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I think it has to do with increased friction of the air/fuel mixture going through the smaller 39mm as opposed to the 41mm.

I know that when you are trying to get the most water through a pipe, regardless of the soucre, a bigger pipe size will accomodate more water because of less friction.

So it follows that smaller bored carbs require richer mains, etc. to do the same job as larger bored carbs.

I have never actually tuned a carb, but I have layed a fair amount of pipe :eek:
 
All else remaining equal.

Same volume of air flow. Same motor. A 450 always gulps the same amount of air, conservation of mass. The only difference is the 39mm vs 41mm and the jetting. Mass air flow equation. The only parameters to change between the two are Area. Until the Reynolds number approaches 750ish, or the speed of sound (ish), or turbulent air flow, these principles should apply.

-Parsko
 
hey fry

i left my answer above my lesson!

whatever it is lineaweaver had to edit his clue six times!! yes six times :twisted: :twisted:

he's on the edge!

regards

Taffy
 
Does the larger bore result in more of the needle jet shroud protruding upward from the bottom of the bore?
dan
 
Re: Answer

Bored on center the smaller Venturi FCR places the fuel discharge farther from the float bowl. Such is akin to lowering the fuel level which does indeed reduce fuel delivery all else remaining equal.

Dale
 
but they aren't bored on a centre line dale. they're bored so that they always have the same bottom edge and all the material is taken from the rest as in 2mm from the top when going from 39 to 41mm.

this can be seen when looking from the end nearest the head. the removable alloy tube has been eccentrically bored and only comes good when opened to 41mm.

regards

taffy
 
Not on all of them Taffy.
It seems now the majority of FCR MX 39 and 41 are bored on center whereas the 37 and 35 are not.

In addition not all 39 and 41 FCR MX units share the same height. Out of three 41 carburetors two had different measurements. 8O

Dale
 
no probs dale and all good fun!

my fully cast 39 had a half moon crescent over the top of material. the 37 i did for scoot and it's in built screw in spigot was bored off-centre- a pig!!!

i think you marked a few cards though...

regards

Taffy
 
FCRMX2.thumb.jpg


FCRMX1.thumb.jpg


Both 39 mm

8O
 
Not the answer I was expecting,but interesting.That being the case, any idea as to the correlation between float level height to discharge height as it relates to MJ size. ie. 1mm height change=1 MJ size.
 
nsman said:
Not the answer I was expecting,but interesting.That being the case, any idea as to the correlation between float level height to discharge height as it relates to MJ size. ie. 1mm height change=1 MJ size.
Click to expand...

Truthfully speaking, "not a clue".

Such can certainly be calculated mathematically, however, empirical discovery would prove more efficient and ultimately more accurate as well.

Dale
 
all you guys in this thread are waaaaaayyy tooooo smart for me to understand!
Bottom line......
If you wack the right hand WFO does she sh*t and get?
Sorry even comprehend what the question was now!
hehe
 
Very interesting Dale.

I was not even thinking about that either as someone else stated. Is this phenomena only on the 39 => 41mm FCR MX carb or is this a general rule across all carbs? Why I ask; Is it common practice for a mfg to use the same casting and molding but always bore according to the "true" centerline or is this particular carb the exception going from 39mm to a 41mm?

Regards,
 
Caught the part about being bored on a centerline, but still not sure as to whether the needle jet shoulder in the carb casting remains precisely in the same place or not. If it was not changed, shroud would protrude upward 1 MM less on the 39. Remember the Yamaha wrench report that had you filing precisely 1 MM off the needle jet shroud to weaken the signal thus leaning the midrange on certain models?
dan
 
Dale,

I respectfully disagree. I don't agree that the only reason is the difference in height. I think it also has to do with the bore sizes. Let me know if you agree on this...

Let us assume the 41mm carb distance to the fuel level is 20mm and 21mm for the 39mm.

The difference in areas btw 41 and 39 is roughly 10%.

The inlet and outlet sizes are the same. Flow is the same. Thusly, velocity must be higher in the 39mm carb. Higher velocity yields higher vacuum.

Since the jet delivery port dimensions are all the same, the higher vacuum must suck more fuel, and thus be able to suck this fuel a higher distance. So, with a 39mm carb, the distance might actaully be 22mm. With the same jet, that yeilds a rich situation, right?

Now, assuming the height to the float is 20mm, and you change this 1mm either way, we are looking at a 5% difference in required vacuum to draw fuel.

Would not one offset the other, to a certain extent? I understand the numbers are still fuzzy, but my point is that I think the size comes into play too.

Any thoughts?? Educate us!!!

-Parsko

PS - I might have to edit this. It's been floating in my head all day, and I need to think about it more.
 
Hi Dan,
The FCR carburetor bodies are multi piece. The fuel discharge block is based on Venturi diameter and placed in the same location with regard to bore centerline between carburetors.

Mikuni Big body 40 MM and 44 MM "Round Slide" carburetors are bored on center. "All else remaining equal" the later most often calling for the same or leaner settings than the former.

Hi Luke,
I am quite certain a multitude of variables contribute to variations between actual jetting requirements. However, "all else remaining equal" a stronger signal will increase fuel delivery. As with most everything , their are exceptions to the rule.

My personal conclusion was derived via empirical logic and reverse engineering as opposed to a mathematical equation.

In short, I noticed "all else remaining equal" when switching from an FCR 39 to an FCR 41 I was most often required to implement leaner settings primarily regarding mid to full throttle. I simply pondered the reason.

Nothing carved in stone mind you, merely an observation mixed with a bit of logic.

I implore the curious to explore all boundaries. :D

Sincerely,
Dale
 
sandskipper said:
Very interesting Dale.

I was not even thinking about that either as someone else stated. Is this phenomena only on the 39 => 41mm FCR MX carb or is this a general rule across all carbs? Why I ask; Is it common practice for a mfg to use the same casting and molding but always bore according to the "true" centerline" or is this particular carb the exception going from 39mm to a 41mm?

Regards,
Click to expand...

Hi keith,
I can only guess for ease of manufacturing as the carburetor has been around far longer than the CNC machining center.

Much like the primary gear issue of Nsman, one must ponder from what reference comes the initial bore centerline?

Indeed, the dreaded tolerance stacking issue.

I find comfort in such matters as the many variables insure a need for the experienced tradesmen. :D

Kind Regards,
Dale
 

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