Carburetted motorcycle stalls when accelerator is applied

Question

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.

Answer 1

A free flow exhaust requires more fuel to the engine

The baffles created back pressure in your exhaust system that restricted the flow of the exhaust gasses. When you removed the baffles it allowed the engine to 'breath' easier due to the lack of restriction.

Restriction in exhaust systems are designed for several things.

• to restrict gasses flowing into and out of the engine. This means that you can use less fuel to operate the vehicle and allows the manufacturer to advertise a better fuel economy for the vehicle.

• Better for the environment. In order to meet EPA regulations and such. Restricted exhausts allow for less exhaust gasses and....better for the environment.

• Better bottom end power. At the lower end of your RPM range you will have more power which makes the vehicle more user friendly driving in the city.

The free flowing exhaust allows more gasses to pass through the engine and thereby requires more fuel to enter the engine to be burned. The condition you are experiencing is a lean condition.

You can drill out the air screws at the tops of your carburator CV cover closest to the head. They are aluminum. Be careful, your drill bit will go right through them and the aluminum plug over the air screws might just come off in the bit. That's ok, just don't penetrate to deep, you will hit a brass air screw if you do and drive it into your carburetor breaking off the the pin at the end of it. Once you have access after removing the aluminum plug blow out the aluminum shavings. Turn the air screw down all the way and turn it out 1.5 times as a starting point. Turning it out makes the pilot/idle circuit operate leaner, more air for more turns out.

This may resolve your issue but not. It will make your lower RPM's richer.

For higher RPM you will need to order the next size up main jet and a jet needle that has notches at the top. The jet needle goes into your throttle. The jet needle rests at the bottom of your CV piston valve.

You will raise and lower the needle by changing the clip position on the jet needle. If you want to make it richer, drop the clip to a lower notch. If you want to make the engine run leaner, raise the clip to another slot to drop the needle down lower into the main jet stand.

This procedure will allow you to tune the carburetor.

K&N makes a jet kit for your bike. Jet kits are typically applied to carbs after a free flowing aftermarket pipe is installed to enrich the system. In your case, it's just a modified stock exhaust turned into a free flowing system. It should work just fine.

Cheers

CV Carburetor Image

Notched Jet Needle Image

Air Screw Plug

Answer 2

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.