new engine & Weber tuning help needed

[chris steineger]

We've just built & installed in a '66 911, a 2.5 liter short stroke motor. 9.5:1 compression, E/S grind cams, Weber 40 IDAs

Webers have 34mm venturis; 150 main jets; 50 idle jets; F3 emulsions

Engine has 20 minutes of time on it, but still pops and minor back fires. It bogs badly when advancing the throttle.

Not sure if its carb, timing, vacum leak ?

What are your thoughts ?

[proporsche]

it is probably silly q from me ?Did you balance them?

Ivan

[MBruns]

Idle jets are pretty small for that setup, try 55 0r 60 and get it behaving and when you get it settled in lean it back to where it's happy, where are you located, altitude,temp., etc
Mike Bruns

[juanbenae]

what distributor are you running and what kind of advance can you achieve with it? I had SC dizzys recurved for both of my carb'd motors to get in excess of 30* advance on them to run well.

the popping indicates a lean condition as you are likely aware. what is the history of the carbs? in regards to vac leaks if the shafts are worn it can pull air in adding to an already lean situation. are you regulating fuel pressure? were the floats adjusted after you installed them on the car or bench set prior to install?

hopefully paul abbot (1quicks) will chime in as he is the mr. miagi of carbs with out the condescending tone and boldened font here on the board. if you've not already visited his performance oriented site on carb theory, set up, trouble shooting and all things weber I suggest a visit before you go any further.

good luck and keep us cavemen (carb guys) posted on your cause and effect efforts moving forward.

[jpnovak]

So many open questions here...

What is an E/S cam? Is this hybrid E intake and S exhaust? What are the actual specs? This will give you an idea of peak HP and torque used to calculate gas speeds for jetting and setup.

Did you check timing? Verify that mechanical advance is working? What is the timing curve? What is the timing at idle and WOT?

Are all 6 plugs firing correctly?

Is it popping through intake side or exhaust side. Intake side usually means it is lean. Exhaust side usually means rich (when hot) or exhaust leak.

Did you use an IR Gun to verify that all 6 cylinders are firing and that exhaust tubes are warm and even?

Did you balance and set linkage side to side?

Did you balance air bleed screws?

Did you set the mixture screws and tune carbs?

[kent olsen]

Mike

Great job on that engine build you just finished.

My engine, you helped me build, now has about 20,000 miles on it. Paul Abbott helped me with the weber's, now 42 mm. She purrs like a kitten and snaps like a rattler. Pulls hard right up to the 7200 rpm limiter.

Good to hear your still out and about.

Chris

As Ivan says check the balance, all 6 pulling the same amount of air. Single plug as I remember is 38 deg. total advance. That's a base line and as Mike says a larger idle jet.
Another thought, if the carb's have been sitting check the idle and main jets for dirt, it doesn't take much to get in those small holes and cause a stumble.

[911pcars]

Kent,
38º total advance sounds like a bit high. Is this with factory compression ratio and premium 91-93 octane? Granted, many factors determine max. ign. advance. The early factory 911S engines were about 30-32º total w/9.8:1 compression (notwithstanding different pistons, combustion chamber, valve size and cams, operating and ambient temps., altitude, etc.).

Sherwood

[kent olsen]

My reference says cars prior to 1970 were 30-35 deg. After 1972 it was 30-38 @ 6000 rpm. Haynes manual