Fix CIS or Upgrade to Weber,PMO?

mdanna · 03-19-2002, 02:21 AM

I have a 81 911sc ROW, MSD ign. The car has cold start problems even after replacing accumulator, fuel pressure ragulator....it also appears that the air box is cracked.

The question is should I spend the money and time to replace the airbox and possibly the fuel pump, hunt down all air leaks, try and stop the embarassing backfires when starting or replace the whole thing with weber or PMO carbs?

Where is a good place to get the carb conversion kit. I would like to use Pelican but don't see the carbs listed.

wckrause · wckrause's Garage

Get a new air box, put in a pop-off valve. Then get your CIS running the way it's suppose to run, and you won't have any more backfires. It will be far cheaper than carbs, and you'll have a better, more reliable system. (There's no such thing as UPGRADING to carbs).

A few questions:

Why do you think you need a new fuel pump?

Why did you replace the accumulator and WUR?

mdanna · 03-19-2002, 04:45 AM

I am suspecting the fuel pump because after driving it to op temp, if I let it sit for about an hour it has a hard time starting. It doesn't sputter or run rough it just seems not to be getting gas. I have to pump the accel to get it up to idle. This seems to point to lack of fuel pressure at start or fuel draining out of the system when sitting. It could also be the injectors leaking.

wckrause · wckrause's Garage

Or it could be that you have a cracked air box and your mixture is too lean for the engine to start, or you might just need to replace the check valve.

Your CIS will not work properly with a major vacuum leak like a cracked air box. There's no point replacing anything that is only suspected of being bad until you replace the air box.

After you get the airbox replaced, get a CIS pressure tester and measure system pressure, control pressure, and pressure retention. I doubt your fuel pump is bad if you can drive it to "op temp" without a problem.

NJJim · 03-19-2002, 06:01 AM

I converted my 3.0L from CIS to carbs.... In fact I have a complete ROW CIS in great condition if you are interested (678-297-1149)

A word on carbs: I upgraded to 40mm PMOs - I also changed the cams to early "S". and the crank to a 3.2. After some fine tuning and jetting - my car is VERY fast. I'm very happy and it sounds like a real 911.

Expensive ? YES - I wouldn't upgrade without makeing at least the cam change. Also - count on never passing emissions again if your 911 is not exempt.

Dependability ? I think carbs are just fine since they are by far more simple then CIS. But remember, there is no "cold start" assistance or choke - so when the wife tries to start your car to move it out of the driveway, floods it and fouls your plugs - be gentle with her ;-)

My opinion ? I love my current setup and it produces in the 275hp range. It's a matter of taste, but I love the feel of carbs and cams in a classic 911. - but it comes at a cost $$

Jim

Vipergrün · 03-19-2002, 08:42 AM

mdanna, do you happen to have a pop-off valve in your airbox? If so, make sure the O-ring is seated correctly and that the flapper can shut all the way and seal correctly. I had my O-ring come loose once and wedge the flapper open causing a vacuum leak.

cheers
-Brad
80SC

Rich911E · Rich911E's Garage

I have a complete 1979 CIS injection system (air box, etc.) that you can have for mega-cheap if you want to come get it out of my garage by next week. I think I also have the fuel pump somewhere as well.

I am in San Diego, CA.

Rich

Formerly Steve Wilkinson · 03-19-2002, 12:15 PM

NJJim, I did pretty much the same thing you did to an SC engine--CIS to PMOs and S cams--but went to 46mms because I went up to 3.3+ liters and twin-plugged it. Love to get some tuning tips from you, as I think I'm still a little too rich. Give me an e-mail if you're willing to spare the time.

Stephan

mdanna · 03-19-2002, 01:12 PM

Pop-off valve is in place and appears to be seating correctly. The airbox looks like it has a fine crack next to where the pop-off valve is (just left of centerline) where two screws are. It was there when I bought it so I can't vouch for install job as I have found multiple things done out of spec. I have been replacing items as I find problems and I'm only left with poor cold starting and hesitant warm starting on occasion.

Hladun · 03-19-2002, 02:22 PM

If you want to spend a ton of money on carburetors, you can stop here, but if you want to fix your CIS system cheap, read on.

