Cutter's Smog Elimination Kit

[73043]

I agree with you. Why remove something that's constructed to make the bike pollute less?
I'm dual plugging my 95 R100RT, but not removing any of that stuff.
Hans
Norway

[hultman]

I agree with you. Why remove something that's constructed to make the bike pollute less?

Simple. Because it made my motorcycle sound like a pop corn popper, which I found really annoying. Also, I cut my teeth on /5's, and removing all that silly, unnecessary plumbing just felt right!

[mneblett]

I don't understand why it is helpful to plug these ports, or remove the balance tube. How would this impact the performance of the engine? What am I missing here?

First, these are not balance tubes. For emissions reasons, the Pulse Air system injects air into the exhaust gas to burn unburned hydrocarbons which enter the exhaust. The system does nothing for performance; its removal has no effect on how the engine runs, as it is self-powered.

The primary reason for removing the system is it tends to cause overheating in the area of the exhaust valves. The problem is the injection point is so close to the exhaust valve that the air reaches the hydrocarbons and ignites them right in the exhaust port.

The injection of air into the exhaust is controlled by the big brass-colored valves in the air box. The valves are controlled by vacuum pulses from the carbs, hence the hoses from the front bottoms of the carbs back up to the big vacuum valves in the air box.

HTH!

[88bmwjeff]

First, these are not balance tubes. For emissions reasons, the Pulse Air system injects air into the exhaust gas to burn unburned hydrocarbons which enter the exhaust. The system does nothing for performance; its removal has no effect on how the engine runs, as it is self-powered.
The primary reason for removing the system is it tends to cause overheating in the area of the exhaust valves. The problem is the injection point is so close to the exhaust valve that the air reaches the hydrocarbons and ignites them right in the exhaust port.
The injection of air into the exhaust is controlled by the big brass-colored valves in the air box. The valves are controlled by vacuum pulses from the carbs, hence the hoses from the front bottoms of the carbs back up to the big vacuum valves in the air box.
HTH!

And, after a while, the tubes can get plugged up. When I took them off of my bike the tubes leading from the airbox to the heads were plugged up. FYI, I had to cut one off, so I know for sure at least one was plugged.

FWIW:
The WebBikeWorld article sez you can put a screw into the vacuum ports under the carbs to block them off. You can, but I don't like the idea of chewing up the brass port tubes. Your dealer can sell you a pair of rubber vacuum port (nipple) covers -- the same ones used on early K-bikes - which just slide over the ends of the tubes.

I donÔÇÖt know the proper part number, but you can get screws of the property size and pitch to not mess up those threads. These screws were standard prior to the crossover tubes.

[walt3022]

Not balance tubes MNEBlett? If the tube connects one carburetor to the other and there is no valve to prevent the free flow of air from one unit to the other, I don't see how one could believe they are anything but balance tubes. There are any number of engines on the market that have their carburetors connected in this fashion. In fact, I recall the old Triumph Bonnies from the 1960s had this feature. I've always understood it helped the engine to run more smoothy at idle, as their is less chance of a vacuum differential between the multiple carbs. But, at wider throttle openings I doubt it makes much difference one way or another.
I've always liked a bike with a smooth, dependable idle....

[AnnapolisAirhead]

Not balance tubes MNEBlett? If the tube connects one carburetor to the other and there is no valve to prevent the free flow of air from one unit to the other, I don't see how one could believe they are anything but balance tubes. There are any number of engines on the market that have their carburetors connected in this fashion. In fact, I recall the old Triumph Bonnies from the 1960s had this feature. I've always understood it helped the engine to run more smoothy at idle, as their is less chance of a vacuum differential between the multiple carbs. But, at wider throttle openings I doubt it makes much difference one way or another.
I've always liked a bike with a smooth, dependable idle....

Sounds like we are talking about two different tubes. I believe Mark was just referring the the pulse air injection system, not the vacuum port connection tube that runs through a notch in the aft end of the starter cover. I thought that the vacuum port tube was an attempt to balance vacuum pull and is completely independent of the Pulse Air System.

