Emulsion Tube/Flat spot

[silicon_toad2000]

Thanks for the help so far people and Lucky I can't thank you enough for the website and the manual i downloaded from it.

Anyway, I've been working with Hi-Po carby's for a while, mainly webbers and Holleys. Quite often in Webbers there is a flat spot and they drive like pigs. There aren't a lot of wrenches (I like that word now) that know enough to fix it, they change the jetting and the idle mixtures, but its like a computer technician trying to fix your microwave. With the webbers the flat spot is mostly caused by the main fuel circut cutting in too early or too late. This is governed by the emulsion tube and the float level.
The carby's on the V are the same. What I want to know is have many people played with the float level or the emulsion tubes to get rid of the flat spot, are there different emuslion tubes available and how hard are they to come by.
The other question I have is with the "stumble" is that generally caused by overfueling or underfueling? anyone know?
They're a great set of racing carbs, take the air box off and you can chage the air jet, emulsion tube and main jet without any problem, almost as good as the DCOE45 webber for quick changes. Cold front comes in, pull in the pits and change the jetting... magic.

hmmm waffle, please excuse the waffle.
Cheers for any help.

[Riche]

  I don?t have a carburetor apart but memory tells me the Vision emulsion tube is pressed in and contains no holes in it?s sides like the Weber tube.  Being pressed in changing it would not be as easy as removing the main jet stack in a DCOE . Removing the stack from the holder and replacing the emulsion tube between the main fuel jet and main air jet is fast and easy. Plus there are no alternate tubes available for the Vision to my knowledge either. If the emulsion tube was the problem Yamaha?s fix for the 1982 model could have been much cheaper than the airbox/carb top fix they used.

I feel the flat spot in the Vision is caused by lean mixture. To me the off idle mixture holes (the idle circuit holes on the atmosphere side of the throttle plate) are to blame. They are just to small or there are not enough of them extending higher in the bore. This is just my opinion. Silicon, you can see them on your DCOEs from the outside by removing the idle circuit assess plug near the throttle plates. Plus note that different models of the same size DCOE may have a different configuration of holes. I believe this was to solve a similar problem with some engines they were fitted to. My experience with the DCOE is limited to just one car I owned and does not encompass tuning many different cars.

[Lucky]

We're talking apples, oranges & watermellons here...
Apple = 82 carb
Orange = 83 carb  (They're completely different animals, almost no interchangable parts)
Watermellon = The Vision Stumble

the 82 carb's e-tube is not pressed in they sandwich between the carb body & gaskett, and then airhorn, but does get stuck due to crud & corrosion. I allways pull them on a rebuild. (& they do have holes, otherwise they wouldn't be emultion tubes...)  

83's however screw in and additionaly have a jet integraly soldered on the top.

The flat spot is caused by too much air entering, & overwelming the carbs under snap throttle conditions. the flapper valve in the airbox closes (not completely) durring hard acceleration, reducing the volume of air entering the carb, and also slightly increasing it's velocity.

--Lucky

[Riche]

I stand corrected on the emulsion tube, freely admitting to being a newbe with this bike. :-[ The bowl gasket on both my carbs are stuck to the bodys. Not wanting to risk damage to them they were left in place during cleaning. I didnt dip but used Gumout spray.

Yamaha's solution for the 1982 to restrict the air flow during accleration sounds much like having a choke in the air horn of the carb. Something to give the slower to accelerate fuel mixture in the carb circuts time to catch up to the air so they could prevent the flat spot. A cheap way to make up for poorly designed circuits. My 1982 doesn't have the air box mod.

[Lucky]

We can fix that, I have a front carb top & airbox top w/flapper for sale if you want. that along with clean & properly tuned carbs make a world of difference.
--Lucky

[silicon_toad2000]

So the emulsion tubes... are there different tubes available that anyone knows about? ???

I had a quick look for a progression circut in the manual but i didn't see one, I'll have a closer look Riche, good advice.

I have an 82 Lucky, it had the airbox mod but some nutter has taken that off aswell as the yics chamber. >:(

Another question, if i was to place a stronger spring inside the accelerator pump diaphragm or a weaker spring on the shaft outside, that would make the pump spray slower and give the fuel circuts more time to catch up with the airflow wouldn't it. Has anyone done this?

