Dellorto Pump jets

[cliveyboy]

I have been checking the Dellorto carbs on my Sprint after they were
overhauled and the carbs have been set up to the Lotus manual for recomended
jets, tubes etc.
In the Lotus manual it states 45 pump jets can be replaced with 35s to "
improve driving".
Can anyone elaberate on what happens with smaller pump jets. The manuals I
have are not to clear.
The pump jets seem to come in on the Acceleration cycle, do the pump jets
cut out once you stop physically accelerating or are they classing
acceleration as anything above tick over upto full throttle?.

Clive

_________________________________________________________________
Want to block unwanted pop-ups? Download the free MSN Toolbar now!
http://toolbar.msn.co.uk/

[elansprint71]

The accelerator pumps on a Dellorto give a great squirt of juice as the pedal is pushed down, it is a mechanical pump, it only reacts as the pedal is in motion downwards, so you could mash the pedal and the fuel would rush inbut maybe not all get burnt and the car would continue accelerating (at a slower rate) without the benefit of the accelerator pump.

I think that the manual is trying to say that if you use your throttle as an on/off switch then smaller accel pump jets might be an advantage. Those with a sensitive right foot, like Jim Clark and Shuman Michaelmacher can feed in the fuel to best advantage.

Off ya go, techno-dweebs............

Pete
----- Original Message -----
From: Clivey Boy
To: ***@***.***
Sent: Thursday, May 27, 2004 9:19 PM
Subject: [LotusElan.net] Dellorto Pump jets


I have been checking the Dellorto carbs on my Sprint after they were
overhauled and the carbs have been set up to the Lotus manual for recomended
jets, tubes etc.
In the Lotus manual it states 45 pump jets can be replaced with 35s to "
improve driving".
Can anyone elaberate on what happens with smaller pump jets. The manuals I
have are not to clear.
The pump jets seem to come in on the Acceleration cycle, do the pump jets
cut out once you stop physically accelerating or are they classing
acceleration as anything above tick over upto full throttle?.

Clive

_________________________________________________________________
Want to block unwanted pop-ups? Download the free MSN Toolbar now!
http://toolbar.msn.co.uk/

[lotuselan2]

Clive
I'll take a stab at this but others may be wiser. Assuming that the jet
size is the fuel going TO the venturi as opposed to the fuel BYPASS that
goes back to the well, then a 35 will give a bit smaller squirt than a 45
and therefore a bit leaner on acceleration. I think "improve driving" in
Lotusese means around town driving versus track. So the 45's might be for
competition and the 35's for street.

The volume of fuel injected from the pump jet does not immediately stop when
you stop advancing the pedal. Because of some deliberately flexible parts
the squirt extends a little past the end of the push. I hope that makes
sense.

Fine tuners say that SMALL changes in overall jetting can be accomplished by
making pump jet changes, that even at steady full throttle some fuel passes
the pump jet to the venturi. If you are asking questions on this board, you
are NOT one of those tuners!

Like all good books on carburetion, the real answer is "Suck it and See".
Try both and see what you get.

If you don't want to buy the extra jets unless you think they will work,
then what is wrong with what you have now? If you are getting a flat spot
on acceleration (that's hell isn't it!), then there is a way to tell if you
are too lean or too rich. If the flat spot goes away like a BANG, then you
are not getting enough fuel from the pump jets (lean). If the power comes
back as a PUP PUP PUP VROOM, then you were getting to big a squirt and you
are temporarily rich enough to wet the plugs.

Hope this helps and I welcome corrections from other posters. Especially, I
have the sneaky recollection that the pump jet is the bypass not the
injection port. So lets clear that up for Clive for sure.

Ken
'69 +2 with BDR

[elansprint71]

Ken,
Sounds right to me and put in a far more erudite form than my earlier reply! I have a feeling that we are going to finish up deciding that a Zetec with injection is the right way to go but I'm sure that it would not be so much fun. Any spare BDR's in your garage?

Pete
----- Original Message -----
From: lotus
To: ***@***.***
Sent: Thursday, May 27, 2004 10:11 PM
Subject: RE: [LotusElan.net] Dellorto Pump jets


Clive
I'll take a stab at this but others may be wiser. Assuming that the jet
size is the fuel going TO the venturi as opposed to the fuel BYPASS that
goes back to the well, then a 35 will give a bit smaller squirt than a 45
and therefore a bit leaner on acceleration. I think "improve driving" in
Lotusese means around town driving versus track. So the 45's might be for
competition and the 35's for street.

The volume of fuel injected from the pump jet does not immediately stop when
you stop advancing the pedal. Because of some deliberately flexible parts
the squirt extends a little past the end of the push. I hope that makes
sense.

Fine tuners say that SMALL changes in overall jetting can be accomplishedby
making pump jet changes, that even at steady full throttle some fuel passes
the pump jet to the venturi. If you are asking questions on this board, you
are NOT one of those tuners!

Like all good books on carburetion, the real answer is "Suck it and See".
Try both and see what you get.

If you don't want to buy the extra jets unless you think they will work,
then what is wrong with what you have now? If you are getting a flat spot
on acceleration (that's hell isn't it!), then there is a way to tell if you
are too lean or too rich. If the flat spot goes away like a BANG, then you
are not getting enough fuel from the pump jets (lean). If the power comes
back as a PUP PUP PUP VROOM, then you were getting to big a squirt and you
are temporarily rich enough to wet the plugs.

