Sputters at 4K RPM - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 16 (permalink) Old 02-08-2009, 04:20 PM Thread Starter
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Unhappy Sputters at 4K RPM

Guys I really need you help on this one. Been troubleshooting every day for like two weeks now.

It idles fine, and if I can get it over 5K RPM's it'll handle the freeway just fine.
The moment it hits 4K RPM's it starts sputtering. Once that begins it'll continue until I get it over 5K or I can get it to idle without stalling completely.


---------------------------------
A little history in case it's relevant:

Within two weeks before this started happening I did an oil change (Mobile 1).

Replaced the O-rings in the deep spark plug tubes.

I removed the sidestand switch because the bike wouldn't start at all. Those wires are shorted now. I've tested this connection at the junction box. Bike starts fine after this.

---------------------------------
Bike ran excellent for about two days after those things.
Then on my way home from work (this is my commuter), after getting off the freeway and lowering my RPM's it sputtered and stalled. And such has been my world every day since.


---------------------------------
Things I've tried:

Checked all wiring/plugs/connections. Cleaned contacts etc.

Changed the oil filter since I hadn't when I did the oil change. Topped up the oil again.

Gaped the spark plugs. Tightened the screw-on connectors (I don't have stock wires).

Checked/cleaned air filters and airbox/hoses/boots.

Pulled the carbs and cleaned them out using gumout.

Blasted the jets with gumout followed by compressed air.

Checked the vent hose, fuel hoses and vac hose for cracks/kinks.

Rerouted the vent hose to face the rear of the bike. I do not believe it was in it's holder when this happened. It's possible something may have been sucked in.

Checked the air mix screws. They were one turn out. I set them to two turns. No noticeable change.

Synced the carbs.

Used excessive amounts of Sea Foam throughout this time (and had on occasion previously).

There may be few other things I did and forgot. Feel free to ask. It's been a long couple weeks.


---------------------------------
A little info about the bike.

It's a 94. I've had it for about 5 years and put about 20K miles on it.

K&N Air Filters, MF Battery, Iridium NGK Spark Plugs, new spark plug wires, coastered (still have the air box though).
The "pumpkin" is off an 86. Had my splines turn to dust about 6 months ago. They've since been replaced and lubed proper.
Headlight device (switches from high to low and vis verse when one goes out) has been removed.
Regulator has been replaced with another used one. It's possible the original was fine. That was years ago though.
Added a handlebar voltage meter.

Think that's about it. All of those mods were done 6 months or longer ago.


-----------------------------------

I'm assuming it's a carb problem since it happens Right at 4K RPM but I've blasted the jets so many times I just can't imagine anything still being in there.
I didn't see anything horrific inside the carbs when I had them out. Some minor tarnish on the needles but no real buildup on anything. Float needles were fine.

I had the carbs off and cleaned a few times when I first got the bike (about 4 to 5 years ago). I've always been good with the Sea Foam since then.

I'm not sure what else would cause this behavior though.


-Bean
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post #2 of 16 (permalink) Old 02-08-2009, 06:08 PM
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Ckeck the gas cap vent, petcock diaphram, fuel filter and make sure the fuel vent is clear.


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
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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
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post #3 of 16 (permalink) Old 02-08-2009, 06:23 PM
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Quote:
Originally Posted by lance328 View Post
Ckeck the gas cap vent, petcock diaphram, fuel filter and make sure the fuel vent is clear.
Along with all of the above check the vaccuum hose to the petcock. edit; I see you done did that.

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post #4 of 16 (permalink) Old 02-08-2009, 06:36 PM Thread Starter
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I've ridden it with the tank's cap open and the petcock on reserve. No change.

How would I go about checking the diaphram?

edit:
Forgot to mention, I've also inspected (with a flashlight) the tank. Didn't see anything scary in there. Also took a swap to the bottom. Seems clean.

I've also drained the crabs a number of times during all this and the gas has always come out clean.


-Bean

Last edited by Bean; 02-08-2009 at 06:39 PM.
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post #5 of 16 (permalink) Old 02-08-2009, 06:42 PM
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The tank would have to be empty or removed, and set on side to keep fuel leaking while petcock is removed and dissassembeld and checked.

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post #6 of 16 (permalink) Old 02-08-2009, 07:35 PM
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Quote:
Originally Posted by niterider View Post
The tank would have to be empty or removed, and set on side to keep fuel leaking while petcock is removed and dissassembeld and checked.
I think you could put a vac pump on the vac nipple to the petcock and after applying vacuum, fuel should flow through the lines to the carbs. I would try this before removing the petcock.

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post #7 of 16 (permalink) Old 02-08-2009, 08:35 PM Thread Starter
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If it idles fine and runs at high speed fine wouldn't that rule out a petcock issue?


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post #8 of 16 (permalink) Old 02-08-2009, 09:00 PM
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Quote:
Originally Posted by Bean View Post
If it idles fine and runs at high speed fine wouldn't that rule out a petcock issue?


-Bean
If it consistantly hapening from 4000-5000 rpm you may have an electrical problem and not fuel. I would start with the side stand wires since you bypassed the switch. If the wires have a loose connection at certain speeds the wires can resonate and temporarily loose connection and cutting the ignition circuit.


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

Last edited by lance328; 02-09-2009 at 12:46 PM.
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post #9 of 16 (permalink) Old 02-09-2009, 10:09 AM
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I think I have a side stand switch in my parts box if you wanna try a fix that won't cost a whole lot.

On my Ninja 250, after I rejetted for a new pipe, I had a stumble exactly like you described aroun 9K (14K redline) that was due to the jetting being too rich. The bike pulled fine up to there, and once past, ran fine as well to redline. Would also start and idle fine. What are your jetting numbers? Also - have you checked the rubber on the vacuum slides for small tears / leaks?

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post #10 of 16 (permalink) Old 02-09-2009, 10:54 PM Thread Starter
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I think lance is on the right track. I've been trying some things. I'll post my findings in a day or two. Thanks for all the suggestions everyone.

As for the side stand. I'm beginning to think that was a symptom of the real source of the problem. i.e. a bad wire/connection or something grounding out, something along those lines.

I got some electrical contact cleaner. Sprayed down all the connections and brushed then with a wire brush. I also grounded the side stand wire right at the junction box to eliminate the possibility of the wire being bad along the way to the side stand.
I haven't had a chance to thoroughly test ride it yet though.


-Bean

Last edited by Bean; 02-10-2009 at 01:07 AM.
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