Kawasaki VN750 Forum banner
Not open for further replies.
1 - 4 of 4 Posts

· Registered
178 Posts
Discussion Starter · #1 · (Edited)
A copy of this thread is also located in the CARBS AND FUEL section of the Vulcan Verses

The sound (popping upon deceleration) is created in the exhaust pipes because of Kawi's EPA emissions design. Kawi designed the system to add cooler fresh/filtered air into the exhaust in order to create a second burning of wasted or unused mixture that may not have completely combusted during the times of closed throttle/decels. The poppings you hear are the secondary combustions in the exhaust sounding out

Couple things. First, everyone else is right about the air mix screws. Turn them all the way in (Clockwise, using that very small screwdriver) and then turn them out about 2 turns. If you experience popping, turn them out another 1/4 turn. If that doesn't work, try another 1/4 turn. Keep doing that, but don't go more than 3 1/2 turns total.

Second, when you replaced your pipes, did you use new crush gaskets where the pipes attach to the cylinders? This is a common place for air leaks which will cause popping. Somebody posted the Harley Davidson gasket part number a few days ago that will work on the VN750. Or you can get a pair from JUDGE on the VROC board for $6.00. Also, those acorn nuts tend to work loose a little after changing pipes - check to make sure that they are snug, but DO NOT overtighten - I think they are only supposed to be 12 or 14 ft-lbs. or something like that.

Third, you might want to consider coasters or marbling to cut off the air to those reed valves if you haven't already done so. Some combination of these things should eliminate the popping, or at least greatly reduce it. Good luck, and keep us posted on your progress. I'm curious to learn what works for you. Jim "Pick" Foster Dixon, IL

· Registered
178 Posts
Discussion Starter · #2 · (Edited)
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 ½" 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

· Registered
178 Posts
Discussion Starter · #3 ·
Q: Since I got the Yuasa, my bike starts right up. I don't care about the popping (I kinda like it). I believe the plugs look OK. Thus, is there any reason to mess with the fuel mixture screws on the carbs?
A: I'm going to say No...if it ain't broke...don't fix it...( it MAY let your bike run smoother at idle however..) Knifemaker

Those copper rings are the crush gaskets. If they're completely flat, they've been "crushed" and should probably be replaced. Someplace I heard that they are pretty much a single-use item, and whenever you loosen or remove the header pipe, they should be replaced. No promises, but I'm guessing there's a good possibility that replacing them with the HD gaskets or the ones from JUDGE would help your popping problem. I got mine from JUDGE ($6.00 for a pair), and they are just hollow metal rings with a slit that will "crush" when you tighten the acorn nuts on the header pipes.

Sorry...forgot to give you the part #..is 17048-98 at any harley parts department.

You say that your gaskets at the cyclinder heads are copper ring flat gaskets. I suspect they might have been "mashed" flat. The cylinder head gaskets should have some "expansion" capability. The gap between head pipe and cylinder head expands and contracts as engine heats and cools. Try new crush gaskets to eliminate popping

· Registered
178 Posts
Discussion Starter · #4 ·
The carb screws come from the factory covered by a little round piece of soft metal - like a plug. VERY CAREFULLY drill or poke a small hole in the plug, then stick a sharp point (like an awl or a pin) in and pull it right off.

If it looks like it has a little metal seal over it.. Yours was sealed by the factory...it's just a lead cap...you can remove it by carefully drilling a real small hole and prying it out with an icepick. I used a dremmel with a small bit...I added a plastic tube to the drill so it would create a stop..so the drill bit only sticks out about an 1/16 th of an inch..( you don't want to drill out the screw behind it) Just be carefull ...the carb body is soft metal...so don't use much pressure prying it out

There is a small diaphragm on the side of each carb that is supposed to richen the mixture during decelleration. Over time, diaphragms get old & crack or develop pinholes in them. Try replacing the one on the rear carb,I beleive that will solve your problem

In reading this thread it seems to me that we need a "standard" way to count turns. When using a round screwdriver on a round screw, the only reference to "a turn" is the slot of the screw (and if it's phillips, there goes the reference). There is some instruments I calibrate at work that I need to reference the # turns in or out, like a carb screw. First, I place reference marks on the screw in a place that wont rub off (the side of the screw head). I use two different color paintsticks (like red and yellow) to make small dots 180° apart. Then I set an "outside" reference - a small piece of wire and some tape or such. You wouldn't believe how far around the circle my eyes will move the starting point without this outside reference. When I start counting, it is now a simple matter of starting the adjustment with the red dot aligned with the reference, then watching as the red dot pass the reference again and saying one. Actually the piece I work on has four scribe marks for reference, and a built in pointer, but you get the idea.

