My '93 vn750 Vulcan dies when I open the throttle - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 36 (permalink) Old 10-07-2011, 11:03 PM Thread Starter
GreatDays
 
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My '93 vn750 Vulcan dies when I open the throttle

Hi -- I just bought a used 1993 Vulcan 750 in pretty decent cosmetic shape for the year, and 25k miles. The bike is un-modified -- 100% stock.

When I start it, it starts fine on the choke -- but when I open the throttle it dies, sometimes right away, sometimes it bogs and stays running if the choke's on and I let go of the throttle. If the choke's off the bike won't start and if I turn the choke off after the bike's warmed up it dies.

I made sure the battery is good and have flushed out the gas tank and cleaned the petcock and installed clear fuel filters just to make sure the bike's getting fuel -- it certainly is getting gas.

I pulled and cleaned the side front air screens -- they were not very dirty.
I pulled and cleaned the plugs and made sure they were getting good spark -- they are.

So it gets fuel; it does spark; it's getting air; and it actually starts right up with the choke on.

I checked to make sure the idle adjuster knob was set correctly.
To make sure the CV carbs were working -- I loosened the carb boots leading to the airbox and twisted the throttle. I noted 2 things:

1) the butterfly in each carb opens correctly
2) and with the engine running (choke on), the throttle slide does move upward when I open the throttle -- so both the butterfly and carb slides are opening correctly.

Why is this turkey doing this?
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post #2 of 36 (permalink) Old 10-07-2011, 11:42 PM
If only it had 6th gear..
 
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Sounds like you might have partially clogged jets. Where did you put your fuel filters? Not a lot of room in there.
Some have had success with loosening the bowl drains on the carbs and spraying carb cleaner up into them to back flush them.

Do you get good flow out of the petcock outlets? I think the 93 might have a PRI position on the petcock. If it does, lets run it on that for test purposes to bypass relying on the vacuum operated valve.

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)
LED tail/plate lights
Relocated helmet lock to license plate

Added stuff:
Customized hard bags w/ lights
Chrome levers
Hellcat windscreen
LED running lights
Home made 'Vita-Volt' meter
Foot rests on engine guard
Tunes

Hmm....she was on the back when we left.... ??
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post #3 of 36 (permalink) Old 10-08-2011, 12:21 AM Thread Starter
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Quote:
Originally Posted by DavesVulster View Post
Sounds like you might have partially clogged jets. Where did you put your fuel filters? Not a lot of room in there.
Some have had success with loosening the bowl drains on the carbs and spraying carb cleaner up into them to back flush them.

Do you get good flow out of the petcock outlets? I think the 93 might have a PRI position on the petcock. If it does, lets run it on that for test purposes to bypass relying on the vacuum operated valve.
Thanks for your help! Very much appreciate it.

I put the clear see-through fuel filters side-by-side, temporarily, they stick out too far for normal use but I had to do it to eliminate fuel flow as the issue -- not the issue. These see-through inline filters are the flat 'flying-saucer' looking ones -- and both fill up about the same amount with gas. I'll redo the gas lines later after I solve this problem.

The '93 750 I have has 'On/Res/Off' -- I'm very surprised -- this is the very first vacuum petcock I've seen that had an 'Off' -- it's bizarre. If the factory thought "we need an 'Off' choice on the petcock for the 1 in ten million chance the diaphragm fails to shut off the gas flow' -- if they thought that they would have just re-designed for a better vacuum-actuated petcock. The 'Off' is handled by the diaphragm -- it is redundant to have a 2nd 'Off' selection on the petcock lever. I've seen quite a few vacuum petcocks from the 'big 4' manufacturers and they ALL had 'Pri' on them.

So the vacuum is working and gas is flowing fine. I guess there's a chance the gas inlet to each carb is obstructed -- but then why would it start even with the choke on?

I will pull the 'bowls' but here's the thing -- there are no 'bowls' on these CV carbs! The bottom of the carb is just flat -- I want to do your suggestion, should I remove the bottom of the carb -- and can it be done with carbs still on the bike?

Last edited by CalifVulcan; 10-08-2011 at 12:24 AM.
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post #4 of 36 (permalink) Old 10-08-2011, 03:29 AM
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On the bottom of both carbs there should be a drain plug, it screws counterclockwise to open and drain the carbs. Do you have these?


Vulcan #1 1993 - 28K
Vulcan #2 1999 - 26k
It All Comes Down To 3 Things:
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post #5 of 36 (permalink) Old 10-08-2011, 10:38 AM
If only it had 6th gear..
 
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Yeah that's right, mine has allen head bolts that will drain. You're right, the 'bowl' is actually up higher where the float is. What you're draining or opening is the jet chamber. To get to the floats, carb removal is needed but it doesn't sound like that's your problem (unless one/both are out of adjustment, not filling enough and starving the engine for fuel.
Another question: When it bogs, does it seem like both cylinders are dying or that it's a partial loss (sputter)? Trying to determine if one carb or both are affected.

