Throttle Body Valve Question

[Mhanis]

This question is specifically about my Suzuki and not my Duc. On a TU250 forum a shade tree mechanic has spouted the virtues of removing the secondary throttle body valve to instantly improve low engine speed performance. I removed mine and I certainly noticed the difference right away. I figured since there were only two screws that needed to be removed and could easily be reversed I'd give it a go.

The question is, is there any chance of damaging the system/engine by doing this? To my simple understanding I cannot come up with a down side, but I thought I'd ask here and see what the collective brain trust thoughts are.


Mark

[Speeddog]

Do you have a link or some sort of information on what this 'secondary throttle body' actually is?

[Mhanis]

Here is a picture of it.



What you are seeing is the first of 2 different valves. The one that is BEHIND this one opens and closes with the throttle, however, this one ONLY opens once a certain RPM is hit. Because the little bike doesn't have a tach and I haven't installed one yet I am unsure of what that RPM number is.

I took the two screws out and removed the valve. That way the mechanism is still in place to open & close the valve (no error light) but the air is more free flowing at lower RPM's. I noticed a significant improvement in acceleration off the line. Keep in mind I'm not trying to race here, but it helps me stay in front of traffic starting from a dead stop at a light.

Mark

[Speeddog]

I imagine that's an emissions control device.

Does it have a manifold pressure sensor?

[Mhanis]

Dog,

I am currently waiting on my service manual so I cannot answer that question, but I did find more information. The proper term is the "secondary butterfly valve". Suzuki uses this system on several of their bikes, the following came from a GSX R 1000 page:

(3) SDTV (Suzuki Dual Throttle Valve) Fuel Injection System

"The new GSX-R1000 uses SDTV (Suzuki Dual Throttle Valve) system, contributing to combustion
efficiency while aiding throttle response and smoothing power delivery.

The SDTV system includes two butterfly valves in each throttle-body.

A primary butterfly valve is controlled by the rider via the throttle twist grip. The primary butterfly
valve is positioned closest to the intake valves.

A secondary butterfly valve is opened and closed by a stepping motor controlled by the ECM. The
ECM opens and closes the secondary throttle valve based on engine rpm, gear position and the
position of the primary throttle valve.

The stepping motor is directly installed on the throttle body.

The purpose of the secondary throttle valve is to maintain the air flow speed, or intake velocity,
when the primary throttle valve is opened by the rider. The primary throttle valve determines the
throttle opening, while the secondary throttle valve maintains optimum intake velocity.

The way the SDTV system works is similar to the way a CV carburetor incorporates a butterfly
valve controlled by the rider and a throttle slide controlled by intake vacuum. In providing what
may best be described as a digital CV effect, the system produces more seamless and more
linear throttle response, adding low-rpm torque as well as significantly reducing unburned
hydrocarbon exhaust emissions."

Obviously Suzuki believes it has its advantages on the small 250 motor, but I swear it pulls better with the secondary removed.

Does this clear things up for the brain trust or just serve to confuse things more?

Mark

[Speeddog]

IMO, the last six words of that are all that's relevant.

The rest of it is BS to make people think that it's doing something more than that.
Possibly on a Gixxer 1000 it's softening the power delivery such that unskilled riders aren't so likely to light the tire up getting out of a driveway.

[ducpainter]

IMO, the last six words of that are all that's relevant.

The rest of it is BS to make people think that it's doing something more than that.
Possibly on a Gixxer 1000 it's softening the power delivery such that unskilled riders aren't so likely to light the tire up getting out of a driveway.

Which 6?

...just serve to confuse things more?

[Speeddog]

Which 6?

...just serve to confuse things more?

Last six of the quoted text.

[ducpainter]

Last six of the quoted text.

I pretty much knew that. I just couldn't resist.

[Mhanis]

IMO, the last six words of that are all that's relevant.

The rest of it is BS to make people think that it's doing something more than that.
Possibly on a Gixxer 1000 it's softening the power delivery such that unskilled riders aren't so likely to light the tire up getting out of a driveway.

This was my line of thinking as well. The 250 really doesn't need the bottom end smoothed out as the power delivery is as about close to flat line as you can get!

So nobody here has a problem with the secondary valve removal?

I am learning quite a bit with this little bike as I am less afraid to jack with it than I am my Duc. For example, after stripping one of the screws on the secondary valve I learned about JIS screwdrivers! I have a small set enroute as we speak!

Mark

[Speeddog]

Does it have a manifold pressure sensor?

If it does not, I'd be a little concerned about it being too lean, as it's getting more air now.

[Howie]

Usually secondary throttles are used to improve low end and/or reduce emissions and increase fuel economy, though sometimes throttle response itself  can be a bit dull.  In the case of your Suzuki, um...well...dunno.

[koko64]

To my knowledge they were introduced to GSXR models to provide more lower rpm torque and tractability, but were removed on race bikes to have an uninterrupted flow through the throttle bodies at high rpm. I guess they might baffle out a bit of intake noise.

[Mhanis]

Does it have a manifold pressure sensor?

If it does not, I'd be a little concerned about it being too lean, as it's getting more air now.

Up a couple of posts ago when I said I couldn't answer that question this is what I was referring to. I am waiting on my service manual to arrive then I can likely answer that.

Mark

[Mhanis]

Does it have a manifold pressure sensor?

If it does not, I'd be a little concerned about it being too lean, as it's getting more air now.

The service manual arrived yesterday, I do have an "Intake Air Pressure Sensor".

So since it has that are you suggesting that I don't necessarily have to worry about running too lean Dog?

Mark