Weber tuning assistance please
 

Hello,
I need some help with tuning my Webers. A little history, I have a '75 ROW 911 slowly in the works of getting up to par. Replaced CIS and original Permatune with Webers and MSD.

Technical parameters:
Stock '75 ROW 911
Stock exhaust (none thermal reactor)
MSD
40 IDT Webers
Fully rebuilt and cleaned
135 Main
34 venturi
F7 Emulsion
55 Idle jet
160 air correction
Float level recently set, tuned to the widely used tuning method on this forum
'69 distributor (must have been replaced at some time) with points
Timed to ~34 degrees BTDC at 4,000 rpm

Symptoms: Bogs at low idle when trying to produce power (for example from a stop and try to accelerate slowly, there is not much power). I felt like this range was too rich--often backfired out of muffler. Also has a noticeable transition when going to 3/4 throttle. At 3/4 or more throttle it really opens up and feels great.

I changed to 180 air correction jets with no real change that I could notice.
Keeping the 180 jets, I also changed the idle jets to 50's and re-tuned the webers. It now seems to produce less power, more of a bog at take off and a bit of a surge at higher rpms.

I'm not looking for optimal performance right now, just enough to feel comfortable as I'm aware of additional changes, cams, exhaust, curve change....

Any help would be greatly appreciated.

Paul


http://forums.pelicanparts.com/uploa...1221276194.jpg

Paul,

When one begin the jetting process, everything needs to be perfect and that includes fuel pressure and float levels. One value depends completely on the other so you need 3.5 psi before setting the floats,...... :) Valve adjustments and ignition system components need to be perfect, too.

A few thoughts,

I'd suggest different emulsion tubes such F26's over the F7's.

Likewise, I'd be using 145 mains and 180 air correctors with 55 idles.

Check those accelerator pump volumes to ensure that all pumps have the same volume and set to spec.

Reset idle mixtures, confirm that both carbs are tracking and get WOT.

Your car should run a lot better as long as there are no other resident issues. :)

Check for clogged idle jets! one clogged jet will throw the motor off.

I suspect Steve is right.

Agree with Steve.

Quick check: Open throttles (engine OFF) and verify each throat is receiving a shot of fuel from the accelerator pump circuit. No squirt = hesitation during acceleration.

As an aside; suggest installing fuel filters before the fuel pressure gauge so the gauge monitors source pressure to the carbs. If filters in the current location clog, the guage will not show the resultant pressure drop.

Sherwood

It's been awhile but I'll update with a question also. I changed to Steve's recommendations and did not notice much change except a surging at higher rpm's. This is not to crack on Steve's recommendations but to confirm that there were/are other issues. Carb on 1-3 cylinders is adjustable and seems to work correctly. Carb on 4-6 has issues. I recently took it off and re-cleaned and found that one passage (the fuel passage to the progressive port holes was clogged on cylinder 5). This is a tricky port as it passes through the bowl, the emulsion tube and such the main WOT fuel port, and the progressive ports so blowing it out may give tricky results unless you verify air or cleaner can come out of each port. I had to take out the soft tapped in plugs from the top and side of the carb to clean it completely. Yes, the fuel adjustment did not adjust properly before. Now I have verified all passages work. The car is running much better but I still need to tune in a little more. I know fuel passes in the progressive circuit on cyl. 4 but I can't get the fuel to adjust (same I had with cyl. 5). Here is the scary question... I can't get good vacuum on cyl 6 with the synch. even opening the idle air all the way. So, I'm guessing the answers will be...find where the fuel is coming from in cyl. 4 (by the way..cyl. 4 was popping out the carb as others were not) and do a leak down on the engine for cyl. 6. Just trying to confirm the next steps.
Thanks so much for the help.

Paul