Calling DCOE Owners -Looking for any progression hole pictures!

Dellorto 40 DHLA from 1978 Alfa GTV 2.0 - there's a tab with what looks like a spec number and serial number, but I can't read it easily (old age).



Same carb from a 1985 Alfa GTV 2.0 - totally different progression holes (different emissions regs?). Don't know if the jetting spec is any different


40 DCOE Type 27 - these were off my mate's 1600 Alfa Giulia Super lump, but Haynes says they should be from a coupe or spider 1600 from the same era
Maybe progression holes are adjusted according to how fast they expect the engine to rev - depending on factors like gearing, weight, etc. That's a total guess, but I though it might be worth sharing. In that case, a 1.8 16v lump might need more progression holes to allow it to spin up faster than a standardish 8v lump (even though the Alfa 1.6 8v Twin Cam produced about 107 BHP in cooking form). I'm wondering if that's why you're getting a flat spot issue.

However, I'd still suggest you analyse the AFR readings before drilling more holes in your carbs

 

Your camera gives far better pics than mine does...

 

Courtesy of Jon Olds, many thanks:

40DCOE2


45DCOE13


48DCOE98

 

First of all thanks Jon and Mike for the photos, it's great to see the big variation that exists. That first set of DHLAs has huge prog drillings! I'm seeing a general trend toward the drillings extending further on a lot of these than they do on mine.

Mike I agree with you about checking AFR before drilling new holes. Looking at the pictures of my carbs I'm sure the first prog hole on one is smaller than the other which I reckon was to do with the engine that they were intended for. I am definitely going to enlarge the smaller so that all four cylinders have equal progression circuits as I'll never get it right with mismatched carbs.

Going back to the vac advance I hooked up the vac gauge today and this was the result:
[video]http://s243.beta.photobucket.com/user/ez-does-it/media/13922796-8091-4175-857F-DABA92044C27-272-0000002BD99E0D96.mp4.html[/video]

This is connected at the ECU end of the vac hose. The figures I've read state there should typically be around 17-22 in Hg vacuum at idle; this is obviously quite a bit lower and fluctuates slightly so the timing wont be great at idle. You can probably hear the rough idle, this is as good as it gets after a lot of effort adjusting idle mix.

Connecting the vac gauge at the other end of the hose, before the 'anti pulse' bodge gives this:
[video]http://s243.beta.photobucket.com/user/ez-does-it/media/ACDC62DB-D11B-403E-B411-CDA6AE813ABD-272-00000030565E9DA3.mp4.html[/video]

Obviously no good at all. I'm hoping to tap all four runners next week and will report back on the difference it makes having a decent vac feed.

 

Just an iPhone They're reasonably good at close up stuff though.

 

Just an iphone? That cost 10* what my camera did. LOL!!!

 

I think you have already found the problem. As you say in your first post the throttle butterfly valves appear , as in the photo, to sit differently in the barrels of the same carb, and as these carbs don't have an air bypass screw ( which should remain closed anyway ) it is not possible to balance barrels within the same carb, you can only balance the carbs to each other. The problem you have is the throttle spindle in the carb connecting the 2 valve plates together is bent and somewhat twisted and weakened , it happens with age and having the full trottle set wrong,but can be replaced, sometimes it can be bent back straight. While you can adjust the idle mix screws at idle to get a good steady idle, as soon as the throttle is applied the uneven movement of the valves flows uneven amount of air and so unbalanced fueling to the engine, so part throttle is poor. no amount of adjustment will cure the problem .As you apply more throttle and move away from the progression holes the problem disappears. If you remove one of the carbs and hold it upto the light, looking trough the carb, the difference in the small light gap around the valve plate will be easy to see.

I have had a similar problem with my webers for some months and after a lot of time found the problem to be poorly fitted trottle plates, my engine raced at idle. After refitting the plates the problem was solved .

my simple test was to use a varible speed hoover to draw air trough the barrels , recreating engine vacuum, and using my balance gauge measure the air flow at different very small throttle openings. This was how i found my problem

 

That's interesting Jamez, I hadn't thought about the possibility of damage to the carbs. The two pictures are actually one set of drillings on one carb, and one set on the other. The confusion I had was that the carbs had been properly balanced using a synchrometer but appeared to have different idle throttle positions. Now you mention this though, I'm definitely going to have a look at all four sets to check that the two on any one carb are the same as each other.
Cheers

 

In any event, the position of the throttle valves should be all the same, best case you might just have a poor linkage.

 

I guess if you've adjusted the idle mixture to balance the carbs, then what you've actually done, is add a bit more fuel, which makes the engine suck harder and masks the problem on that barrel, but when you open the throttle a bit, the adjustment isn't enough any more.

It would be interesting to stick a colourtune on each cylinder and see if it backs up that theory.

 

I have actually got one of those adjustable hoovers and a synchrometer so when the carbs come off to tap the manifold runners for the vac feed I'll do that and post back on the results. Parts are currently turning up for that little project so I'm expecting to get started around the middle of next week.

I'd also like to test the colortune theory, can one get a colortune suitable for the 16v? I have to admit I don't really know how they work.

 

I have a colortune but it dose not really work on a 16v as the plugs are deep in the head the spark arc's to the head from the lead. The colortune is simple, when the flame in the cylinder is white, the mix is lean. As you rich the mix the flame turns to blue and then orange, which is rich. You want a nice gas blue flame

http://www.ralkleuren.com/ral-kleur.php?name=Hemelsblauw&id=5015