Need carb advice

[FiatRunner]

Hello all,

I bought Auto Ricambri's high performance air filter for my '74 Spider, and I assumed it would be an easy, quick way to make the engine bay look nicer, and maybe get a little bit more performance. Sure enough, it didn't fit.

I did a little research, and the carb seems to be a Weber DHS. Is this the carb that it came with from the factory? I'd really like to replace the dirty, ugly, old air filter assembly that requires removal anytime I'd like to work on the engine. I just can't seem to find an air filter that would fit a DHS anywhere online.

Does anyone know where I can find one? Or any information in general? The car runs great and seems to make plenty of power, as low as 2000 rpm and as high as 7000.

[18Fiatsandcounting]

I did a little research, and the carb seems to be a Weber DHS. Is this the carb that it came with from the factory?

No, not for a '74 model. My understanding of the Fiat spiders carbs is this:
Pre-1968 (Europe): 34 DHS
1968-1969: 26/34 DHSA
1970-1973: 28/36 DHSA
1974: 34 DMSA
1975-1978: 32 ADFA
1979-1980: 28/32 ADHA
Post 1980 (1979 in Calif.): Fuel Injection

Of the carbs, the 34 DMSA is generally regarded as the best, while the 28/32 ADHA is, well, awful.

Back to your air filter. I have always had to modify the base of an aftermarket air cleaner housing to get it to fit. This could include expanding the opening (with a file), shifting the 4 mounting holes (with a drill), or adding gaskets to get a good seal between the top plate of the carb and the base of the filter housing. And I always had to rig up some sort of connection for the blowby hose from the crankcase, usually involving trips to the hardware store for metal fittings.

So, I hate to say it, but the issue you describe is fairly common. You could try and find a 34 DMSA carb, and while it might have better performance, there's no guarantee the air filter housing would fit any better.

-Bryan

[FiatRunner]

So, I hate to say it, but the issue you describe is fairly common. You could try and find a 34 DMSA carb, and while it might have better performance, there's no guarantee the air filter housing would fit any better.

Thank you for all the fantastic information!

I think I am going to try and design a spacer in CAD, and the 3D print it. I'd rather not swap out the carb, as this one runs really good, and if I can't get the new filter to work, it isn't the end of the world.

However, would the spaced out filter affect performance? It would only need to be 1/4 or 3/8 of an inch tall, and I wouldn't think that it would make a noticeable difference, but I'm definitely not a professional engine tuner and I don't know for sure.

[18Fiatsandcounting]

However, would the spaced out filter affect performance? It would only need to be 1/4 or 3/8 of an inch tall, and I wouldn't think that it would make a noticeable difference, but I'm definitely not a professional engine tuner and I don't know for sure.

I can't imagine it would affect performance. The only issue might be clearance with the hood, but there should be plenty if you have the original engine.

One other thought that I had after I posted earlier: It is possible that you have a 34 DMS rather than a 34 DHS? "M" and "H" can look similar if the model number stamping isn't all that legible. A 34 DMS was used in some cases in 1974, and I think the difference between DMS and DMSA is some minor emissions mods.

-Bryan

[FiatRunner]

It is possible that you have a 34 DMS rather than a 34 DHS? "M" and "H" can look similar if the model number stamping isn't all that legible. A 34 DMS was used in some cases in 1974, and I think the difference between DMS and DMSA is some minor emissions mods.

I'm pretty sure it is a DHS, as it has a large circular piece extending out of the back... it looks like a large diaphragm of some sort. I'm not sure what it does, but it sticks up higher than the rest of the carb and hits the aftermarket air filter. It's also the only Weber carb that looks the same as mine.

If it makes any difference, my car has no emissions equipment, and no evidence of ever having any.

[18Fiatsandcounting]

If it makes any difference, my car has no emissions equipment, and no evidence of ever having any.

It has a few (ignition advance curves, compression ratios, cams), but Fiat did a good job of hiding them!

I think the large circular thing you are referring to is the secondary vacuum diaphragm. When you floor the gas pedal, if there is enough air flow into the engine through the primary, the secondary butterfly valve will be opened with this diaphragm. Unfortunately, the rubber inside these diaphragms goes bad over time, so the secondary barrel stops working. New diaphragms are almost impossible to find, which is why people convert to other carbs.

Does your carb look like this?
https://classiccarbs.co.uk/product/webe ... -old-stock

If the only thing that's hitting is that vacuum diaphragm, you should be able to make a spacer to lift up the air cleaner housing a half inch or so. Might need longer bolts to hold the housing to the carb.

-Bryan

[FiatRunner]

I think the large circular thing you are referring to is the secondary vacuum diaphragm. When you floor the gas pedal, if there is enough air flow into the engine through the primary, the secondary butterfly valve will be opened with this diaphragm. Unfortunately, the rubber inside these diaphragms goes bad over time, so the secondary barrel stops working. New diaphragms are almost impossible to find, which is why people convert to other carbs.

Does your carb look like this?
https://classiccarbs.co.uk/product/webe ... -old-stock

That is exactly what my carb looks like, and after studying the linkages a bit more, I can see how it operates the second butterfly. Thankfully it works great now, but I'll see if I can buy a backup diaphragm and put it on the shelf in case my current one goes bad.

