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Discussion Starter · #1 ·
Hi All,

I have a 2000 SV650N with I THINK stock carb setup and an M4 muffler. It has ~27k on it, and had full 24k service. Normally it runs well.

Last weekend I accidentally lugged the bike away from a stop sign in 2nd or 3rd gear (chugga chugga chug *squeeze* *click* vroom) and thereafter the exhaust note was somewhat.... 'blurry' for lack of a better term. The sound is normally quite crisp.

At first I thought it was my imagination, but by the time I got home it was clear that the sound was off, power wasn't great, and I was babying it through the gears. It started and idled fine. It won't idle on one cylinder, I know from experience, so both must be firing.

I've pulled the plugs (first time doing that myself), cleaned them - they looked a bit dark, but gapped ok . The plugs are the two-prong high-temp type spec'd in the factory manual to use if your plugs overheat.

While in there, I pulled the air filter out too.
There was some oil in the air box, which I gather is not unusual, so I wiped it up. The OEM filter smells of gas & oil. Can I clean it? Should I? Should I replace it? Is this likely to have anything to do with the weird episode described above?

I'm trying to decide what to do before putting everything back together. Suggestions welcome.

Thanks,

-Daizee
 

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Discussion Starter · #2 ·
I have since re-assembled the bike after cleaning the plugs, blowing out the filter, and mopping up the oil in the airbox. It runs exactly the same, which is a victory of sorts - at least I didn't make it worse.

While it was running I hit the exhaust pipes with an infrared thermometer.
Oddly, the header out of the rear cylinder seemed quite hot, while the rest of the pipe immediately after the step-up joint was cooler, relatively. The front cylinder pipe seemed to have a more even temperature gradient. But I'm not CERTAIN what I observed on the thermometer were accurate measurements due to the tight quarters and difference in surface finish btw the two pipes.

Then I lifted the tank again, pulled the filter, and peered through the carb intakes. The butterfly valves appear to be operating in sync.

While poking around some more from the outside, one of the rear carb boot hose clamps seemed a mite loose. The boot does not appear loose, but the other clamps I can reach well enough to poke at seem tighter. Perhaps a burp from my lug-fest bloated the boot and broke the seal that was otherwise fine with the loose clamp?
I'm going to snug that slightly and see if that helps - if so, I'll assume it was a vacuum leak, and the rear cyl was quite lean, leading to a warm cylinder, but low-volume exhaust. That would account for my fantasy that my bum was warm on the ride back last weekend...
 

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When you say that the carb butterflies seemed to be operating in sync, was the engine running? I'm asking because the the standard carbs are constant velocity, so you really wouldn't see the butterflies unless the carb pistons were open quite a bit. Was it the carb pistons you were referring to?

If the engine was running and it was the carb pistons moving in sync, then it would negate what I though was a remote possibility; if the engine backfired through one of the carbs, it could have ruptured the piston diaphragm, which would disable the piston from raising, or at the very least slow it down as compared to the other carb. That would definitely cause it to run strangely under any throttle application.
 

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Discussion Starter · #4 ·
if the engine backfired through one of the carbs, it could have ruptured the piston diaphragm, which would disable the piston from raising, or at the very least slow it down as compared to the other carb. That would definitely cause it to run strangely under any throttle application.
Wow, ok, that's depressing AND I don't understand it.

What I could see was movement under the incomplete 'disc' that fills the carb intake where it meets the airbox. There's a brass-looking plate, or something, that appears to be moving out of the path of the air. My terminology is likely wrong... The only carbs I'm personally familiar with are the ones off my '72 Honda, so I'm trying to translate to what I can see without taking the entire bike apart. Both of the plates actuated by the throttle appear to be moving off the closed position at the same time, as far as I can see through the opening in the disc that mostly obstructs my view into each carb.

I don't recall any actual backfires, I'm hoping it didn't get that violent in there.

Tomorrow I'll tighten the rear carb boot clamp and report back. My current hope is that the bucking of the engine against the frame loosened it up under the loose clamp and all I'm seeing is a vacuum leak.
 

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This link be helpful, or you can do a search on "constant velocity carburetor" to find a similar diagram:

http://www.omnilex.com/public/bmw78/cvcarb.pdf

And there isn't anything to be depressed about. :) The likelihood of a diaphragm being damaged is pretty small, I think. The only other thing I can think of is the rubber plug that covers the vacuum port on the bottom R/H side of the front carb getting blown off by a backfire, but as you can't remember hearing a backfire, that's also a long shot, and it might not idle if that were the case.

Definitely a strange problem. Oh, what does the " *squeeze* *click* " in the original post refer to? Would that be pulling in the clutch and downshifting?
 

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Discussion Starter · #6 ·
Good news!
Sunday around noon I tightened the carb boot clamp and fired her up. Since it had been about 18hrs since I last did so, it was hard to compare the sounds directly, but it sounded better at idle and under throttle in neutral.

So I geared up and took her out. Ended up doing a good 50mi+ ride, and the bike ran great. When I came back, the infra thermometer showed a smooth heat gradient on both pipes from the head on back to the junction. Power was good. I think I'm getting used to the power as it doesn't seem to stretch my arms as much as it used to.

For the purposes of this thread, I'm going to assume that the bucking of the engine against the frame loosened the carb boot under the already loose hoseclamp, and once the seal was broken it just kept wheezing. I've learned a bit more about the bike, having been an inch or two deeper down than before.

-Daizee
 
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