Carb A/F Issues - Need Help
 

It's been an ongoing saga since I built this motor. The motor runs well, but is rich through the mid-range and too lean up top. I changed the main jets and air jets as recommended and the AFR is virtually the same. I don't have a new dyno sheet, but I'm staring at the AFR Gauge on the dash and below is my recolection.
http://forums.pelicanparts.com/uploa...1184610979.jpg
Here is my set-up:
3.0 10.2:1
PMO 46's
Electromotive HPV-1 Single Pug Ignition
Mixing 93/110 Octane

Original New
Carb Size 46 46
AV's 4.5 4.5
Venturis 38 38
Main Jets 150 140
Idle Jets 55 55
Idle Airs 130 130
Airs 180 170
Emulsion F11 F11
FBVCV's 50 50

Issue #2: After about 5-6 laps the car startes to hesitate and bog at the lower RPM range, shrinking the power band. It continues to get worse. By the end of the race the power band is 500RPM's. I spent yesterday's race short shfting at 6000RPM's and feeling way down on power. I installed $300 worth of Wevo engine mounts as suggested too, but they did nothing.
It's been suggested the fuel gets aerated, or I get vapor lock. There is a fuel pressure gauge in the engine copmpartment, but I can't see it while driving. FP is set at 5.5lbs and I running the suggested fuel pump.

Thanks, John

What type of 02 sensor are using for the gauge if it is one of those light deals they are not real reliable. what kind of shape is the fuel tank in on fuel pumps you can have pressure but not enough volume if the screen or a line is pinched or restricted you could be running out of fuel.

maybe 38 vents are too big?

Maybe 46 carbs are too big?

Maybe try larger main jets and ensure you're getting adequate fuel volume and pressure

I run an LM-1 with a dash gauge so I know the readings are accurate. I matched it to the dyno and they were almost identical.
Fuel cell is in very good condition. Fuel lines have been checked and the pressure and volume is good when run stationary. Can't remember which fuel pump, but it's the one PMO recommended for the set-up.
I actually have a set of Weber 40's and am contemplating putting them on. Hoping for some wisdom I have overlooked.

What about the loss of power? I hate to change the carbs and not solve that issue.

John

Isn't 5.5 lbs. fuel pressure too high? I thought it was supposed to be 3.5?

http://forums.pelicanparts.com/showthread.php?threadid=352685

For a race motor, your carbs are not too large. And your venturis are not too large. I keep track of EGTs to deal with mixture issues, and running 46s with 40mm venturis on my 2.7 works well (38s worked as well). You are running a larger race motor. You are not oversized.

I also wonder if your fuel pressures are not 2 psi too high - 5.5 caught my eye also. What that could maybe cause is overflow in the float bowls - you are overpressuring the floats. At higher RPMs you suck fuel out fast enough so this isn't happening. Just a guess. If these are PMOs and Richard says you should run 5.5 psi, who am I to say no. My experience is all with Webers.

But it goes lean at higher RPMs. Well, maybe your mains are too small for my (limited, to be sure) experience. Maybe the small jets compensate for lower RPM overrichness due to bowl spillover/high fuel pressure, but can't compensate at higher RPMs? Caution: more guessing.

I run mains which vary from 1.45 at low (81) air densities up to 1.65 at high (108-109) densities. Being in Colorado a mile high I tend to leave 150s or 155s in most of the time. I leave the AC at 145 (too much of a pain to change - If I had your nice AF meter maybe I'd do something about that). I run .65 idles just to richen things a bit overall. And I switched from F3 to F26 emulsions, though F11 is pretty standard also and I can't imagine emulsion tube voodo will help you.

One thing I do run, and I've not seen this on other carbed engines, is a Cagle vacuum controlled fuel pressure regulator. At high vacuum (as in idle, or decell) the pressure is lower. This allows setting the float level to the top of the top line on the measuring gauge tube. Some day I'll instrument this and see if it does anything. The non-recycling regulator doesn't have to work too hard, since it is fed by a pair of Holley reds, maybe 7psi max to start with, minus fuel line friction to the back where the regulator is.

My tuning efforts centered on keeping the EGTs below 1325 or so degrees F, and getting rid of a mid-range (about 4000 rpm) bog. The bog was painful, waiting for things to catch on coming out of a corner. It seems something in the design of our 911s creates a normal dip in the torque curve around this general RPM range. Eventually I got the engine to at least feel like it was pulling strongly from 3,000 on up to 8,000, and the (one) dyno test session I had confirmed this - the torque slope slackened in the dip area, but didn't dip and HP kept climbing.

Anyway, those are my semi-educated guesses for a problem I have not personally had to deal with. So try lower fuel pressures (3.5 psi) and maybe some 1.65s for mains to see what that does.

Walt Fricke

Thanks for the input. PMO recommends 3.5-5.5psi for fuel pressure.
I think I am dealing with two seperate issues, the rich to lean situation and the loss of power. I have read and taken some advice, but the main jets and air jets changes did nothing. I need a box of parts and a day at the dyno, and I don't either of those handy.

John

According to your dyno sheet, you have 12:1 a/f ratio at 5000, plenty of mixture for power. However, that's on a static dyno run. Real world track conditions could be different - probably are.

While your fuel pump volume tests okay, I might suspect reduced fuel volume at high rpm, maybe along with higher temperature. Where's your fuel pump located?

BTW, so others aren't confused when they read your chart, you should transpose the A/F and RPM titles.

Sherwood

Fuel pump is up front next to the fuel cell.
Dyno numbers and track numbers are the same. When it is bogging and hesitating the AFR is still following the same path above.

John, did you get this sorted?