I also posted this on bansheehq.com so if it looks familiar, that's why.

My specs:
91 Banshee, FMF Fattys, VForce Reeds, Stock carbs, Thumb throtte, TORS in place but unplugged, balance pipe has been removed and plugged, no airbox, K&N filter, checked everything for air leaks earlier and didn't find any.

Went for my first ride on it last evening. I think it was around 65-70 degrees out. During the first hour of my ride, I had no problems. I ran through quite a few mud and water holes (although I wasn't splashing much, just going slowly) with no problems. I rode a dirt road for a while in 3rd/4th gear with no problem. Stopped a few of times and twice when I shut it off, could hear the radiator boiling or something (probably unrelated). On my way home, it started barely raining but it didn't cool off much outside. I noticed a few times that it wouldn't drop all the way back to idle immediately, it would drop down to "almost" idle speed, then after a few seconds, go to idle. On one steep downhill section, I had the clutch in and was drifting when it started to rev by itself. It didn't go full throttle, but it did go pretty quick. I blipped the throttle and it went away. Finally when I got home, I let it idle and got off, checking over things and it started to rev again. First it went to half throttle and stopped, I blipped the throttle a few times and it started to back off, then it went wide open. I turned off the kill switch and IT KEPT RUNNING. Then finally, I jumped on and dumped the clutch to kill it.

Any ideas?

 

Sounds like you have a vacuum leak....considering the age of the machine, maybe a dried up and cracked line that connects between the carbs. Another possibility could be a clogged up carb.....the machine will rev like that when very lean or out of gas occasionally.

 

I agree with garyc660r

 

Might check the tank also, I have heard of people having crud accumulate and not allow enough fuel to the carbs.

I had a similar problem but it was with the tors boxes, apparently something broke in the box and when I started it it would rev to the moon, but it would not come back on its own, or at least I did not wait to see.

Otherwise the things above sound like good things to check.

Bat

 

How new is the motor?? If you just did the top end & stuff, how much compression did you put in it??

Based on your descripion, it sounds like it might be running a little on the hot side. This could be the reason for it continuing to run after you hit the kill switch.

If you have a hot spot in your cylinder, it can act as a spark plug and ignite the charge in the absence of the spark from the plug.

Of course the other explanation is that your kill switch is not working as it should.

As far as the throttle sticking, I have not heard of this with the stock stuff, only on the TORS eliminator kits. Good research project!!!!

Have fun and stay safe!!

 

Sounds like "lean run on" or dieseling as the others have said, its from being very lean

The air/fuel mixture is just as important for cooling the top of the piston as the water is to the cylinder. With less fuel hitting cooling the piston surface for whatever reason you will get hotspots like stated before that ignite the fuel. It could make you rev higher when off the throttle as it heats up. If left alone you could "hole a piston" meaning left uncooled the piston dome will giveway and blow out your exhaust or down into your bottom end.

Depending on the age of the motor you may want to check your crank seals which can suck air through the crankcase.

When the motor kept running were both cylinders firing??

 

I just got this Banshee so I'm not 100% sure what has been done to the motor as far as rebuilds. The previous owner said he did nothing but the original owner before him had said something about replacing the pistons a few years ago. I want to clean up and check the carbs anyway so I'll rejet if I find I'm running lean. I need to check the plugs and see what I find.

The kill switch is working properly because (except this one time) that's how I always turn off the engine.

Which seals exactly are the crank seals? How hard would they be to replace?

Thanks for all the tips. Hopefully in the next few evenings I'll get a chance to look for problems and I'll post what I find.

 

When I had my idle circuit on my Raptor messed up it would do the same thing. It ran very hot at idle and would "self-rev". All I had to do was adjust the idle mixture screw and that solved the problem. On the Raptor the idle screw are on the side of the carb facing the engine. If the Banshee is like that, turn them out to make it richer (3/4 turn each). If the shee has the idle screws on the airbox side of the carb, I think you turn them in to make it richer. Good luck!

RR1