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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Did you increase the displacement?

I don't think you should change anything. The next step for you is to determine the AFRs through the entire operating range and under load.

What do your spark plugs look like?

That's a pretty good size cam. No power under 2500 RPM, especially with carbs.

Can I get power under 2500 RPM by changing some jets or something, or would that sacrifice my top end?
I've been driving with the current configuration for about 2 years. Like I said, it is not a problem. Just wondering.

And no Shane, I didn't increase the displacement.

I would think your current set-up is lean for your new engine. Bigger cam means more air in and out; more air requires more fuel to have the correct mixture. And the stumbling at low rpm is an indicator to me that it's lean on the bottom. With an 11 : 1 CR you run a big risk of detonation, even with twin plugs.

I would get an AFR meter on it as soon as I could and see where you are. If it were mine, I'd fatten it up to say 150 mains and 60 idles and see what you get. My 3.0 with 10.5 : 1, twin plug, GE 60 cams (similar) and 46 Webers ran 165 mains and 65 idles and was a touch fat on the bottom but perfect in the mid range and top.

I have an AFR meter. What should I be looking for?

You should record what your AFR is throughout the range that you are having issues. First check it at idle, I think that for the best lean condition you are looking at a 14.7 ratio.

Then see what it is in the area you are stumbling. If it is lower that means you have a rich condition, if it is higher than it is too lean.

If it is leaning out you need larger jets.

Also make sure all of your acceleration jets are squirting equally, and that none of your other jets are clogged.

What exhaust are you using? a large up restrictive exhaust can be great for power up high but can rob you of torque at low rpms.

The most important thing you can do is to check your plugs to make sure they are at least all the same, this will at least let you now that your carburetor are equally adjusted and that each plug is firing.

I'll check it out with the AFR. Maybe I used the wrong terminology when I said stumbling.
It doesn't stumble, it is more like a split second delay, from when I punch the pedal to when it kicks in.
That delay is only below 3000 rpm and then it really takes off.
It used to stumble, before I turned in my injection pumps all the way closed.

It might just be it's overbuilt for the gas I have here. Its warming up right now. I'll go check what I have at idle with the AFR meter.

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.

I did some reading and I'm going back to 13. It seems that is a better # with ethanol. 14.7 is for purer gasoline.
I took it for a drive at 14.7 and it sounded real crisp, but it backfired, probably lean pops.

This is very interesting. It explains what all the different jets actually do.
http://www.scuderiatopolino.com/TuningofWebercarburetorsrev2.pdf

I like it.

That is a good read, thanks for sharing.

Do you need to borrow my spark plug wrench?;)

Yeah, bring it over. I'm just across the Pacific. Ha,ha.
Now with that article saying what each jet does, it sure makes things easier. It looks like if I increase the idle jet, it would reduce the hesitation below 1500/2000 RPM.
Then if I knew the proper AFR mixture across the RPM range, I could change individual jets accordingly.
Do you know what is desired at idle, 3000, 4000, and say 6000 RPM? It looks like the main jet would take care of the higher RPMs.
Maybe bump up the idle jet, and if the AFR is not correct for the others, bump that one up too. Maybe it is already correct, that I don't know.

Do you still feel the same way after seeing the numbers I posted a few threads up? Posts 10 through 12.

You shouldn't have to rev your engine that high to get going, it really sounds like you might have a dead cylinder, either a jet is clogged or you are not getting spark.

You have to pull the plugs and compare them. Otherwise you really aren't going to know, that's why I asked you if you needed my spark plug wrench. I saw by your location that we were on the same planet.

If you pull the plugs it will greatly enhance further diagnoses and recommendations.

AFR numbers without a load are meaningless except at idle. The engine has to be under load to show an accurate reading. So the answer to your question is yes. Nest time you start the engine from cold, reach under and touch each exhaust pipe for each cylinder to make sure they're all warming up. This will be tell tale that your firing on all cylinders.

Any time you have a "pop" with carbs it's lean. Have you synchronized the carbs yet with a flow meter? Proper set-up of linkage, air screws, jetting and timing are all essential.

Also, except at idle, all jets contribute to any throttle position run state. The idle jet is always in the circuit no matter the rpm. So if you fatten the idle jet, you fatten the entire rpm range. So it really boils down to finding the right combination of idle, main and air screw setting that suit your particular engine set-up.

The absolute last thing you want is to be lean. Being lean means there is not enough fuel to regulate the burn of oxygen. Oxygen burns very hot and will melt things in a hurry.

If I am at these numbers '13 at idle 1100RPM , 14 at 2000 RPM, then at 3000 and 4000RPM it hovers around the 14, 13.8- 14.2.'
What would you suggest? Do you think I am lean on the bottom with those #s?

What do you think would be a good # to shoot for and does it stay the same across the RPM range?

Maybe 60 idles would be better and then move the mains to 140. Maybe 65 idles and 150 mains?
Is there any formula for what happens approximately with each increase in jet size?
A kit full of all different sizes would be fun to play with, but I am sure it is not cheap.