40 IDF jetting and venturi questions

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fran kelly
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40 IDF jetting and venturi questions

#1 Post by fran kelly »

Had to tear down my 1964 356C engine after blowing out a Heli coil. With heads done (i.e., valves, guides, 2 plug inserts ) and new rings I'm about to start putting it back together. I have 40 IDF Webers set up as follows: venturis, 32 mm, main jets 130, air correction jets 200, idle jets.52 and emulsion tubes F 7. Ran well before the big bang but I am wondering about tweaking the carbs a bit, maybe increasing the venture size to 34 mm. Wondering what people's thoughts might be as I also have a big bore kit.
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Re: 40 IDF jetting and venturi questions

#2 Post by Norm Miller »

Best to leave the venturi at 32, at sea level, I would use 180 air correction and 115 - 120 mains and 55 or 60 idle jets.

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Re: 40 IDF jetting and venturi questions

#3 Post by David Jones »

Fran, big bore sounds like a big increase but reality is that an increase of 30cc per cylinder just ain't that much if we are talking 82.5 to 86mm. Unless you have a dyno to recalibrate the jetting I would leave the carbs more or less as they are unless you experience lean or rich running and then I would only change the idles or mains. Changing anything else without the equipment to monitor the results could get you into some serious trouble. Even with a dyno and experience it is not a quick job to set the jetting.
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Paul Lima
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Re: 40 IDF jetting and venturi questions

#4 Post by Paul Lima »

As an inexpensive but very helpful alternative to a dynamometer, I bought a wide band O2 sensor (mine: https://www.innovatemotorsports.com/products/lm2.php). They make bungs for the sensor, or an adaptor that screws on the tailpipe. Drive around while recording low RPM constant throttle, high RPM constant throttle, full throttle acceleration, etc. and look at the recordings. You will very quickly see pretty much all you need to in order to change your carb jetting. The venturi selection is pretty straightforward; it depends on you camshaft selection (and thus where the HP peak will be) as well as displacement. This pix is from the Weber Carb manual:
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Re: 40 IDF jetting and venturi questions

#5 Post by C J Murray »

32mm is big enough, no bigger.

If it ran well with 200 air jets, leave them alone. They are the key to a smooth transition between the idle/pilot circuit and the main circuit.

130 mains are on the big side but if the fuel mileage was good they may be fine. I would try 125s to see if you like them.

Idle jets are usually 50-55 so you are close there.

Your basic setting sounds good but you could try one size up or down on the main or idle just to see what you think.
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fran kelly
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Re: 40 IDF jetting and venturi questions

#6 Post by fran kelly »

Thanks C J. Switched back to 120 mains for the moment as the 130's were a bit large - definite performance improvement with the 120 mains. Will now try the 125 mains to see how they perform.

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Re: 40 IDF jetting and venturi questions

#7 Post by Pete Indelicato »

Hey--The 'Maestro"' used to have an article about weber jetting--can't seem to find it. I did mine as he recommended and it always runs good, maybe a little rich at idle.
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fran kelly
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Re: 40 IDF jetting and venturi questions

#8 Post by fran kelly »

Thanks Pete. I seem to recall the article but gave the bibles to a friend except for Secrets of the Inner Circle which we used rebuild my engine after blowing out a Heli coil. If I recall the Maestro's recommendations were perhaps not that helpful as I was living at a different altitude than now where I am at sea level - the recommendations from C.J Murray have been the most helpful, Lots of different thoughts from folks and sometimes it just takes tinkering with different combinations as I certainly have a lot of practice doing. Keep the Faith!

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Re: 40 IDF jetting and venturi questions

#9 Post by Tom Tate »

Cliff really knows his stuff and is great about sharing but I hear that he’s been banned from the Talk List for 6 months. I hear that it was something about a political statement but I can’t find it. I heard him say “I like Ike” once at Sebring but I didn’t think the moderator was that sensitive. I guess I better watch my step since I thought that Adlai Stevenson would not have spent so much on the military the way Ike did.
Last edited by Tom Tate on Sun Apr 26, 2020 9:30 am, edited 2 times in total.

fran kelly
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Re: 40 IDF jetting and venturi questions

#10 Post by fran kelly »

Well I don't know about C.J.'s political bent - who is Ike, what sort of Porsche did he own? Oh yes, that Ike, the GOP legend who liked golf and I would guess he didn't have much to say about Lysol as it wasn't on the shelfs back then. Anyway C.J. gives good advice about jets - the 125's are not as good as the 120's.

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