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.
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.
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.
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.
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.
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?
Is the rubber fitting still in place under the cap?
If you mean the seal for the cap then yes.
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
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?
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.
Yeah its driving me nuts. Thanks buddy.
In the meantime...
if the idle is still hanging or running away drop your warm idle speed.
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.
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?
I used brake cleaner. I'm not allowed around acetylene anymore.
Other than for test purposes I would not chuck that little rubber shneckle between the cap and tank. The reason is if you need to raise the tank when full fuel will either not come out of the cap or dribble with the shneckle in place (assuming the check valve works). Without, gas will flow freely.
Checking for vacuum leaks when hot is a good idea. It doesn't matter if you use something combustible or not, you are just looking for a change in speed.
Quote from: howie on March 13, 2012, 10:30:49 PM
Other than for test purposes I would not chuck that little rubber shneckle between the cap and tank. The reason is if you need to raise the tank when full fuel will either not come out of the cap or dribble with the shneckle in place (assuming the check valve works). Without, gas will flow freely.
Checking for vacuum leaks when hot is a good idea. It doesn't matter if you use something combustible or not, you are just looking for a change in speed.
Have to disagree Howie.
If the vent flaps in the cap are working correctly no fuel should come out
No idea on the gurgling though huh? It's the only thing that seems out of place. It still does backfire and sometimes as its just idling. The coughing pretty much went away when I re-synched the carbs. Now it does it every once in a while.
Quote from: ducpainter on March 14, 2012, 04:21:10 AM
Have to disagree Howie.
If the vent flaps in the cap are working correctly no fuel should come out
Did on mine. Doesn't with the shneckle in place. Maybe just coincidence. I'll have to revisit the vent design.
Quote from: Porsche Monkey on March 14, 2012, 05:27:31 AM
No idea on the gurgling though huh? It's the only thing that seems out of place. It still does backfire and sometimes as its just idling. The coughing pretty much went away when I re-synched the carbs. Now it does it every once in a while.
Chances are the gurgling is coming from the check valve in the cap. If you are not developing vacuum I would not worry about it. You can take the cap apart and check the vent valve. If I remember correctly it is a spring or two and a couple of check balls. The valve can be gummed up over the years.
Maybe it is time to go through those carbs.
Been through them twice.
What are you using to synch the throttles?
one of these http://www.jbugs.com/product/7034-11.html (http://www.jbugs.com/product/7034-11.html)
Quote from: Porsche Monkey on March 14, 2012, 07:28:47 AM
one of these http://www.jbugs.com/product/7034-11.html (http://www.jbugs.com/product/7034-11.html)
The problem with those is they affect the running of the motor when they're stuffed into the carb mouth.
Sticks that attach to the intake work better IMO
Quote from: howie on March 14, 2012, 07:16:56 AM
Did on mine. Doesn't with the shneckle in place. Maybe just coincidence. I'll have to revisit the vent design.
Chances are the gurgling is coming from the check valve in the cap. If you are not developing vacuum I would not worry about it. You can take the cap apart and check the vent valve. If I remember correctly it is a spring or two and a couple of check balls. The valve can be gummed up over the years.
Maybe it is time to go through those carbs.
It would leak out the drain tube underneath unless the piton blocked the vent hole.
Quote from: ducpainter on March 14, 2012, 01:00:33 PM
The problem with those is they affect the running of the motor when they're stuffed into the carb mouth.
Sticks that attach to the intake work better IMO
I agree. Plus you can see what is going on with both carbs at the same time. Even two matched vacuum gauges would be better.
It would leak out the drain tube underneath unless the piton blocked the vent hole.
Yep, I looked. Probably coincidence. Had to be debris on the gasket.
Started it up last night and it was misfiring real bad on the horizontal cylinder. Think my new plugs may have fouled out. I'm just gonna buy another pair instead of cleaning them. I'll let y'all know what happens.
Quote from: Porsche Monkey on March 15, 2012, 05:48:25 AM
Started it up last night and it was misfiring real bad on the horizontal cylinder. Think my new plugs may have fouled out. I'm just gonna buy another pair instead of cleaning them. I'll let y'all know what happens.
How old are your needle jets?
Theyre the factory pro ones and show very little wear. But they have around 20k on them. The jets don't appear to be ovaled at all. Probably should just replace em and be done with it.
Quote from: Porsche Monkey on March 15, 2012, 06:08:03 AM
Theyre the factory pro ones and show very little wear. But they have around 20k on them. The jets don't appear to be ovaled at all. Probably should just replace em and be done with it.
If you've been running the Ti needles I would.
It doesn't take much to affect running
Ill get some ordered then and try that. While I'm waiting for them to come in Ill probably go down a notch on them and see if it changes anything.
Quote from: Porsche Monkey on March 15, 2012, 06:19:53 AM
Ill get some ordered then and try that. While I'm waiting for them to come in Ill probably go down a notch on them and see if it changes anything.
The problem with that is at idle one notch won't change much.
If you're having a part throttle issue it might.
Yeah I know what your getting at, it does backfire at low throttle and off throttle settings. As much as I want to believe it is a lean condition, it was flooding out yesterday.
Quote from: Porsche Monkey on March 15, 2012, 07:04:10 AM
Yeah I know what your getting at, it does backfire at low throttle and off throttle settings. As much as I want to believe it is a lean condition, it was flooding out yesterday.
I've yet to see a lean condition foul a plug.
I suppose there's a first time for everything.;D
Exachary.
When you had the carbs open, did you replace the o-rings on the float needle valve?
Where the float assembly goes into the carb body? Yes I did.
Okay, so the spark plug on the horizontal cylinder took a dump. Its a new plug but when I tried starting it this evening it was completely dead. Popped in another plug and it started hitting on both again. It still backfires but not as bad as before. The throttle will also still run away but maybe to 3k instead of 5k. I rode it up and down the street a few times and every once in a while the bike would not return to idle when coasting to a stop. I went looking for vacuum leaks again and found none. The throttle is definetly against the stop when it raises up. In order to get this to go away I have to lower the idle way low, and then the oil pressure light flickers. Do these carbs wear out? Does the main shaft wear out or something similar to that?
Either way, I rode it in to work today. Put a few gallons of fuel in it and got about 5 miles of riding in. It did okay although it did cough and backfire a few times. Im going to attempt to put some miles on it this weekend with the hopes that it will clear out and maybe start running better. Wish me luck.
You might want to check the o-rings on the main jets.
They have been known to split.
As far as the carbs wearing out, I kind of doubt it unless you have 11ty billion miles on it.
also...do you have the vacuum petcock still installed? I think that connects to the horizontal manifold. If it took a dump it could put fuel into the manifold.
As many times as I've been into the carbs recently, I don't recall seeing o-rings on the mains. Ill check again.
Quote from: Porsche Monkey on March 16, 2012, 07:15:38 AM
As many times as I've been into the carbs recently, I don't recall seeing o-rings on the mains. Ill check again.
They may be on the needle jets, but I think I recall them on the mains.
Let me look at the picture from my jet kit...brb.
definitely on the mains...