Who's up for a CV Carb Discussion?

[andyb]

Because you are changing the spec to the unrestricted version, you may do well going up on the mains a bit.  Recommended DJ118 mains per dynojet, but they're likely a little on the rich side.  Might be a good starting point for you though.

Also you have the "wrong" needles, judging by the part numbers it looks like they were developed at the same time that the others were (similar part numbers). 

Personally I'd go up on the mains at least a size or two, and then see how the midrange is; you may end up needing to go to the US needles.

[fj1289]

Holy cow!  Lots of great info.  Between David's and Bill's posts, I'm still working to separate in my mind the whole throttle/airflow/fueling separation.  I feel quite comfortable with non-CV carbs, but still trying to better understand what is happening inside the CV's!  The difference seems to be the CV carb is very much airflow DRIVEN vice airflow DEPENDENT in my mind!  With the non-CV carb it is up to the operator to operate in such a manner as to ensure you retain enough airflow not to bog the engine -- the operator must make sure not to demand more than the carb can deliver.  Whereas the CV carb is going to maintain the airflow and hopefully that results in what you are asking for from it -- hopefully the carb designer (and carb kit designer!) has come up with a balance that responds well to the operator's demands. 

That may not make sense to anyone else, but I think I can live with it!

Still, it's all theory to me, time to experiment!

I'm rebuilding the carbs - just got my David Raforth carb goodies in today!  When Randy gets back from his WORLD championship banquet, the correct sized needle seats, needles, gaskets, and a few main jets (I live at 4000' altitude) will be on their way too.  The plan is put the carb setup back to stock.  Install the Factory jet kit per instructions, and tune for best performance and responsiveness.  Then I will try the  Chinese menu of combinations of heavy spring vs light and stock slide vs double drilled slide.  If the double drilled slide sucks as bad as it sounds like it should, I'll try some JB weld in the stock hole and leave the second drilled hole for another combo of both hole size and location. 

Also, don't forget that Dynojet jet sizes DO NOT equate to Mikuni jet sizes!!  One is based on a flow rate, the other on hole size - and they do not correspond!  There are some charts that give a rough comparison - I'll try to find it and post again - and put it in the carb files too. 

Thanks for all the inputs from everyone on this.  Very informative and thought provoking.       

[SlowOldGuy]

DO NOT double drill your slides!!!!!!!

That's a waste of time and good slides.  SImply install teh kit per recommendations and don't get greedy with the jetting.  Remember, more fuel does NOT necessarily equate to more horsepower.  The CORRECT amount of fuel makes the best power.

CV carbs are all about maintaining the velocity of the air flow through the venturi.  The throttle plate controls the air flow amount, the slide attempts to maintain a Constant Velocity to that air flow (thus the CV). 

You can prove this by holding the throttle at elevated RPM (say 3K), reach in the carb (assuming no airbox) and lift the slide.  The motor will NOT rev up (thus proving that the slide is not metering the amount of air).  If anything, the engine may bog down from excess fuel.

DavidR.

[racerman_27410]

CV carbs are all about maintaining the velocity of the air flow through the venturi.  The throttle plate controls the air flow amount, the slide attempts to maintain a Constant Velocity to that air flow (thus the CV). 
DavidR.

i can verify this by using my 1349cc motor as an example....... you would think a big honker would need big ass jets.... WRONG!   the big pistons make a bigger pump... bigger pump moves air faster thru the venturi  hence drawing more fuel up....

i wound up dropping two main jet sizes and going back to the stock pilot jets in order to lean the bike out.

had to lean the heck out of the keihin's also..... more fuel is not always a good thing..... its all about the big picture. 

KOokaloo!
Frank

[andyb]

DO NOT double drill your slides!!!!!!!

That's a waste of time and good slides.  SImply install teh kit per recommendations and don't get greedy with the jetting.  Remember, more fuel does NOT necessarily equate to more horsepower.  The CORRECT amount of fuel makes the best power.

Hell, if you've got slides that are already drilled up, you're going to have to epoxy them anyhow so...

CV carbs are all about maintaining the velocity of the air flow through the venturi.  The throttle plate controls the air flow amount, the slide attempts to maintain a Constant Velocity to that air flow (thus the CV). 