I've had CIS system's on three cars I've owned (including my 78SC) and they are rugged, reliable and, very importantly, fixable systems. Nevertheless, the Porsche implementation has a major flaw.

The reason your car backfires at startup is the auxiliary air valve. I've made several posts on Rennlist about this topic. (Search by my member number 4493.) My SC never (and I mean never) backfires on startup with the AAV removed. I use the car as a daily driver in temperatures as low as -20F and it always starts. Removing the AAV is a must and I just can't believe that after 30 years this is still a problem.

Besides this it's a given that the basic components must be working. The suggestion to test the fuel pressures is excellent. This is a very comprehensive test and is a must to troubleshoot the fuel side of the system.

wckrause · wckrause's Garage

How does the AAV cause the car to backfire? My car doesn't backfire either, but I still have an AAV.

Hladun · 03-19-2002, 07:12 PM

Reply to wckrause

The AAV is normally open and closes on manifold vacuum the instant the engine starts. (Not to be confused with the AAR.) This causes a very lean mixture when the engine first fires and hence the backfire.

It's great your SC doesn't backfire, but there are a lot of cracked airbox stories around that indicate most CIS equipped cars do backfire. (mdanna's does.) My car backfired before I took the AAV out and now it doesn't.

There's also the cold start surging that the AAV cases.

wckrause · wckrause's Garage

I don't understand how the AAV effects the mixture whether it's opened or closed. Any air that goes through it is metered by the sensor plate, so the mixture shouldn't change.

My car used to backfire. One time it blew the pop-off valve completely off. But I fixed it by getting the WUR and the thermovalve working properly.

FranksPlace2 · 03-20-2002, 05:14 AM

Thank you to Bill Krause for pointing out the airbox must seal.

I just bought a '77 911S and after I drove it home from St. Louis, it was hard to start and would not idle.

Turns out one of the straps broke and the box was loose. Runs great now!

Frank

sammyg2 · 03-20-2002, 07:23 AM

The warm start problem is due to the fuel in the injectors/fuel lines boiling and causing vapor lock.
The system is designed to maintain pressure on these lines for a period of time (at least a half hour, hopefully more). This pressure raises the boiling point of the fuel so that it does not boil. It should be around 40 psi at shut off, climb slightly after a few minutes, then gradually drop off to about 20 psi after a half hour or so.
In order to keep the pressure up, you need the following things to function correctly:
1) fuel pump check valve, located at the fuel pump. About $10.
2) pressure accumulator ($65) you said you replaced it, so never mind.
3) Fuel injectors that don't leak. I paid $21 each for them, cheap, and it will help performance if yours are clogged or leaking.
4) no leaks in the fuel lines anywhere. Self explanitory.

If you plan to race the SC only, then do the carbs. You will pick up a little more HP.
If you plan to drive the car on the street, fix the CIS.
When it is running right, it is a great system.
I bought my car 6 months ago, and it had several problems, one of which was the warm start problem.
I spent a total of about $380 on the engine and it runs perfect now.
I replaced the spark plug wires, injectors, check valve, accumulator, and filter. Starts right up warm or cold, and runs with no stumbles at all.

It is very important to eliminate all vacuum leaks. The air box is a biggie, and the pop off valve has already been mentioned.
When you get it running right, you will love the CIS system. Simple, reliable, and functional. (just my opinion).

Hladun · 03-20-2002, 07:32 AM

wckrause is right, the AAV is after the air meter and the system should properly maintain richness, but taking out the AAV stopped all backfiring in my car. If I had to offer a technical explanation it's that the AAV probably closes quickly after the engine fires and the transient effects may be a factor. There's a saying that in theory there's no difference between theory and practice, but in practice there is. Taking out the AAV works.

alan911 · 03-20-2002, 07:46 AM

Gotta ask.....after taking out or bypassing the AAV what do you do with the vacume lines ??? Plug them or connect them together ???

Hladun · 03-20-2002, 10:58 AM

http://www.pelicanparts.com/911/911_Parts/1978-83/1-7-2.JPG

You remove 39 and block 44 and 48. Do not connect them because you'll get a high idle speed.

alan911 · 03-20-2002, 01:29 PM

O.K. will give it a shot... Thanx for the diagram.

slakjaw · 03-20-2002, 02:30 PM

i have webber carbs in good working order and i will trade them to you for your cis