Julio Rodriguez

Hi Renners,

I've been trying to get my head around fuel/air delivery etc these past few days, & here's why.

The mechanic working on my car said a complete refurbishment (fuel system including the CIS) would be ideal since the car has been sitting unattended for such a long time. He's going to clean the tank out on Monday, and hinted that newer all aluminum fuel cells exsisted for the 911 and would be an upgrade as I could choose to use a Walbro intank fuel pump or stick with the OEM style set up.

I would like to use stainless steel lines with AN fittings all around once we begin the rebuild but right now since things are being pulled apart I keep staring at the old dirty MFI on an engine sitting on the shop floor. The owner said it's not for sale but he might be able to get his hands on some Zenith Carbs that "could" be used on my 83sc 3.0L which brings me to my question.

Carbs should all do the same job and reasonably perform the same way on a stock motor. I understand some units have more RND than others but the main reason Porsche left MFI and Carbs was because of tighter emissions restraints and began using the CIS which was than replaced by more electronically advanced units etc.

My car is in Panama, and we don't have any of these emission restraints what so ever, and probably won't for another 100 years. I could run straight pipes and never be bothered by any form of law enforcement, EVER which makes me ponder as to why I couldn't just ditch the CIS and use the insanely cheap set of Zenith carbs from an earlier 911?

Have others done this? I'm sure 99% will say just go PMO but if I had access to these older Zenith carbs and have access to old school mechanics that know the carbs well why not try it out? I have some doubts though, for example; do the intake ports line up ? Would it be necessary to fabricate parts to make it work?

Lastly is their a member that could help guide me on this path if indeed it can be done?

These questions may be stupid, I know...... But! Given the cuircumstances I find myself in it would be wrong of me not to ask.

Thanks in advance.

Julio Rodriguez

http://forums.pelicanparts.com/porsche-911-technical-forum/810574-zenith-ida-conversion-performance-oriented.html

Turns out there is a small group of tuners that have gone this route and experienced success. I think that with a refurbished fuel system, the Zenith IDA conversion with a pair of OBX headers to a high flow exhaust should really give my 3.0L some room to breath.

Julio Rodriguez

http://forums.pelicanparts.com/porsche-911-technical-forum/792229-zeber-carburetor-mating-zenith-weber.html


All "experimental " issues aside I really believe that a pair of refurbished ZENITH carbs with a few weber parts can make a great alternative (1/3 of the cost) to the increasingly expensive Weber & PMO carbs.


I'm reading so many horror stories regarding the trouble shooting of the aging CIS that sooner or later I'll find myself either going this route, maxing out my credit card and getting PMOs or going ITB with a Haltech. It's amazing how many things go wrong with these older engines. Yet I find myself compelled to try and make it work. If all else fails I can part the engine out and drop in an LS.

All relative information regarding the Zenith install on a 3.0L is greatly appreciated.

GTgears

Zenith carbs are not very nice carbs. If you do this you will want some new pmos are used Webers.

That said your cams and pistons are designed for the CIS. If you do the conversion and want best drive ability and performance you need to rebuild your engine with carb friendly parts.

Julio Rodriguez

Thanks GT for your response, it's alway great to have seasoned vets helping the new to air cooled niners out.
I believe that their may be some misinformation floating around the forums and I hope you can help clear things up. From what I've been told & read, swaping out the CIS for carbs bumps up the cars response time but doesn't necessarily add much HP to brag about "unless" you go with more aggressively grinded cams, higher compression pistons and lighter rods etc.

To my understanding the real benefit from going carb vs staying CIS is the removal of all those possible air leaks which cause havoc on the CIS metering dish and thus causing poor performance on behalf of the CIS. Tunning and timing are always key factors to success with Carbs but with so many guys already walking the beat it would be easier to follow in their footsteps re jets/size etc vs. the expert only Dyno Tunning sessions. 1 Dyno Tunning session will never be enough on a stand alone and EFI. Just my personal experience. But I really think that a well tuned carb would rock on a 3.0

theiceman

i would say CIS is WAAY easier to maintain and get working than fiddle and trying to match 6 carbs in two stacks.

GTgears

Agreed. CIS is self regulating. Where I live, where I might start the day at 5000ft and end up going over a pass at 10 or 11k that makes a huge difference. Just changes in temp, humidity and air pressure can make the carbs on my 914 act funky, and anytime it gets up to 90 they backfire whenever I lift throttle.

For a car that you are going to drive 10/10ths all the time, carbs are fun if you don't mind adjusting them and fiddling. But in general, I would prefer a CIS car.

race911

On an otherwise stock 3.0? Well, including swapping out the exhaust. Eh, had too many of those in the '80s and '90s when gas was decent. All you ended up with was a car that got 14-15 MPG.

(No matter what anyone has done with Zeniths to adapt them to larger engines, I cannot fathom installing them.)

fennlane

There is nothing wrong with Zenith Carbs when correctly set up and they work well with a CIS engines but you do need to slightly modify the ignition distributor advance curve for ideal results but we have suitable springs for 3.0 litre SC motors.

They have individual throttle shafts and importantly accelerator pumps which IMHO is superior to both the Weber and the PMO.

The Auxiliary Venturi is taller and provides a better flow characteristic that the short unit used in the Weber and the Venturi position relative to the jet height means that metering at low rpm is superior.

Both the Weber and the Zenith suffer slightly for having non-adjustable idle jet air correctors but we have a 'kit' to modify and correct this feature.

This carb is much maligned as 'in period' no tuning parts were available and some USA cars had vacuum operated emission controls added which again made the carbs difficult to tune.

Now parts are available and the emissions devices can be ditched and ports plugged these carb can

The problem with old CIS units is that the cost to refurbish Fuel Distributor, WUR units and throttle bodies is very high and some of the pressure switches needed are NLA

There is no real need to muck about mixing Weber and Zenith parts, just modify the Zeniths to suit the application and they work well.

We make Venturis, Jets, Gaskets and range of Zenith parts and have them running well on a number if 3.0 litre cars

The improvement in throttle response compared to a large Throttle Body CIS is huge and driveability is much better.

Balancing carbs does need some experience but it not that difficult and once set correctly will be stable for significant periods of time.