DHSA Jetting

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Special
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DHSA Jetting

Post by Special » Thu Aug 08, 2019 4:45 pm

Hi guys.
I have a Weber 26/34 DHSA carburetor on my 1438 Wagon.
Most likely everything in that carb is original .
I decided to clean it , took everything apart and put all parts and jets in the tray in certain order.
Of course later on I kicked that tray and flipped it over .
Now I have a pile of parts and jets .
I have a diagram . It explains what goes where, but there is no explanation of specific jet # location .
Maybe someone knows where can I find that info.
And also - where does tube-valve connector goes ? #10 on the diagram. It was nothing on my carb . Should I leave it open or plug it ?
Thanx in advance.

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'73 124 Familiare
'74 124 Special TC
West Hills , CA
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Re: DHSA Jetting

Post by bartigue » Thu Aug 08, 2019 5:08 pm

Jetting tables are in the service manuals, in the library of this forum.
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1969, 1969, 1970, 1979, 1980, 1981, 1982, and 1982 124 Spiders
1969, 1970 850 Spiders
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Re: DHSA Jetting

Post by 18Fiatsandcounting » Thu Aug 08, 2019 11:13 pm

For unknown reasons, I can remember Weber jet sizes for my Fiats, but nothing that my wife told me 5 minutes ago. Go figure. Anyway, for a 124 spider with a 1438 cc engine, here's what I recall for the 26/34 DHSA carb:

Primary main jet= 1.20
Secondary main jet = 1.40
Primary air corrector = 1.80
Secondary air corrector = 1.50
Primary emulsion tube = F43
Secondary emulsion tube = F6

I hope this helps.

As to #10 on your diagram, this tube is connected to the "choke unloader", also known as a "choke pull off valve". In a nutshell, it operates as follows. Some Weber carbs are set up so that, under full throttle with the choke still on (either manual or automatic), the choke butterflies are "over-riden" to allow maximum airflow into the engine. So, this tube #10 gets vacuum from the fast idle and secondary throttle diaphragms to override the choke butterflies when the engine is under full throttle. If missing, the car will still work just fine, although perhaps not optimally when the choke is on and you're leaning on the throttle.

-Bryan
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Re: DHSA Jetting

Post by Special » Fri Aug 09, 2019 1:03 am

Thank you guys.
I found the info in a service manual.
Great memory Bryan ! It's all correct. Including the wife part.
I'll just plug that #10 tube - I don't drive my cars with the choke on. So it should be OK.
Well , I was able to put everything together .
The car starts , runs and idles fine too.
But .
The main reason I took the carb apart was :
The secondary barrel throttle shaft was seized up and someone tried to free it with the screwdriver or something like that and bent the round brass plate.
I took it all apart , unseized the shaft and straightened up the plate .
Everything moves the way it should now.
The vacuum hose is OK and connected too.
But , the secondary barrel doesn't open under full throttle .
I can open it with my finger and it makes a huge difference in power , but it doesn't want to open on it's own.
Is there certain things I should check ?
And one more question - what does this screw do ?
And how should it be adjusted ?

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Re: DHSA Jetting

Post by 18Fiatsandcounting » Fri Aug 09, 2019 10:38 am

On this carb, the throttle valve for the secondary is opened by vacuum from the engine rather than directly by the throttle linkage, and the primary has to be open a little more than halfway for the secondary to open (again, going by memory here). There are two reasons that the secondary might not open: 1) the butterfly valve or shaft or the linkages are stuck, or 2) there is insufficient vacuum in the top of that large vacuum diaphragm on the right of your illustration to pull up the linkage rod and thereby open the secondary. Since you can open the secondary with your finger, it would seem that reason (2) is the issue.

The vacuum to operate that secondary diaphragm comes from the mounting flange where the three screws are (#7 in the diagram). Either something is plugged up in that area, there is a leak in the gasket where those three screws are, or you have a hole in the secondary diaphragm itself. If the rubber diaphragm itself is bad, that's a tough one as this part is hard to find. You might need to find another 26/34 DHSA for the parts. But, take it (carefully) apart and have a look. A good carb shop might also be able to find a rubber diaphragm that could be adapted to work, and at one point, I found a commonly-available diaphragm for a Holley carb that appeared like it could be made to work.

