Econo...(Power?) jet carbs.

[rminier]

 In the late 70's, power jet carbs became very popular from the OEM's. Mikuni and the rest of the world called them "Power" jet carbs. Kawasaki used the term "Econo" jet, which was actually a more appropriate term.
  We had moved through the OPEC oil embargo in 1973, and became much more concerned about fuel efficiency.
 Polaris used them...Ski Doo was a little late to the party, but used them in the '93 Mach Z triple/triple 780 top dog lake racer...not exactly a " grocery getter".

  My admiration for Mikuni is huge...not exactly some Chinese, Taiwanese, Mexican, Indian outfit.
 I love the signature of one of the guys over on "Vintagesleds", which is "Midrange carb tuning is for wussies".
  Anyone snocrossing, drag racing, cross country racing is either WOT or hard on the brakes.
 The power jet carb evolved to precisely meter the fuel/air mixture from off idle to nearly WOT.
 The early Mikuni round slides had a needle with a single taper that metered the fuel in the mid-range...then they evolved to a double taper....eventually I think they moved to a triple taper configuration.
  Good grief, Charlie Brown...I would need a chassis dyno to figure out precisely which needle is perfect.
  I'm surprised John Deere didn't dwell on the "Econo" part...they used the term "Power Jet".
  They excelled at some mileage runs in the early '80's with their Trailfire achieving around 40 mpg...considering the fact that a team of engineers must have been devoted to maximizing fuel efficiency...Those JD guys weren't deadbeats...dedicate yourselves to winning this...dial the rear skid to relieve ski pressure, dial back the track tension, squeeze the carb jetting, pop the secondary clutch spring back 1 hole to limit RPM and hold everything in "overdrive".....
  So, I have to believe there is absolutely nothing inherently wrong with a Power Jet carb.
 They have 3 distinct, minor disadvantages.....
  1.  The rubber o-ring, that is not in a "regular" Mikuni. They worked great for several years when the sleds were new, but were a potential wear item. This is compounded by the fact that the o-ring fit in to the bottom of the float bowl, where...if any water had accumulated over the years, it would corrode the aluminum and prevent a perfect seal.
  2.  Power Jet carbs had to cost a few more bucks to manufacture....another jet, another passage in the bowl, additional fittings on the carb to accommodate the fuel tube, etc. Can't be much....but in the absence of a CLEAR advantage... Particularly when Mikuni developed more options for the needle to precisely control the mid-range mixture.
  3.  Jet changes...a regular Mikuni you can remove the plug from the bottom of the float bowl and use a nut driver to change the jet in a couple minutes. A Power Jet carb is much more of a hassle...you 'bout gotta yank the carb and remove the bowl to access the jets.
 
  So, there is absolutely nothing wrong with a Power Jet style carb...it boils down to the condition that they are...no need to assume any rush to replace them with a standard Mikuni...Power Jet carbs can be made to perform spectacularly.
  Mikuni had to be a PITA...there are some variations to the Power Jet carbs...thanks, Mikuni...can you make life any more complicated?? My '83 Yamaha Vmax has an additional fitting at the top of the the fuel tube that has an air jet....to begin introducing air, or emulsifying the fuel flow into the carb....available in different sizes
  Aftermarket kits were available to convert a regular round slide Mikuni to a Power Jet style...I'm sure they sold thousands.....I doubt that they would have sold as much if they had used the more accurate "Econo. Jet" designation!!..
  One thing has me a bit " stymied" frankly...
  I have read on here, and some other sites, that Power Jet carbs are more sensitive to removing the air box than a standard Mikuni.
  I am on here to learn and share...please sound off your thoughts...I'm having a little trouble with this one.
 Carbs operate on a very simple law of physics...The venturi  constricts the airflow thru the carb, speeding up the air velocity...which lowers the pressure of the air...which draws the fuel up thru the jets in the bowl...which is exposed to stationary, atmospheric pressure.
  Are Power Jet carbs really more susceptible to removing the airbox and altering the airflow...or is this just an "Old Wives tale"?
 Removing the air box from any motor, with standard or Power Jet carbs....you better consider the need to change the jetting for the change in airflow.
  Remember...I don't have a clue what I'm talking about, so throw what you think out there!!!

