Rich running

[jono]

My Elan +2 has DHLA 40's. I'm just back from a 1200 mile round trip on the NC500 (wow, fantastic!) and before I went I stripped the carbs and put the bodies and jets through an ultrasonic cleaner and the rebuilt them with new gaskets, valves and diaphragms

The car runs very well expect for a slight flat spot between 2.5 and 3k revs which it's always had (and I can live with), however after about 20 miles from cold it starts to run rough from idle and bogs when floored from low rpm. I'm told that it chuffs soot out of the exhaust when it does this. I'm using NGK 6 plugs. Its runs fine when cold.

The car was tuned with a 4 column manometer and the mixture set with a colour tune before I went on the trip

On the open road it's great when on song, this evidently rich running only occuring at low revs.

I set the floats to 15mm when vertical, measured from the gasket face to the top of the float.

Floats too high?

Any suggestions welcome or this time it really is a FI conversion!

Jon

[bill308]

Hi Jon,

I'm not an expert on DHLA's, but I believe your idle jets are too rich. It may also be your mains are also too rich. High float levels would affect the entire rpm/load range.

Your manometer system is as good as it gets for balance/synch measurements.

At idle, the mixture screws can be set to deliver any mixture from lean to rich with a twist of the mixture screws. Where are you color wise with the Colortune? I would think you would want be at the blue/yellow transition at idle and no load, yellow when you blip the throttle, and back to the blue/yellow at, no load up to say 2k rpm.. This range and no load condition is all that can be expected from the Colortune.

Off idle, mixture screws should have reduced effect and as rpm increases, really no effect at all. Mid range mixture is controlled by the idle jet, up until the mains kick in at maybe 3k rpm or so. It is possible that high float levels cause both the idle and main systems to be rich during this overlap segment.

I think you need a good air/fuel meter to pin down the mixture over the rpm/load range. An O2 sensor bung should be welded into the Y-pipe, as tail pipe probes are unreliable because the exhaust gases can be diluted by atmospheric air.

Bill

[jono]

Thanks Bill,

The curious thing is the carbs are jetted exactly according to QED's spec sheet for the cams and general engine specification (mildy tuned to around 135bhp) - mains 135, idle jets 55 and chokes 33mm.

I took it to an experienced classic Alfa specialist before I went way and he was of the opinion that the jetting was basically 'correct'.

The idle mixture screws were adjusted to achieve a bunsen blue flame at idle.

One of my barrels is flowing slightly more than the others but this model of carb has no air bypass screw - notwithstanding that it idles very well.

Jon

[bill308]

Hi John,

What do the plugs look like after a good run and clean cut?

I seem to remember there being controversy concerning float heights on DHLA's. I believe there are more than one type of float style or weight.

Bill

[jono]

Bill,

I think I have the early 10g floats and I set them at 15mm. Some internet digging however suggests 17mm/17.5mm for these floats - will try that and see how I get on.

Jon

[StressCraxx]

The idle mixture screws were adjusted to achieve a bunsen blue flame at idle.

One of my barrels is flowing slightly more than the others but this model of carb has no air bypass screw - notwithstanding that it idles very well.

Jon

Hello Jon,

Do you have any off idle(tickover) stumble? I found that setting the idle mixture screws where the color of the mixture just turns yellow eliminated that issue.

[William2]

Hi Jon, you might like to read my thread on "Dellorto Idling Frustration" as I have been playing around with float levels. I have noticed that even in Des Hammill's Dellorto book it doesn't make it clear that when bending the float arms whether you should aim to get the floats parallel with the float chamber cover when in the vertical position or at a slight angle. Maybe it doesn't make much difference.

[bill308]

Hi Jon,

Do your DHLA's have a suffix letter like C, D, E,.....?

I believe the original DHLA's have no suffix. I had correspondence with a UK supplier, who provided me a set of DHLA40D's, if I remember correctly. He told me the early DHLA's were designed to get on the mains as soon as possible and featured a 3-hole progression grid. Later C, D, E 's had more progression holes and tended to extend the progression phase over a wider range.

The likely effects of a higher fuel level, all else being the same:

1. Richer across the rpm/load range
2, Idle circuit will come in earlier and last longer
3. Mains will come in earlier.
4. The idle to main overlap is even richer because of 2 and 3.

Bill

[bill308]

Hi Jon,

Other items to check:

1. Could your float valves be leaking, thereby causing a higher than intended fuel level?
2. What is your fuel supply pressure, at the carb? Too much pressure will overcome the float shut off function and result in overly high fuel levels in the bowl.

Bill