Losing Power in higher gears at 5000-6000 RPM

[rscottlow]

Hey everyone...I hope someone can help me with this. I bought my bike early this year, and especially as I start riding a little more aggressively I've noticed an issue that I cannot pinpoint the cause. When I'm accelerating, especially hard acceleration in 4th-6th gears, I lose power between 5000-6000 RPM. Initially, I thought that this was the clutch slipping, but the more I ride, and the more I think about it, I'm not seeing an RPM spike when this happens...it's more like I'm hitting a rev limiter and neither the engine nor the bike will go any faster. I've found that sometimes if I let off the throttle and get back on it, that solves the issue (another reason that I thought it was the clutch). I'm thinking that maybe it's a carburetor/fueling issue. When I first bought the bike I had to replace one of the carb floats because it had a pinhole leak and was filling with fuel...perhaps I didn't adjust the float height correctly...could this be causing my issue? Or does anyone have any other ideas? Any input is greatly appreciated...I need to get this fixed ASAP.

[rscottlow]

A guy on another forum had a similar issue with his bike, and it turned out to be dirty carbs. Since I had mine apart when I replaced the leaky carb float, and they looked pretty immaculate, I don't think that's my problem. I suppose it could still be an issue with float height, but I've not had an opportunity to check it yet. Any other ideas???

http://forum.motorcycle-usa.com/default.aspx?f=18&m=497849

[smokestack]

Could it be that one of your jet needles/slides isn't opening all the way? Curious if you figured this out.

[rscottlow]

Could it be that one of your jet needles/slides isn't opening all the way? Curious if you figured this out.

I had a sick kid over the weekend, so I didn't get a chance to pull the carbs apart. I'll hopefully get around to it sometime this week.

[rscottlow]

Update...sort of.

I rode to my brother's house last weekend so I'd have an extra pair of hands to help with this stuff. We adjusted the float levels...one was about 1.5mm off, but the adjustment failed to solve my problem. By the time we put the bike back together, not only was it dark, but the work light we were using burned out. Long story short, I got it home and took another look (since it still wasn't running right), and I put the float bowls on backwards and we missed one of the hoses when we were reconnecting everything.

It's back on the center stand now (it's been sitting there since last weekend, while I haven't had time to work on it), and I'm setting aside some time this weekend to get it fixed. I just really don't even know where to start at this point, aside from fixing my stupid reassembly mistakes. I know my earlier explanation wasn't all that clear, so here's what I've got going on...as I'm accelerating, when I get to around 55-60 MPH the bike acts as though it doesn't have any more to give. It sounds almost like it's hitting a rev limiter, and even if I'm giving it full throttle it doesn't want to go anymore. After a few seconds of this, sometimes it will lurch forward and continue accelerating...other times it doesn't - which is probably a result of my throttle input, but I haven't experimented quite enough to pinpoint it.

Pretty frustrating, because this is where I feel like I should be getting the greatest amount of power out of the bike, but there's nothing. It's especially scary when I'm trying to pass someone...

Any ideas? Other than cleaning the carbs, I'm not sure what else to even consider as a starting point.

[Big Rich]

Insufficient fuel flow..... next time it does it, do a plug chop. I'd bet your spark plugs are white from being lean at that range.

Search for fuel starvation, and you'll find all the possibilities. I can't remember - have you messed with the jets? If you accidentally put in smaller mains instead of larger ones, that could do it too. 

[rscottlow]

Thanks Rich. I haven't messed with the jets, but perhaps they're clogged? I'll take the plugs out and look at them later today. Have to go get passports for the kids this morning, but I should have most of the afternoon to tinker.


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[HPP8140]

How's the air filter? Correct part for the bike?

[rscottlow]

It's clean, and as far as I know it's OEM.


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[HPP8140]

post the part number

[Big Rich]

Thanks Rich. I haven't messed with the jets, but perhaps they're clogged? I'll take the plugs out and look at them later today. Have to go get passports for the kids this morning, but I should have most of the afternoon to tinker.


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While it is possible your main jets are clogged, that's probably not the problem. I say that because the pilot jets become clogged much more easily than the mains (smaller holes). So if it's running well off idle and thru the midrange, I think you're running out of gas when the engine demands it.

[rscottlow]

I cleaned the jets, even though they already looked clean. I put everything back together and went down the road for a test ride. Now it's doing the same thing, but also in 2nd and 3rd gears. If it's this simple, I might just kick myself, but is it possible that too much slack in the throttle cable is causing this? It looked a little loose, but visually appeared to be operating correctly so I didn't worry about it. I didn't do any intentional adjustment any of the roughly half a dozen times I've taken the carbs out, and it seems as though it would be easy to accidentally loosen or tighten the cables when removing and reattaching them. But could that cause me to be losing power only at higher RPM range? When it's sitting in neutral, I can rev it up just fine, but would being under load make a difference?

