Mysterious rev climbing - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 8 (permalink) Old 06-02-2019, 01:41 PM Thread Starter
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Mysterious rev climbing

Hey guys, please help me solve this mystery...

So less than 2 weeks ago the bike was running fine. I was commuting on the interstate, everything was shifting smooth, sometimes it would jerk upon take off but I'm also new to motorcycling so I thought maybe that was me, maybe I need to practice smoother on the clutch. I had already taken the carbs apart twice, first to clean them from the previous owner, then the second time because an o-ring got pinched and was having vacuum problems. I bench synched them the last time I put them in, but I had to be Mr Perfect and borrow my friends carb sync tool... and that's where the trouble started.

It's one of the tools with the gauges and hoses, not a cheap blue fluid kind. I plugged it into the vacuum ports on the carb and turned it on. They weren't perfectly in sync, so i figured I wanted to adjust, but I didn't have a wrench that could reach. It ran for a couple minutes with the carb sync tool attached while i tried to reach, but couldn't. There's a motorcycle collective/shop a mile away, so I figured why not ask to borrow a wrench. I plug back in the hoses appropriately and take off.

Immediately the throttle feels weird/jerky/different. I make it half a mile and pull over, take a look at it. Maybe I bumped the screw trying to reach it and made a more out of sync? Well I was only a half mile from the shop so I hop back on and continue. As I pull into the shop, my engine revs through the roof. Goes up to about 6k, so I pull in the clutch and shift into neutral as I park, it stays at 6k and didn't shift into neutral easily, but after 5 or 10 seconds I get it into neutral and cut the engine. White smoke comes out of the right-hand exhaust (attached to front cylinder).

I go inside and ask someone who's working on their bike for advice. We both go out and take a look at it, I start it up and it idles completely fine. I borrow a wrench from him and he goes back inside, I plug in the carb sync tool again and then it starts revving weird. So now I'm like **** this sync tool, I return his wrench we talk about it and he suggests it's probably an air leak, but I start again and it's idling fine again. So I go to ride it home, and before I'm even a 1/4 mile away it revs up to 5 or 6k again, but only when the clutch is pulled in. When I let the clutch out and it has weight on it, it goes back down to 3k, but of course that means it shoots my speed way up.

So I pull over, and 2 other riders pull over to help. I let it cool down, talk to them about it. We all agree it sounds like an air leak, after we let it cool down, we start it up again and surprise it idles completely fine. Revs fine while we're parked, nothing's acting weird. I did notice some smoke coming out of the coasting enrichener though.

I climb on, take it home, as soon as I'm driving it again it revs through the roof. So I rode it home like that, cutting the engine off for long stretches downhill, only starting the bike when I need to go uphill, cause I didn't want it to stay at 6k and lean out.

That was a week ago. Today I go outside and open the coasting enrichener, that little tiny o-ring is missing on one side. (swear I put it in there though, but maybe I was wrong). I have extras lying around so I put it in. Start it up and I can hear a hissing from the opposite enrichener, replaced that little o-ring as well, the hissing stops. Well I guess I solved the air leak. I crank it up and it sounds great, much better than 5 minutes ago. It idles for 5 minutes completely great, I put on gear just to take it around the block, I hop on, and the bike shifts backwards about a foot as I put the kickstand up, and before I even shift out of neutral IT REVS UP TO 4K OR 5K AGAIN!!!!

I'm at wits end with this thing, I try starting a couple more times, always revs after a minute or two of running fine. I sprayed it with starter fluid to try and find an air leak maybe on the boot or the carb.... nothing. It didn't change the revs. And then when I just cut it off the white smoke came out of the right exhaust again.


2 other things I noticed that don't seem related but could be: There's a really slow leak from my radiator, small puddle in the garage, but the levels seem fine. And there's some oil leaking out of the left-hand exhaust where it attaches to the cylinder.

Also the day I tried to sync the carbs and all this **** started was one of the first really hot days we've had in Georgia, definitely the first hot day that I tried to ride the bike.... Let me know what you guys think or if you know anyone that wants to buy this thing.
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post #2 of 8 (permalink) Old 06-02-2019, 03:31 PM
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You have an air leak in the carb boots to the cylinder head, bad one too by the sound of it

'99 Vulcan "Victoria"
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post #3 of 8 (permalink) Old 06-02-2019, 04:04 PM
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Quote:
Originally Posted by TonyO View Post
You have an air leak in the carb boots to the cylinder head, bad one too by the sound of it
Sounds like it, and the leak is worse after it heats up. Spritz some water from a spray bottle around the boots while it idling fast, listen, and watch for steam at the exhausts.

