Advice on some Dell'orto DHLA 40

[nadhome]

Greetings,

Noticed on the left carbie there is an open port, but on the right is plugged?

Thought something might be missing or broken off until I found a photo of a new set and it's the same.

Anyone know what it's for (see the attached Pic)

Cheers,

[LukeC]

Vacuum advance/retard for ignition

Only need one port.

These are carbs for a turbocharged engine. The jet cover is unique to them.

[Colin Edwards]

When a complex casting like a carby body is made numerous critical airways need to be machined / drilled in.  The cross section of the hole or gallery is critical.  After the machining / drilling is completed where galleries intersect, the penetration into the cast body must be sealed.  Usually via an alloy plug or similar deformable semi permanent seal.  Looks like this plug is missing!

However, have a VERY CLOSE look at the exposed "hole".  If the area where the "plug" would normally be does not have a machined surface - ie: has the same surface finish as the main carby body, it may not need a plug.  If the hole is part of a gallery that is under engine vacuum it must be plugged.

[nadhome]

When a complex casting like a carby body is made numerous critical airways need to be machined / drilled in.  The cross section of the hole or gallery is critical.  After the machining / drilling is completed where galleries intersect, the penetration into the cast body must be sealed.  Usually via an alloy plug or similar deformable semi permanent seal.  Looks like this plug is missing!

However, have a VERY CLOSE look at the exposed "hole".  If the area where the "plug" would normally be does not have a machined surface - ie: has the same surface finish as the main carby body, it may not need a plug.  If the hole is part of a gallery that is under engine vacuum it must be plugged.

Thanks Colin.  Had a close look & can't really tell if it has a machined surface around the lip of the hole.  Appears to be the same on both carbies due to their age. 



Seems to be more than coincidence that the picture from the UK Dello'orto website showed the same missing plug on the left carbie only?

Reason for the post is I've taken the car to a mechanic to check out an acceleration flat spot.  The mechanic specialises in Webber's & said the missing plug looks to be part of the reason for the issue and if he can't plug the hole I may need a new set of Webber's?

Hence the research & the question.




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[nadhome]

Vacuum advance/retard for ignition

Only need one port.

These are carbs for a turbocharged engine. The jet cover is unique to them.

Thanks LukeC.

So you saying the left should be open/exposed on mine?

Cheers,


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[Colin Edwards]

If the hole in the carby body is able to allow airflow from the atmosphere to downstream of the choke / venturi / throttle, a rough idle and a flat spot would result.  To check this, drip a very small quantity of fuel into the hole with the engine idling.  if the fuel dissapears quickly, and the engine note alters momentarily, the hole is allowing unmetered air to into the engine. 
Jamb some Bluetack into the hole and take the car for a short drive.  Not too long as the Bluetack will melt - dont use plasticene!  If the flat spot is improved, the hole can be sealed with Permetex or something similar. 
New carbies should not be needed.  The Dellorto's are every bit as good as the Webber.

That vintage DHLA does have a design flaw however!  The rectangular plate at the top of the carby allows access to the air correctors / emulsion tubes / jets for both the idle and main systems.  This access plate has a screen cast into the wall that faces the engine.  This screen allows air to flow to the idle and main air correctors.  Problem is this air is unfiltered.  After time the screen will get dirty and less air will flow to the correctors. The calibrated holes in the air correctors will also reduce in area and flow less air.  The dirty air will also contaminate the emulsion tubes if left long enough.  The engine runs progressively richer and richer!  An overly rich mixture will also cause rough idle and flat spots and significant power loss and increase in fuel consumption.  Later DHLA's featured a white plastic cover in lieu of the cast aluminium, however dirty air to the idle and main systems remained.   Looks like Dellorto have now fixed this issue by the addition of a hose fitting to an otherwise sealed cover.  Fair chance this fitting is intended to pick up clean air downstream of the air filter!

[nadhome]

Thanks Colin,

Sounds like a sensible & practical check.

Will check and post the result.

Greatly appreciate the help & very detailed info.

Cheers,


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[carlo rossi]

they look spanish so the hole is not drilled thru yet
great carbs
ill buy them ha ha
seriously put them on,but check all your jettings
in them

[LukeC]

On dellortos, acceleration flat spots can be caused by wear in the linkage/pin for the pump diaphragm at the bottom of the carb. Once refurbished, the accelerator pump delivery requires re-calibrating. 3.5 cc per 10 strokes (done on the push rod). I have jury rigged a system to do this in lieu of not having the original Alfa tools (try and find someone that does....)

Rear/left carb does not have a vac advance port on the later engines, but the casting will be there.

Personally, I think you may need to go to a different mechanic.

[nadhome]

Personally, I think you may need to go to a different mechanic.

Thanks Luke.

Unfortunately the mechanic I've been going to who does quality work is flat strapped.  Had the gearbox & heads done by him and have been really pleased with his work.  But I needed to get the heads torqued along with some other work like having the manifold ceramic coated and the flat spot sorted.  Picked someone nearby that specialises in carbies and has a dyno.
I've passed on the incredibly helpful detail.  See how I go.  If it doesn't work out, I may have to be more patient waiting for my usual mechanic in Leichhardt.

Cheers,

[nadhome]

In post three the photo shows pre-pollution DHLA's
Pre - pollution dellorto's have exposed mixture screws
Post pollution DHLA's and or turbo as in the photo of your original post have captive mixture screws they are in a tube or bunker
what type of carbie do you have

Thanks Sportiva,

To be honest, I have no idea.  The pics in post three you referenced are the carbies on my car.  First was a picture off a Dell'orto website in the UK selling parts.

The Spider is an import from the West Coast in the US and converted in SA before I bought it last year.  I assume as it is a '77 that it was delivered as fuel injected & converted to carbies at some point ?  If the assumption is right, then who knows what they were lifted from?

Cheers,

[carlo rossi]

they are actually all correct if the later photo is yoursyou have pre poll dellortos ...nice!!
so pull them off they will need a kit through them and as Luke c said the accelerator pump diaphrams are probably shot
easy to fix pull them off and we will give you step ny step advice it is quite methodical and simple
dont buy anything yet there is 6 different types

[nadhome]

Update :  The Carbies have been completely stripped cleaned and refurbished using the Classic Alfa kit.  They had many years of gunk in them and one jet near clogged.  The reported hole lead to no-where. The carbies are now back on the car, balanced and tuned.  Flat spot is gone!  The car is on the dyno today, which leads me to another question.

How much HP at the rear wheels should a stock '77 2000 Veloce put out?  From the research I've been doing the figures seem to vary a great deal and certainly lots of conjecture if the figures in specs are at the flywheel or rear wheels.  I've seen anything from high 90's to 126hp quoted as Net??

I'll post the Dyno report when I get the car back.  Just interested to see what others genuinely get on a stock motor.

Cheers,

[Citroënbender]

A 77 Cali-delivered car surely would have been in the thick of dépollution madness? Or did Alfa find ways around this?

[LaStregaNera]

77 Cali car would have been single throttle body Spica injection I suspect.