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.

I just read I should try for 12 to 13 across the RPM range. Maybe if I back out my screws 1/8th it will drop my 13 at idle to 12, and my 14 to 13. Is that the way it works? Probably not, but if that worked would I still need bigger jets?
This is interesting. learning how a carburetor works.

Bigger jets

All right Lindy, thanks. I'll look into it.
You've been very helpful.
Thanks.

You must run the car under load (driving it) to get accurate AFR readings at anything other than idle. It takes more pedal to make 3,000 rpm driving down the road than it does sitting in neutral. I like 12.5 personally, especially with todays fuel. You're way better off being a tad rich (fat) than lean. A lean condition can cause detonation and worse, a piston that grows too big for the cylinder from excessive heat causing galling. Be safe.

PMO's baseline jetting is just an educated estimate based on experience with what others have successfully installed. So it should be considered a starting point. Your engine's state of tune (outside of the carbs) and your driving conditions (street vs. track/race) will dictate where you want to be in terms of driveability. Without question the AFR is what you need to measure to dial it in properly. Get a wideband AFR gauge, an LM-1 or LM-2 meter or take it to a local dyno to dial it in.

Also note that the Weber jetting people have honed-in on successfully is a good guide but don't take it as gospel. I'm not trying to be difficult or stir things up. Just pointing that out because PMO, while very similar looking, is quite different due to the improvements PMO made to their carbs.

PMO main jets (both air correctors and fuel jets) and emulsion tubes are not the same as our Weber 40 series used on a typical Porsche flat six engine. The PMO hardware is same as the two barrel Weber 48 IDF series where the emulsion tube, air corrector and fuel jet are all in one stack. PMO chose this arrangement because he felt it is a better design that Weber used to put the components all in one chamber within the carb. PMO also uses the needle & seat & the float chamber design from the 48 IDF

Not to mention the PMO has adjustable/variable idle jet air correctors that also influence the mix. Like Lindy said, the idle jetting is not just there to manage pure idle conditions. Idle circuit jetting has a heavy influence on the air-fuel mixture all the way up to around 3000+ where the transition occurs to the main circuit. So up to the transition point (and above) the idle circuit is playing a role in the overall AFR.

Typical Weber emulsion tube number designations don't apply either. So don't get those confused, if you're not confused by my babbling already.....

Another thing worth noting, the PMO seems to need a LOT less accelerator pump volume/delivery than the Webers. The accel. pump delivers a shot of fuel into the carb throat when you move the throttle. It helps avoid hesitation between the time you apply the throttle and the time the carb takes to react when the actual throttle plate opens, which draws more air into the carb, which then pulls more fuel from the fuel bowl, to feed the engine more fuel and thereby make more power. Back to the original point..... You may find it worthwhile to reactivate the accel pump delivery. How did you close it, just disconnect the pump rod?

What happens with too much accel. pump delivery is it throws an overly-rich mixture into the combustion chamber and the engine bogs a bit due to the extra time it takes to burn that rich mix. You can see this on a dyno graph or AFR gauge very easily. When you floor the throttle to full GO, the accel pump squirts a big shot of fuel in there and that makes the AFR dip considerably. An example of that is this guy's dyno plot

http://forums.pelicanparts.com/porsche-911-technical-forum/794830-3-0-3-2ss-pics-dyno-sheets-before-after.html

Check out at 3000 rpm where the AFR dives to 10 momentarily. I think that's due to the accel pump delivery volume being too much. Typically you do a dyno run by bringing the car up to around 2500rpm in 3rd gear and then they mash the gas pedal and the accel pump squirts the extra fuel right then & there.

In your case, your difficulty getting it going until 2000-3000 may be the LACK of an extra shot of fuel? Just a guess.

I didn't disconnect the accel pump, I just turned it in all the way. Don't know if it is still squirting a little or not. I never checked it after that. It stopped the bogging though.

I'm going to call Richard and give him my new specs and current LM2 numbers and see what he suggests.
It seems that I am definitely going to get bigger jets. I want to bring that 14 down. I'll see what he suggests.

After that I will revisit the accel pumps.
Thanks for the info.

Update. Went over to PMO today and picked up 36 Venturis and 145 Mains. Put everything in and reset my float levels, since everything was apart. Car runs like a dream! Smooth as can be!
I checked it with the LM2 and it idles at 12.5, runs between 12 and 13. Didn't even have to make any adjustments.
Checked the balance on the carbs at idle and 3000 rpm.
It's nice.
Drove it down the street and checked AFR at load. Then backed in my driveway and ground the cables off the LM2 sniffer. ****!
At least I got the numbers first.

