Motor refuses to go above 4000 rpm. - Wrist Twisters
 
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post #1 of 18 Old 03-28-2014, 01:08 PM Thread Starter
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Exclamation Motor refuses to go above 4000 rpm.

My name is Daniel and I've been rebuilding a bike. The best way to learn about a bike is to rebuild something that is broken. This is also my first bike.

So to the point, I have finished rebuilding my carbuerator, replacing the float, jets, etc... today I started to adjust the air fuel ratio on my bike, there are two screws since it is a dual carb. After putting it to manufacture's specs, the bike starts up right away, no problem. But when I test the throttle to check for any backfiring at higher RPMs, it will reach 4000RPMs, then die off to 2000RPMs, and move back up to 4000RPMs. When I reach the 4000 point, I keep the throttle constant and unchanging, but the RPMs fluctuate consistantly.

1000
2000
3000
4000
2000
3000
4000
2000
3000
etc.

I am looking for suggestions on what it could be.

I ride a 2002 kawasaki vulcan 750. (dual carb)

Thank you for all your help.

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post #2 of 18 Old 03-28-2014, 01:17 PM Thread Starter
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I was thinking that when I adjusted my air fuel ratio, that the two carbs are not properly synced up. Opinions?

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post #3 of 18 Old 03-28-2014, 01:21 PM
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Sync issues, from my little bit of experience, should not be a big factor in the higher RPMs. It sounds almost like the fuel supply isn't keeping up with the demand. Did you set the float height to the factory setting?

Edit: also check any fuel filters/screens to see if they are blocked.

'96 DR650, '01 SV650, '10 Aprilia Tuono 1000R
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post #4 of 18 Old 03-28-2014, 03:25 PM
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I have a small leak in my petcock after looking everything over. Would that cause it to have this problem?

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post #5 of 18 Old 03-28-2014, 04:40 PM
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Vacuum leak? Certainly.

if you love your motorcycle, set it free.. if it comes back and hits you.. you highsided
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post #6 of 18 Old 03-28-2014, 07:48 PM
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Yeah you want to check all your rubber lines running to the carb. Any that are cracked could be a vacuum leak, and obviously the fuel lines would probably be leaking so hopefully there are no issues there. Also, you might want to carefully check that the diaphragms in each carb are seated properly (or have no rips in them). Sometimes when they are being reassembled they tend to not be in the little grooves under the top plate and it will create havoc with acceleration similar to what you are describing.

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post #7 of 18 Old 03-28-2014, 08:30 PM
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Sounds like your ignition coils from my work on older bikes. Check for cracks or crimped loose wires. While you are at it, check plugs and coil wires and last, do a ohm test on your charging system.

I may not have a lot to say but it doesn't mean I don't listen.
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post #8 of 18 Old 03-28-2014, 08:31 PM
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As i might suspect, google turned up that very idea too

http://www.vn750.com/forum/showthread.php?t=75

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post #9 of 18 Old 03-28-2014, 10:33 PM
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Maybe worth double checking for debris in the main jets too.

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post #10 of 18 Old 03-29-2014, 06:37 AM
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I finished sealing up the small vacuum leak last night on my petcock, and I'm testing it out later today. I will let you guys know. I appreciate all of your advice. I will check my coils if there isn't any change. I will keep you guys updated. Thank you again.

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post #11 of 18 Old 03-29-2014, 11:55 AM
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I found two wires leading off the battery that are not connected to anything. And I can not find the other end. Does anyone know where these wires are supposed to lead?

ForumRunner_20140329_145344.jpg



ForumRunner_20140329_145327.jpg

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post #12 of 18 Old 03-29-2014, 11:56 AM
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I also did check the wires to the ignition coil and everything seems to be in place.

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post #13 of 18 Old 03-29-2014, 11:56 AM
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Quote:
Originally Posted by Hindenburg View Post
I found two wires leading off the battery that are not connected to anything. And I can not find the other end. Does anyone know where these wires are supposed to lead?

Attachment 54577



Attachment 54569
That's a quick connect for a battery charger.

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post #14 of 18 Old 03-29-2014, 11:56 AM
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Wonderful. Thank you.

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post #15 of 18 Old 03-29-2014, 12:03 PM
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Can you post a little more back ground history on the bike and yourself

- was it running at all before the carb rebuild
- have you rebuild carbs before
- do you have a FSM (factory service manual)
- what is the RPM range the bike should be able to reach?

You also mentioned a vacuum leak at the petcock, so it has a vacuum actuated shut off built into the petcock .... if the internal diaphragm is leaking it will be swallowing fuel into that cylinder while running and messing up the Air Fuel ratio big time.

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post #16 of 18 Old 03-29-2014, 12:08 PM
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Here is a chart that helps explain the basics of how the different carb circuits work together in relation to throttle position.



Also do you have the factory airbox reinstalled 100%? It can make a very big difference ..... to the point where some older CV setups won't even run beyond idle without them.

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post #17 of 18 Old 03-29-2014, 12:40 PM
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It was not running at all before the carb rebuild. I have experience rebuilding carbs. I do have a service manual but doesn't have any solutions pertaining to this. And the RPM range is 11000 but red lines at 8500.

After reinstalling the carb, I've adjusted the air fuel ratio and has a pretty smooth idle. But it won't reach above 4500 rpms.

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post #18 of 18 Old 03-29-2014, 02:10 PM
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As superdog mentioned, it could be fuel starvation. How clean is the inside of the fuel tank? Older bikes often have some rust in the tank that will clog the filter screen in the tank or petcock. Enough gas will trickle out to idle and run at low speed but as the demand increases, it can't keep up.

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