1970 T with carbs - Miss at high RPMS

OK, another question for my early 911 friends.

1970 911T Targa is running good but, there is one last issue I am trying to sort out.

Engine is basically stock except for installation of new 84 mm high compression pistons. carbs have recently been rebuild, zenith 40 TIN. venturi is 30, idle jet is 60

When first together, car had a significant stumble at about 3000 rpm. originally started with 115 mains. I installed some larger mains and that went away. 125's in right now. larger mains were a BIG improvement.

so, car pulls well and hard in first and second. at full throttle in third car stumbles around 5000 RPM's. stumbles in fourth at slightly lower RPMs. if i let off gas to 1/2 or 3/4 throttle and reapply, stumble goes away and pulls to higher rpms.

So, what is the problem? bigger mains again? or try 120's?

Marelli distributor with points. new wires, new plugs, new rotor, new cap. float level set. timing set per factory manual.

The real question is what would limit and cause miss at high rpms; too rich or too lean? My guess is i should try 130's...

Why you guys are at it I would love to hear you opinion about the 60 idle jets.

What are some of the set ups being run on similar motors with 30 venturis?

When you let up the throttle and reapply, the accelerator pumps are dumping in more gas, so yes sounds like you're still too lean. The use of a A/F ratio meter would be a big help here.

Thank you edmayo! I have never used an air fuel meter but I have always wanted to. Can you clip one on end on exhaust? Ill do some searching...Any recommendations?

We use one by "Innovate" A probe clamps into the exhaust pipe, the display unit will be in the car with you, and you run the cable in thru the rear vent window. I wouldn't be surprised that you would need at least a 130 main. When you go to bigger venturis the air flow doesn't need to speed up as much to get thru the venturi, therefore you don't get as much of a low pressure differential, therefore it doesn't draw as much fuel from the main well. There is a point of diminishing returns on how large a venturi will work on a stock engine. Modified engines that pump more air draw enough air thru the venturi to create the needed lower pressure. You want to shoot for something on the order of 13 to 1 A/F under full throttle.

Good information edmayo. thanks for the information and the knowledge!

If you are running weber venturis, I noticed that they neck down at a different spot than the zenith type. I ended up giving up on the zeniths( excellent carbs for stock motor) for performance but next thing I would have tried is machining my own 30 venturis with the different diam. end to end and the neck down in the same place as stock. S

I thought I would give an update to those who read this. I put some 135 mains in and the problem is corrected. I went up from 125's... I have some 130's i might try eventually, but the 135's are doing well. only remaining issue is a small amount of popping up through carbs when motor is cold... i think ill install some 65 idle jets soon. im acquiring quite a supply of spare jets. only source i have found is on ebay. delivered from italy. any sources closer? thanks!

any comments about a pop here and there on a cold motor. 62.5 idle jets on order from Italy... ill let you know what changes with the larger idle jets.:)

Another thing on the zeniths ,probably worse now with etOH fuel is the tubes on the sides of the carbs(smog) get dry and leak causing pops. Those idle jets seem large for your application. Just ideas, I did mention that i was never able to get my mod zeniths just right FWIW. Steve
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thanks chiroracer. i know what you are saying about the idle jets being on the large size. just trying something... what i wonder, is the popping when cold "just the way it is"?