Author Topic: Smog System Removal ( ST1100 )  (Read 8291 times)

Offline KoTAOW

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Smog System Removal ( ST1100 )
« on: May 18, 2008, 02:18:39 PM »
This article was taken from web page.  It was originally submitted and written by Martin B. in January 2003.


There are three main benefits to removing the smog equipment. First, it makes removing, replacing and working on the carbs much easier. Though it must be said that removing the fuel vapor recycling system is more beneficial in that regard than removing the air injection system. Second, and perhaps most important, is that it removes possible sources of trouble in the future. And third, your bike becomes lighter.

When I had problems with my bike (outlined in the archives under Carb Problems and Jetting), I decided that rather than try to figure out where the problem was, I'd simply remove the two systems and be done with it. It does require, though, obtaining or making some plugs for the various lines to be plugged and obtaining some new vacuum line and hoses. Don't cut up any of your original hoses if you can help it. If you ever want to go back to the stock configuration, you don't want the cost and hassle of ordering new ones. Just put them away in a box somewhere. I went down to my local auto supply store with samples of each size hose I was working with and bought 4 or 5 feet of each size needed. You likely won't find metric sizes but the fractional sizes will work just fine. Buy extra as it's inevitable that you'll cut something too short and need another piece. Besides, the hose is handy to have around the shop.

Removing the Pulse Secondary Air Injection System

Preliminary notes:

When you remove the top of the airbox, there are two large diameter hoses that attach to the rectangular feature at the front of the box. At their other ends, they attach to a fitting between each pair of left and right carbs. These are the source of air to the bottom side of the diaphrams in the carbs and must remain.

When you remove the main, round plastic housing for the air filter, there is a small hose that connects to the bottom of the housing on the right side. That's for the crankcase breather system. That remains also.

More notes:

With the carbs still in place, look at the front of the aluminum casting (air box base) that is bolted to all four carbs and to which the plastic air filter housing was attached. At the very front center, you'll find a large diameter hose attached to the air box base. If you look inside the base, you can see the hole leading to the hose on the inside front of the box base. This is where the air injection system gets its supply of air. The vacuum control for the air injection valves under the carbs is tapped off the left front and rear intake ports.

When you remove the complete air injection system, you need to plug those two vacuum lines off the intake ports. Those lines (also used for syncing the carbs) come off the intake ports in the head below the rubber boots. What I did was replace those vacuum lines with new longer hoses, plugged the ends and ran them up to the frame and tucked them in with the wiring harness so they were easier to get at when syncing. I made small plugs out of aluminum on a lathe. Small stainless steel ball bearing like those used in bicycles will work also. Make sure they?re large enough to not get sucked down the hose and into the engine. Just squeeze the hose to pop them out when you want to sync. You?ll also need to plug that large diameter port at the front of the aluminum base for the air filter. Otherwise, you?ll be sucking unfiltered air into your engine. I used a very short piece of the same diameter hose and made a large plug for it. Last, you?ll need to block off the ports in the head. I used a kit designed for a Suzuki GSXR750 and made by Intuitive Race Products. It works just fine on the ST. 28 dollars or so.

Of course, you can leave the system in place and simply disable it. To do that, all you need to do is disconnect and plug the vacuum line(s) between the intake ports and the valves under the carbs. And you'll need to do one of two things with the large hose that connects to the front of the aluminum base for the air box. One, simply connect the hose up again so the engine doesn?t suck unfiltered air (a hassle). Or two, not connect the hose and plug the hole in the aluminum air box base. Remember, if you are planning to remove the entire system (valves, chrome lines going to the exhaust ports, etc.), you'll need to remove the gas tank (easy), remove the overflow tank for the radiator (easy), and remove the aluminum casting that bolts to the right head and to which a coolant hose is connected (a bit more hassle). That last bit means draining at least some of the coolant (easy but you may need to remove some tupperware to get at the radiator drain), and having a new o-ring seal on hand for the casting when you put things together again. Actually, that o-ring can be a source for coolant leaks. Now would be a good time to replace the o-rings on both the right and left castings. I cleaned up the mating surfaces (no big scratches allowed!) and used Teflon high vacuum grease to goop up the o-rings before installing them.

