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Discussion Starter #1
So I undertook a rebuild of both carbs on this car ('77 Spider). Carbs are believed to be original to car DHLA40F, based on tag serial numbers, and with verification from Dell'Orto dealer here in UK. It is jetted as it would have been from the start, per their records. Rebuild kit was OEM from Classic Alfa, so good quality.
Once installed and balanced front to back, I am now struggling to achieve a decent idle. Below 1200 rpm, the car wants to chug to a stop. 1200rpm and above, I can keep it running, but there is still too much engine rocking for my liking. Carbs were balanced with syncroniser showing 6 kg/h each at 1200rpm.
I keep having an issue with the idle mixture screws. These consists of the threaded screw with needle point, a spring, small washer and o-ring. On 2 of 4 of these needles, when the screws are tightened down, the O-ring gets pinched between ID of washer and the screw body, thus trapping the spring and washer somewhere in the threaded portion of the screw. I've replaced the O-rings twice with these while trying to get the adjustment right. I have greased these liberally in an attempt to get them to work more smoothly / correctly.
At the moment, when throttle is blipped, and revs start to fall back to 1200rpm, there are a series of pops and spits from venturi #1, #3 and #4 mainly. My thinking indicates this is a lean condition. Correct ?
Is there a 'nominal' setting for these idle mixture screws ? They seem to be wound out quite far at the moment, and I'm still getting spits and pops.
Any guidance appreciated. I am at the stage where I think as soon as stay-at-home restrictions lift, I will take it to a specialist who can put his hands on it, and work some magic, as I don't seem to be able to get to that final silky smooth idle I am looking for.
Good news is that it starts much easier now ! That's one win, anyway.
 

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I assume that the idle mixture screws are similar to Webers. If that is the case then yours may have been over tightened some time in the past causing the seats to be distorted. If that is the cae then the procedure of adjusting them all the same number of turns goes out of the window.
 

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Maybe the fuel level in the float chambers is too low, so the idle circuit isn't fed.

Without the float in, the level should measure 27 mm from the top of the chamber (float weight isn't really taken into account this way, but it did work on DHLA 40E with 10 gr floats for me).
1624672
 

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Discussion Starter #4
Thanks for that tip. These carbs run 10gr floats as well. I measured 17mm float top to underside of top cover, with gasket..and as best as I could check, pretty close to 27mm level after I shut off - but I am going to recheck this last point, as it would sure explain the lack of idle ! Thanks.
 

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Discussion Starter #5
Well...I checked them again. Floats (10gr) when at rest, and petrol level within the 2 bowls are even, and within a mm of 27mm. From where does the idle fuel feed from the bowl ? Is it via the 3 passages on the vertical side of the bowl ? Or elsewhere in the bowl ?
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If I have understood your description correctly, you may have the O ring/spring combination incorrect. Holding the idle screw in your hand pointing upwards the order is spring, washer then rubber O ring. The O ring should sit between the washer and the carb body. Before attempting to get the carbs running right, you need to ensure if you have the rubber carb mounts that they are not leaking, they crack. Also ensure all the ignition is spot on, if you are on points ensure they are set correctly and that timing is spot on. earlier this year I went through my tappet gaps and reshimmed to just about perfect, that has improved the running no end. Start by screwing the idle screws in to the stop, then wind out 2 1/2 turns on each, that is the start point.

See how you get on.
 

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Well...I checked them again. Floats (10gr) when at rest, and petrol level within the 2 bowls are even, and within a mm of 27mm. From where does the idle fuel feed from the bowl ? Is it via the 3 passages on the vertical side of the bowl ? Or elsewhere in the bowl ?
Your (low emission) carbs should get fuel through the main circuit, in contrary to mine in the picture, which are the older carbs where the idle jets are directly fed from below in the bowl (+ these idle jets are differently drilled). If you compare the picture to your carbs you'll see a difference in the drillings.
It's all a matter of intelligent routing of fuel and air. To be honest I don't know if the newer carbs will act the same as the old ones in relation to the fuel level.

If your fuel level is 27 mm from the top, than that probably won't be the issue.
 

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Discussion Starter #8
This is the way the mixture needles are assembled:

1625035


The O-ring seats on the surface of the recessed port it threads into, so that looks correct to me !
As far as I can judge, timing is correct, and I checked point gap as well, plus fresh spark plugs.

I can limp the car now, feathering the throttle, so probably going to sneak it to specialist for final tweaks and/or corrections of my work. It's discouraging, after so much work was put into the job. I hate to raise the white flag of defeat...
 

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Idle needles are correct.
 

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Got the same carbs that I’m struggling to tune (next step is an O2 sensor). Ages ago I remember having stumbled on an article or a BB post where they had measured (in mm) how much the mixture screws protruded on similar carbs. I wonder if anyone has this data compiled. It’s not much different from counting the number of turns out but probably quicker and less prone to errors. Stay safe!
 

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Seb: jetting on the F carb is rather different than the earlier Dellortos, so make sure you’re using the right jets. Counting turns is the easiest way to get a staring point.

Probably best to start your own thread if you’re having trouble.
 

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If you have ‘later’ Model carbs (and as a 77 model vehicle, I suspect that is the case), then your fine-threaded idle mixture screws should be set between 5-6.5 turns out to give it a chance of idling. Same rule of thumb applies to later model ‘emission’ Webers.
 

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Discussion Starter #13
Thanks Ranz. That seems a lot more than they are currently wound out ! Might make quite a difference. Will give it a go. Just hope they don't get too wobbly that far out, and vibrate further open through normal use.
 

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Discussion Starter #14
Update. Nearly there !
I reworked the idle mixture screws with different washers, and the O-rings no longer try to wedge between these and the threads, so you can actually adjust them now. Took the car to a specialist who took carbs apart to check my handiwork (all OK) - they also checked point gap and reset timing slightly, before tuning the carbs. Car now goes like a stabbed rat, right up to redline, and idles happily at 850rpm, something it's never done in my 12 years of ownership. Much less engine rocking as well. The ONLY problem remaining...some 'fluff' or stumble when pulling way, and when underway and tipping into the throttle. It takes place in a fraction of a second, but upsets the driving experience. Could it be mixture is still a little too lean ? Any suggestions as to what to try to make this last niggle go away ?
 

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But Mad North-Northwest
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When I got my GTV it had an off-idle stumble. Turns out the fuel pressure was too high (no regulator) and I think it was flooding the carbs a bit at idle. Ran quite well otherwise.

Another possibility is you've got the wrong jets for an emissions carb. If you jet an emissions carb with the standard DHLA emulsion tubes and jetting you will apparently get a big lean spot off idle, though from what you said it sounds like you checked this and have the correct jetting?
 
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