Removed her heart...

[racerman_27410]

  Crazy when this all started with arguably one of the nicest, best sorted FJs out there...  but look where we are now   

Frank

Its not a hot rod until it has a hot rod engine!   


KOokaloo!

[SlowOldGuy]

I DID NOT!   ...   We're talking about Keihin FCRs (or Weber DCOEs) which are not CV
Frank

My bad, I mis-read that as you were describing the behavior of stock FJ CV carbs.

That said, I still don't understand your logic in this statement:

"More dense air produces a stronger vacuum (due to higher velocity for a given manifold vacuum level) and you get more gas flowing, but the density increase is linear and the vacuum/velocity increases exponentially (see Bernoulli's equation for compressible fluid flow, specifically static pressure change), so you get more fuel than you do oxygen as temp drops and density increases."

The velocity and density are not directly related as it appears that you're implying in the first part of your statement above.  An increase in density will increase the vacuum produced, but it has no effect on the velocity.  Yes, the vacuum varies linearly with density and by the square of the velocity.  Which is why velocity is the dominant factor to fuel flow in any carb.

Even with a flat slide carb, any steady state (cruise) throttle position will produce a certain intake velocity that "should" essentially be equivalent to a CV carb at the same throat area.  The accelerator pump equipped F/S carb has a distinct advantage on acceleration which is why they feel so responsive. 

But like everyone is saying, tuning is the key to everything. 

DavidR.

[Flynt]

I still don't understand your logic in this statement...

I was making the counter intuitive point that, although the air is more dense when cold, the dominant effect is the venturi related vacuum increase due to velocity increase of the air.  The initial comment was "the more dense air needs more fuel, so you need a bigger jet"...  that's logical but incorrect.  My counter point is logical, but perhaps poorly worded...

Anyway, colder air requires smaller jets to get same AFR in a FCR carb...  agreed?

Frank

[JMR]

I still don't understand your logic in this statement...

I was making the counter intuitive point that, although the air is more dense when cold, the dominant effect is the venturi related vacuum increase due to velocity increase of the air.  The initial comment was "the more dense air needs more fuel, so you need a bigger jet"...  that's logical but incorrect.  My counter point is logical, but perhaps poorly worded...

Anyway, colder air requires smaller jets to get same AFR in a FCR carb...  agreed?

Frank

Given no other changes such as pilot jet, air screw setting, main air jet, needle height and taper? I asume you mean the mains? We always changed the CR air jets as the day went on getting warmer and warmer when road racing at Loudon to compensate for the heat. But that was many years ago and long before FCR's (though I believe the basic principle is still in existence).
I have run 39 FCR's for over 10 years with large K&N's just so you don't think I am unfamiliar with them. I do like the whistling noise they produce at low speeds when you open them up.
As a side not note....what are you using for a throttle cable mounting bracket and how have you tuned the accelerator pump? The RS pump set up is far better in my opinion as is the TMR's....I also have a set of those for another application. Not many other people in the USA can say that.

[SlowOldGuy]

I was making the counter intuitive point that, although the air is more dense when cold, the dominant effect is the venturi related vacuum increase due to velocity increase of the air.  The initial comment was "the more dense air needs more fuel, so you need a bigger jet"...  that's logical but incorrect.  My counter point is logical, but perhaps poorly worded...

Anyway, colder air requires smaller jets to get same AFR in a FCR carb...  agreed?
Frank

Well, I guess I can learn something new.  I'd always read the dense (cold) air needs more fuel and mindlessly repeated it.  However, working through several scenarios using Bernoulli, the math supports your conclusion.

Assuming everything else is constant, a denser air condition will create a larger delta pressure (higher vacuum) in the carb throat thus picking up more fuel.  The question is will the vacuum increase be high enough to draw sufficient fuel to compensate for the additional (more dense) air?  Your experience says "yes" and actually requires a jetting change to get back to the desired AFR.

It is counter-intuitive, but the math says you are correct.  Excellent observation!  Bravo sir!

DavidR.

[Flynt]

I do like the whistling noise they produce at low speeds when you open them up.

With ya there brother...  these things are about the coolest behaving multi-carb setups I've seen.  Very robust, nicely balanced, and with Frank's help they must be 80+% of full potential now...  and it is already verging on insane.  The whistle and the WOT roar are intoxicating.

As a side not note....what are you using for a throttle cable mounting bracket and how have you tuned the accelerator pump?

