Hesitation with low throttle at cruising speeds - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 17 (permalink) Old 05-04-2020, 10:41 PM Thread Starter
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Hesitation with low throttle at cruising speeds

Hello everyone,

Bought a 2003 750 about two weeks ago for a good price and I've been doing some small things to get it up to spec (as much spec as I can personally get it to at least). One thing that's been the focus of a lot of my attention is at low throttle, it has issues cruising at constant speed. This is particularly noticeable at speeds of 20-50 miles per hour. It'll move fine, but hesitate once in a while, pretty similar to what's described when the carb vent hose isn't going into the right ear or its bottoming out in there. Jerking a bit, hesitating once in a while, almost seems like it's starving for fuel or something for a split second and you can hear the engine skip or whatever when it hesitates. If I open up the throttle, it kinda goes away for a little bit, but will work its way back in. The bike pulls hard and has no trouble accelerating all the way through the RPM range. Things I've done to remedy this...

Cut the carb vent hose at 45 deg angle and seated it inside the air cleaner box. I've run 3-4 tanks of gas with seafoam through it. This comes out to less than one can total so far, but the can is almost empty. I cleaned the foam air filters, I sealed the air filter housing with duct tape. I checked the carbs and they were sync'd up perfectly. The decel cable was stuck so I removed it, and noticed that my accelerator cable has stretched out over time and there was so much slack that it was almost forming a loop at the carb end where it attaches. I pulled it, re-routed it as best as I could, and even with the adjusters all the way out, there's still a bit of slack at the carb end with the throttle cable. I have a mild suspicion this is it, being that that slack seems to influence the throttle response a bit. I don't get much loud "backfiring" although it has done that one or two times since I got it. It does, however, pop a lot when I'm decelerating and it makes it kinda sound like a harley. Doesn't sound like it's a problem but there's a lot of people that complain about popping and stuff on here so I can't tell if it's normal or not.

Should I be adding more seafoam at a higher seafoam:gasoline ratio? Any chance this will help clear it up? The bike does seem to be running smoother and smoother as I've done these tasks, just seems like that low throttle hesitation continues. It almost seems like it doesn't even do it all the time, as I've told myself several times, "I think it's fixed!" only to be disappointed. Stuck something in the carb? Engine messed up? No clue here!

I've got more guts than experience when it comes to motorcycle maintenance, but really don't want to pull the carbs if I don't have to, as this seems like a PITA and difficult based off other posts I've read. The bike is also old and I'm pretty sure I would destroy 50 parts in the process of removing the carbs lol. I put on a throttle cable from motionpro and that thing honestly seemed like it was a foot too long. I routed it all over the bike and it still had too much slack, therefore, I've ordered a different one that I'm hoping is the correct length.

Anyways, sorry to be spitting out words like a fire hose here, just want to have as many details as possible for whomever is reading.
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post #2 of 17 (permalink) Old 05-05-2020, 02:14 AM
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Low rpm hesitant like a dirty pilot jet?

seafoasm on the can, 1 or 1.5 oz per gallon. kepp running it in/out of the rpm where it hesitates

use choke if you neeed to ... to keep it powering/pulling

sorry fore typ-os... more later... got to go9

Muight ghave to go into the carbs anyway, dirty
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post #3 of 17 (permalink) Old 05-05-2020, 02:53 AM Thread Starter
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It doesn't seem to be related so much to RPM as it does to the amount of throttle input. If it's low throttle, it'll start to hesitate and surge, but when I open up the throttle or decel, everything is fine (outside of the decel popping). I feel like I can almost reset it by blipping the throttle, but after a little bit, the hesitation/missing comes back. This only happens at steady throttle.

As far as pilot jets and main jets and jets in general, I have no idea! I've had motorcycles in the past but have only done work on the chassis, never got into the motor or carbs or anything. Any chance it's a spark plug issue?
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post #4 of 17 (permalink) Old 05-05-2020, 11:50 AM
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Sounding more like a dirty, partially clogged pilot jet. Maybe even the right side carb, the one that dries up when it sits a week on the sidestand.

Does it act up when you're turning a slow street corner, big, herky jerky, almost knocks you down, then explodes with power once you get some rpm?

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post #5 of 17 (permalink) Old 05-05-2020, 11:53 AM
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Bog, not big, in post above. Oh no, I m cellphone spell check guy.

Karma has me by the jimmie leg.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
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post #6 of 17 (permalink) Old 05-05-2020, 12:51 PM Thread Starter
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It does fine in tight corners. It’s a very subtle problem. If someone were to take it for a short ride and drive it like they stole it, they would think the bike is perfect. Just that subtle hesitation at constant low throttle. Think I may try some Berrymans B12 and cross my finger!
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post #7 of 17 (permalink) Old 05-05-2020, 02:18 PM
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Maybe a leaky slide diaphragm?

Inconsistent flow from the gas tank?

I'm might not be reading the symptoms right.

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__________________________________________________ ____________
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post #8 of 17 (permalink) Old 05-05-2020, 02:24 PM
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Make sure the petcock vacuum hose is dry after running, or even check before start.

Probably okay right now, but always possible every ride.

Just trying to think of air or fuel feeding from the wrong route or lacking enough.

Squirrel nest in the airbox? Filter foam fell apart!

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__________________________________________________ ____________
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post #9 of 17 (permalink) Old 05-05-2020, 02:55 PM Thread Starter
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Dammit, it's annoying how fast I get "timed-out" on this site. I just wrote a lengthy response and it got deleted. Oh well, small price to pay for the immense knowledge and experience that is accessible here.

I attempted to sync the carbs yesterday after a 40 mile ride (no adjustment required) and the petcock hose looked dry when I pulled it off the carb. I pulled the foam filters and cleaned w/ soapy water and used the K&N recharge kit as recommended.

I was checking the plugs yesterday, but I don't have a long enough removal tool and could only get the spark plug on the front cylinder on the right side off with a wrench (pic included). I visually inspected the other three plugs from the outside and didn't see any oil in the wells, but interestingly, there seemed to be some kind of fluid on the tip of the front left spark plug (the part that inserts into the spark plug cable) and a little bit inside the spark plug cable where it contacts the plug. The spark plug well itself was dry, so if the fluid was coming from inside the engine, it would have to travel through the inside of the spark plug (is this even possible?). PO mentioned he changed the spark plugs recently, maybe he got some anti-seize on the ends where it plugs into the spark plug cable? Either way, I did my best to dry off the plug with a shop towel and then used a q-tip to clean out the plug itself the best I could. Think this could be it?

Will get some Berryman's B-12 later today. I heard it's a bit harsh, but if it ends up blowing my carb gaskets, I would end up pulling them like I would have to anyways and just use a carb rebuild kit, like I would have anyways, so I figure no harm done?

Thanks for all your replies!

ps- also tried riding w/ petcock on reserve, rode the same
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Last edited by Amir Razai; 05-05-2020 at 02:59 PM.
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post #10 of 17 (permalink) Old 05-05-2020, 05:15 PM
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What would the wet petcock hose indicate, besides the petcock was letting fuel get through the front side?

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