As for that screw you circled? That is the fast idle adjustment screw, which keeps the idle at 1500 rpm in certain gears with the clutch engaged. Part of the (very early) emissions controls, and also to prevent backfiring under certain conditions, if I recall.

Anyway, the vacuum for the fast idle diaphragm (located just below that screw you circled) pulls up on the linkage rod held by cotter pin #6, thereby opening the throttle just a tad to get you the higher idle speed. The vacuum for the fast idle comes ultimately from the intake manifold or the base of the carburetor (memory is fuzzy here), and is controlled by a round valve mounted on the firewall across from the carb, and the switch is turned on with certain combinations of gears and clutch position. The vacuum can also be "turned on" by the fast idle adjustment "test switch" which is a simple plunger type manual switch located near that round valve. This vacuum also pulls up on that choke unloader through tube #10 that we talked about earlier.

OK, hope this helps!

-Bryan
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Re: DHSA Jetting

Post by Special » Sun Aug 11, 2019 2:01 am

Thank you Bryan for shearing you knowledge.
I think I figured it out.
Yesterday I didn't have any time to work on the car , but today I spent few hours with it.
I noticed the screws #7 weren't very tight and gasket #9 was too soft a mushy .
Both diaphragms were ok though .
I cleaned the air passages , replaced the gasket and tightened the screws and seems like its working now.
My car is automatic and all smog staff is removed.
I am not sure if I still need that round valve or not , but after adjusting that circled screw ( it was almost all the way in ) RPM does go up a little when I press the check switch.

These cars are definitely amazing.
Half of the jets was plugged , carb filter was plugged as well , needle was stock , some linkages were disconnected , few vacuum ports were open , big vacuum hose what goes to the brake booster had two huge cracks , some disconnected , messed up wires and despite all that - CAR WAS RUNNING OK !!!
Brilliant Italian engineering.

Oh yeah !
I took it for a spin and runs much better , but now it overheats , dew to plugged radiator core.
Most likely it's impossible to find this radiator for an automatic wagon, so I'll take it to the radiator shop and I hope they'll be able to clean it.

Never ending story ...
'73 124 Familiare
'74 124 Special TC
West Hills , CA
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Re: DHSA Jetting

Post by bartigue » Sun Aug 11, 2019 8:50 pm

You can always test the diaphragm without removing it; move the linkage and you’ll hear air rushing out or in as the secondary plate moves.

A lot of time the secondary linkage is just stuck.
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Brad Artigue
1969, 1969, 1970, 1979, 1980, 1981, 1982, and 1982 124 Spiders
1969, 1970 850 Spiders
77 X1/9
http://fiat.artigue.com
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Re: DHSA Jetting

Post by friedman » Mon Aug 12, 2019 8:40 am

A non-auto rad will work fine, just add an aftermarket trans cooler and strap it in front of the radiator.
Carl in Virginia
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83 Bertone
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Re: DHSA Jetting

Post by Special » Tue Aug 13, 2019 7:48 pm

bartigue wrote:
Sun Aug 11, 2019 8:50 pm
You can always test the diaphragm without removing it; move the linkage and you’ll hear air rushing out or in as the secondary plate moves.
I think if it is a small hole in the diaphragm - you'll still hear the air moving , but it will not hold the vacuum.
Likely on these cars it's not too hard to pop up the covers and visually check the diaphragms.
bartigue wrote:
Sun Aug 11, 2019 8:50 pm
A lot of time the secondary linkage is just stuck.
Yep , that was the original problem , it was stock.
'73 124 Familiare
'74 124 Special TC
West Hills , CA
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Special
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First Name: Gene

Re: DHSA Jetting

Post by Special » Tue Aug 13, 2019 7:59 pm

friedman wrote:
Mon Aug 12, 2019 8:40 am
A non-auto rad will work fine, just add an aftermarket trans cooler and strap it in front of the radiator.
Separate transmission cooler is a good idea , I checked and they are pretty cheap.
But so far I can't locate any radiator for sale , auto or manual.
There are few on Italian Ebay , but they are not exactly the same as mine.
I also took my rad to the radiator shop . Guy checked it and said - core must be replaced .
He will reuse both tanks and install new core.
It will run somewhere between $250 and $300.
I think it is to much $ for something like that.
But what other choice do I have ?
'73 124 Familiare
'74 124 Special TC
West Hills , CA
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