jet size question

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dingo
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jet size question

Post by dingo » Wed Jul 01, 2015 8:32 pm

i noticed in the Bentleys table of jet sizes for various stock carbs that the main for 34pict was 125, and the main jet for the dual 32s was only slightly smaller..like 120 or thereabouts. So im wondering, if each carb is serving on ly two cylinders, then it is theoretically delivering only half the amount of fuel...so then should not the jet sizes be half the size.? or is the strength of the vacuum on each side that much weaker that the jet size is barely different ? If that is the case, then the ability to atomize the liquid fuel is much less than a centrally located carb.
Herr Colin..what sayest thou ??
'71 Kombi, 1600 dp

';78 Tranzporter 2L

" Fill what's empty, empty what's full, and scratch where it itches."

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dingo
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Re: jet size question

Post by dingo » Thu Jul 02, 2015 8:36 am

Well i read that jet size should be 4.1 to 4.3 x venturi size....so dual 32s would each get approx 130 mains....tho the physics of it still escapes my brain
'71 Kombi, 1600 dp

';78 Tranzporter 2L

" Fill what's empty, empty what's full, and scratch where it itches."

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Amskeptic
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Re: jet size question

Post by Amskeptic » Thu Jul 09, 2015 8:41 pm

dingo wrote:Well i read that jet size should be 4.1 to 4.3 x venturi size....so dual 32s would each get approx 130 mains....tho the physics of it still escapes my brain
Think of the entire operating map of a carburetor. It has to provide *not only the maximum fuel* for a given carburetor, but an *appropriate flow for all other loads and speeds.

Most of us think of jet sizes like we think of . . . other sizes, and we are stuck on maximums. But what if the jet size has to be proportional to the venturi to maintain the correct fuel-to-air ratio at low air speed as well as higher air speed? Then the above makes sense.

As for dual versus single carburetors, the exact same physics applies. Yes, dual carbs will have lower respective air speeds at idle and low load, but they will and they do flow more air up at the top rpm range.
Therefore, they must have the above proportions to the venturis to draw the correct ratio of fuel to air.

Now ponder that the European dual carburetor 1972 buses had better fuel economy than the 1971 single carb buses.
Colin :flower:
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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asiab3
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Re: jet size question

Post by asiab3 » Thu Jul 09, 2015 10:29 pm

Amskeptic wrote: Now ponder that the European dual carburetor 1972 buses had better fuel economy than the 1971 single carb buses.
What was special about the European dual carb models?
1969 bus, "Buddy."
100k miles with me.
279k miles on Earth.

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Amskeptic
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Re: jet size question

Post by Amskeptic » Tue Jul 21, 2015 10:45 pm

asiab3 wrote:
Amskeptic wrote: Now ponder that the European dual carburetor 1972 buses had better fuel economy than the 1971 single carb buses.
What was special about the European dual carb models?
The fact of this superior fuel economy was noted there in a new car review.
Colin
(it was only 1 mpg, but it was counter-intuitive, thus an interesting factoid)
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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