Twin carb vs single carb heads. What's the difference?

The 4A, 4AB, 4AC, 4AD cars, including the SM 1500 Roadster and SMX
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Twin carb vs single carb heads. What's the difference?

Postby DGreen » Wed Sep 14, 2016 5:42 am

I read through a few old postings regarding twin carbs vs single carbs (basically 4ADT vs 4AD) and I noticed that there was some discussion of a different head being used with the twin carb set up, but no information on how to tell the difference from a single carb head. Are there external differences used to distinguish between the heads without taking them off? My car (a 1953 4AD) has twin carbs, but doesn't have the "T" suffix in the serial number as far as I can tell, so I'm trying to sort out what may have been done to the car over the years. The person who owned the car for most it's life (1950s to 2005ish) had three of them, so he may have mixed and matched parts, but I'm really not sure. I'll check for the Salisbury axle next. It seems to me that there are a lot of twin carb cars out there without the "T" in the serial number. They can't all be cobbled together from parts scavenged from wrecks, can they?

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D'Arcy

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Peter McKercher
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Re: Twin carb vs single carb heads. What's the difference?

Postby Peter McKercher » Wed Sep 14, 2016 10:42 am

D'Arcy
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Re: Twin carb vs single carb heads. What's the difference?

Postby mikeyr » Wed Sep 14, 2016 6:44 pm

the answer on the heads is "it depends"

No, without removing the manifold and its plate between the manifold and the head, there is no way externally to tell the difference.

There are single carb heads, there are twin carb heads, but some twin carb heads can be run in single carb with a proper blanking plate. Last 4AD I purchased had twin carb head setup with a single carb, luckily in those days I had a twin-carb manifold to put on. I think but I have no proof that as Singer got into financial trouble they made only the twin-carb head for the last year and just blanked it off. I have one twin manifold and head left that I am keeping for a project down the road, but the other heads I have I think are all single carb. If I have another twin carb head, I can take some pics tonight next to a single carb head. Since I have not removed the manifold I dont know if my twin setup that I am keeping is 2 hole or 3 hole but it came off a true ADT early car so I assume 2.

Basically the single carb has of course one inlet into the head and the twin has 2 inlets, some heads have 3 inlets, the one in the middle is for the single and they put in a different plate between the manifold and head. I have only seen 2 heads with the 3 holes so they are very rare and they both came from last year model 4AD's. The 3 hole heads are not "true" twin carbs since obviously the middle inlet port goes to all 4 cylinders and the 2 outer inlet ports are joined with the single port so the 2 outer ports can go to all 4 cylinders but they ran fine. The true twin carb, each inlet port goes to only 2 cylinders.

In the old days when I still had the parts, I converted 3 single carb to twin carb, those were the days, now I am running low on heads and blocks but you are right that converting to twin carb was a popular thing to do and pretty easy way to get some extra HP. The California dealer even had a hot rod setup with different camshaft and twin SU-sidedrafts available as a after market option. I still have one of his camshafts, it even changed the firing order and idled super rough.
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Re: Twin carb vs single carb heads. What's the difference?

Postby mikeyr » Wed Sep 14, 2016 6:51 pm

one more thing...The rear axle is of course a quick way to tell if the car was originally a twin carb. Another much less accurate way is to look at the radiator hose housing that bolts to the head. The twins had a flared housing to accept the thermostat, the singles did not have that flared housing since they did not have a thermostat. Many conversions in the old days were made and since they didn't have the fancy radiator hose, just kept the straight hose and single carb housing during the conversions. So if you have a twin with a single carb radiator housing its a safe bet it was a conversion. Of course many people did it right and also installed the flared housing so there is no guarantee by looking at that. I think the rear axle is more likely a way to tell how it came from the factory but after all these years who knows if the rear has been changed.
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1953 Singer 4ADT (sold), 1934 Singer 9 Le Mans, 1934 Singer 1 1/2 4-Seater Sports, 2009 BMW K1300GT

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Re: Twin carb vs single carb heads. What's the difference?

Postby DGreen » Thu Sep 15, 2016 2:07 am

Thank you,

I see now that my engine number is stamped with the ADT prefix, and the car does have the correct Salisbury rear axle. The engine number doesn't match the serial number on the cowl though.

D'Arcy

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Re: Twin carb vs single carb heads. What's the difference?

Postby mikeyr » Fri Sep 16, 2016 4:32 am

look closely, you will see you could fairly easily make a twin-carb head from a single carb, just by machining 2 new holes and making a new plate that plugs the single carb hole. You do have to worry about water since the intake port is surrounded by the cooling jacket but it can be done. The 3 hole heads had a flat section like around the single carb hole. Remember you can click on the picture to make it bigger.
IMG_1587 copy.jpg
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Re: Twin carb vs single carb heads. What's the difference?

Postby DGreen » Fri Sep 16, 2016 8:16 pm

Great information. Thank you!


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