Run away throttle?

Porsche Monkey · February 08, 2012, 06:20:23 PM

Hey guys. I have a 97 M900 with high comp pistons and stage 2 jetting in the stock carbs. The bike has been parked for a bit with charging issues which have now been fixed. The bike backfires quite a bit at low no load rpms which I can't seem to figure out. I believe but am not certain that it is running lean. The valve openers are spot on and the closers are a little on the loose side. After the bike idles for about 15 minutes the engine will accelerate all by itself to around 4000 rpm. The pipes will start glowing, the vertical cylinder pipe glows really bright. If I rev and release the throttle it will go back down to idle for a minute or so then rev up again. I just went through the carbs chasing this issue and its still doing it. Any ideas? The only thing that seems out of the ordinary is the closers being a tad loose but I don't see how that would make a difference.

Speeddog · February 08, 2012, 08:45:03 PM

Most likely it's running away due to it getting up into the ignition advance, carbies have a single-step advance that's pretty significant.

I'd guess an air leak is letting the idle go up high enough that it's catching the advance and zinging up.

Howie · February 08, 2012, 10:08:56 PM

Yep, once it hits 1400 RPM or so, zing goes the ignition timing. More like an ignition curb than curve. Fresh gas and a new fuel filter would be a good idea, do follow Speeddog's lead on a vacuum leak. You know the routine.

Porsche Monkey · February 16, 2012, 06:54:20 PM

It has a brand new fuel filter and vacuum leaks were one of the first things I checked. I was messing with it again today and with the tank up and air filter off it seems like there is quite a bit of fuel being sucked into the carbs around the needle jets. Not necessarily through the jet itself but around it. Is this possible? Are there o-rings that may have degraded? I wouldn't think excessive fuel would cause my problem but I'm at a loss.

ducpainter · February 17, 2012, 05:34:19 AM

Quote from: Porsche Monkey on February 16, 2012, 06:54:20 PM
It has a brand new fuel filter and vacuum leaks were one of the first things I checked. I was messing with it again today and with the tank up and air filter off it seems like there is quite a bit of fuel being sucked into the carbs around the needle jets. Not necessarily through the jet itself but around it. Is this possible? Are there o-rings that may have degraded? I wouldn't think excessive fuel would cause my problem but I'm at a loss.

There are o-rings on the main jets that screw into the needle jets, but usually too much fuel will cause the opposite.

I'd check synch and idle speed. If the warm idle gets up past 1200 or so rpm the advance will take over.

Porsche Monkey · March 13, 2012, 06:56:32 PM

Okay, so I took some time to revisit this tonight. Went through the carbs again, re-synced em, and let her idle for about 15 minutes. Sure enough same thing. I am at a loss. One thing I did notice is that the fuel tank seems to be gurgling quite a bit. It continues even if I open the cap. I am not running a canister and the nipple on the bottom of the tank is open. The bottom of the tank is warm to the touch but not excessively hot. Any ideas?

ducpainter · March 13, 2012, 07:02:22 PM

Is the rubber fitting still in place under the cap?

Porsche Monkey · March 13, 2012, 07:17:48 PM

If you mean the seal for the cap then yes.

ducpainter · March 13, 2012, 07:20:52 PM

Quote from: Porsche Monkey on March 13, 2012, 07:17:48 PM
If you mean the seal for the cap then yes.

No...

I meant the little black rubber 'piton' that connects the cap to the tube in the tank

Porsche Monkey · March 13, 2012, 07:28:53 PM

Yes, just checked it now. It appears it may have been blocking the vent hole a little bit so I repositioned it. It's soft and pliable. But would the tank still gurgle with the cap open?

ducpainter · March 13, 2012, 07:34:31 PM

Quote from: Porsche Monkey on March 13, 2012, 07:28:53 PM
Yes, just checked it now. It appears it may have been blocking the vent hole a little bit so I repositioned it. It's soft and pliable. But would the tank still gurgle with the cap open?

If you have no canister throw it away...

honest.

Tank should not gurgle with the cap open... weird.

Porsche Monkey · March 13, 2012, 07:38:34 PM

Yeah its driving me nuts. Thanks buddy.

ducpainter · March 13, 2012, 07:42:35 PM

In the meantime...

if the idle is still hanging or running away drop your warm idle speed.

Porsche Monkey · March 13, 2012, 07:51:46 PM

When it idles, its around 1000rpm. I can lower it till the bike almost quits, give it a few minutes and it'll accelerate by itself even if I hold the grip against the stop. Blip the throttle and it'll settle back down for a minute or so. I'm at a loss. No vacuum leaks, new fuel filter, valves are in spec. I may swap the fuel pump next.

ducpainter · March 13, 2012, 07:55:01 PM

Quote from: Porsche Monkey on March 13, 2012, 07:51:46 PM
When it idles, its around 1000rpm. I can lower it till the bike almost quits, give it a few minutes and it'll accelerate by itself even if I hold the grip against the stop. Blip the throttle and it'll settle back down for a minute or so. I'm at a loss. No vacuum leaks, new fuel filter, valves are in spec. I may swap the fuel pump next.

It isn't the pump...

but I honestly don't know what it is.

Did you try the acetylene trick around the carbs when it starts to race?

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Run away throttle?