Sputters at 4-5k RPM when riding, 8k RPM in neutral..help? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 12 (permalink) Old 08-09-2015, 10:01 PM Thread Starter
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Sputters at 4-5k RPM when riding, 8k RPM in neutral..help?

Randomly, this started happening Wednesday during my ride home. I opened the throttle up completely to pass a car and as soon as it hit 4k RPM it immediately started sputtering and would not accelerate past 4-4.5k RPM no matter what. This weekend I took a look and was able to repeat the problem easily. I noticed, however, that if the bike is in neutral (parked or not) it will rev up to 8k RPM or so, but also does the same thing where it starts sputtering out. It almost feels like someone came and installed a governor on the bike when I wasn't looking.

I tried a few things, but nothing helped:

1. Ran it with the gas cap open, just in case there was an issue venting.
2. Removed the pod filters and cleaned/re-oiled them - but also took her around the block WITHOUT the filters, no difference
3. Checked the battery voltage, that looked good (12.something, can't recall offhand, this was parked with the engine off)

Here's a video I took earlier, it's 4 minutes long but here's the interesting bits:

https://www.youtube.com/watch?v=yaygAnUWnMs

0:22: Parked, revs to about 5k RPM with throttle fully opened
0:42: Riding down the street, again limited to about 4-5K RPM
1:30: Pulled out onto the major street, 1st gear to 4k RPM no problem, 2nd gear to 5k RPM starts sputtering
2:55: While rolling along, I pulled in the clutch and was able to rev up to the red line without sputtering
3:20: Slowed down to lower gears and gunned it again, 3rd gear it sputtered at 5k RPM
3:40: Coasting into my drive way in neutral, and my dumb ass downshifted into 1st without the clutch. Whoops!

I just replaced the spark plugs a couple weeks ago with new ones (gapped them myself), the charging system was replaced last year with a TPE stator and new R/R. Not sure what to look for or test for aside from what I mentioned...any ideas?

'92 Kawasaki Vulcan 750

Mods:
Rattle-can high gloss black paint job
Frame, engine, Headers/exhaust painted satin black
Ears shaved, K&N pod filters
TPE Stator, MOSFET R/R, Magna Power AGM battery
Pickup Coil mod
New mirrors, grips, cables
VN900 seat
Straight pipes with 8" baffles
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post #2 of 12 (permalink) Old 08-11-2015, 01:58 PM
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sounds like a sticky float try some carb cleaner in the tank
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post #3 of 12 (permalink) Old 08-11-2015, 02:03 PM
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I'd pop the carbs off and inspect. Could be as simple as crud in one of them jets.

1986 Kawasaki Vulcan 750
NGK Iridium Plugs #7803/DPR7EIX-9
Duralast Gold ETX15 AGM Battery
Coastered & Shaved
TOC MCCTs
Metzeler ME880 [110/90-19, 170/80-15]
Balance Dampers Replaced
Tuxedo Mod
Rebuilt Forks w/ Progressive Springs
V&H Cruzers
VN750.com Grill Cover
Meanstreak Seat
Emgo 23-92411 Handlebars
MOSFET FH012AA R/R


1990 Kawasaki Vulcan 750

1998 Honda VFR800 FI

2014 Honda VFR800F

1989 Pontiac Firebird Formula 350
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post #4 of 12 (permalink) Old 08-11-2015, 02:17 PM
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Mine has done this once per season since I bought it.

1oz of Seafoam per gallon of gas always seems to cure it until sometime in the next season.

I suspect dirt getting into the jets, but I've yet to see the inside of my carbs. My bike did sit a while before I bought it, so I'll bet there's crud coming loose from the bottom of the bowls.

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Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
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post #5 of 12 (permalink) Old 08-11-2015, 02:38 PM
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Really should not be revvin to 8grand in neutral.

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post #6 of 12 (permalink) Old 08-11-2015, 04:24 PM
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Quote:
Originally Posted by OleDirtyDoc View Post
Really should not be revvin to 8grand in neutral.
X2, not good

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post #7 of 12 (permalink) Old 08-11-2015, 05:33 PM Thread Starter
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Not making a habit of it, just trying different things to pinpoint the issue.

I'll try seafoam first, if that doesn't help I'll pull the carbs and clean them out. I cleaned them out a few weeks ago, but I painted my tank a couple weeks after that, so there is a chance something made it's way in.

'92 Kawasaki Vulcan 750

Mods:
Rattle-can high gloss black paint job
Frame, engine, Headers/exhaust painted satin black
Ears shaved, K&N pod filters
TPE Stator, MOSFET R/R, Magna Power AGM battery
Pickup Coil mod
New mirrors, grips, cables
VN900 seat
Straight pipes with 8" baffles
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post #8 of 12 (permalink) Old 08-11-2015, 06:32 PM
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I've had this happen on different bikes for different reasons. The easiest fix is to back flush the carbs through the vents as Wolfie so often suggests. A proper cleaning (removal and complete disassembling, manual cleaning not just carb spray) and seafoam can fix it sometimes. Don't get carb spray on the air cutoff diaphragms or slide diaphragms. Remove those first.

Another time, I bought a bike with torn slide diaphragms. OEM slides are pricey, but there's a guy making parts that replace the plastic and rubber with one rubber piece, and they work GREAT for 1/3-1/4 the price. You can find them on the Googles.

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post #9 of 12 (permalink) Old 08-11-2015, 07:30 PM
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Only reason I don't do the backblow.... If there's a pile of dirt in the bottom of the bowl, I don't want to launch it up through the carb. Although the backblow can get you off the trail if you're stuck.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #10 of 12 (permalink) Old 08-27-2015, 09:48 PM Thread Starter
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Looks like it was the carbs. I cleaned them, changed the jets (been meaning to do that for awhile now) and good as new.

'92 Kawasaki Vulcan 750

Mods:
Rattle-can high gloss black paint job
Frame, engine, Headers/exhaust painted satin black
Ears shaved, K&N pod filters
TPE Stator, MOSFET R/R, Magna Power AGM battery
Pickup Coil mod
New mirrors, grips, cables
VN900 seat
Straight pipes with 8" baffles
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