Triumphrat

Unklian

Hi Folks.
Pleased with my Bonnie.
Just want to improve my knowledge.

Why is it necessary to re-jet carbs?
Bear with me......

Standard bike, standard jets. Running 3000rpm.
X amount of air coming through carbs at correct mixture.
Just for simplicity 14:1

Same bike, 4000rpm, say 30% more air coming through,
mixture still correct. More air, more fuel being sucked in with it.

Modified bike...... Open pipes, no airbox, big cam, polished & ported.
Running 3000rpm now breathing much better. say 30% better.
So, there is 4000rpm's worth of air coming through the same carbs,
why isn't it still in the same air / fuel ratio?

Surely, putting in bigger jets will make it run rich?

Why has it now gone lean?

Simple explanation please.

Cheers.

Ian.

sbpark

by removing the airbox and installing less restrictive pipes you are flowing more air. if you dont install bigger jets to allow more fuel to maintain the correct A/F mixture you will be lean, and th esame is true for the opposite, causing a rich mixture. if you have more air but dont change your jets to provide more fuel to balance out the mixture you will be lean. make sense?

not quite following you though on your terminology about rpm's and such though.

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'07 Thruxton, Caspian Blue Checker, FCR's, 813 cams, Pieman ignitor, Predators, ported & polished head...

Grunt

Your standard bike comparison of 3000rpm and at 4000rpm still running a good air/fuel mix is due to the carb jets being tuned right for the amount of air being delivered through the rpm range.

Your terminoligy of a Modified bike now breathing better at 3000rpm by 30%.
With stock jetting is still only getting the standard amount of fuel for 3000rpm, though it is now getting what it would normaly get at say 4000rpm's.

Carburetors don't automaticaly increase the amount of fuel delivered according to the amount of air sucked in at a set rpm, eg. 3000rpm standard amount of air compared to 3000rpm with 30% more air. No more fuel is going to be delivered by the carb.

Therfore to get a good air / fuel mix you must increase jet sizing to match the amount of air being sucked in throughout the rpm range.

That's as simple as i could get it using those comparisons.

Someone correct me if i'm wrong somewhere.

Unklian

30% more air, won't that be like a standard bike at 4000rpm
so the ratio is still 15:1 ie correct?

Thats the bit I don't understand. I thought that was exactly what carbs did.
I thought that regardless of what is sucking air through the carb,
at a given amount of air, the jets would provide the correct amount of fuel.

Say a 250cc angine running 4000rpm
or a 500cc engine running 2000 rpm.

Same amount of air, through same jets should provide the same 15:1 ratio?

Why wont (for example) a pair of carbs off a 865cc jap / italian bike deliver exactly the correct ratio of fuel?

It's not your description, I simply can't get my head round it.
Never have been able to......

Ian.

jonkster

How I see it... after a single malt...

The amount of air the engine tries to gulp through the carb is due to the piston displacement. Each rotation takes one gulp. The gulp draws air through the venturi in the carb. To get same amount of gulp through a restricted venturi you have to make the flow faster than a gulp through a large opening.

The faster the airflow, the lower the air pressure. (Bernouilli principle)

The low air pressure that 'sucks' the fuel through the jets into the venturi drops as the air velocity goes up. If you restrict the inlet to the venturi the pressure gets even lower (air going faster) so more 'suck'.

(I say suck but it doesn't really suck - the outside air pressure as exists in the fuel bowl 'blows' the fuel into the jet towards the lower pressure in the venturi)

restricted airbox = fast air flow = low pressure = more fuel suck = rich

unrestricted airbox = slow air flow = higher pressure = less fuel suck = lean

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sbpark

i think your percieved idea of what you think carbs are supossed to do is in reality, what fuel injection is. carbs arent smart. they dont calculate the amount of air flowing in and then determine the amount of fuel to be mixed. they are fixed/static. a jet does not vary the amount of fuel based on the amount of air, it can only deliver as much fuel as the hole in the jet will allow. the only factor that varies this amount is how much the operator is twisting the throttle.

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'07 Thruxton, Caspian Blue Checker, FCR's, 813 cams, Pieman ignitor, Predators, ported & polished head...

Dave2001

OK - this is where I have a problem (some would say I have many!). I don't understand just how you can acheive a greater airflow into the engine. There is only a certain volume that can be contained within the combustion chamber, so how does increasing the 'breathing' work? Is there more air getting into the same volume due to an increased airflow?

jonkster

same volume in less time = faster airflow

if you have a bigger mouth - you can get same volume but with slower flow

faster flow = lower pressure. Pressure is what drives fuel flow through the jet - lower pressure means more fuel for same amount of air.

NB I am currently drinking Glenlivet - take what I say with grain of salt (but I did have to pass aerodynamics at one point!)

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Unklian

Ahhhhhh..... Never thought of that.
Now I'm beginning to understand why one carb. contains so many different jets.

Would I be correct in saying then that "perfect" mixture is only achieved at certain rpms & throttle openings and the rest is a juggling game where to "switch" from on jet to another.

eg. bike ticking over. only jet used is idle screw.
When the throttle is opened, that jet is still delivering a minute amount of fuel (as you can't get the fuel through a fixed hole any faster) right up to 8000rpm.

Then, as airflow comes into play, the pilot jet will also admit a set amount of fuel?

With me so far?

But as the throttle is opened SLIGHTLY, airflow will increase but the pilot jet cant deliver any more so the engine will run slightly lean. (Is that what a "flat spot" is then?)

Then (I presume) the emulsion tube allows some kind of transition over to the main jet?

Getting There......

Ian.


EDIT:-
just had a thought......with modern small computers and clever engineering, If you had some kind of high pressure fuel delivery system, you could vary the amount of fuel delivered very accurately right through the rev. range. You could call it ......I don't know "Fuel Injection" or something. wouldn't that be simpler in this day & age?

jonkster

flow of fuel through a jet is related to *both* size *and* the pressure pushing it.

pressure is due to velocity of air through the venturi.

The needle also acts as a kind of 'variable sized' jet adjusting mixture as it moves.

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