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48 IDF jetting.
Hi.
There are currently lot's of posts on the 48 IDA jetting, but few on the IDF. The first question is then: will the "ultimate" jettings for the IDA also apply to the IDF? I'm running a 347 stroker and will reach 8000 rpm. According to the charts, the main venturi in my IDF's should be around 45 mm+ for max power. This is not happening. Is the 37mm main venturi the way to go (driveability + top end power) or should it be let out to 41 + mm? What's bugging me right now is that the jetting chart for my carbs have gone missing (I was SURE I saved a copy in this computer..).I'll have to look through my books and find it! Regards Rune |
Assuming your 347 is 3.4" stroke/5.4" rod, 8000k for any length of time is a bit scary when you consider the rod ratio.
700cc per cyl suggestion, 40mm venturis for fast road use/43mm for race. |
48 IDF jetting cont.
OK. I found my book of notes on everything...
Jetting is: Main venturi: 38 (Aux. venturi maybe 4.5 - no notes on this, silly me) Emulsion tube: F11 Air correction: 190 Main jet: 140 Accel. jet: 50 Idle jet: 45 (Float valve: ?, no notes on this, either) This combination has worked well in my 302 which freely spun up to 7500 (until block cracked). Driveability was OK, although there was a flat spot down low- maybe camwise, since it's a high-lift high- rev unit. This combination was NOT run on a dyno, unfortunately. Hope for input on the matter: putting these carbs on the 347. Anyone have the jetting for whoever it was who got 595? Hp from his 347 stroker @ 7200 (?) ???? (Now I cannot remember what forum I saw this persons postings....) Now- I'll go to my workshop in an hour- checking aux. venturi size as a start.... Regards Rune |
Hi, Jac.
Where do you find those jetting info's? My John Passini book isn't as fine- tuned as your data. Sounds like the 41 mm off- the- shelf mains are the way to go. No; it will not run @ 8000 continously, I'd say. But Eagle sayz 8000 is fine with the "2000" rod bolts. I was quite sure the long rods opt'ed for longer dwell period and less rod/ cylinder wall angle which again means it will take more beating. So, what are you refering to whan you say it can't stand the 8000 rpm beatin'? Rotating mass? Regards Rune |
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I have a (very) old setup book from Warnefords in Australia which gives baselines for IDA's etc ( No IDF- they had not been thought of:)). It has been very close to the mark with most stuff I have fitted webers to & the jetting should be transferable to IDF's since they are similar in principle- at least it has been so to date on other guys stuff I have made recomendations on. Book is out of print now ) Weber jetting is configured around two main items- individual cylinder size & chosen choke( main venturi) size. Once you have chosen those with consideration to an honest assessment of how you will operate the car, then you choose your main jets-air correctors- emulsion tubes- pump jets-secondary/aux venturis. Camshaft choice is very important as well- webers dont like close lobe centers such as those used on many modern sprint/circle track motors from todays world. While the 5.4" rod in your 347 is longer than the std 302 ( 5.09" ), your actual rod ratio in the 347 is 1.588/1 where the 302 is 1.696/1. Not saying it cannot or wont rev to 8000, just that its asking a lot with that ratio & rod angle. The rotating mass isnt a concern- piston dwell duration at TDC ( shorter ) is. |
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My combo is on a 304 Holden V8. Choke 38, aux 4.5, main 150, air 180, F2 emulsion tube, 50 idle , 50 pump, 50 pump bleed. Engine 10.5:1, 4.030 bore, 3.025 stroke, 242@.050 solid cam .576 valve lift. |
Hi, Gary.
What RPM do you run @ max, and did you take it to the dyno? Peak Hp@? Your set-up sounds similar to what I run, although the volume of the new engine will be larger. Does anyone have any suggestion to what changes should be done to the IDF's jetting if the main venturis are changed to 41mm? Jac MAc? After looking throgh the posts on this theme, I have found rather inconsistent information- for instance some goes with 140 mains and other 190 or something. I know there are no simple recipee on this matter, but more input would be nice. |
[quote=Caprimaniac;851974
Does anyone have any suggestion to what changes should be done to the IDF's jetting if the main venturis are changed to 41mm? Jac MAc? [/QUOTE] 41mm multiplied by 4 = new main jet size reqd--164 ( 165 ) then your 190 air may need to go up to 200, but as you sound!!:rolleyes: like a bit of a racer 190 should be OK. |
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Mine makes 320 at the tyres at 6900. Although as a street car I don't turn it that hard very often generally around 6300. ;) |
OK; I'll go with the set up as is for now. Maybe take it for a spin on the dyno and then move up to larger sizes if the potential is there... (should be).