Removing the Evaporative Emission Control System

After the plastic part of the air box is off the carbs, you?ll notice that there are some medium diameter hoses (with plastic fittings and clamps here and there) going along both sides and across the top end of the aluminum base of the air box. This hose system goes to each carb and runs along the top edge of the aluminum casting. There?s a T fitting at the front right of this daisy chained hose system where another hose goes off to a vacuum operated valve attached to the frame at the front right of the carbs. (We?ll call that valve the ?right front? valve.) These hoses are connected to the fuel bowl vents and will not be removed but the hose between the chain of hoses and the valve will be removed as well as the valve itself and a second hose connected to the valve and open to the air at its other end. (The open end is held in place by a clamp at the front of the aluminum air box base.) What this part of the system does is collect fuel vapor from the float bowls and directs the vapor through the system and back to the motor to be burned. The hoses stay because the float bowl vents cannot be plugged but need to be vented so that air can go in and out of the float chamber as the fuel level goes up and down during normal operation. Also, they need to remain vented in case a float valve malfunctions and fuel overflows the bowl.


Remove the hose between the valve and the daisy chained bowl vent hoses. (Leave the daisy chained hoses and T fitting in place.)

Remove the hose going from the valve to the clamp at the front of the aluminum air box base. (This hose is open at the end.)

At the ?right front? valve, disconnect the vacuum line coming from the right front intake port and plug it. (Leave this line connected to the intake port as this hose gets used when syncing the carbs. Leave it or make it longer for future convenience.)

Remove the ?right front? valve.

There is another valve at the front of the aluminum air box base that will also be removed. (We?ll call this the ?air box? valve.) This valve?s job is to take fumes from the evaporative emission canister (just above the front end of the swing arm) plus fumes from the float bowls and feed them back into the intake system. When you examine things, you?ll find that there is a larger hose going down from the bottom of the valve with a T fitting and a hose going off left (to the canister) and off right (to the ?right front? valve). There are also some smaller lines coming off the valve to the carbs. The lines that connect to the carbs do the following: The single line coming off the back side of the valve to the bottom of one of the carbs controls the valve. The two hoses coming off the front of the valve and going left and right each branch into two more hoses for a total of 4 hose ends. Each end goes to a carb and feeds fumes back into the intake system.


Disconnect all 5 smaller hoses at the carbs.

Disconnect the hose going back to the canister from the T fitting but leave it in place for now.

If you haven?t already done so, disconnect the hose between the T fitting and the ?right front? valve.

Remove the ?air box? valve.

Using new hose, cut 5 short sections of hose, plug one end and use these to seal the 5 ports on the carbs. Make the hoses as short as possible. Make sure they fit snug and that whatever you use to plug the ends cannot be sucked into the carbs.

Now you should have the following:

The vacuum line from the right intake port is still there but plugged.

The hose running between the two valves is gone.

The hose from the ?right front? valve that was open at its other end is gone.

The five hoses from the ?air box? valve to the carbs are gone and the 5 vacuum ports on the carbs where the hoses fit are sealed with short sections of hose plugged at the end.

Both the valve on the frame at the right front of the carbs and the valve at the front of the aluminum part of the air box are removed.

Note: To make things easier in the event you want to reinstall everything. Leave as many of the hoses as possible connected to the valves and only disconnect hoses where necessary.

At this point the 5 vacuum ports have been capped/plugged and the vacuum line from the right front intake port is also plugged. The only remaining task is to connect the hose from the canister to the daisy chained float bowl vents. There are several ways to do this.

What I did was to move the location of the T fitting to the left side of the carbs and reroute the hose from the canister to the T fitting. I used new hose as needed so I wouldn?t have to cut any original hoses. It was somewhat difficult to find a spot to relocate the T fitting so that it and the hose and its clamp wouldn?t interfere with the plastic part of the air box when reattached to the aluminum base. In hindsight, I think it would have been better to simply leave the canister line where it was and use a short section of copper pipe and some more hose to connect to the T fitting at the right front of the carbs. Or run an entirely new line from the T fitting back to the canister. I may do this next time I tear into things.

