turning the untouchable screw CIS

[handdrawnsquare]

hi you all, i hope that i can get some help here. for reasons that i now deeply regret i played with the throttle stop screw a few weeks ago. no real good experiences since.

i was wondering if any body could check their throttle body and tell me how big a feeler gauge you can slide down vertically between the plate and throttle housing.

i now understand why it is not to be played with. the stop screw does leave the plate open somewhat, enough for a quantity of air to pass. if this is mostly fixed (small variances are made with the larger idle screw) then the variable position of the mixture screw can only have a limited position where the gas flow is right for the fixed air flow.

now that my air flow is not fixed (almost infinite possibilities) the corresponding right spot for the fuel can be found for each position i chose for the throttle plate but the height of the plunger changes in the fuel distributor.

we all know that small changes with the mixture adjusting screw (which changes the plunger height in relation to the air sensor plate) had big effects on the running of the motor.

in short i have ben going through those infinite possibilities with no real luck.

so if a few members with a throttle body kicking around can you give me that measurement. maybe if we had few we could get a fixed or average height.

thanks in advance. by the way i have 1975 911S. it has a relatively early cis system

someone on the forum used to say" nothing like driving blood, sweat, and tears." now i understand!!

[boyt911sc]

The gap between the butterfly valve and the throttle body in the first picture below is so small (air gap) but uniform all around. Hard to see the gap in the picture.



The same throttle body but with some backlit to enhance the gap.



The same throttle body but with some backlit to enhance the gap and overhead lighting partially switched off.



The backlit does a good job of enhancing the gap between the valve and the throttle body housing. A way to set the gap correctly is removed it from the motor and do the adjustment on the bench using a light source (flashlight) in my case.

The position of the camera over the throttle body plays the visual effect with actual gap size specially in a dark environment (last pic). Good luck.

Tony

[john walker's workshop]

In my opinion, the stop screw is there to keep the throttle plate from digging into the aluminum housing bore. Needs to have just enough clearance to not contact the bore. The air bypass screw deals with the idle air flow.

[handdrawnsquare]

hi there, great photo. the gap seems to be much bigger than the 0.004 that i thought it should be. how big is the gap? when you put the gap as you said "to have just enough clearance to not contact the bore" not enough air gets into the engine.

i had a revelation last night and i am going to attack the problem in reverse.

porsche tells me how to do basic setting with the mixture screw. i will try to set my airflow to that instead of setting the mixture to the airflow.

that real measurement between the plate and throttle body would be good though. i would like to know if the cars are sett all the same or if there is a variance.

i feel i am getting close !!! are we having fun yet???

[boyt911sc]

Robert,

I was also wondering what would be an acceptable gap clearance between the butterfly valve and throttle body. To satisfy my curiosity, I cut two (2) strips from an index card and used them as gauges. One (1) strip has a slight drag when you pull it out. Two (2) strips won’t slide @ 12 & 6 o’clock orientation. So, one (1) strip of index paper is a good starting point. The paper strip thickness was approx. 0.0067” (0.17 mm).



Tony

[handdrawnsquare]

thanks fro the measurement. i assume that the throttle body came out of a running car.

if i use the porsche factory setting for the mixture screw, rich until the injectors just start then 1/2 turn back and the car just won't run. i end up leaning out the mixture by almost 1 full turn. the car runs smoothly enough but it seems a lack power. even worse the car runs very very hot.

today i decided to go back to the factory mixture screw setting and open the throttle plate to match.i will keep you posted. i am battling other issues with the cis mainly a persistant hot start problem. i'm hoping a new fuel pump will sort that out.

thanks again

r

[boyt911sc]

Originally Posted by handdrawnsquare

thanks fro the measurement. i assume that the throttle body came out of a running car.

if i use the porsche factory setting for the mixture screw, rich until the injectors just start then 1/2 turn back and the car just won't run. i end up leaning out the mixture by almost 1 full turn. the car runs smoothly enough but it seems a lack power. even worse the car runs very very hot.

today i decided to go back to the factory mixture screw setting and open the throttle plate to match.i will keep you posted. i am battling other issues with the cis mainly a persistant hot start problem. i'm hoping a new fuel pump will sort that out.

thanks again

r

Robert,

Test and confirm that the CIS component/s is defective before replacing it. Replacing one component after the other will not solve your problem/s. Well, maybe if you are lucky. But why are you replacing the FP? Simply because it is old?

