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Main : Misc : Tech Section Optimum Carburetor Choke Size

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Optimum Carburetor Choke Size
Optimum Carburetor Choke SizePopular
SubmitterddgonzalMore Photos from ddgonzal   Last Update2006/2/24 7:50    Tell a friendTell a friend
Hits8315  Comments10    0.00 (0 votes)0.00 (0 votes)
from the Weber book

Compare to the main choke (venturi) on a stock Datsun 1200 Hitachi which is 20mm.

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Poster Thread
b310gx
Posted: 2006/2/24 8:14  Updated: 2006/2/24 8:14
No life (a.k.a. DattoMaster)
Joined: 2002/7/19
From: sydney australia
Posts: 1856
 Re: Optimum Carburetor Choke Size
bugger,this is a different chart than for dcoe's.good thing i've edited all i wrote before.

Poster Thread
Grunterhunter
Posted: 2006/2/24 9:27  Updated: 2006/2/24 9:27
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 Re: Optimum Carburetor Choke Size
Depends where you're expecting to make your power- for 5000 rpm that may be ok, but most A series peak in the 6000-8000 rpm range so you want something a bit bigger ya?


Edit:alternative (but similar) graph of questionable origin here:

weber site

From my (limited) understanding, on twins and fours carby size is related to cylinder size not number of cylinders cos you are only ever filling one cylinder at a time. My Bro's VT1000 (bike) is a twin with 48mm carbies, making 88kW at 9k rpm- About the same as you would use on a 2 L four making peak power at those kind of revs.

Not sure about the sixes though

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ddgonzal
Posted: 2006/2/25 16:21  Updated: 2006/2/25 16:21
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 Re: Optimum Carburetor Choke Size
Yes, for 6,000-8,000 rpm max power, you'd want a bigger carb. This chart is for 5,000 rpm. Notice there is a range of what works best (the shaded area of the chart), as it will depend on the specifics of the engine.

Millions of a-series engines peaked at 6000 or less, like the A12. The A12GX peaked at 6400 rpm. A14s (depending on year) peaked at 5600.

Quote:
on twins and fours carby size is related to cylinder size not number of cylinders
That's what this chart indicatesQuote:
cos you are only ever filling one cylinder at a time
That does seem logical (four cylinder, four stroke), but that 'cos' is doubtful because the same holds for six-cylinder (see the fine print on the chart). It may be related though if the induction doesn't occur for a full 180 degrees.

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nzdatman
Posted: 2006/2/25 20:18  Updated: 2006/2/25 20:18
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 Re: Optimum Carburetor Choke Size
hmmm.. there's something not quite right about this graph. what model carburettor is it for?
Is it supposed to be venturi size for a one choke per cylinder application? Even then it would seem small.
A 120Y SSS has an A12 with a warmer cam and it used 28mm chokes in it's twin dellortos, much larger than what the chart suggests?

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ddgonzal
Posted: 2006/2/26 6:24  Updated: 2006/2/26 6:24
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 Re: Optimum Carburetor Choke Size
It's for any carburetor.

The fine print says "one single barrel carburetor", so the chart won't apply to twin dellortos.

Poster Thread
ddgonzal
Posted: 2006/3/5 19:53  Updated: 2006/3/5 19:53
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 Re: Optimum Carburetor Choke Size
Here's what the text says about this chart, in regards to progressive carbs (like our Hitachi, or the Weber 32/36):Quote:
the secondary choke [venturi] size can be determined from Fig. 5.1 and the primary choke size is normally 1 mm less.
For a 1200cc engine at 6000 rpm, I picked a 26mm secondary venturi based on this chart and interpolating for 6000 rpm based on the DCOE chart.

Now, looking at the A12 specs, it seems Nissan used the same logic:
A12 secondary venturi: 26 mm

Remember that this is just a starting point. Actual test driving or dyno runs will show if slightly bigger or smaller is desired. Like, it depends on what you want, more peak HP at the expense of partial throttle response and low-speed operation, or the reverse. Note the the chart indicates a range of size (the shaded area), but you could go outside that if you wanted to enhance flat-out acceleration and don't care about off-idle performance (example: fitting a DGV Weber 32/36 to a stock A12).

The Weber book also says:Quote:
the difference between the pimary and secondary choke diameters may need to be increased 1 mm or 2 mm in order to give more flexibility at the lower engine speeds.
This would explain why Nissan gives small primary venturis to our A-series.
A12 secondary venturi: 20 mm

That's a lot smaller than the book recommends, but then this road-going car has very good "flexibility at the lower engine speeds".

