Drops out at 4,000 rpms - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 16 (permalink) Old 04-17-2009, 08:23 PM Thread Starter
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Unhappy Drops out at 4,000 rpms

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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post #2 of 16 (permalink) Old 04-17-2009, 09:28 PM
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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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post #3 of 16 (permalink) Old 04-17-2009, 09:38 PM Thread Starter
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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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post #4 of 16 (permalink) Old 04-17-2009, 09:57 PM
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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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post #5 of 16 (permalink) Old 04-17-2009, 10:17 PM Thread Starter
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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.

He who whittles himself down to please everyone will soon have no legs to stand on.

99 750
18K
Coastered
K&N filters
Custom pipes
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post #6 of 16 (permalink) Old 04-17-2009, 10:33 PM
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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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post #7 of 16 (permalink) Old 04-17-2009, 10:37 PM Thread Starter
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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????

He who whittles himself down to please everyone will soon have no legs to stand on.

99 750
18K
Coastered
K&N filters
Custom pipes
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post #8 of 16 (permalink) Old 04-18-2009, 12:54 AM
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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?


85 VN700 "Old Yella"

REBUILT ENGINE
CUSTOM PAINT
VANCE & HINES CRUZERS
EAR SHAVED AND RE-JETTED W/K&N'S
DUNLOP ELITE K591 FRONT & REAR
VOLTMETER
SYNTHETIC BRAKE FLUID
SYNTHETIC OIL & GEAR LUBE
PLEXISTAR 2 WINDSHIELD
SPLINES LUBED
ACCT'S GREASED W/TOC SPRINGS
COASTERD
LEATHER SADDLE BAGS
LEATHER TOOL AND ROLL BAG
PICKUP COILS GAPPED AT .018"
NGK CAP, WIRES, IRIDUMS
BARNETT FRICTION PLATES & SPRINGS
CUSTOM GRIPS, MIRRORS, LEVERS
ORIGINAL STATOR & R/R 14.5v
DEKA MF AGM ETX15L

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post #9 of 16 (permalink) Old 04-18-2009, 07:09 AM Thread Starter
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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.

He who whittles himself down to please everyone will soon have no legs to stand on.

99 750
18K
Coastered
K&N filters
Custom pipes
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post #10 of 16 (permalink) Old 04-18-2009, 07:14 AM
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Quote:
Originally Posted by VirginiaKid View Post
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.


85 VN700 "Old Yella"

REBUILT ENGINE
CUSTOM PAINT
VANCE & HINES CRUZERS
EAR SHAVED AND RE-JETTED W/K&N'S
DUNLOP ELITE K591 FRONT & REAR
VOLTMETER
SYNTHETIC BRAKE FLUID
SYNTHETIC OIL & GEAR LUBE
PLEXISTAR 2 WINDSHIELD
SPLINES LUBED
ACCT'S GREASED W/TOC SPRINGS
COASTERD
LEATHER SADDLE BAGS
LEATHER TOOL AND ROLL BAG
PICKUP COILS GAPPED AT .018"
NGK CAP, WIRES, IRIDUMS
BARNETT FRICTION PLATES & SPRINGS
CUSTOM GRIPS, MIRRORS, LEVERS
ORIGINAL STATOR & R/R 14.5v
DEKA MF AGM ETX15L

Best Auto/Moto Insurance | Motorcycle Protection Today | FREE Trade-In Quote
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