Note: Your bike may differ somewhat in hose routing or even the equipment installed. My bike is a 1992 ABS/TCS California model. Later non-California models may not have a canister or perhaps some of the other equipment. They may also differ somewhat in hose routing. Always refer to a good manual and understand what you?re doing before you do it. I also removed my canister so that now the fuel bowl vents and the gas cap now vent to the atmosphere. To remove the canister you need to put your bike on it?s center stand, disconnect the bottom of the shock at the swingarm and lower the tire and swingarm to the ground. That will give you enough room to get the thing out. Use a board wedged under the rear of the tire to gently raise and lower the swing arm.

Another note: Before reinstalling the carbs, do the following: One, make sure the carb boots are still soft and compliant. If not, replace them. Two, replace the Phillip head screws on the boot clamps with stainless steel socket head screws so you can use a Bondhus Balldriver. Much easier. Three, put a light coat of wheel bearing grease on the carbs and boots where they mate to make popping the carbs in and out much easier.

Sill another note: Removing these systems will have no effect on your jetting. However, it?s always a good idea to check carb sync when reinstalling carbs. You can also try setting all the mixture screws out to 2.5 to 3 turns. The bike is jetted lean anyway.

In the end, I accomplished the following:

The bike is lighter now. (Whooppee!)

Many potential sources of vacuum leaks and other problems have been eliminated.

Working on the bike has been greatly simplified with the elimination of so many hoses and connections.

Cheers and happy riding!

Pictures provided by Craig Severson:

Before removal:

These are the vacuum lines for the #2 and #4 cylinders which drive the smog system air pumps. They are removed from the T-connector and plugged. These are used in the future for carb syncs.

It was a pretty simple job to remove the pulsed air smog system; the chrome tubes are unbolted from the heads, and I cut the rest of the tubes to facilitate removal. The large center rubber tube that goes up to the airbox must be blocked at the airbox, otherwise unfiltered air will be pulled in.

Thread Comments from  Assuming the systems were functioning properly to begin with, there's no performance benefit. Strictly speaking there might be some difference that would be measurable in a laboratory environment, but practically speaking, there shouldn't be any difference in performance.

Thanks for the very good description of this work. I just did it, and it does create a lot more space around the carbs, which may help in the passage of air to cut down on heat in the summer, I think. But in doing it, I took off those two bowl vent hoses, which would have caused much trouble when I start the engine........I do thank you!!

Disconnecting the pulse air injection system should cool down the exhaust headers by as much as a couple hundred degrees, which should help with the heat problem

The weight removed is not much. If you include the canister, then maybe 3 to 5 pounds. I'll try to weigh things this weekend and post the results. The biggest benefit is to make working on the bike easier and to eliminate real or potential problems.

Well, I finally got around to weighing things. I used my bathroom scale so accuracy may not be that great, but here it is. The two air pumps that reside under the carbs along with the chrome tubes that go to the exhaust ports and various other hoses, clamps, etc. weighs just a smidgen over 3 lbs. The charcol canister (after sitting for a year so all gas has evaporated) weighs 2 lbs. So, the combined total is roughly 5 lbs.

As for improvements in mileage after removing the smog systems, I can't really see where it would make a noticable difference either way. The air injection system is completely passive. The fuel vapor recovery system results in a imperceptable increase in the fuel going into the engine (in the form of fumes) so removing the system and the extra vapor would hardly be measureable.

Second, for those of you that have the charcoal canister and attempt Martin's "EVAP-ectomy"....Martin details how he connected the bowl vents together to a hose which vents to the atmosphere. I did this as well, venting them to a hose ganged to the rest of the hoses under the bike.

After completing this job, I had some strange surging problems above 45mph. I tried several things to cure this. In the end, the problem was the bowl vent hose terminating too far under the bike, which picked up air after a certain speed and pushed it back through that vent hose and up to the carb bowls.

If you look at the carbs from an ST *without* this vent system, the bowl vents gang together to a hose that just ends right in front of the carbs, buried inside the belly of the beast. I disconnected my "under bike" vent hose and just emulated the vent from "non evap" bikes, and the problem went away.

This whole job is definitely worth doing for purposes of pre-emptively removing potential problems. It doesn't make the bike any faster or more efficient, unless you had a problem in there to begin with.
« Last Edit: March 25, 2009, 08:13:15 PM by KoTAOW »