Project bike carb sync and bogging at high rpm - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 10 (permalink) Old 06-25-2018, 10:09 AM Thread Starter
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Project bike carb sync and bogging at high rpm

Basically the title. I bought a bike from a guy that had it sitting for a year and a half or 2 years. It started but only with choke all the way on. So I took the carb off and cleaned it. Put it back on and adjusted idle. It starts and idles fine now but I have two issues.

1. At high high rpm when I rev it in nuetral, and it gets to around 4k RPM it bogs and dips back down to around 2500 or so and I have to let off the gas. Is this out of sync carbs or what? It's almost like it misfires and has to recover

2. Lastly I hear a knocking in the engine. I'll try and record it sometime but if I heard it on a car I would think it was bad timing or something in the piston assembly. I'm running seafoam through the gas and oil right now.

Any advice would help a ton. It's a 2004 also.
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post #2 of 10 (permalink) Old 06-25-2018, 10:34 AM
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Did you have the slides removed? Maybe the diaphragm(s) are out of place, not sealing. Or they're cracked? If they can't hold vacuum, they can't lift properly as the throttle is opened. The needles are different front/rear, so you don't want to mix them up.

Can check slide operation by lifting then dropping the slides, should drop back down with some resistance and a woosh sound.

Most often, the knock or tap noise is a loose ACCT. Should be able to feel it on the body of the ACCT. (timing chain tensioner) Look up MCCT here, it's the most popular, longest lasting fix.

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post #3 of 10 (permalink) Old 06-25-2018, 12:05 PM Thread Starter
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I only removed the slides to check the diaphragm seal and it was good. I put them back in and they sound good. Is it easy to replace them and get parts if they need to be? Can I check them without removing the carb again?
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post #4 of 10 (permalink) Old 06-25-2018, 12:13 PM
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Originally Posted by kyaustad View Post
I only removed the slides to check the diaphragm seal and it was good. I put them back in and they sound good. Is it easy to replace them and get parts if they need to be? Can I check them without removing the carb again?
Not sure, but you may be able to get the top plates off with the carbs in place, then the assemblies will slide up and out.

Parts were available last time I looked, but seemed pricey to me. Slide and diaphragm is all one piece.

Many people have had the rubber slightly out of place and not sealing after carb service.

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__________________________________________________ ____________
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post #5 of 10 (permalink) Old 06-25-2018, 01:47 PM Thread Starter
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Awesome thanks. I’ll have to look into that after work. Also, the hoses that come off the center top of the carb into a T and then one hose goes to where? The right air filter right?

Last edited by kyaustad; 06-25-2018 at 02:02 PM.
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post #6 of 10 (permalink) Old 06-25-2018, 02:47 PM
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Quote:
Originally Posted by kyaustad View Post
Awesome thanks. I’ll have to look into that after work. Also, the hoses that come off the center top of the carb into a T and then one hose goes to where? The right air filter right?
correct, if your bike is stock that is where it goes. it is a vent line, so it does not get plugged with anything, but is placed on that right air filter housing (also commonly called an "ear" here). If you modify the bike and remove the stock air box and filter housings, you can tuck that tube into the frame so it can not get hit by the wind. if wind hits it, your bike will act like it is running out of fuel.

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post #7 of 10 (permalink) Old 06-25-2018, 02:49 PM
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Quote:
Originally Posted by kyaustad View Post
Awesome thanks. I’ll have to look into that after work. Also, the hoses that come off the center top of the carb into a T and then one hose goes to where? The right air filter right?
Yep, carb vents, there's a hole for the hose in the back of either filter housing. End of the hose needs to be in dead air space, if it's not, that could easily be the bogging problem you're having, very common problem.


edit: It's bogging out in neutral, so it may not be the vents.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf

Last edited by Spockster; 06-25-2018 at 02:52 PM.
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post #8 of 10 (permalink) Old 06-25-2018, 02:53 PM Thread Starter
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Quote:
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Yep, carb vents, there's a hole for the hose in the back of either filter housing. End of the hose needs to be in dead air space, if it's not, that could easily be the bogging problem you're having, very common problem.


edit: It's bogging out in neutral, so it may not be the vents.
Good to check anyway. I’ll make sure that is in a filter housing. Thanks!
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post #9 of 10 (permalink) Old 06-25-2018, 03:40 PM
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Deleted - Pic didn't work.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
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Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #10 of 10 (permalink) Old 07-07-2018, 02:03 AM
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It sounds very much like what I was dealing with as far as bogging down at about 4000 rpm. Do you by any chance have inline fuel filters on the lines? I did, on recommendations from TonyO, OleDirtyDoc, GoofyFoot2001 and Spockster, removed them to just straight fuel lines. Problem solved, runs like a champ.

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Last edited by bullwinkle; 07-07-2018 at 02:07 AM.
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