Note that most A14 carbs are have venturis of 23/27, so that would probably give a good performance boost for a stock A12, without sacrificing driveability.

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Grunterhunter
Posted: 2006/3/6 2:55  Updated: 2006/3/6 2:55
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 Re: Optimum Carburetor Choke Size
Quote:
Note that most A14 carbs are have venturis of 23/27, so that would probably give a good performance boost for a stock A12, without sacrificing driveability.


I'm not sure if the A14 got a bigger carb than the A15, but ther is no way my A15 venturi's are that big. The barrel size may be close. I will measure tonight if I remember.

It's interesting that for a dual throat carby they almost ignore the primary- "determine the secondary and make the primary 1 mm less" As though the primary is not flowing any fuel/air when the secondary is open Surely you have (almost) double the potential flow with both barrels open and are then seriously over-carbed.

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ddgonzal
Posted: 2006/3/6 5:44  Updated: 2006/3/6 5:44
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 Re: Optimum Carburetor Choke Size
All Hitachi DCG306/DCH306 have barrel sizes of:
26/30
So they have much smaller barrels than the Weber 32/36.

According to my Nissan factory books:
The A12 (and A12A) got venturis of 20/26.
Most A14s are 23/27, but some were 22/27.
A15s mostly use the 22/27, but Canadian A15s had 23/27.

You can look at the size of you carb, the numbers appear as is this photo. You can shine a flashlight down the air cleaner to see them:

7888

From the text, it seems this chart applies to a "single carburetor" feeding all the cylinders (i.e. using a plenum), whether that carb is one or two barrels. So why a smaller primary? Well, the usual goal of a progressive carburetors is to give better low speed flexibility. The primary and secondarys will be matched on synchronized carbs like the Weber DGAS 38/38. I don't see why you couldn't do the same with a progressive carb. But if your goal is max performance, the synchronized carb (or carbs) is the usual choice.

Which brings us back to the first post: it doesn't matter if it's a 4-cylinder or 6-cylinder engine, Weber recommends to use the same size carburetor. And it doesn't seem to matter if it's a one barrel or two barrel carb.

But now I'm wondering about it. Why does it say multiple 2-cylinder engines by two? I wonder if it is a typo and meant to say "for two-stroke engines, multiply the engine capacity by two".

Poster Thread
Grunterhunter
Posted: 2006/3/6 6:24  Updated: 2006/3/6 6:24
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 Re: Optimum Carburetor Choke Size
Quote:
But now I'm wondering about it. Why does it say multiple 2-cylinder engines by two? I wonder if it is a typo and meant to say "for two-stroke engines, multiply the engine capacity by two".


That would definatley make more sense.

As for my 32/36 it is has 26/27 on the venturis, so it seems that I have increased the primary by about 4 mm and done nothing useful for the secondary compared to the original carb. It does run better tho

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benny
Posted: 2006/3/6 7:52  Updated: 2006/3/6 7:52
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 Re: Optimum Carburetor Choke Size
Quote:
But now I'm wondering about it. Why does it say multiple 2-cylinder engines by two? I wonder if it is a typo and meant to say "for two-stroke engines, multiply the engine capacity by two".


From memory I think it may have something to do with the degrees between pistons. i.e anything less than 120 degrees [3 cylinder engine] requires a larger choke.

Benny

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ddgonzal
Posted: 2006/2/24 7:45  Updated: 2006/2/24 7:45
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 Re: Optimum Carburetor Choke Size
The most interesting thing about this to me is ... it doesn't matter if it's a 2-cylinder, 4-cylinder or 6-cylinder engine, Weber recommends to use the same size carburetor.

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benny
Posted: 2006/3/6 4:24  Updated: 2006/3/6 4:24
No life (a.k.a. DattoMaster)
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 Re: Optimum Carburetor Choke Size
Quote:
The most interesting thing about this to me is ... it doesn't matter if it's a 2-cylinder, 4-cylinder or 6-cylinder engine, Weber recommends to use the same size carburetor.


Because it refers to the total engine displacment. 1.2 litres is 1.2 litres whether it divided into 4 cylinders or 6, it doesnt really matter. It does however mention exception for 2 cylinder engines though.........

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ddgonzal
Posted: 2006/3/6 4:30  Updated: 2006/3/6 4:30
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 Re: Optimum Carburetor Choke Size
Quote:
1.2 litres is 1.2 litres whether it divided into 4 cylinders or 6, it doesnt really matter.
Hmm ... there is a different chart, and different sizing, for a carb barrel feeding a single cylinder (e.g. twin DCOEs on a four cylinder).