02GF74
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| posted on 22/10/07 at 03:10 PM |
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snapping open throttle tends to stall engine
as per title, twin delllorto 40s on 1300 crossflow.
when idling, snapping open the throttle rapidly makes the engine stutter before it picks up.
what is the cause of this? I can think it is the carb. pump supplying too much or too littel fuel - any way to diagnose this in a locost way?
need to do more tests but I think it happens more noticeably when engine is not quite up to temp.
need to try this when driving as this may give more clues.
dunno what the jetting is nor if it is right for the engine.
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whitestu
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| posted on 22/10/07 at 03:20 PM |
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Not sure if this applies to your Dellortos, but my old alfa did this if the accellerator pump jet was blocked.
Stu
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02GF74
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| posted on 22/10/07 at 03:22 PM |
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^^^^ well there is a start.
some while back, I did take the filters off and trun the throttle spindle and did see petrol being squirted in; cannot recall if I checked all 4
chokes.
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Fozzie
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| posted on 22/10/07 at 06:11 PM |
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Crossflows like to be woken up gently before they roar! 
Try letting it warm up before giving the throttle some welly.....
HTH Fozzie 
'Racing is Life!...anything before or after is just waiting'....Steve McQueen
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Daimo_45
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| posted on 22/10/07 at 06:35 PM |
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I can stall my Pug if i dab the throttle rapidly, immediately after start up. Just needs warming up.
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chriscook
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| posted on 22/10/07 at 06:50 PM |
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With unknown jetting I don't think I would expect it work very well. Just check each choke get fuel pumped in with the a quick opening with
engine off then get to a rolling road.
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Fozzie
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| posted on 22/10/07 at 07:00 PM |
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quote:
Originally posted by chriscook
With unknown jetting I don't think I would expect it work very well. Just check each choke get fuel pumped in with the a quick opening with
engine off then get to a rolling road.
That'll teach me to read posts properly! 
Totally agree with you Chris re the jetting....
But nonetheless, even when you do get the jetting right 02GF74, you should always warm up a crossflow before trying to give it welly...
Fozzie
'Racing is Life!...anything before or after is just waiting'....Steve McQueen
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lotustwincam
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| posted on 22/10/07 at 08:14 PM |
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First thing to check is that all four pump jets are working as they should.
The earlier pump jets squirt fuel directly down into the carb and can be easily seen by looking into the carb from the trumpets. The later jets,
squirt fuel back at the butterflies and aren't so easy to see.
If you operate the throttle slowly (with the engine stopped) you should see fuel squirting simultaneously from all four jets almost as soon as you
move the throttle.
It is possible, if someone has adjusted the pump rod nuts, that you might be able to move the throttle some way before the jets begin to operate. This
can cause a huge hole in the acceleration to the extent that the engine will almost stop completely if you jab the throttle open.
Of course, on the other hand, you might just have the wrong jets, or as stated maybe you haven't let the engine get up to full operating
temp.
Drew
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02GF74
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| posted on 23/10/07 at 08:34 AM |
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quote:
Originally posted by lotustwincam
It is possible, if someone has adjusted the pump rod nuts, that you might be able to move the throttle some way before the jets begin to operate. This
can cause a huge hole in the acceleration to the extent that the engine will almost stop completely if you jab the throttle open.
well funny you should say that ^^^^^
One of the pump bolts is shorter than the other - a bit is broken off - and I played with one of the springs to make the movement the same on both
sides.
So it seems my bodge may not be perefect & I need to spend a few pennies to sort that out and then start looking elsewhere.
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lotustwincam
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| posted on 23/10/07 at 08:08 PM |
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quote:
I need to spend a few pennies to sort that out and then start looking elsewhere.
Matt Cooper at Eurocarb will supply you with the parts www.dellorto.com
As far as I'm concerned this whole pump rod mechanism is a pile of pants. Essentially, you need to adjust the nuts so that the jets in both
carbs start to work almost as soon as the throttle is moved. You also need to check that they both STOP working at the same time.
In other words, the Timing and the Duration needs to be the same for both carbs. When you get that bit right, then you can start to play with the pump
jet size.
Drew
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darren(SA)
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| posted on 25/10/07 at 10:46 AM |
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I am having the same problem, symtons are the same. Doesn't matter if its hot or cold, the instant you open up quickly, the engine dies. gradual
opening is fine.
my problem however is on the webers not dellortos. I found this diagram
http://www.racetep.com/weber/40DCOE151.pdf
pump jet is no 57,
If I remove the screws and take out the pump jets, on the one carb I can see a squirt into the pump jet chamber. On the other carb, it remains dry. if
I open it up, the chamber fills up but doesn't seem to get to the pump jet.
I'm not sure what your carbs look like, but try take out the pump jets and see if anything is squirting through will the accelerator is blipped.
do this while the car is off.
sorry for hijacking your post but if anyone could point me in the right direction here as well
cheers
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02GF74
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| posted on 25/10/07 at 01:03 PM |
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looking at literature about this I have found that the lever should be adjusted to rest against the pump base so when the throttle moves, the pump is
activated immediately.
that is one thing I will check.
Other thing is to pump the lever 20 times and collect fuel from each barrel and meeasure. Somethwere there is data on how much that should be, then
the next size jets, up or down, should be fitted.
There is other stuff that I cannot recall right now.
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