Rear cylinder problems - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 11 (permalink) Old 05-06-2016, 01:49 AM Thread Starter
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Rear cylinder problems

Still new here... Searched several threads and found a few posts but no resolution to the issue...
86 750
5000miles

The other day I degoated the bike and punched out the screens in the baffles. After putting it back together the right exhaust (front cyl) sounds great. Healthy and loud. Bap bap bap,The left on the other hand is weak and kinda just ,put put put .

When I rev the bike up it reaches 3k and the rear kicks in and the bike jumps to 4K and it kind of backfires and is boggy. Above 3k or on a hard rev the rear kicks in and isn't really to noticeable but through a turn it's unpredictable and I've fallen once already because of it....before the degoat. It did do it before the degoat but figured it was just a touchy throttle now I notice it much worse.

Pulled both plugs off rear cylinder and no change at all.
Idle changes going from kickstand to straight up
Cleaned and reinstalled carbs
New plugs look good. Fuel air screws have no change
Tried to sync carbs. Rear had insane vacum evened it out pretty close

Anyone ?
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post #2 of 11 (permalink) Old 05-06-2016, 08:33 AM
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IMO, the idle changing from kickstand to straight up is strong evidence that your issue is still fuel/air related, very likely float height.

1986 VN750 27k miles
1999 EN500 32k
1983 GL650i in pieces. Someday it will ride again.
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post #3 of 11 (permalink) Old 05-06-2016, 09:49 AM
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You say the new spark plugs look good, does that mean you've actually seen them spark? (ruling out a bad rear coil)
When you run below 3k rpm then pull a rear plug, is it wet with fuel?

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post #4 of 11 (permalink) Old 05-06-2016, 11:26 AM Thread Starter
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I agree the floats are definite possibility I had issues with the front cylinder floating previous to this. By the plugs looking good I mention the color there a brownish. and running at an idle I did pull the plugs and test them the right one sparks blue but slightly slower then the left that sparks strong and rapidly .... I think I'll try to run the bike with the bowl drain open some and see if the back carb is flooding
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post #5 of 11 (permalink) Old 05-06-2016, 11:27 AM Thread Starter
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Maybe a related issue but in order to get idle to 1100 my adjustment screw is almost maxed out all the way in ... Warm no choke 1100 cold start full choke 2500-3000
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post #6 of 11 (permalink) Old 05-06-2016, 11:56 AM
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Originally Posted by 750_rob View Post
Maybe a related issue but in order to get idle to 1100 my adjustment screw is almost maxed out all the way in ... Warm no choke 1100 cold start full choke 2500-3000
Since you have had the carbs off...cleaned & re-installed, make no mistake I am sure you are speaking Idle Adjustment Knob.....
NOT the rear carb "Fuel -Air Screw" all the way out ..... Right ?

Sorry, had to clarify .

'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
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Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
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post #7 of 11 (permalink) Old 05-06-2016, 12:34 PM Thread Starter
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Yes the little black knob is almost all the way in... Maybe 1 turn left. Not the air/fuel screws those are both set at 2 1/2 turns I think I richened up the rear to 3 turns. The bike also sat for almost 30 years so I went through most everything new fuel line and petcock and carb cleaning... No fuel was left in the bike or carbs
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post #8 of 11 (permalink) Old 05-06-2016, 01:34 PM
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If the idle is that high with the screw in, I'd check for a vacuum leak.

1986 VN750 27k miles
1999 EN500 32k
1983 GL650i in pieces. Someday it will ride again.
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post #9 of 11 (permalink) Old 05-06-2016, 02:44 PM Thread Starter
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Ok well if I'm on the right track today I will check for leaks and the pilot jet as well
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post #10 of 11 (permalink) Old 05-06-2016, 03:31 PM
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The pcv off the rear cyl head is right above the exhaust, check for a good seal.

Removing back pressure from the engine can cause it to go lean, adjust the screw on the side of each carb about a 1/4 turn and see what happens.


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