Vacuum leak or something else? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 5 (permalink) Old 02-24-2020, 06:16 PM Thread Starter
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Question Vacuum leak or something else?

So the weather hit 50 where I was so I decided to give my bike a test run down the street after getting it running. One thing I've noticed is when revved up, the RPMs hang and take a while to go down. The idle is also very spontaneous, especially so when the bike is leaned over on the side stand. Sometimes when starting, the RPMs will shoot to 5k and won't go down unless I crank the idle screw all the way out. on my ride, the bike would instantly lose 90% of its power at around 5k or so and be very jerky. Sounds like an obvious vacuum leak, right?

Well, the odd thing is that spraying around the carbs with carb cleaner yields very little results. The RPMs might increase by around 500, but I also have the intake boots off that goes to the black airbox so it could just be leaking through the intake. But when I spray near the top of the carbs where the diaphragm sits, the RPMs actually drop around 500, almost instantly.

Why is it running so bad? I've had the carbs off 4 times now and it's growing old. There's also a lot of gas coming out of the holes at the front of the rear carb intake right where the slide is. (The big opening you can look into that you would put pod filters on if doing the ear shave.) This gas is going straight into the intake, would a bad float needle cause this? are the carbs in very bad need of being synced?

Thanks for any help.
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post #2 of 5 (permalink) Old 02-24-2020, 06:43 PM
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Diaphragms might not be seated right. There's a sort of tab that has to go into the right spot, a notch. Diaphragms could also be cracked, you can hold them in front of bright light and flex them to check for cracks. Should be no leaking at the diaphragm cover to let the spray in. Don't mix up the slides, the needles on those are different sizes front-rear.

Spraying near open carb intakes can give false results, but a hanging rpm is one sign of an air leak. There are marks on the carb boots for alignment, they need to be fully seated.

If the float is sticking, or the float needle leaking, the air leak will use that extra fuel to make more rpm. You shouldn't be seeing any gas, so sticky float or leaky float valve is my first guess, it's overfilling and the gas is gushing out of the ports.

These carbs usually don't work well with the intake ports open, need the airbox hooked up or pods on there.

In short, sounds like it's flooding, and also has air leaks, along with possible problems with the diaphragms.

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 #3 of 5 (permalink) Old 02-24-2020, 09:13 PM Thread Starter
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Originally Posted by Spockster View Post
Diaphragms might not be seated right. There's a sort of tab that has to go into the right spot, a notch. Diaphragms could also be cracked, you can hold them in front of bright light and flex them to check for cracks. Should be no leaking at the diaphragm cover to let the spray in. Don't mix up the slides, the needles on those are different sizes front-rear.

Spraying near open carb intakes can give false results, but a hanging rpm is one sign of an air leak. There are marks on the carb boots for alignment, they need to be fully seated.

If the float is sticking, or the float needle leaking, the air leak will use that extra fuel to make more rpm. You shouldn't be seeing any gas, so sticky float or leaky float valve is my first guess, it's overfilling and the gas is gushing out of the ports.

These carbs usually don't work well with the intake ports open, need the airbox hooked up or pods on there.

In short, sounds like it's flooding, and also has air leaks, along with possible problems with the diaphragms.
So I should get a new float needle, check diaphragms, and use the propane method to check for leaks. Got it, thank you!
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post #4 of 5 (permalink) Old 02-25-2020, 09:39 AM
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Quote:
Originally Posted by rerb View Post
So I should get a new float needle, check diaphragms, and use the propane method to check for leaks. Got it, thank you!
Also make sure the carb boots are seated well.

Sometimes it's just a chunk of dirt stuck in the float needle, I've seen some really high prices on new ones.

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
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post #5 of 5 (permalink) Old 02-28-2020, 01:46 PM Thread Starter
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Quote:
Originally Posted by Spockster View Post
Quote:
Originally Posted by rerb View Post
So I should get a new float needle, check diaphragms, and use the propane method to check for leaks. Got it, thank you!
Also make sure the carb boots are seated well.

Sometimes it's just a chunk of dirt stuck in the float needle, I've seen some really high prices on new ones.
Well, I think I found the issue with the diaphragm. Part of it was peeled back when I took the lid off.
On a side note, I had to remove the slide piston to get to the needle since I'm doing the ear shave anyways. I've heard you shouldn't do this, is there anything I could have messed up by taking it out?
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