Thanks again, you guys were very helpful.

Amazing what a little extra fuel can do!

Update to the update. Was just out in the garage and I found some 55 idle jets. Checked my paperwork and it looks like I put in 60 Idles at the same time I moved the Mains and Venturis up. Ha.
Forgot to mention it last time.

I figure this is still on topic.
I went out today and decided to putz around with the car. After the main jet, idle jet and venturi changes mentioned above, it still didn't feel perfect to me. It was definitely better than before, but after driving things for a while it still needed fine tuning. Plus I had time to kill.

I decided to reset the timing and set it at 26BTDC. I did some reading and that looked pretty good for a twin plug. My idle falls at 14, I probably have about 22 degrees advance in the distributor. If I remember right it was about 28/29 before.
Then I put the LM2 on it and it had changed from last time. I wanted 12.5 or close to it across the board.I assumed changing the timing, changed the AFR readings.
The idle was at 13, but I could not get it out of the 11's while driving. Low 12's occasionally, mostly low 11's, sometimes in the 10's.
I spent the day messing with air corrector screws, synching, balancing, accelerator pumps, mixture screws, everything I could think of. Still in the 11's, 80% of the time.

So I figured I would put the 135 mains back in and see what happens. I think I am at 2 1/2 turns out on the mixture screws, which is perfect.
I am 13 at idle.
Mid 12's at 2000. mid 12's at 3000 and 4000. It drops to 11/11.2 at 5000 and mid 11's at 6000.
This is all under load while driving.

Those #s look pretty good, right?

So it's getting richer towards the top end? Aren't they usually a little fat on the bottom?

Todd

It all depends on the different combinations of jets, ventures, emulsion tubes, engine configuration, everything.
I'm no expert, but learning.

You likely have 22 at the crank, 11 in the distributor;). You can get a little more out of the distributor by bending the stop tabs, that might be a better option which will allow you to set it at the factory setting at idle and still get everything out on the top end, 28 is well advanced and I don't think you want to go much higher.

Another good way to tune is by using EGT or on a dyno and tune it, like Lindy911 has mentioned, you need to run the engine under load to tune it further.

Oh, your right. Those numbers don't add up do they. Maybe I'll take a look inside the distributor
tomorrow. Its a JB Racing 12 plug distributor, fully adjustable. Huh, I can't remember what I set it at.
What do you think would be best at idle and 4000?
I could do just about anything? I could set the advance gap between 0 and
28.

I think what you meant was that you measure at the crank, 7 at idle, 22 at advance, total 29.

Is your distributor advance electronically controlled or mechanically? I have an MSD unit that if digitally controlled and I can program an advance curve, can you do that?

Advance curve
 

From. http://forums.pelicanparts.com/porsche-911-technical-forum/795056-msd-6al-2-digital-programable-install-distributor-lockout.html#post7913316

On mine the distributor is set in full advance and the MSD pulls out timing digitally. So at static 1000 rpms, it is set to 8 degrees advanced at the crank then as rpms increase timing is added back in digitally for a total of 34 at the crank. I have a lower CR so I can get away with more advance.

Please explain your timing process. Are you setting the timing at say 4500 rpm so the distributor is fully advanced or are you setting it at idle and depending on the numbers you were told for distributor advance? I've never trusted the numbers I've been told and always set timing at 4500 rpm then the distributor backs down to whatever it's going to back down to. The important setting is on top not idle. Idle AFR readings are also not as important as mid and upper rpm range readings under load. The engine won't melt while idling.

I set the timing at 4000 or above and let the idle fall where it may. I have to use a digital timing light and set 26 degrees of advance on it. Then with the light on, rotate the distributor, rev the motor and move the notch to TDC.
I have a serpentine belt system and the lower pulley only has one notch at TDC. I wish it had more notches so I could use my MSD timing light. I heard it is better, but they seem to work the same.

That's nice.

I just looked up the information for your distributor, it has an adjustable spring weight driven advance plate that can be adjusted from 0-28 crank degrees.

According to the literature with stock springs the advance starts at 1500 rpms and is complete by 3500.

I don't understand if you are able to adjust the JBR distributor base, like the original, to adjust the initial timing or if that can only be done within?

If you can adjust the base to say 7 degrees at idle than that leaves you another 28 degrees of advance internal. That would be 35 degrees total. It would idle smoother and you might get a bit more pep at top end.