You can prove this by holding the throttle at elevated RPM (say 3K), reach in the carb (assuming no airbox) and lift the slide.  The motor will NOT rev up (thus proving that the slide is not metering the amount of air).  If anything, the engine may bog down from excess fuel.

Having had slides physically get stuck in the topped out position, I certainly felt that it was horrifically too rich.  Now, thinking about it, it's possible that it was actually too lean from the 1/8 throttle and further open until around 6k and WOT.  With the throttle blades closed, the mix is coming from the idle circuts on the far side, but once you let air in, it's just having massively too big of ports in a way; a total lack of velocity.

Tuning WOT jetting is easy!  Go to the dragstrip, assuming you're relatively consistent on launches, and compare your back half mph gain (comparing 660' mph to 1320' mph).  When you stop seeing gains, back down to the smallest jets that gave the best numbers, and you're done!  It will be interesting to see what happens with the new motor when it comes time to start dialing it in; stock displacement but loads more airflow and higher static/dynamic compression, depending on where the cams decide to end up at.

CV's to me seem a little like some of the newer FI bikes with the secondary throttle plates.  You can snap the primary butterflies open, but the secondary ones only open a given amount depending on RPM and load, as programmed into the ECU.

Dynojet sizing is based in mm.  A DJ100 jet is a 1.0mm orfice.  I believe this is the way the Keihn's are also.  The Mikuni sizes are based on flow, so a 100 jet flows 100cc/min of fuel (at what pressure I have NFI).  Conversion that I found online was (orfice size * .9375 = mikuni size).  You end up in between regularly, though.  I also may have copied that backwards (may be division rather than multiplication!).  Don't have a set of jets in front of me to compare.

When running the FP emulsion tubes, according to their site they atomize better and you can run leaner jetting becuase you're getting a more complete burn.  Dunno!  There's a load of factors that change what optimal is for that motor, so it's always preferrable to set your jetting at the track (IMO.. some will say a dyno is better, but I don't ride on a dyno as often as on pavement, so tune with what I know...)

[fj1289]

Randy,

not to worry - I have a set of slides that were already double drilled.  Only thing I can figure is someone didn't read the directions very well, and drilled a second hold instead of enlarging the existing one.  The way the dynojet instructions show a drawing of the needle "cover" in the slide, I guess it could lead you to think there should be another hole -- if you don't read the instructions!

I saw a while ago somebody using carb jets to "reclaim" slides that had been "ruined" by  drilling.  They basically drilled the slide hole large enough to tap it for a jet.  Don't remember any more details than that though. 

Putting the carb bodies back together tonight, again, for the fourth time!  Don't forget the float bowl vent "T"'s nor the fuel transfer tubes!  Don't forget to rotate the float bowl vent  "T"'s where you want them before you put the carb bodies together (not enough room for them to rotate when it is all assembled).  Finally, don't forget to put the starter plungers back in before putting the carb bodies together too!  Oh, and probably best not to do all this tired too!

Big thumbs up to Randy for an excellent kit!   

Chris

[SlowOldGuy]

Someone must be really tired......

DavidR.

[andyb]

Indeed.  Tommorrow, he asks himself how he got the big side of the carbs in the intake boots...

[fj1289]

Indeed.  Tommorrow, he asks himself how he got the big side of the carbs in the intake boots...

...I thought it would improve performance, as the airflow expands going into the head, the pressure will go up and make a "supercharger" effect right?!   

Now I've got to get a 2mm allen wrench - the T-handle wrenches that were not in my "trackside" toolbox were in the trailer too... 

This little simple carb "cleaning" has gotten way out of control.  Since the carb bodies looked so good coming out of the chem dip and I had Randy's carb kit with all the pretty allen head bolts, I decided to clean up the carb tops too.  Well, they were stained so bad that by the time they were "clean", I had gone through the cad plating.  So, I took the rest of the carb tops down that far as well as the rails, and painted them all black with some Color-Rite truck bed liner paint.  Has a very slight texture and is supposed to be tough as hell - I had bought it to do the dragbike frame and swingarm.  Looks great with all the stainless steel hardware.     Best looking part of the entire bike will be hidden from view!

Chris

[andyb]

Looks don't make your bike quicker...

(Or your ugly mug would never get anywhere at all!)