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Discussion Starter #1
This is a problem that's only manifested itself in the last 2 weeks or so. When I'm riding around town, keeping it below 5,000 RPM, the engine always runs smooth with enough low end power, like it always has. However, sometimes when trying to pass in top gear at highway speeds, or running through the gears hard, the engine bogs considerably between 5,000-7,000 RPM. Sometimes, at 75 MPH, it will barely accelerate at 6,000 RPM. Above that, and much of its normal power returns. Even when it's bogging, the motor still seems to be running smoothly; it just won't pull hard.

This only happens sometimes, and it can come and go a few times during the duration of a ride. The rest of the time, the motor runs just as nice as it always has. The bogging seems to be independent of engine or ambient temperature. Plugs were done at the beginning of the season. Gas tank was drained and the fuel valve was cleaned a month or two ago while trying to diagnose another problem (turned out to just be a faulty fuel gauge - I was running it out of gas. Doh! :bitchslap ).

Anyone have any ideas? I'm thinking maybe a carb synch is in order, but it would seem that if they were badly enough out of synch to cause the bogging I'm experiencing, I probably would never see normal throttle response. I also considered that it might just be a bad tank of gas, but I've been through a few tanks now - all from different places - and the problem is still there.

Your input is appreciated.
 

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second chimp in space
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I'm betting on dirty carbs. Especially since you've been running out of gas recently, which can suck up some extra dirt from the bottom of the tank.

Since it's not too bad, I'd get a bottle of berryman's carb cleaner, the kind you put in the gas. Do that at like twice the concentration they say. Hopefully that will clean it out within a few days, and you won't have to take the things apart and properly clean them.
 

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Bogging is a typical sign of a rich mixture. Lean would cause a notiable ping or knock...dirty carbs would create a lean enviroment.

I would suggest you clean the carbs...something is probably sticking. Then synch them. I had a problem just as you describe with my FZ600 and that turned out to be the cure.
 

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Discussion Starter #5
Thanks very much for the replies. I think I'm going to try the easiest thing - and incidentally something I'd considered doing already - first, and run some carb cleaner through the bike. We'll see what happens from there. Thanks a lot for the advice.
 

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Discussion Starter #6
I ran some carb cleaner through the gas tank, and it seems to have helped. Unfortunately, in the meantime I managed to run the tank out of gas again, and since then there was another loss of power. More carb cleaner, and things seem to be looking up again.

However, something else is really troubling me. As I rode to work yesterday, I switched my tank over to reserve because I figured I'd be needing to soon. Normally I get up to 50-60 miles out of my reserve. I rode about 5. Then I left it at work for the day. At the end of the day, I went to leave, and got about a quarter mile before the bike stumbled and died on me. I could crank and crank, and didn't get that flooded-engine smell - no fuel to the engine. A quick call to my dad and he was on his way with a jug of gas. Filled the tank, and the motor fired right up. So where did my gas go?

Well, for as long as I've had my bike, the oil has always been overfilled. This didn't bother me too much, because the dealer I bought the bike from is actually known for overfilling bikes. But then I changed the oil myself at the beginning of season, and put in 3 liters off a completely empty engine as per spec. Again overfilled. Not long after, I had to get my clutch changed. Got the bike back, and it was overfilled. So a few days ago I decided to drain out the excess, which actually turned into a full-on oil change. I was very careful to fill the oil so it would be below the full mark, running the engine for a few seconds and then topping off. Once I was finished, I had the oil sight 3/4 full, exactly where I wanted it.

Not long after, I noticed again that the sight was completely covered. Now I have to tip the bike a fair bit to the side in order for the sight not to be completely filled. Hmm... missing fluid from the gas tank, and excess fluid in the crankcase. This does not look good.

Is it possible that, as the bike is sitting, gas is leaking into the crankcase from the gas tank? Recently, I've been sure to turn my fuel valve to the off position, despite the fact that it's a vacuum-controlled valve, and when I go to start up the bike after leaving it overnight - with the valve still turned off - the bike fires right away. This leads me to conclude that either the valve is faulty - not that likely, seeing as how I recently had the tank off the bike, and no gas leaked out - or else gas isn't leaking through the carburators, seeing as how they're still full of fuel.

This problem has me really confused. The old oil from my oil change doesn't smell of gasoline... at least not to my nose. My coolant level stays constant and there's no milkiness in the oil, so that's a fluid ruled out. Aside from some recent carburation problems, the bike runs well, although it does and has always leaked a fair bit of oil, moreso while riding than sitting. Yet despite this leaking, and my never topping it off except when changing the oil, the level is always too high.

Anyone care to take a stab at this?
 

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second chimp in space
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First thing's first. Your tank is filthy. Drain it and flush it out. Take the petcock off while you do it, and drain it through that hole. And stick a fuel filter on it. That filth can also keep the float valves from closing, possibly dumping gas in the motor. Drain the carbs after rinsing the tank out.

The overfilling might be from oil from the top end of the motor trickling down after the change, but i'm not sure about that.
 

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second chimp in space
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On, and it's possible that you have water in the bottom of the tank. That might seem like you're running out of gas. I had that happen on my '87 zx750 when the gas cap started leaking during a big ass rain storm.
 

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sounds exactly like a stuck float to me. i think enos has it right, drain and clean the tank, add filter and completely disassemble and clean the carbs and then a new oil change. if you skip any of these steps, you'll probably be back to square one in a couple days. let us know if you need any help.
 

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Discussion Starter #10
Actually I drained and rinsed my tank less than two months ago. It was quite clean for a 16 year-old tank then, and I don't see any reason why it would be different now, unless maybe if I got a bad tank of gas. I suppose that's possible and maybe worth looking into.

As for the oil coming from the top end of the motor, I left the motor with the filter and drain plug off overnight before refilling, so I imagine it should've been quite empty. I also cycled the motor on for a few seconds and then waited for the oil to settle before putting in small increments of oil. As near as I can tell, whatever oil could have been in the top end, should have drained to the bottom. But I've been wrong before, and I'm sure I will be again.

Thanks for the replies. Any more ideas as to the likelihood of gas getting into my oil? Would it be easy to identify gas in the oil by looking at/smelling it?
 

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Does your bike have a vacuum operated fuel petcock? If so that could be part of your problem. If the vacuum line is cracked or the diaphragm leaking, it will not open fully, therefore not allowing enough fuel to flow the carbs. Or your carbs might need cleaning.
 

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roadracer319 said:
Does your bike have a vacuum operated fuel petcock? If so that could be part of your problem. If the vacuum line is cracked or the diaphragm leaking, it will not open fully, therefore not allowing enough fuel to flow the carbs. Or your carbs might need cleaning.
I didn't read your last post completely, you do have a vacuum operated petcock. Also, even with the fuel valve in the off position, the bike will start and run for a few minutes on the fuel in the carb bowls.
 
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