I've removed both on my '83 R100RT, dual plugged, no worries and it has been running well for almost 20,000 miles both highway and city. For whatever its worth, I get 38-45 MPG usually cruising at 4200-4400 RPM.

[lkchris]

Not balance tubes MNEBlett? If the tube connects one carburetor to the other and there is no valve to prevent the free flow of air from one unit to the other, I don't see how one could believe they are anything but balance tubes.

That's easy.

The carbs were connected solely to provide an even source of vacuum to the air injestion system components in the airbox.

On NO other BMW that did not have these components was there ever any vacuum connection between the carbs, including other-market Airheads built in same time period.

Yes, if you remove the air injestion system there is ZERO reason to keep the cross-over vacuum line.

[lkchris]

FWIW:

The WebBikeWorld article sez you can put a screw into the vacuum ports under the carbs to block them off. You can, but I don't like the idea of chewing up the brass port tubes. Your dealer can sell you a pair of rubber vacuum port (nipple) covers -- the same ones used on early K-bikes - which just slide over the ends of the tubes.

Well, the ports have internal threads and if you use the correct screw you don't "chew anything up."

[mneblett]

These screws were standard prior to the crossover tubes.

Correct -- the R75/5 in the garage has them. I prefer the rubber caps because the tiny screws and their even tinier fiber washers are a minor pita to deal with, and there is the (admittedly small) potential for them to back out/fall out. Not so with the rubber caps. The downside of the rubber caps is they need to be replaced once a decade or so -- I can deal with that.

Not balance tubes MNEBlett? If the tube connects one carburetor to the other and there is no valve to prevent the free flow of air from one unit to the other, I don't see how one could believe they are anything but balance tubes. There are any number of engines on the market that have their carburetors connected in this fashion. In fact, I recall the old Triumph Bonnies from the 1960s had this feature. I've always understood it helped the engine to run more smoothy at idle, as their is less chance of a vacuum differential between the multiple carbs. But, at wider throttle openings I doubt it makes much difference one way or another.
I've always liked a bike with a smooth, dependable idle....

I'm not aware of any BMW boxer which used balance tubes, and these are too tiny (inner diameter) and too long to provide any signficant volume of cross-flow. They are only there to provide a vacuum source for the Pulse Air valves.

Sounds like we are talking about two different tubes. I believe Mark was just referring the the pulse air injection system, not the vacuum port connection tube that runs through a notch in the aft end of the starter cover. I thought that the vacuum port tube was an attempt to balance vacuum pull and is completely independent of the Pulse Air System.

Nope -- if they were there to serve as balance tubes (i.e., as an "improvement" introduced at the same time as the Pulse Air system in 1981), you would expect that the same "balance tubes" would have been present on all of the non-US market boxers. They weren't. The non-pulse-air bikes didn't have starter covers with cut-outs for the Pulse Air vacuum hoses.

Well, the ports have internal threads and if you use the correct screw you don't "chew anything up."

I'll have to go check my '94 Mystic -- I was not aware of that -- learn something new every day! That said, for the reasons above, I'll stick with the rubber nipple caps.

[toooldtocare]

I'm not aware of any BMW boxer which used balance tubes, and these are too tiny (inner diameter) and too long to provide any signficant volume of cross-flow. They are only there to provide a vacuum source for the Pulse Air valves.

I agree, if my memory is right my now very long gone Triumph Bonneville had a balance tube between the carbs, and again if my memory serves me right it's inside diameter was around 1/4 to 3/8 of an inch. The tiny tubes on our airheads could never provide enough balance effect to do much of anything.

My bike is a 78, thus never had the emissions stuff under the tank, but the ports are still there. I use them to adjust the carbs. When done I use a tube running between mine, not for balance reasons, but it is easier to find a small tube to stop vacuum leaks than those small rubber caps.

