Timing and Weber 40IDA 3C Carbs

[techguy]

History

The car is a 72T with a 1975 2.7S transplant (unknown miles). The motor is configured with with dual Weber carbs, older Blue anodized Permatune box (recently added when my Bosch unit died) The Permatune came with a used Bosch coil (bonus)! I have set the plug gap after several experiments with 0.028, 0.030 and finally settled on .045 gap as per my recent call to Permatune tech support. I have also recently installed a new 6500 RPM rev limiting rotor, changed the points, fuel filter as well as checked the resistance on all the Bosch wires. Car was backfiring when I bought it last year but probem cured with new muffler.

Results

Car runs as well or slightly better than it did before my CD box died! Lots of power thru the RPM ranges and smooth idle.

*Problem/Question*

Over the course of the last month I have noticed a bit of popping thru the left bank of carbs. I have checked the air flow with a borrowed unisyn and number one stack on my webers is a bit lower than number 2 and 3. I have played around just slightly with the mixture and problem is mostly gone.

Now for the dilema! I set the timing today as per 101 Projects (5 degrees before top dead centre using the second mark on the pulley). I noticed the engine is a bit quieter and of all things the tach went from bouncy/eratic especially over 4000 RPM to perferctly normal. The tach has bounced since I bought the car!!!

HERE IS WHAT I NEED TO KNOW:

#1 Is this an odd set of results or what? Why does the tach work so well now?

#2 Should the timing be set to where the #5 mark would usually be with Webers or should it be more advanced? (this car has 2 marking on the pulley, no Z or 5 marks). The #5 mark was at about 11:00 o'clock before I set the timing, I moved it clockwise about .25 of an inch to align with the timing mark on the fan pulley housing.

Thanks Guys!! I know its a long one but I wanted to eliminate as many configuration questions as possible. Yes, I will adjust the valves next spring and I also intend on rebuilding the Webers soon also. I also have a new Unisyn on order to perform a more accurate job of carb balancing.

[CamB]

Bouncy tach can be because of poorly adjusted points - did you readjust them?

I think you want to time it for 6000rpm maximum advance of whatever the spec book says.

[73911guy]

I agree - check your dwell to make sure it is set correctly, then set the timing at 6000 RPM. As BA says, we don't care what the advance is at idle we only want to know at 6000.

Also, my '73 with Webers was dramatically improved in performance and eliminated spitting, popping, and missing after I had a distributor curved for the carbs by Barry at IEC in Detroit. Do a search and you'll find more info.

Jim

[Early_S_Man]

OK, a bit of info is still lacking, and some clarification needed ...

Are you using the CIS distributor without the vacuum module connected?

The picture in 101 Projects ... is of an SC pulley and is NOT correct for your car! The diagram below from the '72 - '73 Spec Book shows your crank pulley marks ... correct timing for your car without vacuum attached would be 5° BTDC, which is to the right of the second mark -- Z1/TDC ...

I suggest reading the following thread ... all of the 'T' data applies to your 2.7 distributor, which had the '73 911T production distributor:

http://forums.pelicanparts.com/showthread.php?s=&threadid=5432&highlight=distributor+lube

[techguy]

As far as the points go, yes I did set the gap as per the specs for a 1975 911S motor (Haynes Manual) p.80 Bosch Distributor (CDS) 0.012, as far as dwell is concerned I did not bother checking it since the Permatune tech instructions mention that it is no longer a factor.

"The Perma-Tune system uses the points only as a sensor so they will generally last the life of the engine. Dwell is not a factor simply set the points to stock gap specifications and then adjust the engine timing using a timing light. "


As far as the distributor in 101 Projects is concerned I figured it was the wrong one and incorrect picture but projected the correct concept hence the post. My pulley actually just has 2 marks with no info or writing beside or under them.

Again p.80 in Haynes 1972 models onwards...5 deg ATDC at 900+/- 50rpm. But no mention of carbs.

IF I GO WITH DIAGRAM #1 ABOVE IT LOOKS LIKE I MUST NOW BE SET AT TDC BANG ON...SO BEFORE I ADJUSTED THE TIMING I MUST HAVE BEEN ABOUT 5 DEGREES BTDC AND THE CAR FOR A 75 SHOULD BE 5 DEGREES ATDC WITH VAC LINE CONNECTED. I TAKE IT MY CAR MUST HAVE BEEN CORRECT BEFORE I STARTED IF THE VAC LINE ACCOUNTS FOR 10 DEGREES AND MINE IS NOT CONNECTED. MAYBE I SHOULD PUT IT BACK? WOULD BE INTERESTING TO SEE IF MY TACH STILL WORKS PROPERLY WITHOUT ANY BOUNCE.

Now for the good stuff, the distributor advance is not connected...Webers do not have anything to connect to it as far as I know. But I am the 1st to admit I am no expert...been reading a lot but thats it

I am also not sure if the distributor is from the original 2.4 or is the one for the 2.7..how can I check?

Thanks again guys!!!

[techguy]

Just for information sake I gave PMO a call and here is what they said:

Timing with Webers and no vac advance:

5 degrees to 7 degrees advanced @ idle
aprox 35 degrees @ 6000 RPM

Plugs

NGK BP5ES over the stock Bosch
Gap with the Permatune and Webers 0.035