Hope this helps and I welcome corrections from other posters. Especially, I
have the sneaky recollection that the pump jet is the bypass not the
injection port. So lets clear that up for Clive for sure.

Ken
'69 +2 with BDR

[Esprit2]

From: "Clivey Boy" <***@***.***>

Sent: Thursday, May 27, 2004 3:19 PM
Subject: [LotusElan.net] Dellorto Pump jets


Clive,

The pump stroke determines the amount of fuel that is delivered by the
accelerator pump, and the pump jet determines how quickly that shot is
delivered.

The pump's diaphragm is a thin rubber sheet that can bulge a bit under
pressure. Snap the throttle wide open and the accelerator pump will move
through it's full stroke distance right now. However, all the fuel won't
be delivered instantly... ie, with the speed of your foot. The diaphragm
will bulge a bit like an inflated balloon, and then continue to deliver
fuel over a small period of time as it deflates. How long that takes is
determined by the pump jet size. Given a full stroke and held, the pump
will deliver the same amount of fuel regardless of pump jet size. It will
either dump it all in quickly through a large jet, or meter it in more
slowly over a longer period of time with a smaller jet.

Assuming the OEM spec had the pump jets and the pump stroke both properly
set up in the first place, I'm at a bit of a loss as to why going to
smaller pump jets would "improve driving". I have no special insight
there. But the stock set-up is a lot of carb for a little 4-banger in
street use. If you pop the throttles wide open quickly, it does require
a pretty big shot of fuel "right now" to keep the engine from sucking air
(going lean) and falling on it's face. If the choice of 45 pump jets was
made to facilitate a lot of aggressive throttle use, then it could very
well be that a slightly more modest squirt of gas delivered over a longer
period of time would be more appropriate for street driving.




Show me a Lotus manual that is clear.

Regards,
Tim Engel

[lotuselan2]

Tim
Thanks for getting my approximate explanation more clear.
Ken

[lotuselan2]

Pete
No spare BDR's hear; it took some doing to find the one I have. Bean will
take a big chunk of your money to deliver one and Cosworth will spend hours
talking you out of the BDR but then sell you what you want.
Ken

[cliveyboy]

Ken,Pete & Tim
Thanks for the advice, I guessed it was for initial accelleration, I have
been swotting up on dellortos reading all the different manuals and tuning
guides I can get hold of. The trouble is they all say slightly different
things and can end up contradicting each other.
I was at Eurocarb Dellorto specialists yesterday buying a proper carb
balancer and when I asked them they just said 45 pump jets are too big. I
would need a bit more of an argument than that before I start changing
anything as it helps to know what it does and how much it does it.
The manual I am using at the moment is "Setting up dellortos" by CB
performance in the USA.
It deals mainly with DRLAs but has sections on DHLAs and the theory is the
same(ish) for both.
What I am basically doing at the moment is setting the carbs up so they do
not keep sooting the plugs up whilst I run the engine in.
It is a slow task as I will take the car for a run and she will start to
misfire a bit. I will take the plugs out check to see which plugs are
sooting up and lean that carb slightly.
The only problem now is to stop the plugs sooting up to the point of
misfiring I end up leaning them off to the point where I now get popping
out of the exhaust especially at tickover which indicates running lean. This
also seems to cause problems starting up when hot. If you dont fire her up
cleanly the plugs seem to get wet and will not dry even after a few minutes
of running.
It would be good if I could just get her running resonable enough to run her
in and then think about getting her set up professionally.
Cheers
Clive

_________________________________________________________________
It's fast, it's easy and it's free. Get MSN Messenger today!
http://www.msn.co.uk/messenger

[steveww]

Beg, borrow or steal a CO meter to set up the mixture. You can get the
Gunston types from Halfords, just get the basic one. They are also to be
found regularly on ebay. I use the SUN diagnostic centre at my local
garage, but the owner is a drinking budy of mine

You want between 2 - 5 on the CO with 2 being lean and 5 being rich. I
found mine runs best at 4.3

I hear all the problems people have with Webers and Dellortos and I
think may be Stromergs are not so bad after all

Clivey Boy wrote:

[elansprint71]

Also borrow Gunsons Colortunes, one is ok, four are better, you can actually watch what is going on in each cylinder at the same time if you have four!

Pete
----- Original Message -----
From: Steve Waterworth
To: ***@***.***
Sent: Friday, May 28, 2004 9:19 AM
Subject: Re: [LotusElan.net] Dellorto Pump jets etc


Beg, borrow or steal a CO meter to set up the mixture. You can get the
Gunston types from Halfords, just get the basic one. They are also to be
found regularly on ebay. I use the SUN diagnostic centre at my local
garage, but the owner is a drinking budy of mine

You want between 2 - 5 on the CO with 2 being lean and 5 being rich. I
found mine runs best at 4.3

I hear all the problems people have with Webers and Dellortos and I
think may be Stromergs are not so bad after all

Clivey Boy wrote:

[lotuselan2]

Back to Clive

BEWARE, do not lean out your mixture do to sooting while running in unless
you are sure you are really rich. Setting 'by the book' should not be very
far off. You can end up with a burnt piston if you do not watch out! CO
meter is a good idea.

BUT then you need to look into why the sooting. If sounds like oil rings
are not bedding in. Are you sure you have not used modern 100,000 mile
emission-free rings. They are hell to bed in in an older type engine.

Ken
'69 +2 with BDR