I'm using the pull only of the stock pull-push dual cable setup.  Wrapped the grip end around the back of the part originally meant for the return cable.  The Keihins have a very strong spring and no facility for the other cable.  The cable needed some adjustments, but is now engaged immediately and about 1/4 turn to full throttle.  The carb end mount was already in place to accept the FJ cables...  The accelerator pumps are as shipped...  they are close I believe as this thing is a tractor off the line and in roll-on.  I'm sure the dyno will right the wrong eventually, but it is sweet for now.

Frank

[JMR]

I do like the whistling noise they produce at low speeds when you open them up.

With ya there brother...  these things are about the coolest behaving multi-carb setups I've seen.  Very robust, nicely balanced, and with Frank's help they must be 80+% of full potential now...  and it is already verging on insane.  The whistle and the WOT roar are intoxicating.

As a side not note....what are you using for a throttle cable mounting bracket and how have you tuned the accelerator pump?

I'm using the pull only of the stock pull-push dual cable setup.  Wrapped the grip end around the back of the part originally meant for the return cable.  The Keihins have a very strong spring and no facility for the other cable.  The cable needed some adjustments, but is now engaged immediately and about 1/4 turn to full throttle.  The carb end mount was already in place to accept the FJ cables...  The accelerator pumps are as shipped...  they are close I believe as this thing is a tractor off the line and in roll-on.  I'm sure the dyno will right the wrong eventually, but it is sweet for now.

Frank

I modified the throttle bracket (cut/rewelded) to accept 2 Motion Pro cables for positive return with the stock throttle housing. It also eliminated the stock junction box associated with FJ's.
The accelerator pump comes in very early and has a long duration.....it gives that "kill" in the 5,000 RPM range that is oh so familiar with RS's.
You will find the dyno interesting if not frustrating. I've become more used to flat lines from FI at this point.
Also....check your float height...it should be 9mm.....try to find that number anywhere. It is rarely right as arrived and will ultimately effect all throttle readings.
  I'd also recommend John Robinson's "Motorcycle Fuel Systems Techbook" published by Haynes #3514, year 2000. Full of Physics and real world applications. John worked for Performance Bikes for years and was the British equivalent of Kevin Cameron. I believe John was a bit more adept at fuel and brake systems but that is IMO. Truly missed.
I remember tuning the Weber's from RC Engineering for the CB750's. That's all I have to say about that.

[racerman_27410]

+ 1 on checking the float height  =   very important. 

gotta love the noise....i love the way the slides jingle at idle.....like a pocketfull of quarters.....once the tach swings past 6 grand WFO all i can hear is the supertrapp and my own screams 

only real issue i've found with the FCRs is lack of a choke..... it can be a real bitch on a cold morning trying to get some heat in those big pistons.

three full twists of the throttle , hit the starter....and repeat.....and repeat.... until a stable idle is achieved

its well worth the effort though and i would do it all again in a heartbeat.




Kookaloo!

[Flynt]

...check your float height...it should be 9mm...

Checked when changing jets...  they we're right at 9mm measured per directions I have on FCR tuning.

Frank

PS - took my daughter (fav pillion) out to dinner last night on the bike...  she commented on how smooth and strong it feels compared to before, and compared to the K13S.  She's not a fan of the beemer...

[dixiethedog]

Good write up.
Because earlier i spotted the word weber mentioned ,can i slip a pic in please.
Weber'd FJ.

[Flynt]

Looks to me like speed would work against you with this thing.  Those beautifully curved, angle cut pipes about the position of your right knee are your air intakes...  It is the opposite of Ram Air concept and likely would create a vacuum in the carb intakes.  Dip Shit Design must have sorted this concept.  TURN THE FUCKING PIPES AROUND!

Frank

[FJmonkey]

Wouldn't that suck rocks and other debris directly into the engine? Form vs function. I don't see any filtering device but maybe I am missing something....

[Flynt]

Wouldn't that suck rocks and other debris directly into the engine? Form vs function. I don't see any filtering device but maybe I am missing something....

good point...  need a deflector shield or an intake screen of some sort.  Or maybe a brain?

Frank

[Klavdy]

Hmmm, someone's posted a few familiar looking bikes here






Some odd looking ones too,











Looks like "Black Sunshine" is there too,

[dixiethedog]

Hmm...keep up the good work,as i enjoy all forms of modded FJ's. Good bad and indifferent. The FJ chop used to have the intake pipes set in that postion for shows,as they looked better. Its not my bike,but a friends. Or it was his until he became bored one day and chopped it up with an angle grinder. So it doesnt exist anymore. Mad Englishman and all that...