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Discussion Starter #1
I have a set of DHLA 40Hs, with 5 progression holes. I've noticed that the first/smallest hole is only covered halfway by the throttle plate. What would the consequences be of soldering over that hole (or any of them, really)? Think the problems inherent to tuning emissions carbs on small displacement motors could be overcome by soldering over a couple holes?

There aren't very many places one could ask a question like this, so does anyone have any input?
 

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Don't think solder would stick to the aluminum. You might want to try a test run with some suitably bent pieces of piano wire in the holes.

Uh, is the first hole half covered on ALL of the throats? Or just some of them? And, when you say half covered, does that mean half of the first hole is on the vacuum side of the throttle plate? Or, on the upstream side of the throttle plate?
 

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Altering progression holes

I've modified 40 & 45 DCOE's in the past with the correct amount of very carefully applied J B Weld epoxy. There are a couple of problems here. You must know which hole to plug ( and why) and just plugging some is not the whole story. Some others may need to be of reduced size, or even made larger. This requires a good metric plug gauge set, as well as the correct metric pin drills.
The epoxy must go all the way through the holes and leave a shallow mass above, so it will not dislodge over time, living in a gasoline atmosphere. After the clean up of the throttle bore area, ( I use a surgical blade, followed by a fine Dremil tool SS brush.) The location of any smaller (or larger) hole can be a problem. I have used a pin drill, by hand, with the correct metric bit. Through the J B, is not a problem, but to cut through the aluminum, even with a starter hole, will break the tiny drill bits. Another thing to consider is that the location of the holes, as well as the sizes, are critical in relation to the throttle plate. This has all been covered here in the BB posts by many much more knowledgeable than I. I have a pro tell me where, and how big or small, and then have a time consuming go at it.
The nice thing about the J B is that if it does not work correctly, after you have spent hours fooling with the progression holes, you can dig it all out (after a while) and try again! What fun:cool:! :D Gordon Raymond
 

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Richard Jemison
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What!

:eek:
The reason that Delorto`s are much nicer as street carburetors is their progression hole enrichment system. Before reingineering with with whatever, you need to examine the reason that when "closed" the hole is partially uncovered. Are they all off the same amount? Why??

Is it because the idle mixture/speed is misadjusted.

Or is it a bit more technical as with a twisted shaft. If so twist back the other way. No rocket parts here! Idle stops should be on the same side as the throttle pull. Remove the one on the opposite side if there is one.

Or could it be that the butterflys are not centered in the shafts at all? They can be re-centered by loostening the screws and moving them up or down in the shafts to correctly center them.

Or they could have the wrong butterflys in them. They are available for fitment to fully close at different angles.......
 

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Discussion Starter #5
Don't think solder would stick to the aluminum. You might want to try a test run with some suitably bent pieces of piano wire in the holes.

Uh, is the first hole half covered on ALL of the throats? Or just some of them? And, when you say half covered, does that mean half of the first hole is on the vacuum side of the throttle plate? Or, on the upstream side of the throttle plate?
All 4 of the 'first' (downstream) holes are exposed the exact same amount. I haven't run these carbs yet, so I don't plan on filling/drilling anything until I find out if it will actually be an issue or not. Mine are emissions units (H), and have some VERY large progression holes. Knowing that it might be possible to fill some of them is very handy (and the answer to a question that I've been asking myself for a while now). I am going to reset all the throttle plates with a feeler gauge (as per the Dellorto Superformance book) and see if that closes off the 'first' progression hole at all. Heck, maybe it's a good thing it's half exposed, it's going to be a couple months before I get to find out (winter + uninsulated garage).

Thanks again for all the info, it really is tough to find a forum with good carb info, it's much appreciated! Millions of EFI forums, and not a single 'sidedraught/sidedraft one :(.
 

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Discussion Starter #6
:eek:
The reason that Delorto`s are much nicer as street carburetors is their progression hole enrichment system. Before reingineering with with whatever, you need to examine the reason that when "closed" the hole is partially uncovered. Are they all off the same amount? Why??

Is it because the idle mixture/speed is misadjusted.

Or is it a bit more technical as with a twisted shaft. If so twist back the other way. No rocket parts here! Idle stops should be on the same side as the throttle pull. Remove the one on the opposite side if there is one.

Or could it be that the butterflys are not centered in the shafts at all? They can be re-centered by loostening the screws and moving them up or down in the shafts to correctly center them.

Or they could have the wrong butterflys in them. They are available for fitment to fully close at different angles.......
Hmm, you raise some points here I never figured on. I'm going to go check them out again today. It'll give me a chance to install my 32mm chokes :D.
 