[20774]

I wonder about this linked-tube for balance...seems to me it defeats one aspect of the system. Each cylinder is basically a separate engine connected through the crank. It doesn't really care what the other cylinder's vacuum is...it just needs to create power and pulses in such a way that minimizes the imbalance of the explosions. So cylinder one needs to do it's job using settings on it - carb settings, valve settings, etc. - but really doesn't need to share its vacuum with the other side.

Am I missing something?

[toooldtocare]

I wonder about this linked-tube for balance...seems to me it defeats one aspect of the system. Each cylinder is basically a separate engine connected through the crank. It doesn't really care what the other cylinder's vacuum is...it just needs to create power and pulses in such a way that minimizes the imbalance of the explosions. So cylinder one needs to do it's job using settings on it - carb settings, valve settings, etc. - but really doesn't need to share its vacuum with the other side.

Am I missing something?

I do not know, but I assumed it was done on the Triumph to enhance the performance. The carbs were very close together, thus limited in size. With a 360 crank like they had, one carb is sitting idle while the other provides all the fuel to the cylinder since one is on exhaust and the other is on intake. Linking the carbs together allows the cylinder on intake to pull from both carbs, thus effectively increasing the gas/air going to that cylinder over using one carb alone. Again, this is guess a guess.

[8ninety8]

But, it's possible the emission experts really didn't know what exactly they were trying to acheive considering the extremely low number of these vehicles on the road for what is a very short time of any given season. Stop me before I get carried away.

Many of us Airheads are getting up in years and really don't run our bikes that much anymore. Wonder how many miles in the aggregate these older airheads actually rack up and how it would effect air quality, in the whole picture? It's really a hobby of sorts.

Had my bike into the State Inspection Garage last Aug. for an "Vehilcle Inspection." Don't know what they did, checked, tried to ask some questions after I pulled it in the stall, the tech. simply told me to leave the area and wait outside? 15 minutes later he came out and said I passed? What it passed, I don't to this day know, and wasn't about to ask. And that was with accelerator pumping Dels on. It's all a mystery to me that I don't want to know about. As for plumbing, if the exhaust valves run cooler, it would seem to be a good thing as it would save natural resources in the production of new exhaust valves instead of conserving the old ones? 3sense.

[AnnapolisAirhead]

Nope -- if they were there to serve as balance tubes (i.e., as an "improvement" introduced at the same time as the Pulse Air system in 1981), you would expect that the same "balance tubes" would have been present on all of the non-US market boxers. They weren't. The non-pulse-air bikes didn't have starter covers with cut-outs for the Pulse Air vacuum hoses.

I think it'd be more definitive if pre-pulse air system, US bikes didn't have that tube and we had a clear explanation as to what it's purpose was. Perhaps it was part of the air pulse system, I've never seen any diagrams and have always heard the dealers refer to it as a carb balancer (that wasn't really necessary).

I'll have to go check my '94 Mystic -- I was not aware of that -- learn something new every day! That said, for the reasons above, I'll stick with the rubber nipple caps.

The proper vacuum port screw also uses a fiber washer. I've always thought a rubber cap would have been a better idea.

[mneblett]

I think it'd be more definitive if pre-pulse air system, US bikes didn't have that tube

They didn't.

and we had a clear explanation as to what it's purpose was. Perhaps it was part of the air pulse system,

From Haynes' 2-valve twin service manual at p. 5.17-5.18:

"The second part of the system was introduced on 1981 models but is available as a kit for fitting to 1980 models. It comprises two rubber tubes which lead from each carburettor inlet tract to a connector in front of the air filter casing; a rubber tube leads from this connector to the left-hand valve of the two valves situated in the filter chamber itself. Another tube links the two valves inside the filter chamber. The function of this part of the system is to use the vacuum created in the inlet tract to shut off the air supply to the exhaust ports by closing the valves when the throttle is closed during deceleration, and to prevent the severe popping and banging in the exhaust system that would result if air were still injected into the exhaust on overrun.

Sorry, don't have a direct quote from BMW at hand, but this is consistent with the explanation from the Mothership.