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My father has 1995 Kawasaki VN 800 Vulcan with low kilometrage (18 000, 11 000 miles). We had installed new Vance & Hines pipes on it (in hindsight, way, way too loud, but that’s beside the point, and we’ll have to live with it). When I put it together, it was about 0 °C outside, maybe a bit lower, but it started right up, without a choke, and responded nicely without need for warm up. Next day, I tightened things up, and went on the short ride (20‒25 min): again, it worked fine, in similar temperatures.

The day after, however, it started to run rougher. I had to warm it up a bit before going during day time. And at night I had to wait couple minutes before I was even able to drive away. It idles mostly okay, but misfires every so often, barely identifiable. However, when the accelerator is applied it instantly bogs down, even if a little bit, and misfires a lot. Due to incomplete combustion, the fuel would even ignite in the exhaust system, as visible by blue flames at night inside the pipe. It is possible to drive after warm up, but it requires unhealthy application of accelerator to beat the misfire and get it to go. After that, at steady pace on the road, it works like a charm.

Below is the list of things I am suspecting:

  • Bike was not winterized, as we did not expect not to use it for that long (procrastinated for months to spend four hours it took me to get it ready), so the fuel breakdown and hygroscopic issues are a concern (although, from my experience with less-demanding boat two-stroke, even couple-years old fuel can be used sometimes).

  • Carburettor (single Keihin CVK 36) may needs tuning, or even re-jetting. We would have to wait until we find and install K&N filter on it. As far as I can see, there is no vacuum actuation on it, so it should be run by plain mechanical linkages.

    I do not know if change of back pressure due to straight-pipe exhaust could affect the carb, but I had tried to run with airbox completely off, but nothing had changed.

  • Electricals:

    • Ignition timing may be off. As I understand, it has electronic ignition, and the control unit cannot figure out proper timing. That is just a guess, for I know not exactly how it works.

    • We are going to check spark plugs for combustion symptoms and quality of the spark tomorrow, along with the battery. I shall update.

Those are few guesses I have. None of them explain, why it worked fine one day, and stopped the next. I cannot think of anything that we could have done between now and then that would cause these issue. All we did is just a bit of cleaning and tightened few bolts. I would like to know which causes are more likely given the symptoms, and where should we start.

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1 Answer 1

The back pressure and flow change of the exhaust will have an effect on the combustion and mixture. Usually with a free flowing exhaust, you have to rejet the carb a little richer and with a free flowing airfilter, doubly so.

Running it without an airbox is making it worse, the mixture is already lean and stuffing more air down the bores isn't going to help with that.

Rejet the carb appropriately, normally the exhaust manufacturer should have some suggestions for the correct jetting. Alternatively get a range of jets and do plug chops. You might also want to consider adjusting the needle position if the carb has one.

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I have read, however, that incomplete combustion and misfiring might, on the contrary, indicate rich condition? Again, I would have to confirm by checking the spark plugs for “wetness”. And still, that does not explain, why it worked fine for two days, and then decided to quit. –  theUg Mar 1 '13 at 21:31
    
Ride the bike, do a plug chop. If the engine is running rich, the plug will be black, if it's running lean the deposits on the plug will be almost white. "Bambi brown" is usually close to a good mixture. If the mixture is really rich, you'll also be able to smell it. For the symptoms you describe, it's either a little lean or really rich. –  Timo Geusch Mar 1 '13 at 21:45
    
I'd err on the side of rich - given it starts at 0c with no choke. rich/lean though, you'd need to set up the carbs again with any drastic change to backpressure/airbox removal, so check that out anyway. –  Mauro Mar 5 '13 at 12:55
    
So, turned out that the spark plug wire on one of the cylinders unsettled. They are hard to get to, because they are not to the side, as is on most bikes, but hidden deep inside, directly from the top, and to access them one has to take off and drain the fuel tank. Should I post that as an answer, or what do I do? –  theUg Apr 1 '13 at 17:31

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