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Discussion Starter #1
Hey guys,

First time poster, but I've been spying for a while. Lots of great info. I bought a 99 vn750 a couple months back. Immediately ran a couple tanks of Seafoam. No real problems, except the decel popping. Did the coaster mod. It helped a little, not much. Last week I took a ride and at about mile 50 the bike starting acting like it was out of gas. Had to pull off the road. Opened the fuel tank and there was no "swish". Tried to start it back up and bingo. Everything was fine the rest of the ride. Two days later I went out to start it up and it's been messing up since. Starts fine, idles fine, but when you get up to around 3500 to 4000 rpms, in any gear, you start losing power like your starving for fuel.

So I pulled the fuel stem: its clear and clean. No vacuum on the tank. But I drilled out the holes in the cap just in case. I pulled and cleaned the carbs, but everything seemed clean, no rust, no gunk, no debris. Put the carbs back on, sync'd, etc. and still have the problem, and have more popping now. If I keep it under 3000 rpms it runs great. But if you push it, it just starts to jerk and hesitate.

The plugs are almost new and no fouling. The float level of the rear carb is square on, the level on the front is about 1/8in low. Is that enough to matter?

Petcock inspected. All the parts look fine. I'm stumped at this point and pissed, 'cause its above 70 this weekend and my bike is sick. I'm looking for ideas if anybody has time. Thanks.
 

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Check the air vent line coming off the carbs. I see you coastered the bike, but where's that line going to now? Often at around 4K rpms, if the vent line has bottomed out against something (like the air filter housing...or in your case, whereever you set the line) it'll cause exactly what you're describing.

Few have described the alternative, that the line is blowin' in the wind and causing the bike to sputter, but I actually have had that happen as well. Without the air filter housing, you'll want to cinch it down with the open end of the line facing towards you when you're riding. A zip tie will do the trick.
 

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Discussion Starter #3
the vac line off the carbs is into the right ear, not bottomed out. But I have had it sort of free underneath there as well. Was playing with it again earlier today, and if I put my finger over it, basically nothing happens. Seems like before my carb rebuild that if I did that there was noticeable loss in performance.
 

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There's some stuff in the verses about cutting it off at a 45-degree angle so it sits in the box without bottoming out...or even coming out of the airbox. But I had a line that kept popping out of the airbox and it was picking up the draft just from driving along. If I turned it towards me or pointing down, the bike regained power. If i left it floating around, at 4000 rpm the bike was sputter and kicking. See if you can cinch it down and take it out again tomorrow. At the very least, you can rule that out as the culprit if you know it's in a fixed position and the bike's still giving you fits.
 

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Discussion Starter #5
Okay, will try that tomorrow. But I'm skeptical because even sitting in the shop, when I rev up it falls out at the higher RPMs. There's a lot of after fire, popping, etc. going on. When the bike is warmed up there's less popping than when cold.

I'm going to put my tester on the coils tomorrow and see if there's a problem there as well.
 

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and the Adventure Cycle
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I've had problems at that RPM range when running with the covers off the air filters.
Have you check the condition of the air filters? They are known to deteriorate after awhile.
 

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Discussion Starter #7
I've had the ears and filters off a lot recently. The filters are K&N and seem to be in good shape and moderately clean. This afternoon I had the filters completely off at one point and on at other points and still had the same problems. Didn't seem to effect it at all.

I'm beginning to wonder if my problem's on the electrical side????
 

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The Professor
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What condition are your plugs in? Check the red coil wires and make sure you are getting 12v. Did you clean the main jets?
 

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Discussion Starter #9
thoroughly cleaned the mains. When I got into the carb it was really clean. Previous owner had really kept it right. The rubber boots on that big vacuum valve in both the carbs looked almost new to me. I did an inspection, and because everything looked so clean and didn't go get a kit. Just soaked, rinsed, and sprayed Gumout through everything.
 

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The Professor
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thoroughly cleaned the mains. When I got into the carb it was really clean. Previous owner had really kept it right. The rubber boots on that big vacuum valve in both the carbs looked almost new to me. I did an inspection, and because everything looked so clean and didn't go get a kit. Just soaked, rinsed, and sprayed Gumout through everything.
Check the other two. :smiley_th
 

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I misunderstood your earlier post - didn't realize that in the shop you were having similar issues. At this point I might be looking for a vacuum leak, especially since you've had the ears off. The boots that connect the ears to the airbox can look like they're on, and not be snug enough for th ebike to run correctly. On the end of each boot, there are TWO sections that need to be inserted into either the airbox or the cleaner housing. Make sure that you've got each boot fully inserted on each end; it's particularly easy to displace the one in the airbox when you try to put the other end into the housing.
 

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Discussion Starter #12
the plugs are almost new, dry, with a bit of sand color, more white than brown.

The red wire at the coils measures frp, 11.79 to 11.89.

I measured the resistance across the contacts at the R&R and didn't get what the Clymers says. But I don't know if I'm doing it right. It doesn't show a short, but doesn't show the resistance that the manual says should be present.
 

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The Professor
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the plugs are almost new, dry, with a bit of sand color, more white than brown.

The red wire at the coils measures frp, 11.79 to 11.89.

I measured the resistance across the contacts at the R&R and didn't get what the Clymers says. But I don't know if I'm doing it right. It doesn't show a short, but doesn't show the resistance that the manual says should be present.

What kind of battery do you have and what is the voltage at idle and 3500 rpm?
 

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Discussion Starter #14
Ok, I checked the coils, plug wires, plugs, etc. and all is okay. I tried the burping the carb thing too, by shutting off the petcock while going down the road, then turning it back on when out of gas. No effect.

though I had checked the carb vac line that goes to right ear multiple times, I decided to pull it out of the ear while cruising. Whammo. If the end of that hose gets within 1/4 to 1/2 inch of anything, at higher rpms it starves the carbs. The line has a 45 degree cut in the end and I know it was set in the ear to not push up against anything. I tried putting it back in the ear and there is absolutely no position that it works in if I cruising above 3500rpms in any gear.

so what's up with that? right now I've got that hose duct taped behind the ear, out of sight, pointing directly forward and it works fine.
 

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Old Truck Junkie
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Since I have removed the ears on mine, I have a small air filter (that may not be needed) on the hose and shoved up under the fuel tank. Works for me. Try to keep it away from direct air flow. BTW, you might check the hose for a bugs nest in it.
 

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though I had checked the carb vac line that goes to right ear multiple times, I decided to pull it out of the ear while cruising. Whammo. If the end of that hose gets within 1/4 to 1/2 inch of anything, at higher rpms it starves the carbs. The line has a 45 degree cut in the end and I know it was set in the ear to not push up against anything. I tried putting it back in the ear and there is absolutely no position that it works in if I cruising above 3500rpms in any gear.

so what's up with that? right now I've got that hose duct taped behind the ear, out of sight, pointing directly forward and it works fine.
Like I say, I ended up moving the hose behind the ear and used a zip tie to position it so that it no longer caused any problems. A couple of years ago, I ran into a Forum member who didn't have the vent hose coming off the carbs at all - no "t", no line, no nothing. He reported no difficulties - which sorta makes sense, because then the carbs are venting under the airbox, which is reasonably still air to begin with. I wouldn't recommend yanking the vent line altogether, but it's an option, I s'pose.
 
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