On 2cyls with CV carbs, I never synch the carbs with a manometer. I just make sure both butterflies are opening at the same time, visually. ... but mine is earshaved, so that's easier to see.

You should also check the petcock vacuum line at the left carb after it's warmed up. Make sure it's dry and not wet with fuel. The air leak at the boot may be making use of the extra fuel through the vacuum line, seems like your rpm is about 2000 high for just a vac leak by itself. Could be the source of the slobbers at the exhaust manifold.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
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post #4 of 8 (permalink) Old 06-03-2019, 04:13 PM
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I synced my carbs on the bike using a pair of vacuum gauges. I couldn't reach the adjustment screw either. However, I found if I shut off the bike and pull the throttle fully open, I can easily reach the screw for adjustment. So I made an adjustment, started it up and checked it. Shut it off, made an adjustment, started it up and checked it. After about 3 or 4 tries, it was perfect.

Make sure you fix the air leak first, or your synch will be WAY off.


'89 Vulcan 750 Bought 4/12/18 @ 17805.9 miles $800
Purchased stock. Wrecked once
22490.0+ miles

Completed modifications:
Removed mufflers, straight pipes out of GB
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Pick-up coil re-gapped to .020"
Compufire 55402 r/r series regulator
USB Outlet + Voltmeter
Repainted Metallic Blue w/ new tank bages
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post #5 of 8 (permalink) Old 06-03-2019, 07:26 PM Thread Starter
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Thank you everyone for the swift responses. I was reading up on different problems with white smoke and I had convinced myself that I must have blown a head gasket. Good to hear y'all just think it's an air leak. I'll try and fix that this weekend and see if that takes care of it. Also thanks for the tips on carb syncing!
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post #6 of 8 (permalink) Old 06-03-2019, 07:39 PM
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I misread the part about the smoke, thought it was close to the head.

If it does it again, see if it smells like gas or antifreeze. When these carbs have problems, they can dump enough gas to cause vapor at the exhaust.

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 #7 of 8 (permalink) Old 06-14-2019, 02:33 PM Thread Starter
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Hey! So I've been working a lot and I've only had a chance to look at the bike in the evenings. I tightened up the boot between the carb and the front cylinder and tightened up the boots some more, and I'm pretty sure I've fixed the air leak. I test rode it about a half mile and let it idle for a little bit, and it didn't rev on its own. But when I got back it was starting to smoke a little again, specifically from the exhaust manifold on the rear cylinder; only a little but I only rode like a half mile or a mile.

I adjusted the air-fuel mixture screw and set it back to stock, but didn't crank it yet. Pretty sure the previous owner had it at 3.5 turns? And I set it back to 2.5.

I THINK the smoke is burning oil? It might be a long shot, but I think the oil level is a little high and causing too much oil pressure? This weekend I'll get a syringe and grab some oil out of it and see if that helps, along with setting the air-fuel mixture back to stock. Maybe the exhaust manifold gasket isn't sealing correctly and causing problems tuning the carb? Any thoughts?
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post #8 of 8 (permalink) Old 06-14-2019, 03:11 PM
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Syringe probably won't help remove the oil, if I recall, the rotor is directly under the oil cap. Maybe some tubing on the syringe would bypass the rotor enough, but I'm not sure you'll access enough oil to make a difference. If you're able to get some out that way, restarting the engine would refill the area so more can be drawn out.

Loosening the oil filter and letting some oil drain will work, might have to bump the starter to get more if needed. You may not see a change in the sight glass until the engine is started briefly and shutdown again, refilling the oil filter.

Often, working around an engine gets oil onto the manifolds and causes some smoke, Depends how bad it is and how persistent. Just not sure of the cause at this point, shouldn't be the oil level or oil pressure. Any chance it's coolant? Gasket leak, valve cover, etc? If the cylinder was pumping oil, it should also have smoke from the exhaust tips.

If the engine warmed up enough when you ran it, sounds like you fixed the air leak.

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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