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Should they be completely closed? I had a look at some old DHLA40 and on none the smallest hole was closed, also looked at DHLA45 and same thing, not closed. But on Weber 45, they were in fact closed. In my opinion, I think there should be a small gap.
(It was easy to see from "engine side" that progression hole was not closed)
Seems easier to change idle jet or emulsion tube?
/Frank
 

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Discussion Starter #8
Good to know, I was concerned about mine because they came from a running car, but I had no idea how well or poorly it ran. I can see how having one of the progression holes just slightly visible could help the transition from part to full throttle, but not being able to run the car currently doesn't help :(.
 

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Discussion Starter #9
Another question, as opposed to starting another non-alfa-related thread...

Does anyone know why more progression holes makes for a leaner transition? This is according to the "How to tune Weber and Dellorto..." book. I'm confused, I'd have figured that fewer holes is leaner.
 

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But idle and progression is the same circuit? The fuelmixture comes from idle jet and emulsion tube, going by the same channel to both progression holes and idle mixture screw.
WHen progression start you still get some fuel from the idle mixture screw. Look at www.dellorto.co.uk for good drawings.
This is not an answer to the question, but I can't think, and why is the progression so important? The fun part is the main circuit and acceleration pump!:D
 

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Discussion Starter #11
True...but I've been tuning EFI for nearly 10 years now, and am making my first foray into carbs. Old habits die hard I guess, I've got to know 'why' it works, can't help it :D.

I'm really looking forward to this setup...now if winter would hurry up and end...
 

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Discussion Starter #12
Hmm, I'm beginning to think that the leaner progression phase isn't so much a side effect of the 5 progression holes as it is the large, fixed idle air-bleed. In the "How to Tune..." book, he makes reference to closing off the air-bleed holes and making them smaller (1.2mm), effectively creating a .2 idle air. I would think that this (without knowing the exact fuel circuit of the H carbs) would create a pseudo-clone of the non-emissions carbs that run the 5 holes. I wish I could find a later 5-hole Dell to inspect, I'm obsessed!
 

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The most commonly used Weber idle jets are F8s and F9s which have 1.2 and 1.0 mm air bleed holes, respectively. I don't know about Delortos, but going to smaller air holes on my DCOEs primarily affects richness up around 2500-3000 rpm.
 

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Discussion Starter #14
Exactly, the built-in idle air-bleeds on my Dell 40Hs are 2-2.20mm (some unclear info there), which means a larger idle jet is needed to compensate for the huge air-bleed. I wonder if shrinking that hole can make emissions carbs more of a 'worthy' choice for tuning's sake. I'm in no position to talk about tuning these at the moment unfortunately, since the car isn't running yet...but I'm so used to having all the EFI info at my fingertips, I can't help but dig as deeply as I can before running these. The more I learn about their function, the better prepared I'll be come tuning time. I much appreciate any info that I'm able to glean from folks accustomed to running carbs, and the more I learn, the more my attitudes towards these mechanical devices changes (for the better ;) ).
 

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You need to be well equipped with different sizes of idle jets, emulsion tubes,
air corrector, accelerator..... And after changing jets go back and adjust mixture screws.
I think it is interesting but very time consuming. Change something, test drive, change again. What I like with carbs is the immediate response. And the sound. And to mount air horns, but then after test drive you realise you must also change the main jets..... it never ends!
 

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Discussion Starter #16
Since I acquired these carbs, I've been buying as much stuff as I can from Gabriele David on eBay. I've got some 7772.6 emulsion tubes, richer and leaner (than my current) main jets, leaner idle jets, extra chokes (32mm), you name it :D. I'm really looking forward to playing with them, I'll probably spend more time fiddling than driving...which will annoy my wife to no end ;).

My christmas gift to myself will be a few sets of that wonderful Colortune I keep hearing about :D.
 

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Since I acquired these carbs, I've been buying as much stuff as I can from Gabriele David on eBay. I've got some 7772.6 emulsion tubes, richer and leaner (than my current) main jets, leaner idle jets, extra chokes (32mm), you name it :D. I'm really looking forward to playing with them, I'll probably spend more time fiddling than driving...which will annoy my wife to no end ;).

My christmas gift to myself will be a few sets of that wonderful Colortune I keep hearing about :D.
Take care, you may end up like me, having a box of carburettors next to me in the living room. They are nice to look at..... :eek:
 

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Discussion Starter #19
I'm not sure the wife would appreciate me keeping them in the living room...although I usually work on them on the coffee table when she isn't around ;).
 
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