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Strange loss of power

  • Thread starter Thread starter Anonymous
  • Start date Start date
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Anonymous

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Hey,

I was biking home from school the other day when I encountered a problem. I had been going about 15 miles, the last 10 of which I was probably running at 6000 rpm. Suddenly, while going up a slight hill, I started to lose power drastically. I tried hitting the throttle more, but it had little effect. Gradually I downshifted and limped to the side of the road (decelerating the whole time) and killed the bike (didn't give it a chance to stall on me).

When I tried starting it back up, it'd hardly cough at me. I let it sit for about 30 minutes while making some phone calls at a nearby gas station to try and figure things out. Then I came back and tried to get it going again. It had some trouble starting, but finally started. I then ran it another 60 miles or so at 5000 rpm, with no problems. Then, I had to run it much faster for a mile or so while merging with traffic, and the symptoms came back such that I couldn't hold it over 40 mph. Again I let it sit, and then drove it home.

I know my bike is not yet in great condition, and I've constantly been working on it, but haven't encountered this problem before. When I drove it to school, it ran fine the whole way there (100 miles) at about 6000 rpm. I have recently swapped out the spark plugs and one of the spark plug caps that had gone bad, but have done little else. What might cause the bike to do this?? Overheating (when I brought the bike down to school at 6000 rpm, it was hotter than the other day)?? None of the fuel lines seemed to be crimped or anything, so I'm not sure what the cause would be. Suggestions greatly needed!!

--Tyler
 
Possibly a vaccuum lock in your fuel system? Than would be your fuel tank not venting...
 
sounds like what happens when the bike runs out of gas in the ON position. it'll cough and not do much because its only getting a little fuel...

if it happens again...switch it to reserve, or prime as a last check, and see what happens.
 
Such a loss of power and so suddenly, makes me think you lost 2 cylinders. You may have a coil going bad.
If it does it again, as soon as you stop and turn it off, quickly feel the headers for a difference in heat. A bad coil will make 1&4 or 2&3 cylinders run much less hot to the touch or even cold if no spark at all.
It's also possible you only lost 1 cylinder due to a poor connection at the coil/plug lead/cap. In this case, you should still feel one of the pipes running much cooler than the rest.
For it to happen so suddenly, I think it's electrical. Electrical connections commonly work when cold but fail when they get hot. I'm not familiar with your ignition but it may also be a failing sensor/pick up in your ignition.
Clean or replace the leads/caps and tighten all connections and then test.
 
Sounds like a brutal commute. Or at least it would be in a car...

Careful about feeling the exhaust. Them things is hot. You should also take a whiff of the cold cylinder(s) if you can find it. At least you'll know whether that one's not getting gas or not getting spark.

Other possibilities include any variety of fuel starvation problems. Perhaps a fuel line is intermittently plugged. Check for debris at the fuel filter between the tank and carbs. If you don't have one, that's bad. Get one. It might take some tracing of the fuel path to find any blockage

Which model and year is your bike?

--

Funny story:

When my GS was new to me I had a similar sounding problem. It sputtered for a mile, then totally lost power and I couldn't start it again. I came back with a trailer and hauled it home, but I eventually found the problem. It was nothing a few gallons of gas wouldn't fix... :lol: :lol: :lol:

In my defense, I have no gas gauge and I was rightfully expecting to go much farther on the tank. A little spark plug gap problem was causing poor gas mileage. I really should have tried the reserve setting before I hauled it home, though. Live and learn. :wink:

--

Best of luck, Tyler.

-Michael
 
To add to what I suggested earlier, if the bike does it again, keep riding for at least a couple miles before checking. If you pull over immediately, the pipes will still all be hot. Giving the no sparking cylinder(s) time to cool a little will make it easier to tell the difference and troubleshoot.
 
Keith - I don't think any of the cylinders weren't firing. When it died the first time, it was still running smooth as it slowed down. If I'd lost one or two cylinders, I think I would have felt quite a noticable difference in how smooth it was running. On the first time, it started dying such that I couldn't continue (just kept slowing down until it hit zero), sort of suggesting that it was some sort of fuel starvation thing.

The gas tank was well over half full, so it wasn't due to an empty tank. I don't have any fuel filter on the bike, other than the screen just before the petcock. Is that enough, or should I look into getting something more serious? If so, any suggestions?

