Hesitation (again) on acceleration but this time - DELLORTOS

[terryp]

Hello
When I first start the car its perfect , but as it gets warmer I get a slight hesiation when I come off idle. Is this normal or can I cure it !!!
Is it the pump jets ...perhaps a smaller size?
The plugs look fine when I take them out, and the idle is smooth
I'm running : -
98 unleaded with octane boost/lead substitute
BP6ES Plugs
K&N in the Nose filter (Cliveyboy)
Lumenition Electronic ignition
Lucas Sports Coil

Thanks alot

Terry

[hatman]

Similar off-idle hesitation on my Dell 'Orto car was cured (virtually) by increasing the pump jet size, as suggested (and effected) by the tuner who was setting it up on his rolling road. Been fine since.

[terryp]

Thanks for that
But if it needed a richer mixture , wouldn't it be worst when the car is first started?

Terry

[hatman]

Don't quite follow your logic there. As it's the pump jets you're changing, not the idle jets, how might that affect start up?

[terryp]

Hello Hatman
No what I'm saying is that when the car is first started there is no hesitation as all, runs perfect , which to me indicates that the mixture is rich?
When the engine is warm , the hesitation comes?

Cheers
Terry

[hatman]

Could the starting richness merely be the result of your pumping the accelerator pedal two or three times to achieve a cold start? (I assume that, in common with the rest of the Dell 'Orto using world, you don't use the choke on start up.) Other than that thought, I'm out of ideas I'm afraid.

[terryp]

Hatman
Its funny the car is fine for about the first 5 minutes, basically until just below working temperature I suppose so it can't be the pre start pumping
Thanks for having a think about it though
I've ordered some 35 pump jets to see if that cures it. It does say in the Workshop manual "use 35 if necessary to improve driving" which could mean use 35 if you get a ****** hesitation!

Best Regards
Terry

[peterako]

Could the starting richness merely be the result of your pumping the accelerator pedal two or three times to achieve a cold start? (I assume that, in common with the rest of the Dell 'Orto using world, you don't use the choke on start up.) Other than that thought, I'm out of ideas I'm afraid.

An aside.

I too used to pump the throttle a couple of times to cold start my +2 Dell'Orto Elan.

That was until I read Miles Wilkins Twink book.

The Dell'Ortos engined cars should be started, when cold, using full choke.

The pumping action is correct for the Webber cars.

Because the Dell'Ortos and Webbers look similar on teh outside there is a assumption that they are teh same on the inside.

BUT, pumping the Dell'Ortos on cold start can 'fill the airbox with a near neat fuel mixture' .

I've been using full choke on my Dell'Ortos for cold start since reading this and the car starts perfectly every time. Choke only stays on for about 50 - 100 metres:).

Sorry for the (not so short) aside.

The hesitation issue.....could be jetting, could be adjustments. Does sound like a mixture (too rich issue) or another accelerator pump issue...

Take care guys,
Peter

[terryp]

Peter
I think that part of that problem (described by Miles Wilkins) is that early or not so early pump jets have the outlet at the bottom which just dumps petrol onto the horizontal bit of the carb. Later or much later or new pump jets have the outlet on the side so sprays at least towards where the petrol should be.
Terry

[alaric]

Hi Terry. I had similar symptoms before I'd tuned the carbs i.e. balanced them and set up the mixture. It was fine on warm up, then wouldn't pick up once warm. Post tune up it was fine. I reasoned that during warm up the engine just wasn't as sensitive to the inbalance. Maybe when cold the air is saturated with fuel droplets, whereas once it's warm, the fuel vapourises. I used a 4 column carbtune to get the barrels balanced.

Sean.

[peterako]

My car deffinitely behaves differently in different weather conditions.

Dry, cool and high pressure - A1
Wet, cool, low pressure....not so great. (Still good, but noticably less performance)

Back in my 'hot rodding' days I seem to remember reading about this. Unfortunately can't remember the science but accepted it as a phyiscal limitation of a normally aspirated non-ECU engine.

Peter

[Robbie693]

My car deffinitely behaves differently in different weather conditions.

Dry, cool and high pressure - A1
Wet, cool, low pressure....not so great. (Still good, but noticably less performance)

Back in my 'hot rodding' days I seem to remember reading about this. Unfortunately can't remember the science but accepted it as a phyiscal limitation of a normally aspirated non-ECU engine.

Peter

I think it's something to do with air density - cool air is denser.

My hesitation problem has subsided since the weather went cold, still want to get it tuned soon though, if only I didn't have so much diy on the house to to

[terryp]

Hello All
Peter sent me an extract from the Bible (the one by Miles Wilkins not the holy one) and the pump jets for Dellortos for a Big Valve engine are specified at 35 anyway??? (with no improve driving comments)
I've just got to ambush the postman everyday with "o? sont mes jets"

Best Regards to all

Terry

[terryp]

35's in the car and hesitation cured ! (even in damp conditions)
Oh .....but whats going to happen in the dry now? (!!!!!!!!!!!!!!!!!!!!!!)

Thanks to everyone

Terry

[johnsimister]

What pump jets were in there before? My car has 33s and has a hesitation like the one you described, so I'm wondering if a change to 35s might fix it. It also has, I think, Cosworth L2 cams so I might have to go the rolling road route if the pump-jet change doesn't do the job.

Thansk

John