues that may cause stuttering? Or will it just take a while to rectify the issue? I ask because while riding (even > 1/2 hour) the bike will still stutter when in the lower gears/lower RPMs - just hold the throttle at ~4K in 3rd gear and give yourself a seat rub.

thoughts?
thanks again for all of your help!

 

[Tdubb]

I just got my bandit from a friend, and he said he had just replaced the petcock. It also had a little studder in the lower RPMs, but once it hit 4K it is good. So I had it dynoed and the guy says its running rich because the needle seat is worn, and letting fuel by. He also suggested getting a 5 degree timing advancer.

So I got the advancer from Holeshot.com and it has made a big difference. I also ordered the seat and some new gaskets for my carbs. I'm dropping the bike off on thursday and hes going to tune it and re Dyno it for me. I wont get it back untill the 11th cause Im going on vacation, but I will let you know how it goes.

Oh if you wanna see the dyno chart do a search for woooohooo

 

[mgevans]

Thanks Tdubb - was reading your 'busa shock thread -looks fun!
yes - i'd be really interested in what you find after the carb rebuild.

found this write-up interesting too.

not sure what an advancer would do for the studdering?

 

[Tdubb]

FYI:That is actually Iamgumby's thread.

 

[mgevans]

petcock in - will be replacing tonight and will let the group know how it goes later this week!