To me, it seems like the fuel isn't getting to the carbs due to a blockage or something. I plan on taking out the petcock and looking at it/cleaning it, but am a little hesitant to do so at this point until I have another O-ring to replace the current one (just in case it gets damaged or falls apart when I take it out).

Lartross - from what I can tell, the fuel tank is venting just fine. I know in the past, there has been no trouble for air to get in (and fuel out #-o ).

--Tyler
 
This happened to me once, going over a windy pass on the freeway. I got the bike to the shoulder, fiddled with the petcock, and finally got it started after about 5 minutes.

When I got it home, I took the petcock apart, but there was no noticeable blockage, and nothing functionally wrong with the petcock or fuel/vacuum lines. The only thing I did was grease the switch o-ring.

That was 3,000 miles ago, with no repeat. The only thing I can attribute it to was cheap gas. I almost always run major brand premium now, with no problems.


Not much help, but thought I'd relate....


-Q!
 
What you are describing sounds like a weak or failed condensor on a standard ignition.

Do you have standard ignition on yours?
 
I had a similiar problem back in June on our 2nd Hill county ride on my 750. After discussing several possibilites with several other GSers we removed the fuel tank, dissembled the petcock and found a small piece of debri partially clogging the outlet. Removed the debri and my bike has run fine since.
 
Ok, I had the opportunity to test the bike again today as I drove it back down to school with me. The bike went out two different times. In both cases, it started losing power.

First time, I went just a little way, then drifted to the side of the road and killed it. Let it sit for a little bit, then tried it again and got it going.

The second time, I was able to ride it out for about a mile, but it got continuously slower. I started at about 60 mph, then within a mile couldn't hold above 40 even with full throttle. I drifted to the side (clutch in), and it immediately stalled. Took some time and working with the starter to get it going again.

One thing I noticed was that these times also seemed to correspond to some smoke being given off by the left exhaust pipe (oil slipping into cylinders and burning??). I don't know if this could be related to the problem, or if it's an entirely different problem. I plan on working on the petcock soon, as so far I think it's related to some fuel clog. I'm a little worried about the smoke coming out of the pipe though.

I also noticed again that as the bike lost power, the ride didn't get any rougher. Also, I felt the heads of the exhaust pipes the second time, and they were all the same temperature. These lead me to believe that all the cylinders are firing, and that isn't the problem. Thanks for the help!!

--Tyler
 
Was the smoke black/sooty or bluish? Black means not enough air for the fuel. Blue means oil burning.

I would agree that fuel path inspection should be the next step.

Michael
 
You can loose 1 or 2 cylinders and still run "smooth" as you lose speed and pull over. It may very well be fuel related, but I don't understand why the bike would run OK and then suddenly starve for fuel, again and again.
A clogged fuel passage usually stays clogged,or passes, instead of what you're describing.
Feeling the pipes will only work if you can ride the bike a couple of miles after the problem starts. If you pull over too soon, the pipes will still all be hot. I know you may not be able to continue running a couple miles.
You can also check the plugs for spark at this point.
I still think it's electrical.
 
Michael - smoke was a bluish, which I guess means oil burning. Would this be related to my other problems? or a totally separate problem?

Keith - I don't understand why it would happen repeatedly either, or why letting it sit a little seems to solve the problem for a short while. Where is the best place to start for checking the electrical? With the spark plugs?

Also, I said I thought I'd notice it running less smooth because in the recent past, I ran with it for a while with only three cylinders firing. Had to replace a spark plug cap to fix it, but there was a noticable difference having the fourth cylinder instead of just three. Also, I'd think even with two cylinders, I'd be able to keep it going, although not fast or well. However, the bike basically forces me to stop, and stalled as I pulled off the road with the clutch in.

--Tyler
 
I can't think of a plausable connection between the blue smoke and the loss of power other than engine mechanical stuff, and I'm the wrong guy to talk about that (until I get myself edjumukated).

Can you think of any way that the loss of power is following a cause pattern? Is it at a certain time or distance in the ride? Is there any relation to the level of fuel in the tank, or switching anything in your fuel supply (choke, reserve, throttle)? Is it only on inclines, declines or after blinking twice?

Just for brainstorming, is there any impediment to your air flow? Is your airbox clear and have you cleaned the air filter recently? Who knows, maybe your seat is suffocating it somehow, when you put your weight on it just so.

If you can safely manage the bike, I might suggest that you not hit the kill switch unless something terrible is going to happen. Maybe there's a clue hiding in the low rev environment.

