CIS Decel Valve Adjustment - 75 2.7L

[fanaudical]

What is the proper adjustment for the CIS decel valve on a 75 with the 2.7L?

I have found some anecdotal mention here on the forums that the bypass should crack with 20" Hg vacuum on the actuator side, but can't find anything definitive in the manuals that I have.

Does anybody have a real Porsche manual that states a definitive adjustment value or procedure?

Thanks for the help.

[fanaudical]

bump - Is this not documented anywhere?

[ClickClickBoom]

I did my 2.7 seat of the pants. Used a brake bleeder vacuum gun, made sure it was opening. Then installed and warmed up the car, started blipping the throttle up to 2500 RPMs or so and when the idle was almost dropping to stall, I loosened the screw on top of the decel valve about a turn or turn and a half until the throttle blipping came down to 400 RPMs or so and settled at the 650 RPMs that was normal. Took it out and drove it till it was smoking hot, 220f or so and did the throttle blip test and adjusted until I was satisfied with the results. Took maybe 1-2 hours including warm up and the drive. My 2.7 ran like a top fueler, I was sad to see it go. The CIS system is actually pretty good at what it does. I regularly fiddled with mine to compensate for season and fuel quality, just 1/16th of a turn on the fuel screw made a world of difference!
Good luck

[fanaudical]

Thanks for the tip. I've got mild bucking at part throttle and am trying to rule out the decel valve. I guess the easy thing to do is disconnect/plug the vac source for the decel actuator and see if things magically get better.

[boyt911sc]

Originally Posted by fanaudical

Thanks for the tip. I've got mild bucking at part throttle and am trying to rule out the decel valve. I guess the easy thing to do is disconnect/plug the vac source for the decel actuator and see if things magically get better.

fanaudical,

After disconnecting and plugging the vacuum line to the decel valve, keep us posted if the problem went away. There is a diaphragm/membrane inside the decel valve that could deteriorate and cause vacuum leak. If you want to determine if your decel valve is leaking or not, test it for vacuum. If you don't have the set-up to do the test, I'll be more than happy to do it for you for FREE. Well, you spend for the postage.

Tony

[ClickClickBoom]

Originally Posted by fanaudical

Thanks for the tip. I've got mild bucking at part throttle and am trying to rule out the decel valve. I guess the easy thing to do is disconnect/plug the vac source for the decel actuator and see if things magically get better.

Bucking can be caused mu a couple of things motor mounts is one, and the throttle bellcrank on the left side of the engine is another, check the left top of the engine for another relay assy:

Throttle bellcrank!


Idle bounce/hunting

Idle bounce/hunting


CIS question?

[fanaudical]

Thanks for the note on the bell-crank; I hadn't considered that throttle mechanical linkage could be causing some bucking. This is a new build and so almost anything is possible. I'm going down the path of validating that I don't have a vac leak somewhere first. System/control/WUR pressures are good. Filter is new with blown out lines and a cleaned tank. I haven't checked spray pattern yet. I'll update after some more troubleshooting later tonight.

[fanaudical]

Confirmed that throttle linkage is not sloppy. Motor/trans mounts are new with the rebuild. I tried to shake the engine around just to make sure that nothing is moving and all seems tight. Ignition is good (don't see any issues with erratic timing or advance).

Bucking is there at lower rpm/low throttle right from cold. I warmed up the car and then disconnected/plugged vac line to the decel valve with no improvement. I reconnected decel valve vac line and then disconnected/plugged vac line to the WUR - and that made almost all the bucking go away. It also made it seem like the decel valve got adjusted (more "hang time" on rpms). I verified the decel valve diaphragm is intact.

Check my thinking here:

Disconnecting my vac line to the WUR essentially reduces control pressure and enriches the mixture. For that to help, I must have extra air getting in someplace (i.e. vac leak that I haven't found). The bucking I'm experiencing at part-throttle is a too-lean condition because I've got air bypassing the air flow sensor (which also negates an issue with the decel valve bypass circuit, as all that air goes past the air flow sensor).

Sound right?