[Lucky]

I seem to remember Jason Morris swapped out a bic pen spring on the pump rod on his 83, but 83's also have an additional adjustment.

[Riche]

I saw something about making a change to the pump spring in one of the service bulletins or on one of teh Visions sites help areas. It may be a part of the airbox up grade, not sure.

[Lucky]

It was a seperate TSB (dated Monday, April 12th 1982)

Paraphrased:
--Measure the rod from the bend to the pump lever, see illustration.

--Using a pair of pliers turn the collar to increase the pump rod length by 5mm.  secure with a drop of blue locktite.

unfortunatly, there is no mention of the original length, so there is no way to know what the length of a rod that has not had the procedure is...

--Lucky

[Riche]

At first opportunity mine will get measured. I believe it?s virgin.

Lucky I don?t want to go for an airbox kit right now. Some threads have reports of good results without. After all the other tweaks have been exhausted that will be something for me to look at. With so little time to ride or work on the bike there is no telling when that will be. But I did ride it to the Walmart yesterday  ;D, of course it?s been at least two weeks sense the last time it was run. :( The bike starts and runs great... execpt for that flat spot. The last time I did anything to it except for fluid change was at least a year ago. Plus I've never sinked the carbs and that should help some.

[Lucky]

Carefull tuning should help a lot, but you probably won't be able to eliminate it all.
BTW, I found my TSB's and it's noted (twice) that 60mm is the Max length.
--Lucky

[Ken Williams]

I believe the vacuum operated air box flap is the best single fix for the Vision flat spot.  Increasing the fuel pump delivery should help also.  

The emulsion tube is another area that can be investigated for potential improvement.  To richen the low midrange try soldering shut one of the upper holes in your emulsion tubes.  You can do this with the quick application of a small bit of rosin core solder with a low power soldering iron.  Done as I intend, it?s more like sticking bubblegum over the hole.  Don?t get the tube warm enough for the solder to flow.  If you like the result or can?t tell the difference, try closing the second top hole or one of the second from top holes.  You can always remove the solder if you don?t like the results.  

[silicon_toad2000]

I hadn't thought of doing anything with the holes in the emulsion tube. That's a good idea. I'm thinking though that with any given air jet, less holes would still flow the same volume of air through the emulsion tube, just faster through each hole.
If less holes did mean less air, I don't think it would have too much effect on the low rmp fueling, but rather enrichen the high rpm fueling.
What I was looking for was an emulsion tube with a higher step than standard as this would enable the main fuel circut to come into play earlier.
Maybe I'll have to buy a few spares and find someone with a nice accurate lathe. ???

[Riche]

Silicon I hope you get the emulsion tube mod to work. But my opinion is the main metering circuit is not coming in soon enough without the airbox flap restricting the air. Restricting the air into the airbox should decreases the pressure between the flap and the throttle plate. The same pressure inside the float chamber (atmospheric) brings the main circuit in quicker. By quicker I mean less/slower air flow is needed at the booster venturi to cause a low enough pressure to get fuel through the circuit. If Yamaha had more off idle holes or maybe larger holes above the throttle plate maybe there wouldn?t be a flat spot. Again just my thinking which could easily be flawed. I?ve little experience with this machine as you can see from my earlier posts. What I know about the airbox mod is only what I have read about it. Good luck

[Ken Williams]

The following is my understanding of Vision carburetor function relevant to the Vision stumble.  Please feel free to comment if you see errors or omissions.  

Midrange fuel mixture is controlled mainly by the carburetor emulsion tube and vacuum present in the airbox and venturi.  Air is drawn from the airbox, through the center of the emulsion tube and through the emulsion tube holes where it mixes with fuel.  The fuel air mixture is then drawn through the venturi and into the carburetor body, below the throttle plate.  

Carburetor vacuum drops precipitously when the throttle is suddenly opened at low RPM. Under low vacuum conditions, the small amount of air that flows through the emulsion tube can move through the top holes and flow directly to the carburetor venturi, picking up very little fuel.  This transient lean mixture condition is the source of the Vision stumble.  The extent of this effect is quite dependent on float level and exact placement of the topmost emulsion tube holes.  The vacuum operated airbox flap richens mixture under this condition by increasing vacuum in the airbox and carburetor throat and bringing the two vacuum levels closer together.  Another way to richen the midrange is to close off one or more of the top emulsion tube holes.  This will richen the entire midrange a little bit.  The biggest effect will be when the throttle is opened from low RPM and also during operation at low midrange throttle.  The mixture change at constant throttle becomes progressively less as throttle position is increased.  