Sorry for my lack of knowledge, but this is my first bike, and I've never worked on a small motor before I bought this thing.


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[rscottlow]

post the part number

It's dark and raining now, but I'll look tomorrow. I assume based on the symptoms worsening, though, that it must be related to something that I've done.


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[rscottlow]

I made some adjustments to the throttle cables. If nothing else, it started up easier, and is idling better than it was. It's raining here, so I can't go test it out, but as soon as it dries up, I'll report back.


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[rscottlow]

Took it out, and the issue is not resolved. Same thing as before, but I rode it around a while longer since it was only raining a little bit, and I was trying to collect as much information as possible. So here's what I have: when I open the throttle all the way, the bike doesn't run well at all. I mean to the point where if I'm going up hill and I open it up, RPMs will actually drop. Partially closed, I can get enough power to make my way up a hill, but very slowly. Going downhill, the bike almost operates as normal. If I really get into it, I can still hear and feel the loss of power when I open it up, though. Again, my knowledge is really limited, but doesn't this point to fuel mixture? Something in the carbs? I just don't know why it got worse after I cleaned the jets, unless I messed something else up while I was in there...


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[rscottlow]

Other things I noticed...once I get it up past where power initially drops off (usually by letting off the throttle some), I can keep accelerating...but only to a certain point. For example, I couldn't get it above 10,000 RPM in first, 8,000 in second.


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[AK Baller]

Hope you've figured out the issue, but I'm curious - I don't know much about this area, but it does seem like a mixture issue, I'd assume that it is only tied to the RPMs and in that case the issue could be emulated without riding - just revving up the bike? Did your bike have the same issue w/choke on ? I wonder if fuel lines are clogged or if there are gremlins on the vac side - although I guess those show themselves even at idle...

[rscottlow]

Hope you've figured out the issue, but I'm curious - I don't know much about this area, but it does seem like a mixture issue, I'd assume that it is only tied to the RPMs and in that case the issue could be emulated without riding - just revving up the bike? Did your bike have the same issue w/choke on ? I wonder if fuel lines are clogged or if there are gremlins on the vac side - although I guess those show themselves even at idle...

Unfortunately, I haven't figured it out yet. I'm sure it's simple, but I haven't found time to work on it much. I agree it sounds like a mixture issue, but the issue cannot be replicated by just revving up the bike. I even put it on the center stand and went through the gears to see if I could recreate the problem, but no luck. So this is only happening under load at higher RPM's. Based on what I've read, this points me right back to the carbs, but all I know is what I've read online.

So just to recap...I replaced one of the floats early this riding season due to a pinhole leak causing it to fill with fuel and not float. When I installed the new one, I don't think I adjusted the float level properly, which caused my initial problem - bogging down in higher gears at higher RPM's. A few weeks back, I pulled the carbs out and adjusted the float height correctly, but ran out of daylight and my work light burned out while the bike was still apart (I was at my brother's house, and my bike was my ride home so I had to get it put back together). We finished reassembly with flashlights, but put the float bowls on backwards and missed a couple of hoses when putting the carbs back on. It rode somewhat poorly on the way home - part of this was due to my brother fiddling with the idle screw when the bike didn't seem to start up just right. The next day, I pulled the carbs back out, flipped the float bowls, put everything back together (I think correctly), and took it out for a quick test ride. This is when it started bogging down in second and third. My only remaining hypothesis is that when I was switching the float bowls around, I think I pushed down on the floats when I was examining them to make sure the needles were seated correctly and whatnot - perhaps I pushed too hard, bending the tabs, and throwing off that adjustment. Sometime within the next few days I'm hoping to get the carbs back out and measure the float height (or use the clear tube method if I can find some clear tubing that is the correct size...any suggestions on where to find this stuff? I've tried a couple different sizes, but none that fit snugly on the drain holes). If it's not the float height, then I honestly don't even know what to check next. I'll hopefully report back with a fixed GS sometime this weekend...then again, Oktoberfest is this weekend, so who knows when I'll find time 

[rscottlow]

This is mostly for my reference, but this may have something to do with my issues...so many hoses. I marked them when I initially took the carbs apart, but not very well, and much of the paint has come off since I first took the carbs out.

http://gstwins.com/gsboard/index.php?topic=56491.0

[smokestack]

Have you already ruled out a slipping clutch?