1972 2000GTV DHLA40H jetting questions - Alfa Romeo Bulletin Board & Forums
 
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post #1 of 4 (permalink) Old 12-08-2009, 09:51 AM Thread Starter
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1972 2000GTV DHLA40H jetting questions

Hi,

My 2000GTV (Euro spec, Malaysia) is now back on the road after a longish rebuild. I am having trouble getting it to run right (I didn't drive it much before, so can't say if it ran well pre-carb rebuild).

Idle has been adjusted with a Colortune and it keeps a steady speed that only drops about 100rpm when the A/C and lights are on). Above 2500rpm it runs well, accelerating strongly. The issue is when the throttle is opened above idle and below 2500 rpm- it 'bogs down', won't accelerate except in first gear.
With the idle mixture screws set at 2.5 turns as recommended as a starting position, the mixture was too weak, giving pops and bangs when decelerating with a closed throttle. Now they have been set with Colortune they are 4+ turns out, and the pops have gone.

The mixture at 1200-2500 rpm looks a little weak with the Colortune under no load- there isn't a big rich pulse when blipping the throttle, and the engine misfires under load until the Mains come in.
The pump diaphragms are new.
Timing is correct and stable (new distributor)

The carbs are DHLA40H, probably not the originals. Jets are:
Main 150
Main air corrector unknown (no marking)
Main Emulsion tube 7772.10
Idle 58
Idle air corrector unknown/no marking (in fact, there is no hole at the top of the jet holder at all- though I don't think all four are blocked with dirt)
Pump jet 35

Carbs have been cleaned and rebuilt, all four pump rods set to the same turns on the nut as before rebuild, but balanced to give the same stroke measured with a caliper
Balance between carbs set by measurement to give the same throttle opening looking through the progression holes

My questions are:
- All the jets seem larger than standard (150, 58, 35 against 135, 50, 33)
- The main emulsion tube (I have read) is 7772.8 as standard
- The lack of marking on the air correctors and the apparently closed Idle air corrector seems odd. The car idles, so there is fuel going through the jet, but where is the air coming from? The Main circuit ? (also no marking)

Where do people think I should start first? Replace the idle jet holders (7850.1 seems to be the consensus)? Reset the accelerator pump stroke longer? Move the jet sizes back towards standard?

Can anyone offer some advice?

regards
Emulti

Emulti
1973 2000GTV, Malaysia
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post #2 of 4 (permalink) Old 12-09-2009, 09:32 AM Thread Starter
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Did some more research.

These carbs are Emissions DHLA40H type 5304P and 5305A, and were fitted as new to Alfetta 2.0 Berlina and Alfetta GTV. The original DHLA40E (non-emissions) which were fitted were replaced.

The standard jetting for these carbs on Alfetta was (mine in brackets):
Main 150 (150)
Main Emulsifier 7772.11 (7772.10)
Main air corrector 210 (210)
Idle 58 (58)
Idle Emulsifier 7842.3 (unknown)
Idle air corrector 220 (in the carb body, not the jet holder)
Pump 40 Horiz. (35 Vert)
Pump volume 8cc (20 strokes)
Float 7298.1 (7298.1) 8.5g
Float height 14.5-15mm (14.5)

The jetting for DHLA40G on a GTV2000 with emission carbs is the same, except Main 145, 7772.10, 35-40 depending on model, pump volume 9cc.
DHLA40G is supposed to be near identical to DHLA40H.

I checked the pump stroke, mine is set near minimum.

The jetting seems to make sense, particularly as I have an Alfetta distributor. It could be that the issue is with the accelerator pump not delivering enough fuel.

So, I am going to try resetting the pump to 9cc of fuel per 20 strokes, and consider enlarging the pump jet to 40, horizontal spray type, if that doesn't work.

I took this information from DHLA Dell'Orto perpetual leanout part II - Sideways-Technologies Forum

There are factory jetting tables there for these and many other models, as attached.
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Emulti
1973 2000GTV, Malaysia
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post #3 of 4 (permalink) Old 12-10-2009, 09:35 AM
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As these are emission carbs I think they are jetted from the factory to be on the lean side. I have improved the low speed running of my cars by using 60 idle and 40 pump jets but as with all carb settings it depends on the individual engine, good luck.

Sean
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post #4 of 4 (permalink) Old 12-13-2009, 04:33 AM Thread Starter
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Thanks for the advice!

Well, I set the pump to make 9cc on 20 strokes (which seems to be the maximum on one carb) and the pickup is much improved under 2500 rpm. However, there are still signs of running lean on a steady throttle after the pumps have finished their work, a second or two.

I'll try increasing the idle jets to 60 or 62 next.

Emulti
1973 2000GTV, Malaysia
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