You said in the first post that it had some trouble starting after your 30 min wait. What did the bike do and not do during the starting trouble?

If there is a chance you bring a laptop with you on this commute, perhaps you could audio record the situation once you come to the first stop. My dad's a wizard at hearing what's going on in an engine.

Perhaps it's gremlins... or maybe gnomes...

Michael
 
Sounds like you're running into a lean condition, fuel tank vent, petcock, vacuum hose to petcock, or carbs. Also everything to do with the air filter, its box and associated hoses. Also the orings between the carbs and the head if this bike uses screws insted of clamps. Are any of the header pipes turning gold or blue? Check the plugs also. Most of these bikes sat a long time at some point during their life, the carbs definately needed tanked.
 
Just for giggles, try replacing your vacuum hose from the #2 carb to the petcock. I had an intermittant problem once where the vacuum line would get hot and crimp off the vacuum. It didn't happen all the time and took me forever to find.

Just a guess, but its cheap 'n easy..........
 
I have to agree with Kieth on this one - I also think it's electrical. In your case I think it's the regulator-rectifier and/or stator (or bad/dirty/corroded wiring connections to either). I say this because my old GS450 would die at random and then start up fine after sitting for 10 minutes. When a bad RR starts to heat up it can become a big resistor, impeding current to the coils, leading to gradually weakening spark as the resistance in the RR increases. After replacing the regulator-rectifier that problem went away. Then I had a different problem - the bike battery (brand new) would drain after a few rides - In this case it was a bad stator.
Another problem I found on my friend's GS450 only after a very tedious process-of-elimination was that his signal generator had frayed wiring, preventing a strong spark to the spark plugs. This one was especially hard to find because the bike would run all day but with reduced power. As soon as I replaced the wiring to that signal generator, the bike ran perfectly, but I spent at least 10 hours checking tons of other things.
It can also be faulty coils (gradually building resistance as they heat up).
I think the only way to pinpoint your problem is the time-consuming "process-of-elimination" method.
First of all, I'd ride the bike without the gas cap on, to make sure it's not a venting issue.
Then I'd carry a spark plug wrench with me so I could check the plugs immediately after the stall. Verify that they're getting strong spark. If they're not, then wait a half hour till the bike cools down. Then check the spark again. If the spark looks good at that point, I think your problem is narrowed down (namely, faulty electrical components preventing strong spark when heated up). If so, run the bike till it stalls (again) and measure voltages for each component (while the bike is still hot) to pinpoint the weak link in the electrical circuit.
At this point, if you've verified that all electrical components are functioning properly even after the stall (bike is still hot), then it has to be a fuel problem (because all cylinders stop firing). The only think I can think of that would gradually worsen the fuel flow to all carbs (besides the venting - which we've eliminated by riding without the fuel cap) is some kind of blockage at the internal petcock screen that worsens with heat.
Sometimes, just taking that petcock off the tank can reveal the truth.
Let us know if you try my troubleshooting steps and what your findings were.
Good luck - you'll figure it out.
 
I have a couple of ideas that might help.
When it does it again, try turning the petcock to prime as you're riding.
If this stops the problem, then you know its either a bad vacuum tube, or bad diaphragm, or a dirty petcock.
If the problem remains, then at least you know the vacuum tube and diaphragm are not the cause. You could still have a dirty petcock but I don't think so.
Another easy fuel related check would be to pull over as soon as it does it and check for fuel in the bowls. Put the bike on the centerstand and drain each bowl into a small cup. If you can see that one or more of the bowls had less fuel in it, you may have a blockage at the float needle valve/seat.
If you try this, be sure to take a rag with you. Use a cup that is "see through' and mark the level of fuel, dump it out and go to the next bowl.
If you see different amounts you know you have a low fuel level problem in that carb.
 
Monday night I so nearly set my bike on fire when the vaccum hose wouldn't quit pouring gas all over my exhaust pipe. I called 911 but fortunately, the thing never caught fire. The gas boiled off the pipe, though. Sooooo close. The bike sat, not running, for a bit before this happened, so the pipes weren't as hot as they could have been. Thirty seconds earlier and I might be a former GSer. The fire engine gave me a ride home and I came back the next day with a trailer. One of the firefighters was a Harley rider, so you know they're not all bad. :wink:

Anyway, be careful if you're going to have loose gas around a hot bike.

Michael
 
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