Access to the adjusting screws is not good. I could not get my smallest screwdriver in there, so I made one from some heavy wire. I put a nice flat tapered end on it and bent the other end 90 degrees. Now I know I have positive connection with the screw, and I can count the turns accurately.

I'm currently happy with 3.5 turns from the beginning, or adjustment screw's bottom. That half a turn made a difference for me. I also found the screw will fall out of it's boring with just a little more than 5 complete (360) turns, on my bike. I was thought if I go close to that setting, then I would need some putty to hold the screw from falling out fill in the hole with, similar to someone's idea of using tape. But, I'd need to find a putty that wouldn't make a mess while trying to remove it. It'll still pop, esp. when I twist back the grip quickly, but not loudly

With this subject coming up again I'd like to offer a word of caution for those that are going to turn the mix screws in first. The metal is soft on the seat so don't bottom it out too hard, a light touch should be enough. If you don't think you've hit bottom but the screw has stopped, back it out a half turn and try again. If it stops in the same place your probably there. Also count the turns you make in, as a point of referance. For what it's worth.

Use some RTV gasket silicone over the screws to hold them in. When you pull it off it should come out in one piece.

Don't know if you may be referring to the popping that comes from the 'Coasters' or not. Any way here is a link to some info on it. There is more than this one but this is start. I'm sure others have more info as well. http://members.tripod.com/diverex/coaster.html Also check out www.shermscycleproducts.com

I think the original discussion is now talking about apples AND oranges. Apple: The Vn750 exhaust *POP* is created by air induction to the exhaust system as designed by Kawi's EPA junk...period. Orange: After installing my Jardine exhausts, I immediately took it to the street before readjusting the carb screws out. The engine produced a new sound like a "kak-kak-kak", or maybe some would say a slight "pop". I believe this may be "the pop" the fella at the store was referring to, suggesting we use a richer jet. But, that's exactly what we all have been doing, more or less. Usually, the suggestion given by all us Vn750 riders is to just readjust the carb's pilot screw outward, and I believe this works well enough, and there's no urgent need for a larger jet. However, I have bumped up my main jet and raised my needle using a thinner needle. Now I have an exhaust that rumbles nicely, doesn't sound like a cat coughing up a furball, and still burns correctly during all the different engine speeds. Lance
I beg to differ...a few posts have mentioned that they still got occasional popping after they "coastered"...which is bypassing the Kaw Epa junk I believe. After turning out the airscrew...it seemed to vanish. You also mention that one doesn't need to rejet the carbs...but then say you did and the bike runs better....??? Knifemaker
Well most of this talk about mixture settings and carbs is above my pay grade but I do know I had popping on decel before coastering and after the coasters on decel no popping. I have V&H Cruzers with no rejetting and bike seems to run fine. Dats about all I know. Phil
For the complete lowdown on coasters, see Ray Warriner's excellent write-up:

Bob, I'm pretty wrenchless myself. Don't worry, you can do it! You don't even have to get coasters. All you need is a couple of marbles of the right size. Pop the hoses off of the reed valves, stuff a marble in the hose to block it, then re-attach the hose. Five minutes max. No one will know, and you've accomplished the same thing as coasters! And if for some reason you ever want to go back to stock, just remove the marbles. Jim "Pick" Foster
I used copper caps from the plumbing dept. at home depot. I didn't want the marbles to shatter and get in my engine. It's still a 5 minute job. Warren
I plugged my reed valves with two 5/8" diameter plugs I got from Advance Auto. They fit perfect.

You will NOT have to remove the airbox IF you 'coaster' using 5/8" good quality PLASTIC chair leg end covers found at any good hardware store. Black plastic... Just remove the hoses, Permatex on the cover, and voila'... Should cost ~$1.49, almost invisible on the left side, totally invisible behind right ear, no hassle removing reed covers..takes all of 5min, works as well as anything out there, and leaves your reed switch covers stock in case you want to revert. Tech tip o' the day...
1 - 4 of 4 Posts
Not open for further replies.