No idle on rear cylinder again - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 9 (permalink) Old 12-23-2011, 07:22 PM Thread Starter
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No idle on rear cylinder again

After about 300 miles of flawless operation on my newly overhauled Steve Reed carbs I was sitting at a top light and the rear cylinder dropped out, never to participate in idling again. I've done the following

1) While idling pulling the rear plug wires has no affect on idle (pulling forward plug wires obviously does)
2) Pulling plug rear plug wire, installing loose extra plug in wire and placing it against block results in very strong blue spark (i.e. they're sparking)
3) Pulling rear plugs after reving shows nice looking plugs
4) Pulling rear plugs after 10 minutes of idle shows soaking wet plugs
5) Moving plugs around, swapping extra plug, etc has no affect
6) Spraying massive amounts of brake cleaner around the rear cylinder's intake boot results in no change of idle.

When I accelerate the rear cylinder seems to come back online resulting in jerky throttle operation.

So, it really looks like the rear cylinder is, once again, getting flooded with fuel.

So what could have happened since? I really don't want to have to send the carbs off again but I really want to someday feel comfortable riding more than 10 miles from the house. This was the problem that initially prompted me to overhaul the carbs in the first place.

-Robert
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post #2 of 9 (permalink) Old 12-24-2011, 02:16 PM
If only it had 6th gear..
 
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have you looked at the petcock screen and how's the condition of the inside of the tank? seems like something is getting into the carbs sticking the float needle valve open due to the wet plug you mentioned.

Dave

Mods:
Jetted, shaved & coastered
Home made under-tank single filter
Converted ACCT's to manual (thanks wib714)
Dual vac lines on petcock (my cure for ticking)
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Added stuff:
Customized hard bags w/ lights
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Home made 'Vita-Volt' meter
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Hmm....she was on the back when we left.... ??
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post #3 of 9 (permalink) Old 12-24-2011, 05:11 PM Thread Starter
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I overhauled the petcock when I reinstalled the carbs when the carbs arrived. I probably only did a light job of cleaning the tank. However I did inspect the pickup screen and it has no holes in it. I've not used the reserve.
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post #4 of 9 (permalink) Old 12-24-2011, 06:34 PM
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From your description, your rear cylinder is definitely flooding out, which can only be caused by the carb. Be careful running it like that, because if the rear cylinder gets enough liquid gas in it, you could wind up with a condition known hydrostatic lock. Liquid will not compress, and when the piston tries to compress it, something is going to break, usually the piston or con rod. It can also crack the cylinder and/or head, and if enough force is transferred through the con rod, it can also destroy the crank and crank bearings. I don't like the thought of pulling the carbs on a Vulcan 750, but in your case, I don't see any choice, since there is no way to tell what is going on in there without taking them apart, or at least the rear one.

Considering you recent had the carbs rebuilt, and it ran fine, there are few possibilities. You can eliminate the float level being off, as it does not change by itself. Either the float is sticking, which is unlikely if the carbs were professionally rebuilt (seems a number of Vulcan 750s had defective floats from the factory, with a small piece of flashing on them that hit the side of the float bowl, causing the float to stick, I found this on the rear carb of my then new '93), the float sprung a leak and sunk, ot the float needle is leaking, either due to damage to the needle/seat itself, or something stuck in it.

One thing you might try before removing the carbs, is to drain the float bowl and leave the drain screw open, remove the fuel line to the carb, and shoot some compressed air in there. With the float bowl empty, the float should be all the way down, and the needle will be open, so it shouldn't hurt anything, and might clear out anything that might be causing the needle to not seal properly. It's a long shot, but easy to do.

I am a motorcyclist, NOT a biker.


1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike
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post #5 of 9 (permalink) Old 12-24-2011, 06:48 PM
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Might want to ck also to see if gas has gotten into the oil!

04-3,200 miles bought July 2011 with 928 miles. Loaded with acessories to many to list here, see my albums for more details! I also make a custom tank bib specific for the 750 if anyone is interested, ck my albums for some pics.
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post #6 of 9 (permalink) Old 12-25-2011, 01:28 AM Thread Starter
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Don't the front and rear share the same float bowl though? Would it make sense that the front runs great but the rear doesn't if the issue is a sticking float?

-Robert

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Originally Posted by VN750Rider/Jerry View Post
One thing you might try before removing the carbs, is to drain the float bowl and leave the drain screw open, remove the fuel line to the carb, and shoot some compressed air in there. With the float bowl empty, the float should be all the way down, and the needle will be open, so it shouldn't hurt anything, and might clear out anything that might be causing the needle to not seal properly. It's a long shot, but easy to do.
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post #7 of 9 (permalink) Old 12-25-2011, 02:30 AM Thread Starter
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I should add that I think this only affects the idle circuit. At higher RPMs both cylinders fire. When I first get to a stop light it will often idle correctly. It takes 10-20 seconds at idle before the rear drops out.
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post #8 of 9 (permalink) Old 12-25-2011, 03:18 AM
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While the float bowl casting for the front and rear carbs is one piece, they are totally separate in operation, there are 2 separate floats, needle valves, 2 main jets, 2 pilot jets, and 2 pilot screws. It all works the same as if it were 2 pieces. If it is your rear carb that is flooding out, or "loading up" as it is commonly called, then the problem is with the rear carb. The fact that the plugs are wet tells me that you are getting gas into the cylinder, otherwise I would suspect a plugged pilot jet. Have the plugs been removed from the pilot screws (idle air screws) and if so, how many turns out are they set to?


From your description of the symptoms, it almost has to be gas leaking past the needle valve, and overfilling the float bowl. When you ride at full speed, it burns up this excess gas, and runs ok. But when you are idling, gas is still filling the float bowl at the same rate, and your air/fuel mixture is way to rich to burn. Just for the heck of it, with the engine idling on one cylinder, pull the vacuum line off the rear carb (on the right side) and see what happens. Also, remove the fuel line for the rear carb from the petcock, and plug up the fitting on the petcock. See if it will start to idle again on the rear cylinder as the fuel level in the float bowl drops. If it does, that confirms what the problem is.

I am a motorcyclist, NOT a biker.


1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike
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post #9 of 9 (permalink) Old 12-25-2011, 12:58 PM
..have a vulcan good day!
 
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I agree with others, your problem appears to be a fuel issue.
Also note: I lost the spark to rear cylinder (at all rpm's) ..... replaced the Ignitor Box (CDI Box) plus performed +12 volt from battery directly to coils via an added relay _modification _ ... which corrected my problem.
Good Luck
WilliamTech

'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
SilverStar Ultra 9003/HB2 H4 Headlamp, Jardine Fwd Controls,
Iridiums DPR7EIX-9 & Wires, Tuxedo Mod, Coil Mod, P/U Sensor Mod, Fork Seals,
Splines Lubed - 11/4/2012 - MF AGM Battery
Additional Flashing LED Brake Light on Trunk
Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!
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