Test Your Knowledge

Are the slides and carb bodies identical?
dan

I'm going to guess the pilot signal is enough stronger on the 39 to require a smaller pilot jet hence the need to richen the needle to balance things a bit.
dan

I am going to guess that the 39 is woefully undersized for the needs of the 450 and actually has a weaker venturi signal, due to a choking effect, than the 41.

Dunno,But Think It Might Be The Venturi Affect,BIG-small-BIG Works Better On The 41 Than The 39???
Or The 450 And Larger Engines Is Designed With The 41 In Mind,Then They Half Arsed Make An Enduro And Chuck In A 39 In Order To Gain Some Low Down Torque Without Proper Testing :!:

I think,
the fcr 39 and 41 we're made in the beginning for road bikes,when you take the smaller carb you must enlarged the jets or change your timing on the squirt,for the reason I think to compensate the signal loss during acceleration or speeding up,from low rpm 's.

My experience give me the following answer on my bike.(xr 680);
The venturi signal will remain strong,also there is rapid throttle opening the reason there for is by the increased displacement.

I am curious for the right answer

Size
"All else remaining equal"
Same size gulp of air and Jets.
39mm= smaller area => higher velocity => higher vacuum
higher vacuum = more fuel required
smaller jet = less fuel capacity

Right?

-Parsko

Parsko said:

Size
"All else remaining equal"
Same size gulp of air and Jets.
39mm= smaller area => higher velocity => higher vacuum
higher vacuum = more fuel required
smaller jet = less fuel capacity

Right?

-Parsko

Click to expand...

But 41 Is The Bigger Carb,So I'd Have Thought Bigger Carb=Bigger Gulping?
The Sock Is Restrictive Anyways,Then The 39 I Suspect Would Restrict It Slightly More??????????

all i can think is that the air speed is slower in the 41 and there is reversing of the charge slightly. thus the air picks up fuel twice on the 41. so we jet it down. the airspeed on the 39mm is very fast so there is less or nn reversion and so the bike has to be fueled properly or as one would have thought in the first place before i brought this up!

we await o jetting superhero!

(i can't believe i just put that dribble!)

regards

Taffy

It would be logical to assume that the volume of air comprising the intake charge would be travelling at a higher velocity through a 39 than through a 41.Since the carb is smaller and the air velocity is higher the engine will be operating more efficiently and will be able to atomize and burn more fuel allowing for a larger main jet etc.

nsman said:

It would be logical to assume that the volume of air comprising the intake charge would be travelling at a higher velocity through a 39 than through a 41.Since the carb is smaller and the air velocity is higher the engine will be operating more efficiently and will be able to atomize and burn more fuel allowing for a larger main jet etc.

Click to expand...

High Velocity Is Good,But That Means It Pick Up More Fuel Through The Main/Pilot Jets,Thats Why Taffy And Dale Always Say To Start Low Then Go Up? Slower Velocity Means You Need To Have a Larger Jet/s To Get The Same Affect???
Or Have I Got This Completly Arse About Face :?

what you want is the most power and there is usually a mixture ratio that defines this. valve overlap, exhaust gases mixing in etc knock out any CO readings so it's the dyno that you go by-it's the bible.

the faster the air, the stronger the signal down to the MJ. and the amount of fuel picked up by the engine goes up many fold over the air that it gets. so you jet down as power increases if you still have the original carb. the only exception i know of are the things that make it easier for the air to get in and out thus dropping the signal strength and they are: removal of air filter lid/air filter and an open exhaust. for these you jet up. after that (well i trust you'd do these things first, right?) you jet down. it's like an inverted 'tick' from school.

an engine is an enormous air pump and the more power the more it can move. the more it can move the stronger the signal to the MJ so the jetting comes DOWN.

we all start high for safety sake but work down. bikes don't run badly when rich, they usually run fine whereas a narrow lean condition can cause a cough or splutter which we often wrongly diagnose.

so the skill is to come down until we have the correct jetting and deal with each cough and splutter on the way. it's a slow process and over at TT especially (on my old stomping ground of the WRs) they have gone back to 175-180 jet since i left because rider after rider after rider went on and cured a hiccup by jetting up and up. faced with such overwhelming evidence everyone suitably falls into line and jets up!

i could cry! :cry: :cry:

but over here we will fight this! dale and i run very low MJ numbers and boss and fry run very high. top enduro riders both and an ISDE to boot for fry. GULP!!!

regards

Taffy

8O Oh, my God. I think I just hurt my brain reading all of this! :shock:

but over here we will fight this! dale and i run very low MJ numbers and boss and fry run very high. top enduro riders both and an ISDE to boot for fry. GULP!!!

Taffy

Not to argue or try in vain to defend myself, but a berg doesn't need a main jet, Its midrange is strong enough!!!

However, as you say a rich main is safe and for the most part easy enough to ride with, and then are often forgotten. I just ride the darn things.

So in all that Taffy, what was your answer agian?

I've got to go with the venturi effect.

The smaller diameter increases air speed over the jet/needle. With the increase of speed also comes a drop in pressure, which requires a "larger" hole to suck the fuel from to satisfy the higher speed of the air to maintain a proper air/fuel ratio...

If the velocity of the intake charge is higher for a smaller carb,then the volume of that charge would also be smaller.This would be dictated by the smaller cylinder volume of the 450 engine,since valve sizes and cams are equal...it would be reasonable to assume that the 450 engine breathes better than the larger engines as the carb size as a percentage of its displacement is larger....soooo...even though the charge volume is smaller than the larger engines it is in fact drawing more air into its cylinder as a percentage of its displacement which should require a richer setting to maintain the proper a/f ratio.

Please shoot me now before my head blows up. :roll:

I know very little about carburation except for what I have read over the last hour or so. With that being said I will take a stab at it from the clues that have been given and a theory that I have.

I would have to agree mostly with nsman's explanation but with a little bit added to it. My guess is that it has something to do with the placement and location of the idle jet circuit just behind the slide (after reviewing Dale's clue). I think that the richer main jet and metering rod aid in the A/F mixture and atomization during closed throttle positions. Now I know that main jets and metering rods are for the function of 1/4-full throttle positions but I a guessing that it also has some function in the idle of the engine. Maybe the small amounts of atomized gas that does make it past the metering rod assist in the idle of the engine to some degree and as you use a richer jet and rod this also gives you the added benefit of opening the window of the 1/4-full throttle range as the venturi allows larger flows of air in the opened ranges. :?

Anxiously awaiting the answer.....

It must have something to do with the slide being the same for all three. Perhaps there is too much slide material for the 39 contrast to the 41 and a smaller richer needle makes up for the venturi limiting/restricting slide mass.