[Lucky]

That sounds reasonable in theory, the proof is in the pudding (and the 83's, which have different tubes) if you want to try it out, I have one set of 82 tubes i'll donate to whoever wants to stick some solder in their carbs.

I'd REALLY like to see dyno numbers befor & after, with CO levels, that would be exceptional, because as we all know, what works on one Vision, defenatly does not apply to all, especialy when it comes to carb tuning.

Keep in mind, in places like Calif, they have to pass emissions tests (although you can allways swap out tubes)

Hmm, it occures to me that a coupla o-rings on the upper part of the tube would cover a few holes, and allow for some experimentation.

stuff to chew on...

--Lucky

[Glyn Pickering]

Bit of a change of tack - has there been anyposts in the past about completely blocking off the YICS circuit and all it's associated tubing. If so what is the effect? Also have any other carb types - like from a 650 Honda Hawk been used on the Vision. Wouldn't trying this be a worthwhile project to try and rid these problems??

Regards

Glyn

[Lucky]

I don't think anyone has really persued different carbs because it's not needed. With clean and properly tuned stock carbs, these bikes haul. (changing the 82 pilot air jets to 135 is the only change i'd reccomend)

Several people have just capped the YICS at the heads, and like the resuls that way. I prefer it on, I think the bike starts easier & I think my fuel milage is better with it on.

--Lucky

[ArrrGeee]

Hi Ken,

 

his transient lean mixture condition is the source of the Vision stumble.

depending on the model, dyno charts that have been posted here generally show the vision carbs as being too rich in that area.
personally I think it's more a matter of airflow and velocity than rich or lean conditions as a generality, meaning the venturi is a bit oversized for the vision, However this is why they keep putting out power up to almost 10k and yet still ridable around town.

http://www.ghetti.net/cgi-bin/ROVBB/YaBB.pl?board=general;action=display;num=1082243899;start=4#4



http://www.ghetti.net/cgi-bin/ROVBB/YaBB.pl?board=general;action=display;num=1081632639;start=1#1


also, if just modifying the emulsion tube would fix the vision stunble, why wouldn't the engineers have done it when it was new instead of the funky flapper in the airbox.  They easily could've issued a tech bulletin to fix it and saved thousands.

it's good to experiment though and look at things with a different perspective.  btw my bike has virtually no stumble problems.

ymmv

-Ron

[Ken Williams]

Hi Ron,
Thanks for your response.  I certainly agree with most of what you state.  The carburetors are big for the displacement they are feeding.  When the throttle is snapped open at low RPM, venturi velocity and airflow drop to low values.  Vacuum drops likewise unless the airbox flap creates a sufficient obstruction to airflow.  On some Visions, this results in stumble.  During stumble the mixture must be incorrect, that is lean, rich or perhaps not adequately emulsified.  My best guess is the mixture is lean during stumble, at least for 82 model, before vacuum operated airbox top and modified accelerator pump stumble.  Both of these fixes increase delivery of fuel when the throttle is snapped open at low RPM.  

I also believe the vacuum operated airbox flap is the most effective device we have available to eliminate stumble.  I never intended to imply any other easily applied fix was more effective.  My discussion of the emulsion tube was intended to suggest a potentially fruitful area to explore if one did not have the vacuum controlled airbox flap.  I understand Ducati Paso stumble has been successfully treated with this technique.  

I had not previously seen a Vision dyno chart showing fuel mixture.  Although I do not know the status of the test subject, my bet is that it has the vacuum operated airbox flap.  If so, it would be interesting to see what the flap was doing as a function of RPM.  Probably, the overly rich low RPM indicates the airbox flap is working too well.  There may be a lot to be gained with careful timing of the airbox flap and jetting to match.  If someone successfully implements dual throttle plate throttle injection I am sure all our stumble problems will disappear.  My bike also experiences virtually no stumble problems.

Thanks,

Ken