CIS vs webers
 

Hi All,

I have a 1977 911 which has the standard CIS / K-jetronic system on it. It has two problems, one being a hesitation or stumble after about 2000rpm, and also it has a hot start problem.

I am wondering whether or not to even bother with the CIS system, i.e. are they problematic but can be fixed?

Or, should i just switch straight to carbs? I am in Aus and there are no emmissions regs that i know of.

The car is new to me, and i am in the process getting it ready to be registered.

All advice greatly appreciated.

Chris Jensen in Aus

They're pretty reliable. Both of your problems sound like they could be fixed by adjusting the mixture. It takes a 3.5mm allen, there's a small hole between the fuel distributor and the intake boot on top of the CIS. I can't remember which way is which, lean or rich but I always seem to be able to remember when I get my head in the engine compartment.
It's easy to say that the injection is the problem but so often it seems that the problem ends up being something basic and common to most engines, cap & rotor, spark plugs, vacuum leaks, whatever. Something not so mysterious, something you can see and hear and feel.

Your cold start problem may be that the 7th injector (the one inhte throttle body) is not being activated when you start he car. This can be bacause it has become unplugged or the microswitch that is powered by the starter is not working.

OK thanks guys.

I have only just bought the car and i hope it is an easy fix solution. If the cis is fairly reliable then i will give it a go for a while. I do like the simplicity of carburettors, and the home maintainability. I like the fuel efficiency of the cis.

The car needs a full service anyway.

Chris

If it is hard to start when hot, then the mixture is probably too lean. The hesitation at 2000 revs can possibly relate to the mixture as well. You have to rotate the mixture-adjuster to the the right to fatten it up.

My car will start oscillating at idle if the mixture gets too rich. The best ting is to get it adjusted to the correct co2 by a mechanic. You have to find the correct values for your car. They may be higher than you think...

Adjusting carbs is probably harder than getting your CIS right.

Thankyou all again... I love these BBS's!!!

Can someone also tell me where to find the dipstick?

I know this is a silly question but i can't find an answer from previous posts.

TIA<

CHris

The dip stick is in the oil tank underneath the oil filler cap, if it is not there someone has stolen it lol. Remember check the oil at operating temperature with the car at idle.

Goodluck if you go the weber route, I tried to find a set here in Aus, $2500Aus and they were worn out.

I'm in Melbourne also Christojen, are you in the Porsche Club Vic. ?

CIS is very reliable, not to mention better gas mileage. on the other hand, carbs sound sooo good and a little better power and throttle response. both have negatives, cis can be intimidating and is very sensitive to air leaks. both need special tools if you plan on doing all repairs and maintenance yourself.
CIS you will need a fuel pressure tester and you will need a synchrometer for carbs. carbs have 6 air volume, 6 mixture and 2 idle screws and linkage adjustments. CIS has 1 mixture and 1 idle screw.
get the bosch book and read about CIS if want to keep CIS.

your hot start could be
1 check valve
2 fuel accumulator
3 WUR (warm up regulator)

stumble could be mixture, timing, the vacuum enrichment to the warm up regulator(WUR), tune up or vacuum leak or bad control pressure.

Checking the oil, remember 3 things-
Warm
Idling
Level
This is a dry sump engine so the oil level is checked in the oil tank. It holds approximately 12 quarts so the oil must be warm, it expands as it's heated. If it's not running the level on the dipstick means nothing. All that oil can slosh around quite a bit so the car must be level.

Switching to carbs(Webbers?) will cost big $$$$$ compared to getting the CIS correctly adjusted.

Then, the carbs will continue to cost you more $$$$ than CIS, because the carburators will get roughly 2/3 the fuel milage of the CIS injection system.

Moot point if you are wealthy.

Carbs are far more diffucult to tune up than CIS. As mentioned above, but well worth emphasizing:
carbs - 6 air volume screws, 6 air/fuel mixture screws, 2 idle screws and linkage adjustments.
CIS - 1 air/fuel mixture screw and 1 idle screw.

It only sounds like you need to find a competent Porsche mechanic who knows how to correctly tune the engine up; valve adjust, ignition settings, fuel pressure check, air/fuel adjustment.

It's too bad CIS always takes the blame for ignition problems. Even a hot start problem which is usually caused by a bad fuel pump check valve is blamed on CIS which somehow seems unfair.

Really? I've spent more than $AUD7,000 trying to sort out my CIS issues, between half a dozen so-called CIS specialists and even Porsche Brisbane. One well known Brisbane specialist finally got the car running reasonably well, but it still does not run as well as other CIS cars I've driven. I wish I had bitten the bullet and bought a set PMOs instead, which would have "only" cost about 6 grand installed.

:rolleyes:

I switched to Webers a few years ago when I gave up on my CIS and it's been trouble-free and reliable since. My CIS was cobbled-together and was so jury-rigged that to fix it would cost more than the Webers conversion. Specifically, the fuel distributor was shot and need to be rebuilt and the injectors leaked, throttle microswitch was bad, etc...

I'm very glad I switched to Webers and would recommend it to others whose CIS is too far gone to repair.

Sheldon, Art Z.,

Sorry to hear about your CIS mechanic troubles. Our 2 CIS cars have forced me, by their shear reliability and economy, to be a CIS stalwart.

'73.5 2.4T-CIS engine has 280K miles on it now, 90K on it when bought in 1987.
'79 3.0-CIS engine has 318K miles on it now, 67K on it when bought in 1989.

We have put a combined 441K miles on the 2 CIS Porsches since buying them.

I just checked through my maintenance records spread sheet for CIS issues over the past 21 years, and found CIS related entries for:

tune ups
replace 2.4L fuel distributor (engine sat unused for a few years before I bought it)
replace manifold rubber boots (twice both engines)
replace injectors (once both engines)
replace fuel pump (once both engines)
replace fuel accumulator (3.0L engine)
re-glue air box flapper (once both engines)

That's it. I am genuinely dumbfounded by how trouble-free and bullit-proof these CIS engines have been.

Now, that stupid smog pump on the 3.0L sure has been a different story!

cis also starts well in cold weather

it was great 1970s technology for everything except a sports car

CIS is great 70's technology. Webers are great 50's technology.

I've got 170,000 on my CIS car, but I did have two problems. One was an electrical short on a microswitch. Even bigger was was vacuum leak caused by an aluminum tube around the back of the engine that chafed agains the oil cooler fiberglass cover. Eventually had to do a partial engine drop and pull the injection off to get to it.

CIS engines have very mild cams with no overlap, so they make good daily drivers. Not the most sporting thing in the world though.

Regards,

Jerry Kroeger

Agree. Most common CIS hot start prob is the fuel pump check valve.

You might check this out - Pelican Porsche Tech articles.

http://www.pelicanparts.com/911/technical_specs/911_cis_troubleshoot.htm