engine sputters at speed. - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
Carbs and Fuel System
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post #1 of 12 (permalink) Old 08-02-2007, 12:39 AM Thread Starter
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engine sputters at speed.

Just bought this bike. 6100 miles. starts fine, idles fine, runs real well around town (under 45 mph). Plenty of torque at bottom end. Occasional backfiring when off throttle and coasting in gear to a stop. Get up to highway speeds (55-70) and it starts sputtering like it is running on watery gas. I have filled it with 93 octane twice and added gas line dryer. I am going to check plugs next. Any other thoughts?
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post #2 of 12 (permalink) Old 08-02-2007, 08:02 AM
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A common problem is that the vent hose for the carbs gets stuck up against the inside of the air filter housing. Look on the right side (sitting) of your bike for a small (1/4") hose that's coming from the carbs and sitting loosely in the housing. If it's not sitting loosely, that may be your problem.

Is it just sputtering, or are you having trouble even getting the bike up to higher speeds?

C
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post #3 of 12 (permalink) Old 08-02-2007, 06:10 PM Thread Starter
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Thanks for the suggestion If I grab a handful, it responds just dandy to accelerate to speed. I am going to try opening the gas cap while I ride. If that smooths the engine out, then I will re-do all vent lines. I would not be surprised if the carbs do however, need a good cleaning. This bike must have sat for a while. 6100 miles in 16 years?
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post #4 of 12 (permalink) Old 08-03-2007, 01:00 AM
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Welcome to the site. Always nice to see new members.

Forget the high octane stuff. The 750 Vulcan was designed to work just fine on the low octane. Infact, the owners manual recommends the lower octane.
And instead of a gas drier additive, try to find some Seafoam. It's available at alot of auto supply stores anymore. It works wonders on getting the carbs cleaned out. A double or even triple shot (normal is 1 oz per gallon) wouldn't hurt. It's sometime called the 'miracle in a can' around here.

The popping on decel is kinda normal. I'm one of those that kinda like the Brrrr pop, pop, pop, Brrrrrr, pop, pop.
There are some carb adjustments that can help reduce it, or what's referred to as 'marbling'... adding a couple marbles to the lines to the coasters (the coasters direct unburned gas into the exhaust to reduce emissions)

The hose Cindy mentioned is also a common problem. It's a good idea to cut the end of the hose at an angle to stop it from bottoming out.

Also, how are the air filters? Dirty ones can cause issues.


AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC


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post #5 of 12 (permalink) Old 08-16-2007, 08:54 PM Thread Starter
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still sputtering

Okay, I found the vent hose and it was directed back near the battery. I managed to get it into the right ear, slant cut end. I haven't tried it again yet. I am wondering about the vent on my tank. There is a single tube near the seat with no hose on it. I also have the hard warm start issue. I will ride it again and see what happens. The only way I can describe how it acts is like when you are low and gas and need to flip to reserve. It seems worse with a full tank. I am also going to try riding with a low tank and the gas cap open to see what happens. Next, the R/R relocation. Thanks for your help so far everyone!!
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post #6 of 12 (permalink) Old 08-16-2007, 11:11 PM
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In theory there is supposed to be a vent line coming off that tank piece... cheap to replace, so might as well start there. Have you Seafoamed the carbs yet? It's possible one of the mid-range jets is a little clogged and causing some problems, too. Again, regardless, it's a cheap intervention and offers all sorts of other positive benefits.

good luck taking her out again!!

C
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post #7 of 12 (permalink) Old 08-17-2007, 05:29 AM
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I had the same problem. On my bike it was the boots that connect the carbs to the cylinders. There is a little nipple on the boots and the cylinder that have to line up. I searched for months and spent a lot of$$$$ to find this problem and it cost nothing to fix. USE SEAFOAM it is the nector of the fuel gods.


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post #8 of 12 (permalink) Old 08-23-2007, 07:25 PM Thread Starter
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Talking Thanks to all who posted responses.

I found my vent hose tucked towards the rear of the motor, not blocked but hanging there. I put in in the right ear after cutting it diagonally. Just in case, I put seafoam in the gas. It started running much better right away!

Thanks again! Now, it seems like the next mods I should do is relocate the regulator/rectifier, adjust the gap on ignition pickups and get a maintenance free battery! Anything else I should do? I am fairly new to the site so I am still poking around.

Has anyone had a bouncy tach? I thought I fixed mine with adding rubber pads to the vibration isolation mount but, it is coming back.

I also have an electronic copy of the factory manual. I am an electrical engineer so if anyone has questions on the electrical system feel free to ask!

Frank
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post #9 of 12 (permalink) Old 08-23-2007, 07:42 PM
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Congrats! Seafoam to the rescue once again!

Let's see: lube your rear splines (check Fergy's sig for a link to how to do this without tears)...I might be drawing a blank, but the r/r relo and MF battery are the other two biggies.

For the tach, depends upon how bouncy it is. Could be the carbs need sync'ing, or could be the leads just need a good cleaning. Start with the connecting pins and see what's going on there (that'll be child's play for you!).

C
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post #10 of 12 (permalink) Old 08-24-2007, 11:40 AM
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Quote:
Originally Posted by piromedicx View Post
I had the same problem. On my bike it was the boots that connect the carbs to the cylinders. There is a little nipple on the boots and the cylinder that have to line up. I searched for months and spent a lot of$$$$ to find this problem and it cost nothing to fix. USE SEAFOAM it is the nector of the fuel gods.
I'm having a similar issue since I got in a tough spot and had to let a local Kawa stealership work on it for totally unrelated work (Stator, ouch!). Got the bike back and it started running on just the front cylinder. Took it right back and they say it's the rubber intake manifold (and they want to charge me to repair it.) I think I'll try to do it myself rather than give them one more penny of my money. Anyway, I'm trying to visualize what you mean by "lining up"

86' Vulcan, Stock & Lovin it!
Stator, R/R, splines lubed, ACCT's repl. at 25,200
Just turned 29,000 mi.

95' Vulcan, all stock, 9,700 mi.

John Spangler
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