This is a brief outline of a carb mod I did today.  The results were significant for low-midrange  response and power.  The UFO is made by Thunder Products
http://www.thunderproducts.com/u_f_o.htm, the same company that makes the Dial-a-jet.

These pics are a brief sequence of the steps involved in the installation.  I spent a few hours doing this, but was also playing with a few other things on the bike at the same time, since the tank and seat were off.  If done as a dedicated job it could be completed in less than an hour.
Keep in mind that this is a mod for a Mikuni VM roundslide carb only... not a stock CV or an Amal.  It may be possible to adapt this to an Amal if the measurements are close enough.

Turbulence created under stock slide.



Flow under the UFO modified slide has a higher velocity and is much smoother, resulting in much better atomization of fuel.




Locater disc used to figure where to drill hole for locking screw.



Hole drilled in slide



Countersink for screw



Checking to see if the countersink is deep enough for the screw to come through far enough


Yep, it is pretty deep.


UFO before installation


UFO after installation...note the shape of the bottom of the slide now...much better than the deep dish shape before.  It is very aerodynamic now.



This modication made a noticeable improvement in throttle response from just off idle to 3/4 throttle.  It was almost as if another lower gear had been added.  It pulled much stronger than before.  The UFO device is designed for low-midrange performance, not full throttle power, and it does it very well.

**Insert: According to the company info this carb mod will make a remarkable improvement..."Typical power increase is 1 HP per 100cc of engine size or more".

That is quite a statement.  One of the data sheets that came with the package shows results from 4 dyno runs on an 800cc 3 cyl engine.  There were 2 runs at half throttle and 2 at full throttle.  One eace was in stock form and the other with the UFO.    
By the way, with the UFO in place the pilot jet needed is drastically changed.  The 36mm VM  uses a stock #35 pilot jet, after I made the mod I started with a #20 pilot and it is still a little rich.  The instructions with the UFO recommend going HALF of the stock jet, so when starting with a #35 it should go down to a #17.5.  I did not have one, but did have a #15 and #20, so I used the #20.  It still puffs a little at idle when the throttle is cracked hard.

At 1/2 throttle the hp went from 105 to 117, and torque from73 to 82.  Full throttle hp went from 150 to 154, torque from 96 to 98.
If this holds true for most engines, then we should expect a 5-6 hp increase for the savage ...and that is a LOT.
*Not bad at all for such a simple and cheap change.
By the way, the part cost $30.

From the info on the website it doesn't sound like they will be making the UFO for any carbs except the VM's.  The UFO  was designed specifically for the VM in order to correct a specific weakness.

babbalou wrote on 12/31/69 at 16:00:06:

Lancer,

For countersunk screw, any chance we could get away w/ a flat head (e.g. button head)?  or is there some performance reason to use a flush mounted screw?

Thanks,

V

So, the idea is to smooth the flow by plugging the end of the slide. Why wouldnt a guy be able to fit something in there himself? Like, a piece of one of those plastic cutting boards?
I noticed some shaping of the parts in the pics, not just smooth plastic. Are those to fit something in the bottom of the carb? Would a flat hockey puck do the job?

Yep, at just $30 the UFO is cost effective as is, and is pretty easy to install.
A performance increase could be achieved with most any material that can be cut/drilled/shaped to fit the space in the bottom of the slide, but as mentioned would take more time.  It all depends on what you want to do; spend a few bucks or take the extra time to custom make a similar piece.
The point is though, when installed on the slide of the VM carb this thing does a remarkable job of improving performance.  Thunder Products claims (backed up with dyno tests as proof) that the UFO will increase power by 1 hp for each 100cc of engine displacement, which is amazing for such a simple mod but it does work.   Most of the power increase comes in the low-midrange too, which is great since this is where we ride most of the time.
For the Savage/S40 engine this means a potential 6-7hp increase by just adding the UFO to the VM carb.  I really wish I had the money to do an extensive dyno evaluation of the all the assorted parts available for the Savage engine.  It would be very interesting to see how it responds to the different setups.

If you want to see  the data behind the UFO performance, you can go to the Thunder Products website and read the dyno test results.

As a point of interest, the UFO is not just a smooth shaped filler for the slide.  It does have a unique shape on the bottom which has been developed through a lot of dyno testing to maximize air flow and atomization of the fuel air mix during engine operation.  I do not know what the performance increase would be on just a smooth filler piece but it would certainly be less than the UFO as developed currently.

Before taking everything apart for the rebuild I had the #15 pilot jet and had drilled the hole to just slightly larger than the max hole size that they mentioned in the instructions.  With that setup I had the adjusting screw out at about 2-2.5 turns out.  I felt like it was just a hair on the rich side but I had no black smoke and it idled fine.  I could turn it down lower if desired.   I did not get into solder anything.

I assume your throttle cable is not pulling on the slide and keeping it from setting down normally.  
What exhaust setup do you have ?

I went back and checked the drill bit size I used for drilling the hole and it was as close to a #40 size drill that I had, and it was just a hair under 2.5mm.
You might try that and see how it goes.

spacepirates wrote on 07/18/12 at 20:01:07:


On the VM carb, the pilot screw will lean the mix when turned OUT.  Turning in IN will richen the mix.  This is opposite from the pilot screw in the stock carb.
You still have 2 7/8 ths to turn out and make it more lean.