Balancing Dellorto DHLAs - Alfetta 1.8

[RLeT]

Merry Christmas Everyone,
I have a 1977 Alfetta 1.8 Sedan with twin Dellorto DHLA 40 (H I think).  I've been getting a shudder when accelerating at lowish revs, which clears further up the rev range.  I also can't get the engine to idle.  A synchrometer shows the flow not balanced (cylinder 1/2 about 8 kg/h, cylinder 3/4 about 3 kg/h).  I have adjusted them so that cylinder 1/2 get 7 kg/h and cylinder 3/4 gets 6 kg/h, which is the best I can get for the moment.

The engine seems to run better, but takes a very long time to return to idle, and runs at higher revs a lot of the time.  It can sometimes idle at 2000 rpm, and I need to let the clutch out to get the revs to drop, which they do, and get towards 600 rpm, at which point it nearly stalls, which also makes it difficult to set the idle.

The carbys were rebuilt ~12 months ago.

Any tips on what to look at next?

Thanks in advance,

Richard.

[Paul Byrne]

G'day Richard and best for the season!

I have only worked on this kind of issue with Webers, but they are very similar.
The high speed idle problem seems to be almost always because the butterflys are open too much and the edge of the plate is exposing part of the first progression hole to airflow at idle.

With the Weber this can be confirmed by removing the plug over the holes and observing the plate edge. While you are doing this you can back off the stop screws so that none of the progression holes are uncovered. To get a fast enough idle speed you will probably need to adjust so that the edge of the plate is only just blocking the first progression hole.

    Very carefully trying to get the same plate positions for both carbies will get the balance close also (You can get that accurate later with your flow meter).

If it will not idle or is too slow after that then it is probable that the idle 'mixture' screws are not open enough. Open them a good two turns and then close them a bit at a time to get smooth running. After you have balanced the two carbies you can get a very accurate setting for the 4 idle screws if you have an IR temperature 'gun'. just point it at each exhaust pipe near its port and adjust screws to get even temps across the whole four. Otherwise just go for smoothness.

Probably worth disconnecting the accel linkage while doing all this in case it is part of the problem!

[carlo rossi]

the basics are once you think you have no leaks air or fuel
screw one of your idle (mixture)screws gently in ( clockwise ) and count the turns
once you have established that screw it out again to the same number
do the same to the other 3 so they are all the equal to the first one
You have established a base should be between 4 and 5 turns out assuming standard and jets are normal
start the engine with a higher than standard idle 1200rpm
get a piece of tube put one end in your ear the other near the opening of number 2
then compare to number 3
failing this screw the idle speed screw out unitl it does not touch the throttle and screw the balance screw out as well then with a screw driver you should be able to push one of the throttles back to a zero point
adjust the balance screw until they touch again
adjust the central balance screw ( on the throttle ) until they sound the same at this point it should be only a 1/4 turn either way
now you have balanced carbs
now adjust each idle screw 1/4 turn anti clock wise to each one whilst running
did the idle go up or down
if it went up go again if it went down turn 1/4 turn the opposite way and then 1/4 again ( clockwise)you are now 1/4 turn clockwise from start
until you get the highest idle then turn them 1/4 turn Anticlockwise so that it doesnt run too lean
so play around equally until you find the best idle
then adjust you idle screw down to 800rpm
carbys 101
this assumes it was going originally

[bonno]

Hi Richard and a very Merry Christmas to you as well.

The setting up and balancing of carbies as detailed above by Paul and Carlo need to be applied. However if you have cracked rubber carby mounts (air leak), then no amount of balancing and tuning will get it right.
Find attached links to Dellorto shop site for the Alfa Romeo parts and Youtube video on how to balance carby.
http://www.dellortoshop.com/contents/en-us/d136_Alfa_Romeo_Carburetor_Service_Kits.html
https://www.youtube.com/watch?v=g579jJhRQis&t=10s 
cheers
bonno

[Henry Goodman]

Was it running fine immediately after the reconditioned carbies were reinstalled or are these symptoms recent? As Bonno suggests, start with the simple stuff like checking for air leaks (common cause) including retighten the rubber mounting nuts and manifold given the carbies were reinstalled 12 months ago. Check the fuel filter and make sure there is no crap in the fuel which could block off jets. I've seen these symptoms before on both counts. Otherwise Carlo's advice is spot on.
Please let us know how you get along so others can benefit.
Cheers.
Henry.

[Paul Byrne]

Just a quick correction to my suggestions....

I said to start with the idle 'mixture' screws at least two turns out. This is right for Webers, but I forgot that Dellortos have much finer threads. Carlo has pointed out that you need 4-5 turns out as a starting point with your units.
Cheers. 

[giulia_veloce]

All good points above

Usually a slow return to idle is too much front carby on Weber and Dellorto + Solex twin Carbys.
ALL adjustments should be done at operating temperature

Just try turning the balance screw anti clock wise 1 turn and see what happens
If it is an improvement..turn more.
Due to turning the balance screw anti clockwise to fix this problem,,,idle adjustment screw will need to be turned clockwise to keep the revs up.

Some carbys idle mixture screws like a quarter of a turn from all the way in to idle
Some like 5 turns out
Usually more than 3 turns out means a too small idle jet.
More on that another time if required.

This is just the beginning of getting the carbys right.
Also take into concideration= point gap correct ?
ignition timing correct ?
Air leak from brake booster ?= all models 105 + 116.
Carby mounts leaking ?
Needle + seats leaking ?
Does it still have the fuel pressure regulator filter ?

Going by what Richard first post says= My opinion= Too much front Carby
Please dont use the balance screw as an idle screw
Balance screw adjusts the carby front to rear balance only
Idle screw adjusts the idle revs only.

More tips and tricks available later if required.

[RLeT]

Hi Everyone,  Made Great Progress:

- Got better balance by determining the relative loading of the carbies by disconnecting the spark plugs one at a time and seeing how much the revs dropped.  I certainly had too much front carby - which was delaying the return to idle, and making it run less well than it could. 
- Idle was definitely too lean.  The idle mixture jets are now out a major amount, so I will be looking to see if I need bigger idle jets.  I've had it suggested previously that I may need bigger idle jets, and this seems to back that up.
- Idle is now much better.  Engine idles at ~1000 rpm (or just under)
- Better response at low throttle also.
- Car has also been difficult to start in the past.  It's better now, but I've had carbies rebuilt and also installed a 123 ignition and new coil in last 13 months.

Thanks again for everyone's advice and tips, and to Giulia Veloce for talking me through the adjustment procedure over the phone.  It's greatly appreciated.

Cheers,
Richard.