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Discussion Starter #1 (Edited)
My '74 750 GT originally came with 30mm Dellorto carbs. After I put in Sport pistons I went to 36mm which gave me great mid-range. Now, many years past 1974 and it's really time for a new pair of carbs. Since the 36mm weren't original, also, rebuilding them is just a PIA. (Still have the original 30s -- and even a pair of Super Sport 40s I tried). But trying to sort the number codes as my originals say PHF 36 AS. I believe PHF 36 DD are a rubber to rubber mount. But what are DS and BS? Or, which version (if any) will bolt onto my stock 36mm manifolds?

Thanks
 

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Yeah, it's not entirely clear :). However, to do the easy bit first, let's start with the second letter.

It is always either S or D; S is 'sinistro' = left. D is 'destro' = right. So this letter tells you whether the adjustment screws are on the left or the right.

Simples!

Now for the first letter. I believe this describes how the carb is intended to be mounted on the engine - at least, that's my reading of what is written in the Dell'Orto "Guide to the choice, setting and use ...etc"'. But I concede there seems to be some inconsistency /complexity.

Anyways, on this basis:
A is a male, clamp fixing
B is a female, clamp fixing
C is a flange fixing

I can't find an explanation for D.

Not very useful, I know. But it's the best I can offer for the mo'. Hopefully someone more knowledgeable will come along soon!

Craig
 

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I'm using a pair off of an Allazura on my Darmah, bolted right on. Parts are common Dellorto pieces available many places. Ed at ducpower.com probably has a pile of carbs lying around.
 

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I'm using a pair off of an Allazura on my Darmah, bolted right on. Parts are common Dellorto pieces available many places. Ed at ducpower.com probably has a pile of carbs lying around.
So just out of interest then will the PHF36mm carbs on my Alazzurra slide (not bolt) on to the intake manifolds of my SS with 40mm PHMs? I suppose I could make the swap but both bikes are running well and I don't want to experiment.

Bruce
 

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I don't think so, I believe that the 40mm manifolds are larger.
 

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Discussion Starter #6
To run the 40 PHMs I did indeed have to go to a larger manifold. Moreover, at least on my bike, I had to grind out the holes on the 40mm manifolds to fit on the "tighter", stock studs. I have separate manifolds for stock 30mm, then the 36mm (currently running) and the 40mm. The 40mms gave me the best top end, but mid-range was limp.
 

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Are there different heads to match up with the different manifolds? IE: if you are grinding the stud openings to accommodate narrower studs, do the bores of the larger intake and the intake bores in the existing head match up? Or do you have to hone those out to match the larger intake?
 

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Are there different heads to match up with the different manifolds? IE: if you are grinding the stud openings to accommodate narrower studs, do the bores of the larger intake and the intake bores in the existing head match up? Or do you have to hone those out to match the larger intake?
Yes, some heads have wider stud spacing than others.

When I had a pair of bevels ('80 900 Darmah and an '80 900 SS) I found out that my Darmah had the narrow stud spacing like a 750GT or an 860GTS. The 900SS had wider stud spacing. The Darmah had a smaller intake port than the SS, but I don't recall how much smaller.
.
 

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Discussion Starter #9
As far as bore match-up, if I recall, the last year (s) of the 750 GT the factory mounted 32mm as opposed to the original 30mm. Don't think they altered the heads. I'll check the bores in the morning, let you know.
 

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The GT and Sport had the same heads, even though the Sport ran 32mm carbs.
 

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Yes, one of life's small pleasures was matching up the inlet port to the manifold. Funnily enough, I never noticed that it made any difference!
 

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My Darmah has 36mm Amals which were installed many years ago by a previous owner who was a bike mechanic. Why, I don't know, but it runs great. (The originals were 32mm Dellortos) I noticed this when I restored the bike, and the fact that everything was so nicely ported to match up, made me hesitant to put the smaller Dellortos back on. So it's interesting to hear that matching the ports doesn't really make much difference, Duccout.
 

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Well gas flowing is such a black art; maybe the step between the manifolds and ports helps gas swirl! does anyone here remember 'Cycle' magazine's Yamaha SR500 tune -up guide? They took the cylinder head to Jerry Branch's flow bench and tried to improve flow by porting the head. After a day's work, the best flow was achieved by sticking a stalactite of clay in the centre of the inlet port, which improved gas speed.
 

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Discussion Starter #15
That explains why my 36mm carbs run so well on my stock 750GT heads because... they just do. Thought about doing something about the mismatch between the manifolds and ports, and about playing with jets. In the end, decided I'd rather be riding.
 

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Thought I'd add a little more to what I wrote in reply to the original thread title ... :)

Now for the first letter. I believe this describes how the carb is intended to be mounted on the engine ... <snip>
Ok, here's a slightly different explanation of what these letters signify:

A = the carb is designed to be solid mounted, i.e. using a manifold.
B = the carb is designed to be mounted using a flexible connection.

Well, that's a little different to what I wrote before. And kinda makes more sense. However, what is really different is the suggestion that Dell'Orto used any other letters to signify that the carb was in someway a special.

This suggestion would explain why the carbs that went on the later Dramahs were designated PHF 32 Cs 'cos these carbs were 'specials', being standard PHF 32As modified internally in order to meet US emissions regulations.

Still dunno what 'D' meant, mind ...

P.s If anyone wants to know what all the preceding letters - VHB, PHF, SHB etc - mean, please ask. It's much simpler ... well, kind of.
 

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And to add to that Craig, I believe that the CS carbs had their pilot mixture screws glued in place, so that they could not be adjusted and produce extra pollutants!
 

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Discussion Starter #18
All in all, sounds like I'm better off just rebuilding my current carbs.
 

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And to add to that Craig, I believe that the CS carbs had their pilot mixture screws glued in place, so that they could not be adjusted and produce extra pollutants!
Two other 'explanations' I've heard:

1. Kevin MacKinnon (RIP) correctly pointed out that:

"There are two types of PHF 32 mixture screws. One for the AS/AD models, and one for the later CS/CD models. The C series have a fine cylinderical point at the end."

And in support of this, Steve Allen provided a very clear picture that I attach below:

(Source: New mixture screw, PHF 32 - -=- Club Bevel Heaven -=-)

2. Alternatively, I have this note, reproduced here verbatim. As to it's worth, I cannot say ...

"There was a long discussion on the Old Oz Guzzi Form about these ‘Smog Dells’. They have a shroud-like brass bit fitted in the throat of the carb where the needle passes (above the main jet).

The jetting was a lot different to non-smog Dells. Blokes converting the jetting to non-smog settings were getting really poor running."


As always, if anyone knows, do please chip in!
 

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Two other 'explanations' I've heard ... <snip>
And here's a third - plus a bit more rumination from Kevin concerning that long mixture screw :).

In another posting, Steve Allen made the observation:

"(The PHF32 C) has a tiny little jet installed down deep under the pilot (idle) jet. From what I have heard, it's for DOT emissions (SMOG)."

To which Kevin added the following:

"Steve is correct: there is another "jet" underneath the A series idle jet.

There is one other potential difference: the idle mixture drilling may be different to accomodate the longer and different shaped idle mixture screw, with the intent of provisioning some finer adjustment to reduce emissions. There may well be no difference at all in the drilling and it's all down to the needle, but it might be worth checking.
"
 
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