I also talked with a friend of mine who is a wizard with CAD and owns a CNC milling machine. I might have a billet aluminum filter spacer soon! And even if the CNC one doesn't work out, I have a 1/2 inch thick sheet of PVC plastic that I can easily cut to shape with a jigsaw.

Thank you again for all the awesome information. I'll post updates about the spacer.

[18Fiatsandcounting]

...I'll see if I can buy a backup diaphragm and put it on the shelf in case my current one goes bad.

If you find a source, let us know. I've done some searching, and all I came up with were Ebay vendors in Eastern Europe. I have tried to make my own diaphragm with some thin nitrile rubber sheet, and while it worked, it didn't duplicate the bellows action of the original rubber diaphragm.

One aspect of the vacuum operated secondary is that it tends to be "all or nothing". That may be fine for some folks, but the vacuum operated secondary doesn't give the smooth progression on the opening of the secondary barrel that you would see on carbs with a mechanically operated secondary.

-Bryan

[FiatRunner]

One aspect of the vacuum operated secondary is that it tends to be "all or nothing". That may be fine for some folks, but the vacuum operated secondary doesn't give the smooth progression on the opening of the secondary barrel that you would see on carbs with a mechanically operated secondary.

I've definitely experienced this. If you floor it below 3000 rpm, you can audibly and physically feel it open, between 3500 and 4000 rpm. You can feel the car start accelerating faster, and it sounds different too. I don't normally experience this, as I typically shift to a lower gear if I need to accelerate quickly.

However, would it be possible to convert my DHS to have a manual secondary, like on the DHSA? I'd like to keep this carb if possible, but I'd hate for the diaphragm to crack and leave me with a car that won't run well, especially if I can't find a replacement.

Also, to get the car to start I sometimes have to play with the manual choke like it's some fine musical instrument. Is this normal? Or is there some sort of choke adjustment?

[18Fiatsandcounting]

However, would it be possible to convert my DHS to have a manual secondary, like on the DHSA?

Not sure I understand. The DHSA also has a vacuum secondary. Other carbs like the DMS, ADFA have a mechanical secondary.

Also, to get the car to start I sometimes have to play with the manual choke like it's some fine musical instrument. Is this normal? Or is there some sort of choke adjustment?

Ahhh, yes, the joy of finding the sweet spot with the choke and starting the car.

Each engine seems to be different, but my method is: Pull the choke all the way out. Pump the gas pedal once. Start the engine. As soon as it fires, push the choke back in about halfway. Don't touch the gas until it runs a bit. It may take several attempts. As it warms up, push the choke all the way in, and the time for this depends on your climate. In the summer, I hardly need the choke at all. In really cold climates, you might need the choke for a few minutes.

-Bryan

[FiatRunner]

Not sure I understand. The DHSA also has a vacuum secondary. Other carbs like the DMS, ADFA have a mechanical secondary.

Oh. I guess I thought that it had a manual secondary. Is it easier to find parts for the vacuum secondary on the DHSA? Would I be able to swap the secondary from the DHSA to the DHS?

In the summer, I hardly need the choke at all. In really cold climates, you might need the choke for a few minutes.

When I bought the car about 6 weeks ago, it was typically 65-75 degrees outside, and I barely needed to use the choke, but now that the outside temp is as low as 35 or 40 degrees during the day, the choke is definitely required. My current procedure is to pump the gas one or two times, and while cranking, pull the choke out until it starts. Once it is started, I control the idle speed and prevent it from stalling with the choke only. Full choke is too rich, and it either won't start or won't run for long.

Either way, I need to put the car away for winter. It's too cold, and the car takes too long to warm up for it to be practical. The wood steering wheel gets quite cold too...

[18Fiatsandcounting]

Is it easier to find parts for the vacuum secondary on the DHSA? Would I be able to swap the secondary from the DHSA to the DHS?

I think it's challenging to find a diaphragm for any of the DHSx series. I don't know about swapping the vacuum "plenum", but it might work.

Full choke is too rich, and it either won't start or won't run for long.

There is a device called the "choke unloader" and its function is to open the choke plates slightly when the engine fires, even if you have the choke knob pulled all the way out. However, it is VERY common for these to stop working, and so you have to experiment with different combinations of pedal pumping, choke pulling, and so on. In cold weather, I have had Fiats where I had to pump the gas pedal several times, then wait 10 seconds, then pump a few more times, then wait, then try to start. The wait time allows the gas in the intake manifold to vaporize a bit so it starts easier. One aspect of modern gas is that it's less volatile due to emissions requirements, so it can be harder to start than what was available 40 or 50 years ago. At least that's my theory, and I'm sticking to it.

The wood steering wheel gets quite cold too...

Gloves, man, you need gloves!

-Bryan

[FiatRunner]

I think it's challenging to find a diaphragm for any of the DHSx series. I don't know about swapping the vacuum "plenum", but it might work.

I'll keep an eye out for a diaphragm for sale. I might do a dual IDF 40 swap in a year or two, so hopefully the current one will last long enough.

Thanks again for the help!

[GTRaptor]

DONT use Aluminum for the SPACER... will transfer heat to the carb, better to use a phenolic spacer (Phenolic Resin)

[FiatRunner]

DONT use Aluminum for the SPACER... will transfer heat to the carb, better to use a phenolic spacer (Phenolic Resin)

How would the spacer transfer heat to the carb? I can see how that would happen if it was between the carb and manifold, but I need a spacer between the carb and air filter.