Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums - View Single Post - Reduce or Eliminate Popping / Backfiring
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post #2 of (permalink) Old 01-15-2006, 05:21 PM Thread Starter
Vulcan Verses
Join Date: Jan 2006
Posts: 178
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Symptom: 'Popping' upon deceleration- common with Vulcan's. -

Reason: Emission control.

In addition to tuning the bike lean from the factory to reduce emissions, Kawasaki also induces 'fresh air' (as Kawasaki calls it), or oxygen, into each cylinder during the exhaust cycle. It's the manufacture's good intentioned design efforts to comply with EPA Air Quality Control emission standards to promote cleaner combustion resulting in reduced air pollution. So if you care more about the environment than the your bike backfiring, stop here. -

Problem: It has long been established by riders of Vulcan's that this system is flawed. While introducing fresh air into the exhaust port does reduce unwanted emissions, it has a negative impact on performance. All engines have a small amount of unburnt fuel that makes it to the exhaust manifold. It cannot burn completely and then leaves the pipe without a making a peep- unless you provide oxygen. Now you have heat, fuel, and oxygen. Presto! It results in small explosions or backfiring/popping. This is more evident during deceleration, as more fuel gets momentarily 'loaded' into the exhaust pipe. Most would agree a well-tuned engine does not backfire. Aside from the sound, this can also cause your pipes to blue due to excessive heat, and your engine will run hotter. -

Fix: An air suction "reed valve" is attached to each cylinder head with a large hose attached. During each exhaust stroke, those valves allow fresh air to flow into each cylinder's exhaust port from the air filtered surge tank (large plastic plenum under the gas tank). These reed valves also block air from moving back into the surge tank. There is also a second valve in this system... an air-actuated valve located near the back of the gas tank (take off the seat first). It's several inches in size and has three large Ĺ" hoses and one small hose connected to it. The small hose is connected to the vacuum port on the right side of the bike (rear cylinder carb). When the throttle is opened (accelerating of just cruising), there is no the vacuum and the air-actuated valve will remain open and allow air to flow from the surge tank into the large hoses going to the air suction reed valves on the cylinder heads. During coasting (hand off the throttle), there is a vacuum present, and the air-actuated valve is closed and will not allow air to flow into the reed valves.

The modification to fix this is easy and has great results. You will greatly reduce the popping, reduced pipe bluing, and your bike will run cooler.

method #1: (I like this option, you can always return it to stock which may be needed to pass emissions) - pull hoses off reed valves. On left side (rear cylinder) the reed valve is on the top of the cylinder about where your knee is. On the right side (front cylinder), it's under the back side of the air filter. Plug both hoses with a marble (they seem to be the perfect size), or a steel ball. Do not worry about the marble breaking, never heard of that happening. It probably has the added benefit that it does not heat up like a steel ball would. As an alternative, you can also plug the hoses on the other end, right before they enter the air-actuated valve under the seat at the back of the tank.

method #2: - replace reed valves on each cylinder head with a 'coaster'. This is a metal plate that replaces the valve and plugs the port (same as a marble would do). Some people prefer the look and even polish their coasters. You can then remove all the hoses and even the air-actuated valve if you prefer. Be sure to plug (the now unused) vacuum line that goes to the rear cylinder on the right side of the bike. Some say this method saves weight, but it's not much at all, probably less than a pound. You can buy coasters here http://home.flash.net/~sanco/coaster.html and Also see the Yahoo VN750 site: Files > FAQS > Drews Coaster Installation.htm

I suggest implementing this fix in any event- it will make the bike run better. However, if you still experience popping, it can be caused by air getting into the exhaust or too lean a mixture.

- Back your carburetor mixture screws to about 2 1/2 turns (they are factory set at 1 5/8 turns out- too lean!). I have gone as far as 3 1/2 turns to reduce the popping (just make sure the screw does not fall out). Do each cylinder separately, they do not have to be the same !

- Figure out which side is popping. Check for any leaks where that exhaust pipe meets the engine header. With little to no wind around, hold a cigarette several inches below where the pipe meets the engine. Rev to 3K RPM. If ANY smoke is sucked into the pipe or is disturbed in any way, loosen the acorn nuts and reseat the pipe (note: do not over-tighten the nuts, tighten them evenly, and use anti-seize compound) .If ANY amount of smoke still gets sucked into the pipe or is disturbed after reseating, replace the head gasket. It only takes a VERY small leak to cause popping. Now throw out the smokes. They are not good for you.

- Check for loose boots on the input and output of the carburetors. This will cause a lean condition which leads to popping. Idle the bike (to create the highest carburetor vacuum), then liberally spray each one with WD-40, waiting between sprays. If there is a leak, the engine will stumble as the WD-40 gets sucked in (this will not damage the bike). Please do not use a cigarette around the carburetors. There is fuel in them, we are trying to fix your bike, not blow it up.

- Check float-bowl levels per manual. This can cause a lean (or rich) mixture.

- The coasting enricher gasket may have a pin-hole in it, or the vent hole is plugged. It's the small round plate held in place by two phillips head screws, above the float bowls on each carburetor. Be careful when removing, it's spring loaded inside.

FYI - for clarification of terminology, what you are calling "coasters", are the reed valve assemblies. Coasters are a generic term for "something" used to replace the reed valves, to block-off the fresh air from the air filter surge tank. Other ways of blocking the air, are putting a 5/8" (.625") marble or ball bearing in the hoses coming off the reed valve covers. Reportedly a penny will also work, but a penny is .75" diameter (a dime is .705"), and 1/8" seems to be a lot to stretch the hose!! The "reed" is attached to the reed plate by 2 screws. Some have suggested flattening the reed stopper bracket with a hammer. If this is done, I would recommend removing the screws and the bracket, then straightening the bracket. Knifemaker also sells ($15 ?) plates made to either replace the reed valve assemblies, or with shorter bolts, replace both the reed valve assemblies and the reed valve covers.

Removal of Idle Mixture Screw Caps Webpage describes removal of caps/plugs of Idle Mixture Screws

In Jax's photos at (Click) there is a picture of the left carb air mix screw (photo 12&14 of 21) and right carb air mix screw (photo 13&15 0f 21) under "tech pics"

Turning out would be counter-clockwise (towards your left when looking at the air mix screw) You know, usually there are a few guys out here that have done the adjustment and know just how much.. and if you have turned these they may suggest going back to start and then turning them back to somewhere around 2 and a quarter turns or so.. Jax, Pick, JR, and a few others might be able to walk you through the finer details on this. *S*

also see HERE
Don't think of them as AIR mixture screws since they adjust the fuel and not the air. Since they are FUEL mixture screws turning them in (CW) restricts the fuel and out (CCW) allows more fuel in. This fuel is used at idle only. If you aren't sure where to set them, screw them all the way in then back out 2 1/2- 3 full turns.

Number of turns out should be from the "bottom". Factory settings are supposedly around 1 1/2 to 1 5/8. Mine were 1 5/8 and 2 1/4. Now running 2 3/4. If you turn them out 2 1/2 from the factory settings, you are in serious risk of them falling out! <g> You will know if that happens, though. grambo
I have to agree with Bruce. Anything past 3 is pushing it. 2 1/8 here on my screws
Check ALL your exhaust bolts and junctions. Even a small leak will degrade the system and cause popping. Check the carb boots to make sure they are on tight, etc

Last edited by Vulcan Verses; 01-15-2006 at 05:45 PM.
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