DHLA - hesitation

[ericbushby]

Bob,
Instead of setting the float gap on Weber DCOE, I check the actual fuel level in the chamber, therefore removing any variable factors, as recommended by Keith Franck on 'sidedraft_central' . If you remove the jet cover and unscrew one main jet stack you can see the fuel with a torch. Keith tells us to use a strip of paper with a pencil mark on it as a dipstick to check the fuel level is 25mm below the fuel chamber top. As that surface is 16mm below the jet cover rim then the correct level is 41 mm below there.
I prefer to use the depth guage end probe of a vernier caliper and a torch. When the probe touches the surface the fuel 'shivers' and is easy to see. Keith insists that this is critical to within 1mm.
Sorry, can`t help with Dellortos.
Eric in Burnley.
1967 S3SE DHC ( waiting for new petrol pump)

[europatek]

Had a chat today with a very experienced Alfa tuner. He's had much time spent tuning Dellortos. We discussed the symptoms I had and his feeling was the accelerator pumps. Not the diaphragm but the linkage the operates it. He said that he had to repair many on Alfas 20years ago and suggested if my Elan had never been touched to check this area. Also he said to check and set the "cc" per pump. Looks like carbs off for a good inspection and repair kits.

[europatek]

Its been a while but I'm back onto these DHLA's. First I rebuilt the carbs whilst I had the head off recently. New Webcon rebuild kits. Once assembled and running the same old problem persisted - hesitating when opening the throttle once warmed up. I bought myself the accel pump measuring tool. Took the carbs off to check the accel pump discharge. Well.... I cannot get a consistent measure with the tool. First check one side way more volume than the other - see photo. Took the carb apart and blew it through. Reassemble and check again. Similar inconsistent results. SO... whats the go with this tool? Any expert advice? I had the same inconsistent results with both carbs so I won't be making any changes until I know what the go is with the tool.

[billwill]

When I cleaned my Dellortos, I found that one of the most tricky bits was the tiny ball-bearing valve hidden in the accelerator pump system. If you failed to clean that I suspect you might get your inconsistent results.

I didn't bother with any measurement tool, I just made sure that the squirt of fuel went nigh on 4 to 6 feet from the exits of the carbs.

[europatek]

Had another session with the carbs tonight. Still no success with any kind of consistency. For instance - 5/6, 6/5, 7/6, 6.5/7.5, 5.5/6.5, 6/5 etc...... Perhaps this is just how things are. Not sure how/if any adjustment can be made when the results are so inconsistent. Same results for both cards. Any suggestions from any DHLA experts?

[Craven]

Hi,
Definitely not a Dellorto Expert but over the years plenty of time spent on Webers.
I take it the car was at one time running as it should?
Not mentioned so far, have you set up the balance between the carbs, using some form of gauge, vacuum or air flow, the balance between each half of the same carb can sometimes be out as well.
Accelerator pump should not work under light pedal pressure, on a Weber excess fuel in the pump jet system is returned to the float chamber, does the Dellorto have this is it working OK.
For what it?s worth
Ron.

[rgh0]

Trying to solve this problem starts with understanding why carbs such as DCOE Webers and DHLA Dellortos have accelerator pump jets.

When the throttles are opened rapidly the pressure in the inlet passages rise rapidly and the mixture goes lean as vaporised fuel deposits on the inlet passage walls or forms large drops of fuel that don't burn in the combustion chambers. The acceleration pump jets compensate for this effect. It is a very crude compensation and not all that critical as to how accurate an individual pump stroke delivery is.

When you open the throttles slowly the acceleration pump and jets make no significant contribution. When you open them rapidly the acceleration pump jet is the major contributor for a second or so of the needed fuel supply. For a mid range opening the mixture provision is a combination of the normal jets and the acceleration jet.

When the engine is hot less acceleration pump volume is needed as the fuel vaporises more easily when the pressure rises in the inlet tract and thus less deposits on the walls or forms large drops.

Based on the above I suspect you accelerator pump in combination with the normal jetting is too rich during midrange acceleration when the engine is hot but ok when the engine is cold as more fuel is needed then and ok when doing rapid acceleration and relying mainly on the acceleration jets or accelerating slowly and relying mainly on the normal jetting

What's the solution ? - a few suggestions

1. Change to a smaller accelerator jet and or pump stroke
2. Change the other carb jetting
3. some combination of the above

The above is easier to do and get right if you have a dyno to do the work on or a exhaust gas analyser to see what's going on with the mixture during real life driving. Neither of these are cheap

Another alternative is to replace the Dellortos with a set of Webers and go to the yahoo group sidedraft_central for setting up and jetting. Not a cheap option either unless you have a spare set of Webers ( which I luckily did have for my Plus2S 130/5)

cheers
Rohan