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Carb jetting question

Hi, I have a 1975 911S with a rebuilt motor. I ordered PMO carbs for it before I had it rebuilt, when the engine was stock.
Richard and his guys at PMO set it up for my stock motor. I'm just wondering if I should make any changes? Here is what the spec sheet says I have. This was for a stock 2.7. I'm in California with 91 Octane gas.
Size 40,
AV's 4.5
Vens 34
Mains 135
Idles 55
Idle Airs 130
Airs 190
Tubes F11
FBCV's 50

Now I have the motor changed a bit. It has JE 11.1 compression pistons, it is twin plugged with MSD boxes and a JB Racing distributor.
The cam is a Webcam with these #s.
A lobe center of 101, the valve lash is .004 intake and exhaust, A valve lift of .475 at intake and .440 Exhaust. A duration of 280* at Intake and 262* at Exhaust. The Duration at .050" is 256/Intake and 238/Exhaust.
It's the 464/465 grind. I have no idea what any of this technical stuff means.

The car runs really well, it's tuned, balanced, everything. I'm not really having any problems with it. Just wondering if there are some changes that I should make to the carbs considering they are now on a differently configured motor?

The only thing I notice is that I really have to rev it up to get it going from a dead stop. Maybe 2000,3000 RPM. It used to stumble a bit at low RPMs if I put my foot into it, but since I closed the accelerator pump that has gone away.

I'll probably contact Richard too, but just wanted to see what you guys/gals thought.

Thanks

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Last edited by dipso; 03-15-2014 at 06:47 PM..
Old 03-15-2014, 05:24 PM
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I'm going to guess you want bigger mains, 140 at a minimum, probably 145 if you track it. You shouldn't need to rev it much to pull away from a stop.
My PMO's have 40mm bore. Auxiliary venturis, 4.5. Venturis are 36mm. Main jets were 140’s for years, now 145’s. “Airs” (air correction jets) are 190’s. Emulsion tubes are F11’s (which they generally use for 2.4—2.7 street engines without radical cams.) Idle jets are 55's “Idle airs” are 130’s. FBCV’s (float bowl check valve’s) were 50’s originally. ( I tried 00’s, returned to 50’s, then settled on 80’s.) Cams are Elgin modified S which have specs rather close to yours. Only big difference I see is your much higher compression and twin plug setup.
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Last edited by moneymanager; 03-16-2014 at 12:11 PM.. Reason: add info
Old 03-15-2014, 08:23 PM
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As far as I can tell, there is more camshaft duration, for both intake and exhaust, than a 911S cam. Is this a street car or a track car? For a track car, I'd go up to 36mm venturis then work the mains up a notch or two. For a street car, the venturis and mains are probably OK. Do they still make F3 emulsion tubes?
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Old 03-15-2014, 08:50 PM
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Your carbs sound about right but may be lean at idle. I have a mild 2.7, single plug and not as high of compression. My carbs are the old Weber 40's from 1969 so the idle air bleed circuit is different that the PMO's. Here is how I have mine set:

34mm venturies
57 Idle jets
140 Main
180 Air Corrector
F3 Tubes
BP6ES Plugs


If you have to run it up to 2,000-3,000 to get it moving I would think it is either lean or too much timing at the low RPM. Will the car cruise at 1,500 RPM or does it seem to stumble?

I would suggest checking the timing and get an O2 gauge to verify if it is lean. I put a wide band O2 on my car and it sure makes tuning the carbs a lot easier.
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Old 03-16-2014, 08:12 AM
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It's a street car, I have been driving it this way for about 2 years.
Maybe I don't need to make any changes?
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Old 03-16-2014, 08:14 AM
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I have an AFR meter. What should I be looking for?
And Keith, Yes it will cruise at 1500. It doesn't stumble at all.
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Old 03-16-2014, 11:34 AM
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Keith, I have an AFR of 13 at idle. I moved it to 14.7 and it lean popped, but sounded crisp. I also read that 14.7 is an ideal ration for purer gasoline, but the ethanol infused stuff we have here runs better at an AFR of 13/ give or take.
An 1/8th turn got me 14.3, a 1/4 turn got me 14.7. But like I said, I put it back to 13.

Money Managers carb set up and mine are about the same. He has 145 mains but used to run 140's.
I wonder if I should try 140 mains? What would that do? His was single plugged though.
Might be worth a try. What exactly does increasing the mains a size do?
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Old 03-16-2014, 03:48 PM
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This is interesting. it actually explains what all the different jets do.
http://www.scuderiatopolino.com/TuningofWebercarburetorsrev2.pdf
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Old 03-16-2014, 04:26 PM
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A bigger main lets more gas flow, enrichening the mixture from maybe 3K to redline. In general the idle jet does the same thing below about 3k. There are lots of situations where both are contributing something to the mix.
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Old 03-16-2014, 04:59 PM
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Do you know what the desired AFR readings are across the RPM range. I am currently close to this

"Well, I'm 13 at idle 1100RPM , 14 at 2000 RPM, then at 3000 and 4000RPM it hovers around the 14, 13.8- 14.2.

ill look up some AFR threads since everything is hooked up.


Turning in each mixture screw 1/4, brings me to 14.7. An 1/8th brings me to 14.3.
Humm, now I've got 14.7 at idle and it stays around the 14.7 mark at 2, 3 and 4 RPMS. Little higher, little lower."

That is from the same topic I have going on in 'the rebuilding' forum.

If I knew what Ideal AFRs were across the RPM range, Then I think with that article and a little more serious time with the LM2. I could really diagnose it.
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Old 03-16-2014, 06:13 PM
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12-13 everywhere when accelerating hard. Leaner when cruising, 13-14. Do a little reading; there is a lot of information and a lot of different opinions here.
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Old 03-16-2014, 07:42 PM
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Great. That sure makes it a lot easier.
Thanks.
Old 03-17-2014, 07:05 AM
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dipso,

I am trying to tune my AFR to be low 13's during idle and would like to get it a little leaner. However when I get into the high 13's it does not seem to run as strong.

Wide open it runs in the high 12's which seems to be just right. I have been tinkering with the idle jets, mixture screws and air bleeds every since I put the AFR on. It does help.

Since you can cruise comfortably at 1500 then I assume the timing is close. I had some problems early on with the car bucking at 1500 and it did not want to run right. I had the distributor reworked and that helped. The advance in my old distributor was hanging up and giving too much advance at low speed which caused my trouble.

You sound like you are in the same situation I am, the car does work but after installing the AFR I noticed areas for improvement and better tuning.

Based upon what you state here it sounds like the jump to a 140 or 145 main could help. I will caution you that when bumping the main the idle mixture does seem to change a bit. At least that was my experience. It really comes down to trial and error but once dialed it the car will be smooth throughout the entire rpm range.

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Old 03-22-2014, 09:23 AM
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