What size carb jet for ‘67 N15cs

texasSlick

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The number refers to the number of cc's of fuel passed per minute under strict test conditions. Thus, there is little difference.

Dropping your needles one notch will likely make your needle jets 106.

Slick
 
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The number refers to the number of cc's of fuel passed per minute under strict test conditions. Thus, there is little difference.

Dropping your needles one notch will likely make your needle jets 106.

Slick
That’s all wrong.

The needle jet number refers to the internal diameter of the metering section, in thousandths of an inch.

The metering of the mixture is done by the space between the needle and the jet. At small throttle openings, when the parallel section of the needle is in the jet, that is a very small space, so changing from a 106 to a 107 jet makes a big difference to the mixture.

Trying to compensate for a worn or too-big needle jet by dropping the needle to a weaker notch can dangerously weaken the mixture at bigger throttle openings.
 
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Anyway, to sort of answer your question, the manual specifies (for Monoblocs) 1065 needle jet for all heavyweight twins and needle clip position 3.

So I suppose I’d start with a new 1065 jet, unless others have different recommendations, based on their experience.
 

texasSlick

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Well Triton Thrasher is only partially correct.

My statement that jet number refers to flow rate is true for pilot and main jets. Bring up the Amal website page for jets (ex. http://amalcarb.co.uk/main-jet-size-180.html) and you will find the statement:

"All genuine AMAL jets are calibrated to a flow rate. The number e.g. 100 indicates that it will flow 100cc`s of fuel in one minute at a specified head. Beware of imitations as they are just drilled to a size. All our jets are branded and packaged with the official AMAL logo."

Since needle jets have a tapered needle in the orifice, this statement obviously does not hold as the flow relates to the position of the needle. TT is correct that a 106 needle jet has an orifice of 0.106" .... I just checked that with a #36 drill bit and a new 106 needle jet.

While I agree that manuals for heavy weight twins specify 106 needle jets and clip position 3, I beg to disagree with TT that a #107 with render a BIG difference.

Slick
 
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My experience may not be your experience, but I changed my Concentric needle jets from 106 to 107 and the mixture went from apparently correct, to obviously too rich.
 
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texasSlick

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"That doesn’t translate directly to a 44% richer mixture, because the pilot jet, the slide cutaway and the air bleed drillings in the needle jet also affect mixture at these small throttle openings."

TT, you left out needle clip position.

Only trying it both ways ... i.e. new #106 vs the OP's existing #107 with a lower needle position will definitively settle it.
I always like to go with factory settings, so I would go with #106 in clip setting 3.

BTW ... from the Amal co. " Something that crops up all the time is whether you count the needle clip slot from the top down or bottom up. With all Amal carburettors the clip position is top down."
 
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I try to remember that the lowest clip position number is the weakest mixture.

Needle clip position is used to adjust mixture at over 1/4 throttle, up to 3/4 throttle. That’s why I left it out of my little maths exercise about the parallel part of the needle being in the needle jet.
 

texasSlick

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Yeah, I knew that.

My math says there is only 11.9 % difference in the NET area of the two examples. As fuel flows thru the net area, that is the proper metric.
Area of a circle = Pi x D x D /4
area of 0.106" circle = 0.00882 sq in.
area of 0.107" circle = 0.00899
area of 0.097" circle = 0.00739
net area (for #106) = 0.00882 - 0.00739 = 0.00143
net area (for #107) = 0.00899 - 0.00739 = 0.0016
difference in net area= 0.0016 - 0.00143 = 0.00017
and % difference in net area = 0.00017/0.00143 = 0.1189 = 11.9% = not a big deal

We have spent too much time on this .....

Edit: "11.9% = not a big deal" In your defense TT, I am going to make a concession... Perhaps 12 % IS a big deal. I see that Amal now offers an "in between" needle jet of 1065. Never heard of such a size before you brought it up. I guess Amal would not bother to make one if there was not a significant difference.
 
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Thanks for the help with my dodgy maths- yes yours looks more realistic.

I better delete my dodgy answers in case someone reads them and takes them seriously.
 

texasSlick

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IF the OP procures #106 as we both have recommended, and as he already has #107, .... just think, ..... he has 10 needle jet/clip positions that he can fettle with. That should keep him busy awhile.
 
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