Tony

[handdrawnsquare]

hi tony, i am changing the fuel pump for two reasons. one, i have never changed it and two it makes quite a bit of noise.

i just rebuilt the engine. the injection system worked fine before. now no hot start.

many years ago i bought a second hand cis system. it was cheaper than the two hoses i needed at the time so now i can swap out parts to check components.

here is where i am now :
-rebuilt fuel dist.
-new wur
-new injectors
-new accumulator
- verified the cold start valve
- verified and cleaned the non return valve at fuel pump.
-all fuel lines verified and blown out with compressed air

still no hot start.
i have read and studied all the information i can find on the cis system and i have the porsche factory manuals.

the fuel pump is sort of a "hail mary"

i have a strong feeling that it is my intervention that has caused the problem but i don't know where.

i don't have pressure test equipment. i'm doing this by sound and intuition.

i can pull the cis system off the car in about 20-30 min now if i need to.

i like to think that i am reasonably intellegent but i'm at the wall here. i just don't know what to do next.

as i am not a rich guy, you might even say i'm the inverse i have been working on this car for over 18yrs. i'm at the end now and the car is beautiful. it just doesn't work.

today i will again pull the cold start valve to see if it is leaking (i have already tested it) but other than that i'm at a loss of what to do next.

any help would be great.

thanks for all the help

[boyt911sc]

Originally Posted by handdrawnsquare

hi tony, i am changing the fuel pump for two reasons. one, i have never changed it and two it makes quite a bit of noise.

i just rebuilt the engine. the injection system worked fine before. now no hot start.

many years ago i bought a second hand cis system. it was cheaper than the two hoses i needed at the time so now i can swap out parts to check components.

here is where i am now :
-rebuilt fuel dist.
-new wur
-new injectors
-new accumulator
- verified the cold start valve
- verified and cleaned the non return valve at fuel pump.
-all fuel lines verified and blown out with compressed air

still no hot start.
i have read and studied all the information i can find on the cis system and i have the porsche factory manuals.

the fuel pump is sort of a "hail mary"

i have a strong feeling that it is my intervention that has caused the problem but i don't know where.

i don't have pressure test equipment. i'm doing this by sound and intuition.

i can pull the cis system off the car in about 20-30 min now if i need to.

i like to think that i am reasonably intellegent but i'm at the wall here. i just don't know what to do next.

as i am not a rich guy, you might even say i'm the inverse i have been working on this car for over 18yrs. i'm at the end now and the car is beautiful. it just doesn't work.

today i will again pull the cold start valve to see if it is leaking (i have already tested it) but other than that i'm at a loss of what to do next.

any help would be great.

thanks for all the help

Robert,

You have done a lot with the rehabilitation of your CIS. You have the patience and perseverance of a dedicated person. Working on this car for the past 18 years is a serious commitment and longer than some of the marriages of some couples. Hope you don’t celebrate the silver anniversary (25 years) for troubleshooting your CIS problems (just kidding).

Let’s get to the point. It is not how hard you work but how smart you tackle the job. Don’t interpret the last sentence literally. I am just trying to get your attention. On the contrary, you are a smart and intelligent individual that would need some convincing and persuasion to take the right direction to attain your goal. If you want to be successful in CIS troubleshooting, stop doing guess-work diagnosis. That will make a big difference in your problem solving process.

If I were in your shoes, I would stop what you were doing and ask yourself why am I not able to get this damn CIS to run like most people do in this forum? You could have all the books you want but if you don’t have the hands on experience you will struggle along the way.

This is my advice:
Get a decent set of CIS pressure gauge and start using it. A pressure gauge is a mandatory tool for fuel injection troubleshooting. Without one, you are wasting a lot of your valuable time.

You mentioned that you had the fuel distributor and warm up regulator rebuilt. If I ask you now what are the cold control and system pressures? Would you know? Then how do you know that both FD & WUR were done correctly? These are just the basic information you must know. Otherwise, you are hoping everything is OK. And if you are lucky, you’re good. But you can not rely on luck all the time.

Good luck.

Tony

[handdrawnsquare]

hi boyt911sc,
i know you are right for the pressure gauges. so i don't know if i have the correct pressures but i am assuming that i do. with the hand throttle i can feel the change in control pressure.

the wur is simply a type of variable bleed off valve. i have spare WURs but changing them out doesn't make a difference and they seem to be working fine. i have added a few shims as a test (and then taken them out) to boost control pressure. other than a micro change with the mixture screw everything was the same.

i did recheck the start valve as a place where i was loosing pressure but no leaks. in fact i can find no fuel leak anywhere.

a few years back i changed the accumulator and have tested it with compressed air and it seems fine too.

i just can't seem to find the problem. i just might have to invest in some pressure gauges

thanks for all the input