Has anyone gone back to CIS from Weber Carbs?
 

I am considering reinstalling CIS onto my 1974 2.7 911, which has had Webers on it since my Day One. I have the CIS gear, but have never done this and don't know what I should consider before launching. Any advice Thanks!

CIS redux
 

My 74 has been down for extensive work for about 7 years. I dropped the engine to have the trans done, but primarily to go over the CIS, fearing some or all of the system might be gummed up/potentially leaky, or out of calibration.
Most threads recommended pulling the injectors, including the cold start valve for leak/pattern testing. The FD and WUR could benefit from cleaning, testing and adjustment. Some threads recommended pulling the engine to facilitate the work.
I was considering replacing all the rubber boots and hoses, including the injector lines.
The prior switch from cis to carbs may have had a fuel pressure regulator installed, which will need to be removed. More likely, they installed a lower pressure pump, so you will need a new pump.
If the tunnel lines haven’t been replaced this would be a good time to do that and consider going to a front pump location.

I did the tunnel lines and front pump, (many thanks to Len), but decided against restoring the cis and am installing the X Factory PMO ITB fueling system, for a little more power, throttle response, sound and maybe a little more mpg.

I suggest calling some of the folks that do CIS work to get an idea of their recommendations and cost estimates, for a system that has been down for a long time.

Good luck,
chris

Or use the CIS components to install the Bitz EFI system.

Thanks for all of that. One question, based on your note: You mentioned getting more mileage out of PMO [carburetors?].

Doesn't CIS improve mileage? That is the primary reason I'd go back to CIS.

I already have the fuel pump just behind the tank in the front. Thanks!

if during the carb install the CIS engine wiring harness was thinned out you'd likely need a complete unit to reinstall. ive taken CIS off two 3.oL motors for carbs and had a large web of CIS wiring that I removed that would need to be in place to revive.

IMO you'd be better served going with one of many contemporary individual throttle body EFI systems instead of attempting to revive a 70's era system that has long since been obsolete. if you have weber carbs that are in decent shape you they could fetch as much as $3k that would get you on the way for EFI cost.

Once CIS is gone, don't go back. It works "fine" (for some value of "fine") but it is a dead end. If you must change out your carbs, skip CIS and jump to EFI.

CIS cannot handle aggressive cams. If the cams were changed when the carbs were installed, then you'll have to replace them as well. EFI can handle the aggressive cams.

So to return to CIS you might need a replacement loom and/or less aggressive cams. (Might not?) Moving to EFI is probably easier. Might be cheaper? Definitely more bang for your buck.

EFI is it...
 

I'd say that it depends on your goal. The CIS systems work pretty well, when all of the control parts are working to specification. However, all of the systems are now 35-40 years old.....and require a bit of testing, maintenance, refurb, replacement...tuning etc to get them working properly....

You can replace the CIS with carbs.....which provides simplicity and better throttle response, but the gas mileage is not great.

Or you can replace the CIS with EFI......Tbitz kit or an EFI w/Independent Throttle body Injection and a modern ECU/controller. If you also upgrade to SSIs/headers and a sport muffler....you'll achieve the throttle response of the carbs, but also gain the means to control fuel and ignition, resulting in more usable power throughout the range and fuel mileage comparable to CIS.

It's good to have a reasonable goal and budget and then move forward to get as close to your goal that the budget will allow. Look at your engine/pistons/intake/exhaust/cams, etc as a system and consider how individual changes affect the system. While hotter pistons and cams etc are typically part of the high performance build ....I've found that just replacing the restrictive intake/exhaust systems of the CIS and Motronic era 911s with ITBs and sport exhaust makes a very big difference.

And...they are the bomb.....

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Compared to caveman carbs, yes ! If you already have the components, should be "cheap" as well compared to an EFI setup.

But check if you still have the original pistons. The heads are designed for CIS fuel mixing.

This is mostly true - You'll definitely need to find out if the other key components were changed when the carbs went on - cams and pistons. If so, you'll not be pleased with the result of returning to CIS. If those parts have been changed you'll like the results better with an EFI system that can be tuned to match the cam profiles and P/C combo - one of the better ones is Rasant, but be prepared to spend thousands.
If the original CIS cams, p/c are still in place you'll like your engine much better with the CIS back in service. They are quirky and very high precision systems, but when properly maintained deliver very usable street power / torque and excellent mileage. Word of caution - do not attempt to break open the fuel distributor or other parts yourself. They're tricky and need the right touch. Send it to CIS flow tech or similar for the restoration work. Also consider upgrading the WUR to the later vacuum type.

If I had an engine set up with a system other than CIS, and I even contemplated for a second intentionally installing that dinosaur fuel delivery system, I would bischslap myself silly and give myself about ten swift kicks to the nutz (different than pNutz :eek:) in the hopes of shocking some sense into my cranium! :D

Caution!
 

I’m seeing a lot of “maybes”.
Do you know what cams are installed? Big cams can cause reversion in the intake system, confusing the CIS fuel distributor causing running problems.
Check your wiring harness to see if wiring is there for the WUR, thermo time switch, throttle pressure regulator and start up injector. I’ve probably left a couple more out, but you get the idea. Take a look at the general condition of the harness. I had to replace mine because my dad had the car in SoCal, with AC, and the rubber insulation was crispie and cracked.

The PMO set up I was referring to is a PMO individual throttle body fuel injection system, using a Megasquirt fuel and ignition control and it should get better mileage than the old CIS, (if I can keep my foot out of it).

Do you know how the car ran before the carbs were installed? The CIS May have had some problems, so, sending it out to someone like CIS Flowtech, would be a good idea. The system is almost 50 years old, and as has been mentioned, changing to a later Warm Up Regulator would be a good idea as well. The injectors could benefit from cleaning, testing or replacement.

You might get very lucky and drop the CIS in, as is, but IMHO, iffy.

Or, sell the carbs, go with the Tibits/CIS electronic fuel injection, OR, go with one of several EFI , ITB systems out there.
Good luck,
chris

I'm a big fan of CIS, especially the early non-lambda setup. It's dead simple once you know what you're doing and very reliable as there's very little electronics or moving parts. With a few tweaks you can make decent power with great throttle response.

With that being said, if you can swing it financially, go with ITBs. All of the awesomeness of carbs (HP/sound) with great fuel mileage and tuneability.