 Sorry for the long rant...but the snow is gone until next year
 

 

[Tory944]

My understanding is that the Kawi econo jet carburetors need the vacuum the air box creates to work effectively.  Not just a lean or rich situation.  You can run standard Mikuni rounds w/out the box...  just need to rejet, but fuel delivery with these works w/or w/out the box (unlike the econo's).

[Tory944]

  The Econo Jet carburetors need the air box in order to provide the pressure drop for the Econo Jet circuit to work properly.  Pods or air box delete will not provide the proper vacuum.   Tory

[Checkmarks]

Lets start with the advantages.

The PJ carburetor has an additional passage that can be metered exclusively.  How does that help?  Think Turbo.  When the rpm us under no boost pressure jetting needs no manipulation.  When the boost happens than you can just exclusively manage manipulation for that sit.  Nitrous is another example.

Saves fuel mid range, then my throttle knows two positions idle and WOT

That is all I can think of for advantages

With respect to RM disadvantages.  The o ring and water??  Drain your fuel EVERY years end.  Put it in your car.  Changing jets a pain in the butt? I agree when racing with PJ changing jetting sucks.  When trying to squeak out 4 more horse to go out with your buddies, that is more about drinking the two beers before you go out in the cold.

AIRBOX, but why
So you can have the correct metering of fuel when you use the "stock" jetting.  The jetting is manipulated by the partial vacuum created by the airbox.  If you have no understanding for two stroke jetting (and when you do you are only dangerous) and do not want pistons with holes, stick with what works.  Example:
Son, "But dad why wont my sled start I want to go sledding with my friends."
Dad, "you are going to have to replace the piston and cylinder, that sled is only for water cross in the summer, I told you, not to take that sled."

Sorry was worked up.  Air box if you do not have it on your sled, it will be LEAN.  Lean means BAD. Even if you followed the book and had their (Saki) "correct jetting" for that altitude and temp, LEAN.

Now there is argument for MUST have air box to the degree Tory points out.  Then I do not subscribe to the must have air box.  14.7psi air pressure at sea level 12psi at altitude.  Pressure in the air box...  Food for thought.  My broad share is run the air box it draws colder air into the motor and that makes 4 more horse.  Besides carburetors spit out raw fuel with the box off (fire hazard) run the air box.  A person could drive the sled with no gas cap and slosh fuel when they take off, run the air box.  Two strokes are dependent on air box and all the pieces between, to the exhaust exit.  Run the air box.  But I take my air box off and it runs fine.  Run the air box change your jetting.  My v8 motor runs fine with seven spark plug wires, run the air box.

[Tory944]

Ha, ha...   good read!  I am not sure if my post was answering the original question...   There is very little information about jetting and running air boxes through books I've seen/read.  The information I obtained concerning air box "do's and dont's" when using PJ carburetors was taken from a Yamaha service manual using the same style carburetors.  Just wanted to clarify that my info was not taken from my "book of bs"...   lol.

As far as the air box theory alone, I agree with Checkmarks info.  I run standard Mic round slides, but I also use the stock Kawi air boxes.  I modify the box (if running drags or radar runs) and leave the box internal tube/baffle configuration alone with stock sleds (for debris purposes).  When gutting the box for racing, I also make a dual snorkel box which basically complements the snorkel on the right side of the box.  I take a trashed box and rob the plastic elbow and add to modified box (left side).  I run longer hoses off the elbows and position them closer to outside air (vents).  I've seen a noticeable improvement in this setup on my 4/6.

Pods rob h.p. and K&N's do "straighten out" the airflow which contribute to hp gains.  Problem...  unless you run under hood ducting to relieve hot air and bring in the cold your intake temp will be "warm" at best.  No hp advantage there! 

"Dad?"   

"Sorry son, you can't take the Interceptor...  but the Fanner Drifter is o.k. to use..."      lol.

[Boomologist]

Back in the day we would start our run at the base of the mountain with Airbox on. Near the top of the mountain I would need to stop and take the airbox off to maintain power.