Engine surging

[Metalefty]

On my '02 250 MC the bike has run great for the last two seasons, a few weeks ago we were climbing a lot of steep loose hills, at the end of the day I noticed the idle surging and a slight loss in power under acceleration. A few days later I just took the carb off and went through it cleaning out everything, the only thing that was clogged was the pilot jet, when I got it back together it started on the second kick and idled nicely. I made a slight air screw and idle adjustment just to get it perfect. This past weekend I went for a ride with some friends and about an hour into the ride I climbed a small hill in first gear and at the top when I was letting off the throttle it just kept accelerating and I blew a turn and busted through a dead juniper tree falling on the other side, I noticed that the bike was idling nicely on it's side, then I shut it off. When I picked it up I thought maybe the throttle had stuck open but every time it snapped back strongly. I nervously rode for another hour and noticed a slight surging when letting off the throttle at times and the surging idle and loss of power under acceleration. We took a 10 minute break and I told the guys I was going back to the truck to start drinking beer till they were done, on the way back to the truck the bike ran perfect except for the surging idle. I haven't been back into the pilot jet yet but what would make the bike accelerate like a stuck throttle for several seconds if my throttle cable is perfect ? (yes, I checked the routing). Would an airleak in the cases make the bike surge like a stuck throttle ?

 

[lankydoug]

If the pilot is too rich and you had to raise the slide to get enough idle air it will idle fine but surge after a good clean out wide open throttle. This could also be exaggerated by weather changes to colder air. The solution is to go to a smaller pilot and close the slide some and try to get the combo to work when the air screw is closer to 1 turn out rather than 2 turns. I don't know if this is your issue, jetting info may help. There are many other possible causes of surging like a cracked boot between the carb and reeds or reed plate gaskets leaking but these usually don't come and go as you described.

 

[PEB]

I think Lanky is onto something with the carb boot. Check the rubber piece that goes to the case from the carb. They a prone to cracking. You will likely have to take it off and look it over carefully and or flex it to find the crack.

 

[GMP]

+1 on the carb boot, and possibly the ignition side crank seal. Also, check your float bowl for debri. Sand/grit can be drawn into the carb through the vents, and will jam the needle/slide when drawn through. An easy diagnosis is vertical scratches on the neeedle. This has happened to me in the past.

 

[Metalefty]

Thanks for the replies guys,

Lankydoug, I don't think it's a rich condition since my pilot was clogged after I noticed the surging on the first ride.

PEB, I'll revisit the carb boot, one thing I did try was that when it was idling I sprayed starter fluid around the carb boot area to see if the rpm's changed and they didn't.

GMP, I will check that ignition side crank seal, also, I don't think there is any grit in the carb since I did a pretty thorough cleaning of the carb before that second ride and I noticed the surging after my big surge fall but yet the throttle snaps back strongly when we stopped for breaks.

The bike has never been to rich, so if anything it has to be some kind of lean condition, I've just never had a bike surge so much to feel like a stuck throttle, I should have time this weekend to work on the bike again, hopfully I find something because my confidence with riding it is low...

 

[Metalefty]

Just some closure from last years problem. So about a month ago I was riding and I noticed the bike surging a little so I was coming to a stop and luckily I was finding neutral when the engine revved to red line and didn't stop, kill switch wouldn't work and I didn't want to pop it into gear with an auto clutch to stall it, wasn't sure what would happen so I turned the gas off and tipped it over on it's side until it stalled. A little poking around and I find a huge tear in the intake boot, it must have been smaller before and then just ripped bigger on this ride, funny thing was is that I checked the boot when I had the carb off last year, maybe I didn't look good enough. Now a new boot and no more surging. Thanks for the replies.

 

[Jakobi]

Lucky you didn't fully dust the engine. Thanks for the update. Always good when the thread comes to a closure.

 

[GMP]

When you do a top end and/or reed replacement, replace that boot. As you know they are cheap.

 

[Neil E.]

So if you have a lean surge / runaway engine, what's the best way to stop it?

Plug the exhaust outlet? Could you hold the pressure back for the 20 seconds it might take to die? New gloves would be cheaper than a rebuild if it's really gone wild.

It would probably have to be in top gear to stall it with the brakes. As stated not very practical with an auto clutch equipped bike. Even without the auto clutch, it would be tough to shift up while it's screaming. It's also hard to fight the instinct to gear down while attempting to stop the bike.

Jump off and run away?

 

[Fred1956]

So if you have a lean surge / runaway engine, what's the best way to stop it?

Plug the exhaust outlet? Could you hold the pressure back for the 20 seconds it might take to die? New gloves would be cheaper than a rebuild if it's really gone wild.

It would probably have to be in top gear to stall it with the brakes. As stated not very practical with an auto clutch equipped bike. Even without the auto clutch, it would be tough to shift up while it's screaming. It's also hard to fight the instinct to gear down while attempting to stop the bike.

Jump off and run away?

In the trials world we occasionally have a runaway due to the 'glow plug' effect if a throttle gets hung open and you can't get to the bike quick enough. Standard rule is plug/cap/cover the exhaust, gloves are cheaper than engines