Since Gary is taking his machine up to 6900, it is good enough for me as a starting point. I feel the idle jet could be larger; I ran an AF- meter some years ago and I remember the reading was low when idleing. Also (embarassing as it is): I cannot find the PUMP BLEED!!! Not on any ofthe drawings I have around. Not in the parts list. Is it the one in the bottom of the floater well? |
Rune,
The pump bleed is an orifice on the side of the pump inlet valve which is fitted to the bottom of the fuel bowl. It determines the volume sent to the pump nozzles. A larger bleed sends more back to the bowl and leans the accelerator pump VOLUME. Nozzle size determines DURATION of shot. |
The idle jet will be too small.
I run 55's with a Mild 331 stroker. 65's came stock on my 48mm IDF's. |
Hi.
As I thought, then, Flat- Tyler. OK, Gary- as I thought here as well- the ones at the bootom of the fuel well=bowl. I think (as far as I can remember) the pump bleed can be blocked (no nozzle). Which will mean maximium duration/ volume to the pump. Hmm. Is this the type I have in my 48's? I think so, but will need another look. Thanks for the input. Will be back after I've been to the dyno's (if I'll ever get that far....) Rune |
I have some #65's Idle jets if you want to try them.
Frank. O-wait a sec where's ( No ) at? |
Oh No....
HA, HA. No= Norway. Yes, across the Atlantic. Anyway I got hold of 65 idles- works well with 3/4 turn out on the CO's. Unfortunately the chokes I got was wrong. They fit the DCOE's, not the IDF's. So - I tried to use the 165 mains with my 38 chokes and 200 airs- and this resulted in an all to fat mixture. The sooted plugs and rough running were clear indications that something was wrong, but I'm fortunate enough to have a broadband AF meter at hand. I now have 12- 13 AF at idle and slow running. It looks good at the higher revs as well, but I have a timing problem that makes testing- the right way- a problem. (Result of a brain malfunction that made me forget to earth one of the wires from the Unilite- at last; I hope this is the reason.) Looks good, so far. Rune |
Hmm.
Can someone solve this: I use 65 idle jets. I had sooty plugs and the AF read rich, around 11 on low revs/idle. I then turned the mixture screw in 1/4 turn and the AF showed 12. I had some misfire under higher revs and thought that this might be because of sooty plugs, so I turned the screws 1/4 in once again. (They're about 1/2 turn out, then.) Then the AF is around 13 at idle/ slow speed- the"correct" air/ fuel mixture. BUT then I have another issue: the carbs start sneezing/ popping. So: Is this because the screws are not enough out? Should I change back to smaller size idle jets and turn the mixture screws more out to get around 13 AF and stop the carbs from popping? Weber book sasy the sneezing is caused by a to tight mixture screws and hence lean condition. The condition I have is by no means lean, but the screws ARE tight. ....? RS |
I'd try size 60 idle jets. That should get your mix screw out to 3/4 ( Where it should be.)
Whats your fuel pressure at? You can also take out some acc pump on low end. ( Tightning up nut on linkage rod.) |
Thanks, Flat-Tyler. Smart tip on the acc pumps.
I'll give them a go as a starting point. I know they open very early with the current setting. Rune |
New season, new settings
As the roads got snow- and icefree this week, my 347 will be out of the garage and hopefully also on the dyno very soon.
What changes have been made? - Idle jets up from 45 to 65 - Main venturi up from 38 to 40,5 (were 40, but after polishing/ reworking, they grew...) (Larger main jets were cancelled as mid- to higher throttle showed a rich blend.) Hope this will make the midthrottle- driving, AKA the transaction ,better, as I hope the larger venturis will start sucking fuel from the main (accelerator?) jets a little later. Hopefully, the larger venturis will help deliver the full potential of the engine as well. And while we're at it: can someone give a estimate of CFM of these carbs? Happy easter holiday RuneS |
Rune,
I've read that a 48 IDA flows about 330 per barrel, as factory set with a 37mm venturi. |
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