Max fuel screw question?

[Rabbit TD]

Does the max fuel screw {which is the one with the stop collar if I am right} control the max amount of fuel only at wide open throttle or all the way through the rpm range and how sensitive is it as far as adjustment.  This is for the 1.6 N/A and T/D pumps and I have never adjusted one yet trying to gain anything.

[regcheeseman]

The short and accurate answer is that it only affects the maximum pedal and is the same as installing or removing shims under the pedal.  It also affects idle speed.

I don't think that is accurate at all - if it only affects max pedal, why the effect on idle? 

Linked in with a previous post on lda operation, I understand the fuel screw directly affects pump stroke length and therefore affects fuel delivery at all throttle positions.

[8v-of-fury]

The short and accurate answer is that it only affects the maximum pedal and is the same as installing or removing shims under the pedal.  It also affects idle speed.

I don't think that is accurate at all - if it only affects max pedal, why the effect on idle? 

Linked in with a previous post on lda operation, I understand the fuel screw directly affects pump stroke length and therefore affects fuel delivery at all throttle positions.

Idle is a throttle position 

[regcheeseman]

Fairy nuff.

I didn't read it as that - and I'll guess the OP wouldn't either?

Your answer seemed to say 'there is no effect on fuel except at max throttle position'

but what you are saying is that it has the same effect as wearing a set of platform soled shoes!

That being the case, why the black smoke? Pump timing at odds with delivery???

[regcheeseman]

OK....

given that you've moved the fuel map sideways....

hypothetically...I give the throttle pedal the required pressure for 1500rpm - however the pump now delivers the fuel for 1800rpm. Why doesn't the engine simply rev to 1800 - why the black smoke?

There must be some other influence on this, more than just the translation of the fuel curve?

I'm not being facetious or awkward, just that this is bugging me now - must be something I'm just not getting?

[Jettage1]

Black smoke usually indicates insufficient air for the quantity of fuel injected.  Smoke at partial pedal usually indicates a timing or atomization issue and usually is not black.

I happen to have some of both of the above.  I assume that turning up boost (or down fuel) would cure some of the first issue.  Can you expand (expound?) a bit more on the second (partial throttle) situation?

[regcheeseman]

Either I'm not asking the question right or am missing something very simple.

The situation is PURELY hypothetical and I'm trying to understand diesel operation more fully - I'm well known for asking pain in the a questions until I understand things (ask my old teachers!)

Ok......

take a simple NA diesel engine - the more fuel you add, the faster it spins until a point where there is excess fuel for the aair that the intake can flow and you get smoke.

oooooo hang on.....cogs are whirring as I type....I'm gonna have a play with my AAZ pump later and try some constant throttle and constant load runs and play with the fuel screw. I think I've just answered my own question!

[Baxter]

Either I'm not asking the question right or am missing something very simple.

The situation is PURELY hypothetical and I'm trying to understand diesel operation more fully - I'm well known for asking pain in the a questions until I understand things (ask my old teachers!)

Ok......

take a simple NA diesel engine - the more fuel you add, the faster it spins until a point where there is excess fuel for the aair that the intake can flow and you get smoke.

oooooo hang on.....cogs are whirring as I type....I'm gonna have a play with my AAZ pump later and try some constant throttle and constant load runs and play with the fuel screw. I think I've just answered my own question!

lol!

[Baxter]

Think of it this way.

A diesel can induce as much air as it needs, a Diesels engine speed is set by the amount of fuel you give it.

Imagine a nice Caddy 1.6D, nowt special.

You give it a turn of max fuel screw.

Engine idle speed has now risen.

You go down the road, and at 2000rpm, your engine will be using the same amount of fuel as it did before, as a Diesel is throttled by fuel! your fott will be in a slightly different position as to where it was before.

But, it will get to a point where it can supply that little bit more fuel at the top end.

Personally, I think 1/8th of a turn is enough on an old engine to stabilise the idle and give it a little bit more pep at the top end, anymore and they tend to smoke.

Messing around with the fuel screw is all relative, as with most things on a car, when you mess with one thing it affects something else.

cars are built to budgets and criteria set by accountants at the factory, the pumps are set up to deliver reasonable power, reasonable economy and to get them through the warranty period without failling.



Giving  the pump a bit more fuel will require you to back off the throttle stop a bit to get it to drop back to where it once was, all fine but you have achieved more fuel at the top end of the rev range, fairly pointless if you have stock boost pressure as it will just smoke.

I think one of the essential things about the black of of dicking about with Diesels is understanding what other things that you actions will affect.

[Jettage1]

THE #1 cause of problems is solutions.

...is (ill-informed, hasty, or otherwise half-arsed) solutions.     (hence all of my dumb questions)