Only one cylinder at startup - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 10 (permalink) Old 02-15-2010, 07:55 AM Thread Starter
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Only one cylinder at startup

This problem has only shown up this winter, if the VN has been stood for more than three days.
One cylinder fires as soon as I touch the button, the other cylinder behaves like I've run out of fuel. Nothing for the first few spins, then it'll cough a few times, then it starts. Once its started it runs perfectly.
All spark plugs have got really healthy sparks, and are a good colour (a little rich but very close to perfect).
Once the bikes been run it works perfectly day after day, until its had a long break.
Its like one carb is holding fuel and the other is slowly draining off. I've looked around the carbs and there's no signs of external leaks.
Any ideas?

'89 VN750
14000 miles on 20th April 09
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post #2 of 10 (permalink) Old 02-15-2010, 11:56 AM
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One float valve is not sealing properly and that carb is draining into the cylinder, clean the carb and replace the float vlave.


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post #3 of 10 (permalink) Old 02-15-2010, 04:52 PM Thread Starter
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Thanks Lance

'89 VN750
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post #4 of 10 (permalink) Old 02-15-2010, 11:04 PM
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Float needles are cheap. Might as well replace both while you're in there.

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post #5 of 10 (permalink) Old 02-16-2010, 05:03 PM
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To me it sounds like one of the carbs chokes is not working properly. As you said it only showed up this winter, and I know if the bike is real cold you need to use all the choke to get it started...

And if only one jug is firing up it sounds like the choke is not shooting enough fuel for a cold start into the other cylnder.

Once the bike runs a bit it finally starts up...guessing enough fuel had been shoved in that it gets it going.

If the bike fires up on both cylnders when the bike is hot...I kinda doubt it is a float valve and sound more like a clogged choke.



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post #6 of 10 (permalink) Old 02-16-2010, 06:19 PM Thread Starter
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I'm going to let the bike stand until Friday then try pulling the plugs after the first try at starting. I figure if one of the carbs is empty the plugs fed by that carb should be totally dry. I'll check the operation of the chokes as well, but with the fault taking a few days to develop, a carb that is slowly emptying itself makes sense.
I've got a "prime" position on my fuel tap. Does that fill the carbs without the need for the vacuum that usually starts fuel flowing? If it does I could try that to check Lances theory.

'89 VN750
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post #7 of 10 (permalink) Old 02-16-2010, 06:31 PM
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Yes, the "Prime" setting flows fuel to the carbs (via gravity) bypassing the vacuum valve.

As each carb does have a drain bolt on the bottom of the float bowl it should be easy to see if the valve is sticking closed. But I hink Lance was saying the valve was stuck open, causing the carb to flood the motor...

Sorry, but I forget here, do the carbs on the Vulcan have overflow vents?

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post #8 of 10 (permalink) Old 02-19-2010, 07:50 PM Thread Starter
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The bike had been stood long enough for the fault to develop again today, so before I did anything else I decided to drain both the carbs. I used two identical jars to catch the fuel so that I could compare it. The front carb only had half the amount of fuel that the rear carb had and it was cloudy. I then refilled the carbs using the prime position on the fuel tap and started the engine. It went first push of the button. So it looks like Lance is right and its time to strip and clean the carbs. I should have done some more investigations but we are forecast snow for the next few days and I really wanted to get out and try my new fork springs, so I just kept the engine running and went for a ride. It ran perfectly, as always.
I was hoping I wouldn't need to take the carbs off, but you can't win them all. Thanks for all the help.

'89 VN750
14000 miles on 20th April 09

Last edited by RetroGaz; 02-19-2010 at 07:54 PM.
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post #9 of 10 (permalink) Old 02-19-2010, 10:50 PM
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RetroGaz,

One thing you can do that may clear up the problem without removing carbs.

Connect hoses to the carb drains and put in a container and set the petrol tap to prime. Open the drains and drain ALL the petrol. Fill the tank with fresh petrol and repeat the draining process until the petrol runs clear.

Sometimes debris will stick to the float valve and this process can clear it, it's worth a go.


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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post #10 of 10 (permalink) Old 02-20-2010, 10:01 AM Thread Starter
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Definitely worth a try.
Thanks again.

'89 VN750
14000 miles on 20th April 09
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