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Discussion Starter #1
Hello All,

New to the forum, and hoping someone can suggest something that may be an issue on my bike.

96 Vulcan 800
7700mi

Exhaust is debaffled, and I did the "Scooterize mod" Attached is a picture of the result.

Factory needle, using a 52 pilot and a 170 main jet. I have adjusted the air/fuel screw as few as 1 turn and as many as 4 or 5, this issue doesn't seem to go away.

I have gone through the carb again and again, but I have one issue at this point that is blowing my mind. In different gears and at different times, I roll onto the throttle and when I am at roughly 25%, the bike starts breaking up. popping/spitting/sputtering and if I hold the throttle at that point, it just continues to do so and never clears up. It also wont really accelerate anymore until I go to probably 75% throttle, at which point it clears right up and pulls like a bat out of hell.

Additionally, when I am on the highway it seems to be even worse in 5th gear, which is problematic because I find that 25% is sort of a sweet spot for 70-75mph.

Thanks in advance.
 

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There are two ideas:
1) Vacuum leak or a fuel petcock issue.
2) Too small fuel level in a float chamber which could be the result of first idea.
 

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Discussion Starter #3
Last night before making this post I took the carb off and checked the float level, as it was the one thing that I had not yet checked.

It was set around 17-18mm so I adjusted it down closer to 16.5.

Also I would think that if it was starving for fuel that I wouldn't be able to go full throttle and have it clear right up.

I will investigate the fuel petcock problem.

Thanks
 

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Discussion Starter #4
Just an update:

As a trial to see if the petcock was the issue, I pulled the vacuum line off the petcock and plugged it, and went on a short ride with the petcock in the prime position.

My thought is that if the petcock not flowing correctly was the issue, being the prime position should just supply fuel nonstop independent of any vacuum.

Results were basically the same, rides decent around town, but when you get on the highway the issue becomes more pronounced and it just runs horribly.
 

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Discussion Starter #6
I may go ahead and replace them. I installed new ones about 1400 miles ago when I first got the bike, perhaps with all the changes I've made to the intake/exhaust I should have replaced them already.
 

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You may have some spark scatter due to vibration.
Try jumpering the two wires at the sidestand switch together and try again.
 

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I may go ahead and replace them. I installed new ones about 1400 miles ago when I first got the bike, perhaps with all the changes I've made to the intake/exhaust I should have replaced them already.
You may be right, but I was asking about the color and condition of the plugs as a clue to whether this is, in fact, a carburetor issue.
 

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Just an update:

As a trial to see if the petcock was the issue, I pulled the vacuum line off the petcock and plugged it, and went on a short ride with the petcock in the prime position.
* * *
Results were basically the same, rides decent around town, but when you get on the highway the issue becomes more pronounced and it just runs horribly.
I had similar problems on highway when I had a vacuum leak at manifold. The rubber holder which connects the carb to the manifold was not sealed. (The rubber gets old and hard. I ordered new one.). The engine got warm and started to suck outside air. I got lean and loss of power and extra heat in pipes. Try to check any vacuum leak on hot engine. Take some carb cleaner or starting fluid and carefully spray over Your manifold when the engine is idling. If rpms are changing, everything is trivial.
 

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Discussion Starter #11
Thanks for the tip Tehnis

I went ahead and ordered a replacement rubber coupler and clamp, hopefully this upcoming weekend I can get it sorted out.
 

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your carb stock needle should be a dynojet DNO106 needle 2nd notch, and your 52 pilot jet is too high it should be 48PJ 2.75 turns
 

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Discussion Starter #14
You are welcome! :)
But did You make a test as I said before ordering?
I bought some carb cleaner tonight, and did the test, the only area that seems to be leaking is around the air cut-off valve.

I have a replacement valve set, so I will try and install it and see what happens.

Thanks again for all the tips.
 

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Discussion Starter #15
Update:

Fixed the air cut-off valve vacuum leak and tested it afterwards and the issue was definitely gone. I took the bike for a test drive, and it still did the same thing as far as breaking up.

When I arrived back home I went ahead to spray it down with carb cleaner again, and I hit the "Intake Manifold" where it mates with the cylinder head, and I'm thinking that maybe when it heated up it unsealed? Bike cut off immediately.

Ordered a couple replacement seals, will continue when they come in.

Kickstand switch jumper didn't seem to make any difference.
 

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Seems like You have a real technical disaster there :(
As i noticed on my experience in most cases the vacuum leaks appear (or increase) only when the engine gets warm, because of old rubber and seal part uneven deformation.
My air cut-off valve was also bad but the result was only a huge backfire in air filter and pipes when decelerating. It should not have any problem with a steady cruising on a highway.
As btom said, also check the needle size and washer setting by that pilot jet.
Sure, the kickstand switch has nothing to do with vacuum leaks ;)
No more ideas by now :(
 

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Discussion Starter #17
Update:

I pulled off the intake manifold, and it was hard to tell whether the O-rings had melted a little or at the factory they used some black RTV. I cleaned the mating surface off real well and reassembled using some orange high temp rtv just as an insurance measure. Rode the next day and it was still doing the same thing after heating up. Sprayed some carb cleaner on the IM side of the carb and it was still leaking somewhere.

Today I replaced the rubber boot and clamp that couple the carb and IM together, took it for a drive around town and short burst on the highway, and it ran well the whole time. While reassembling I also put some duct tape on the front side of the air cleaner, as I was wondering if perhaps the carb didn't like air blowing across in front of it. I wouldn't expect it would make much difference because when riding there doesn't seem to be much air turbulence in that area, however like I said it ran GREAT!

Thinking I will pull of the air cleaner back plate and weld in some sort of baffling behind the air filter to break up the air flow. It will still look good, but will be functional. Mountain ride coming up this weekend, so trying to be ready.
 

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Discussion Starter #19
Update:

So I drove the bike to work this week and its run really great.

Tonight for science I removed the duct tape that I had placed over the front half of the air cleaner.......what do you know, but the issue can right back.

Not sure exactly why, cant say that I understand it, but my plan now is to install a baffle on the front side to break up the air flow.
 

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Discussion Starter #20
Update:

Decided to pony up the money and replaced my modified intake with a Kuryakyn Hypercharger.........

Best investment ever. They recommend a factory pilot jet paired with the intake, but I stuck with the 52 jet I had already installed. I had also installed a 170 main jet, which was what was included and recommended for the installation. I did install the adjustable needle with the included 3 shims, and I have to say night and day difference.

Bike runs great now, and I've yet to notice any negative side effects. The whole assembly is well made and functions great.

I cant recommend this mod enough.

Cheers!
 
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