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post #1 of 9 (permalink) Old 01-05-2010, 08:50 PM Thread Starter
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86 750 problems

hello. i got a 86 vulcan 750. this is my first vulcan also have a 76 kz900 a4 .the 750 is giving me problems that im having a hard time sorting out.when i start the bike it only fires on the front cylinder but when i bring it to 5000rpm the rear fires up.the when rpm`s drop back the rear cylinder dies again.checked coils res good,checked ignitor res good according to book #`s. took off stator cover to check timing, started bike and oil went everywhere. is that right or not. need help. bacon
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post #2 of 9 (permalink) Old 01-05-2010, 08:59 PM
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The oil going everywhere with the outer stator cover off is "right" - not pleasant, but it'll happen. Did you check the resistance on the pick-up coils? Check the book (electrical) -it gives good instructions on testing those.

Welcome to the Forum!

C
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post #3 of 9 (permalink) Old 01-05-2010, 09:30 PM
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Hey Bacon..welcome to our world!

My '03 did exactly the same thing earlier this year. It turned out that the "air cut off valve" cover on that carburetor had worked loose and allowed the valve diaphram to slip out of place. That caused a lowered vacuum which prevented that carb to draw fuel at low RPM. The gas will not flow from the tank without that vacuum. At higher RPM, the vacuum is increased enough to draw the fuel. The covers on the carbs are easily accessible with the carbs in place. The right carb runs the rear cylinder. The cover is about 2" in diameter with two small mounting screws. Be carfull as there is a long spring and a small o-ring that may fall out. Don't worry, no signifacant amount of gas will come out. The cut off valve is the rubber diaphram with the metal plunger. Pull it out and make sure there are no cracks or tears. Also inpsect the spring and o-ring. Don't forget to check the vacuum line from the carb to the fuel tank petcock.

When reinstalling the cover screws, use a mild thread locker and you won't feel the need to over tighten them.

The air cut off valve closes when you release the throttle creating a richer mixture in the carbs. When I replace it, the infamous deceleration "popping" was also cured! BONUS!

Crobins is right. The stator relies on oil to keep it cool (although insuficient) so it will definately come out with the cover off and engine running.

Good Luck!

Marty

2003
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Last edited by IronMan750; 01-05-2010 at 09:33 PM.
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post #4 of 9 (permalink) Old 01-06-2010, 02:09 AM
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I agree, the carb vacuum thing is the most likely cause. If you are new to the Vulcan 750, it has a number of peculiarities. The very first thing I would do after getting the engine running right, is to check and lube the rear driveshaft splines. Hopefully they are still ok.

Also keep an ear out for cam chain noise, as the tensioners tend to fail, and for noise from the left front of the engine, because the rubber dampers in the balancer assembly tend to fail after several years, allowing the balancer to dig into the case, causing a lot of damage.

When you change the oil, use the drain on the left dide of the engine, not the one on the bottom, and check the screen for any signs of debris.

Just so you know, if the stator fails, you have to pull the engine at least part of the way out to replace it. This isn't actually difficult, but very time consuming.

The VN750 is a great bike when everything works right, but I found out a long time ago that you really can't compare it to anything else. It's definitely one of a kind. Jerry.

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 01-06-2010, 02:28 AM
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Quote:
Originally Posted by bacon View Post
hello. i got a 86 vulcan 750. this is my first vulcan also have a 76 kz900 a4 .the 750 is giving me problems that im having a hard time sorting out.when i start the bike it only fires on the front cylinder but when i bring it to 5000rpm the rear fires up.the when rpm`s drop back the rear cylinder dies again.checked coils res good,checked ignitor res good according to book #`s. took off stator cover to check timing, started bike and oil went everywhere. is that right or not. need help. bacon
Sounds like you have a clogged pilot jet in the rear carb, carbs may need to come off and cleaned. If you are getting fuel to the front cylinder then the vacuum to the fuel petcock is working.


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Last edited by lance328; 01-06-2010 at 02:37 AM.
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post #6 of 9 (permalink) Old 01-06-2010, 02:36 AM
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I agree. I was about to edit my post to say that, but you beat me to it. On an '86, I would definitely remove the carbs and give them a good cleaning. They are bound to be dirty after all these years, unless someone recently cleaned them. I would think that if it were the pilot jet, it would start firing well before 5000 RPM, BUT, I admit I am not that familiar with the VN750 carbs. Jerry.

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 #7 of 9 (permalink) Old 01-06-2010, 05:22 PM
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Welcome to the madness... sorry no advice...its all been said
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post #8 of 9 (permalink) Old 01-08-2010, 03:27 PM Thread Starter
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thanks guy`s will be in touch
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post #9 of 9 (permalink) Old 01-09-2010, 03:26 PM
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Welcome to the Vulcan madness. Since you have the bacon, I`ll bring the eggs and hash browns over for breakfast. LOL. Opps, you will have to fill out your profile first so we know where you are. You have already discovered that the family here at the Vulcan house will be happy to help whenever we can. Enjoy the cruising here and and on the vn.

Gordon

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

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Kury Offset Hiway pegs
July 13, 2016, Riding on the DARKSIDE now, Classic Radial 165/80-15


TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
https://www.vn750.com/forum/11-vn750-general-discussion/9127-top-ten-items-you-would-suggest-new-owner-do-his-new-ride.html
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