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)
LED tail/plate lights
Relocated helmet lock to license plate

Added stuff:
Customized hard bags w/ lights
Chrome levers
Hellcat windscreen
LED running lights
Home made 'Vita-Volt' meter
Foot rests on engine guard
Tunes

Hmm....she was on the back when we left.... ??
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post #6 of 36 (permalink) Old 10-08-2011, 11:56 AM
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You are not getting enough fuel to the cylinder. When you open the throttle more air flows more fuel is needed. The jets and small fuel holes are clogged. There is a small bowl where the jets are. It has two small bolts that hold it in place. It is not easy but it can be done. Remove it and spray the jet and into the jets with berryman B-12. I would also spray into the fuel lines let it set a few minutes and resarts engine. This is the best that can be done without removing carbs. You may try seafoam in the fuel tank also.

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post #7 of 36 (permalink) Old 10-08-2011, 12:06 PM
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Quote:
Originally Posted by DavesVulster View Post
I think the 93 might have a PRI position on the petcock. If it does, lets run it on that for test purposes to bypass relying on the vacuum operated valve.
The prime (PRI) position was only found on petcocks for the model years from 1985-88.
However the petcock from any year can be substituted for any other year.

Gordon

1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)

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TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
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post #8 of 36 (permalink) Old 10-08-2011, 02:02 PM
If only it had 6th gear..
 
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Quote:
Originally Posted by OlHossCanada View Post
The prime (PRI) position was only found on petcocks for the model years from 1985-88.
However the petcock from any year can be substituted for any other year.
Thanks man. I bought one for my 2000 that i found on ebay and it has the 'pri' position but I didn't know what year it came from. I sort of wish it had the off position also, for draining the carbs at storage time but I'll just pop the vacuum line off and plug it to run it dry.. or of course use the drain outs. (Always seem to forget about those).

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)
LED tail/plate lights
Relocated helmet lock to license plate

Added stuff:
Customized hard bags w/ lights
Chrome levers
Hellcat windscreen
LED running lights
Home made 'Vita-Volt' meter
Foot rests on engine guard
Tunes

Hmm....she was on the back when we left.... ??
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post #9 of 36 (permalink) Old 10-08-2011, 03:12 PM
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Before you do any mechanical how about a good dose of SeaFoam? That stuff is great and it may clean out your jets. Just a thought

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post #10 of 36 (permalink) Old 10-08-2011, 06:46 PM Thread Starter
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Thanks everyone for the great recommendations. By the way -- yesterday I had loosened and removed the clamps from the back of the carb where the rubber airbox boots connect so the carbs could get extra air, I did that because yesterday the plugs were fouled with gas. Now that didn't change anything, loosening the rubber airbox boots at the carbs. But the carbs are flowing lots of air.

I've never used SeaFoam for the gas tank so I bought a can -- I put 2 ounces in 1 gallon of gas which is twice the recommended mixture and sloshed the tank around.



I started it as usual with the choke on and let her run on the choke for awhile (5 minutes) then switched her off and cut some weeds out of my front hedgerow for 25 minute.

Came back to the bike and started her up -- no change, I have to start the bike with the choke on and as soon as I try to rev it, the engine dies, and likewise if I try to turn the choke off it dies (and I have checked that the idle adjust is set OK).

So I put 2 more ounces of Sea Foam in the tank and sloshed it around, and started it up on the choke and let it run (it runs around 1200 to 1500 rpm on the choke) this time for 10 minutes. I wanted the Sea Foam to work into the carbs.

At the end of the 10 minutes all of a sudden the RPMs suddenly started to climb!

Good ole' Sea Foam apparently!

So the revs kept climbing and I went to the bike and NOW she responds correctly to the throttle -- when I open the throttle she revs UP and doesn't die!

Okay so after 5 minutes of "could this be true, lets try revving it again, yep it still revs!" joyfulness I got bold and started to take the choke off -- the motor stayed running!

So finally after 4x the recommended Sea Foam concentration the bike's revving at idle with NO CHOKE ON. Now the rubber airbox boots are still loose so the motor's pulling lots of air still.

I drove the bike around the block tears in his eyes I guess with the choke off at least at the beginning. To keep it running around the block sometimes I had to turn the choke on though.


Only running on 1 cylinder consistently.
Back home I pulled the spark plugs.

The front plugs are still fouled with gas. The rear plugs look great -- dry and tan-colored.

So I put the clamps back on to attach the rubber airbox boots to the carbs.
I started her up -- now with less air getting into the carbs (rubber boots are back on and tight) the bike won't start without the choke and dies if I take the choke off.

Any suggestions? I feel great about the Sea Foam, thanks dudes there is a light at the end of the tunnel
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