2.7l with weber 40 IDA 3C/3C1

[ydderf911]

Dear specialist !!!

I'm starting my brand new motor.
This is a 2.7l (ex 2.4l with bigger cylinders) equiped with 2 WEBER 40 IDA 3C/3C1.

Questions are simple:
What are the inlet needle size ?
What is the "set up" needed for the 2.7l ?

Regards.
Freddy

[1QuickS]

Carbs are set up based upon displacement and at what RPM the engine makes its peak HP; a 2.7 with S cams and 9.5:1 compression would like something like this: 34mm main venturis; 150 mains; F3 emulsion tubes; 55 idles; 180 air corrections.

Float levels need to be set with the engine running (use the fuel level vial that screws to the side of the fuel bowl) and the fuel pressure needs to be set at 3.5 psi (2.5 meters of water) and verified while the engine is running.

[ydderf911]

Thks a lot Paul

I'll do those adjustement and if needed give back some other question.
Rgds
Freddy

[Todsimpson]

Hi Paul-What impact does compression have on the jetting? If the motor were as described but 10.5 to 1, single plug, what would be a good starting point?

Does the use of race gas make a difference?

thanks

[1QuickS]

Hi Todd - I'm out of my real element here as my service is to return Webers (Solexes and Zeniths) to my customers so they may be tuned and will perform like new. I don't have the depth of REAL knowledge about tuning parameters with engine dyno time someone like Steve Weiner would have. I do keep notes of what works based upon my customer feedback.

With that having been stated; the compression ratio question provides an insight as to what the nature of the engine and its application may be. It helps skew a jetting recommendation toward or away from performance and helps me get a clearer customer response. As for a 10.5:1 motor; this motor would ask for fuel better than pump gas (twin plug with a smaller piston diameter and lower peak RPM might be OK with pump gas) which would like a slightly smaller jet size than what pump gas would require. If the application is a race engine then larger venturis (in 44mm or maybe 46mm throttle bores) would be good along with all jetting revised for a race application leaning toward a little rich at peak RPM to avoid engine damage. Of course the RPM at which the peak HP is developed and the displacement demand a main venturi size that can flow that air adequately. Then there is the relationship between main venturi diameter and the throttle bore diameter to help the carb flow correctly and provide some throttle response; 44mm or 46mm Webers or even 50mm PMOs. Also, if the application is "race" with high speed at WOT then even larger venturis would be good but if a more broad band performance is wanted then the venturi size would be decreased to get more driveability across the power band.

Any recommendation I make should be considered as a starting point with final jet/venturi/emulsion tube/air corrector selections subject to performance testing for making final selections. Many of my customers have some variation of engine/intake/exhaust which is WAY beyond OEM (carbs used on OEM engines from 1964 through 1972) and those engines were 2.0 to 2.2 liters. Even the jetting for those engines is being tweeked today due to the quality of pump gas.

[Todsimpson]

Thanks for your thoughts Paul. The motor was previously run on the street on pump gas and I suspect was set up rich with relaxed timing to compensate for the compression ratio. Just a guess.

I am now running race gas and using the motor on the track. I'm also considering moving to an open exhaust. Before I make any changes though I'd schedule some dyno time, bring the muffler and the open exhaust and jet based on the dyno runs.

Any general thoughts on the use of leaded vs unleaded race gas?

[1QuickS]

Only what I remember; leaded gas was good for engines that didn't use hardened valve seats so it isn't required for that purpose with our engines. Hopefully there will be more erudite responses than mine.

[jeff eelkema]

As Paul has stated, lead was used to cushion valves when valves and seats were soft, eg old cast iron heads. Lead was also a cheap way to boost octane. Win win for the oil industry back in the day.

However, it is not good for child brain development or general human health. These days there are plenty of other ways octane can be increased if needed. There is a bit of debate around some of the health impacts of some of these additives, benzine, toluene etc.

It really is a case of using a suitable fuel compatible with the materials used in engines and state of tune, CR etc. Basically, high CR and high output engines need high octane. Our 911 engines don't require lead for valves so it really comes down to ensuring octane is right. High octane doesn't make more power on its own but can often be used to tune to a higher state.